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IntroductionIn electronics, what is clipper? A circuit which removes the peak of a waveform is known as a clipper. Clipper circuit is designed to prevent a signal from exceeding a predetermined reference voltage level. The clipper circuit can be designed by utilizing both the linear and nonlinear elements such as resistors, diodes, or transistors. The diode clipper, also known as a diode limiter, is a wave shaping circuit that limits positive or negative amplitude, or both. In electronics, diode clipper circuits are commonly used to process various signals. It is is a circuit designed to prevent a signal from exceeding a predetermined reference voltage level. Clipping changes the shape of the waveform and alters its spectral components.Clipper Circuits IntroductionCatalogIntroductionⅠ Clipper Circuit Types1.1 Positive Clipper Circuit1.2 Negative Clipper Circuit1.3 Combinational Limiter CircuitⅡ Clipper Circuits Analysis2.1 Clipper Circuit Structure2.2 Clipper Circuit ProblemsⅢ General Forms of Clipper Circuits3.1 Clipper Circuit Description3.2 Common Clipper Circuit ExamplesⅠ Clipper Circuit TypesDiode clipper is a limiting circuit which limits the output voltage. In electronics, a clipper is a circuit designed to prevent a signal from exceeding a predetermined reference voltage level. A basic diode limiter circuit is composed of a diode and a resistor. It is divided into three types: positive clipper circuit, negative clipper circuit and combinational clipper circuit. The positive clipper circuit produces a clipping effect when the input voltage is higher than a certain upper limit value; the negative clipper circuit produces a limit effect when the input voltage is lower than a certain lower limit value; the combinational clipper circuit produces a limit effect when the input voltage is too high or too low. In a positive clipper, the positive half cycles of the input voltage will be removed. During the negative half cycle of the input, the diode is forward biased and so the negative half cycle appears across the output. The clipper circuits are described as following.1.1 Positive Clipper CircuitThe diode in clipper circuit is connected in series to the input signal and that attenuates the positive portions of the waveform. The positive clamping circuit blocks the input signal when the diode is forward biased. During the negative half cycle of an AC signal, the diode is forward biased and allows electric current through it. In following figure, when the input signal voltage is lower than a preset upper limit voltage, the output voltage will change with the input voltage, however, when the input voltage reaches or exceeds the upper limit, the output voltage will remain at a fixed value, so that the signal amplitude is limited at the output.1.2 Negative Clipper CircuitThe diode in clipper circuit is connected in series to the input signal and that attenuates the negative portions of the waveform, is termed as negative series clipper. For the figure below, the diode is series to the input and output. If the diode has ideal switching characteristics, when iu is lower than E, D will not conduct, ou=E; when ui is higher than E, D will conduct, ou=iu. The limiting characteristic of this limiter circuit is shown in the figure.1.3 Combinational Limiter CircuitThis kind of circuit combines the positive and negative limiters together which shows in the following figure. Ⅱ Clipper Circuits Analysis2.1 Clipper Circuit StructureIn the circuit, Al is an integrated circuit (a common component), VT1 and VT2 are transistors, Rl and R2 are resistors, and VDl to VD6 are diodes.Analyzing the effect of VD1 and VD2 in the circuit mainly explains the following points.1) It can be seen from the circuit that the circuit structure of the two groups of diodes are the same. Both play the same role in this circuit, so the working principle of them are the same.2) The pin ① is connected to the base of the transistor VT1 through a resistor Rl. Obviously Rl is a signal transmission resistor. The signal output on the pin ① is added to the base of VT1 through Rl (there is no DC blocking capacitor between pin ① and VT1). From this circuit structure, it can be judged that the pin ① is an output signal pin, and it outputs a composite signal of DC and AC. The purpose of determining that the pin ① is to figure out the specific function of the diode VD1 in the circuit.3) The DC voltage output by pin ① is not high enough to make the external diode in a conducting state. The analysis is: if the DC voltage output by the pin ① is high enough, then VD1, VD2 and VD3 conduct, and the internal resistance becomes small. This will shunt the AC signal output by the pin ① to the ground, so the signal will be attenuated. However, this circuit does not need such attenuation. Therefore, the conclusion drawn from this: VD1, VD2 and VD3 are not turned on by pin ① DC voltage output.4) The output from pin ① is the superimposed signal of DC and AC, which is added to the base of the transistor VT1 through the resistor Rl. VT1 is an NPN transistor. If the amplitude of the positive half-cycle AC signal added to the base of VT1 is very large, which may burn the VT1. When the negative half-cycle signal added to the base of VT1 is large, which has no effect on VT1, because the negative signal on the base of VT1 reduces current.Follow the above circuit analysis, it can be judged that VD1, VD2, and VD3 in the circuit has clipper function, to prevent VT1 from burning out. 2.2 Clipper Circuit ProblemsIn the figure, Ul is the DC voltage in the output of pin ①, U2 is the limiting voltage value.When the AC voltage in the output signal of pin ① is relatively small, the positive half cycle of the AC signal plus the DC output voltage does not make the VD1, VD2 and VD3 conduction. Therefore, all diodes are cut off, which has no effect on the AC signal output by pin ①. Assuming that the positive half-cycle output AC signal by pin ① is very large during a certain period, as shown in the signal waveform, at this time it plus the DC voltage can conduct VD1, VD2 and VD3. If the conduction voltage of each diode is 0.7V, then three diodes is 2.1V. Since the tube voltage drop after conduction is basically the same, that is, the maximum voltage of pin ① is 2.1V. So the excess part of the positive half cycle of the AC signal is limited by the resistor. When the DC and AC output signals at pin ① is less than 2.1V, diodes will not conduct and keep cutoff state, which has no clipping effect on the signal.For the specific details of clipper circuit, there are several explanations as follows.1) The negative half cycle large signal output by the pin ④ will not cause VT1 overcurrent, because it will decrease the base voltage of the NPN transistor and the base current, so there is no need to add the limiter circuit.2) The one-way limiter circuit mentioned above, it can only limit the large signal part of the positive or negative half of the signal, and does not limit the signal in the other half. The other is the combinational limiter circuit, which can limit the positive and negative half-cycle signals at the same time.3) There are many reasons for the abnormal increase of the signal amplitude. For example, the fluctuation of the power supply voltage cause it to increase a lot at a certain moment, and the large-scale interference pulse from the outside into the circuit also causes a certain increase.4) After the three diodes VD1, VD2 and VD3 conduct, the sum of the DC and AC voltages on pin ① is 2.1V. This voltage added to the base of VT1 through resistor Rl is maximum, so as to the current of VT1.5) Since the pin ① is the same as the external circuit of pin ②, the working principle of the limiter circuit is the same. So only one circuit needs to be analyzed when analyzing the circuit.6) According to the characteristics of the series circuit, the current in the series circuit is equal everywhere. It can be known that the three series diodes VD1, VD2 and VD3 are turned on at the same time, or they will be turned off at the same time. Therefore, in the series circuit, a diode is turned on and other diodes are turned on.Ⅲ General Forms of Clipper Circuits3.1 Clipper Circuit DescriptionThere are two types of clippers namely series and parallel. In series clipper, diode is connected in series with the load. In parallel clipper, diode is in parallel to the load.1) Series clippers: if the diode is connected in series with load resistanceUnbiased series clipper: in that case the circuit diode is connected in series with load resistance and no external voltage is applied to the circuit.+ve unbiased series clipper: if the +ve portion of output is clipped its called +ve unbiased series clipper.-ve unbiased series clipper: if the -ve portion of output is clipped its called +ve unbiased series clipper.Biased series clipper: if in the circuit, diode is connected in series with load resistance and external voltage is applied to the circuit+ve biased series clipper: if the +ve portion of output is clipped its called +ve biased series clipper.-ve biased series clipper: if the -ve portion of output is clipped its called +ve biased series clipper.2) Parallel clippers: if the diode is connected in parallel with load resistanceUnbiased parallel clipper: in that case the circuit diode is connected in parallel with load resistance and no external voltage is applied to the circuit.+ve unbiased parallel clipper: if the +ve portion of output is clipped its called +ve unbiased parallel clipper.-ve unbiased parallel clipper: if the -ve portion of output is clipped its called +ve unbiased parallel clipper.Biased parallel clipper: if in the circuit, diode is connected in parallel with load resistance and external voltage is applied to the circuit+ve unbiased parallel clipper: if the +ve portion of output is clipped its called +ve biased series clipper.-ve unbiased parallel clipper: if the -ve portion of output is clipped its called +ve biased series clipper. 3.2 Common Clipper Circuit ExamplesIn general, clippers circuit are classified into two types: Series Clippers, Shunt Clippers, and Dual (Combination) Clippers.Series Clipper: The diode is connected in series with the load resistance. 👇Figure 1. Series Positive ClipperThe positive amplitude waveform is cut, and the negative amplitude waveform is retained, as follows:Figure 2. Series Positive Clipper with Positive BiasThe positive amplitude waveform is cut, and the offset positive voltage is retained on the negative amplitude waveform, as follows:Figure 3. Series Positive Clipper with Negative BiasThe waveform of positive amplitude is cut, and the negative voltage is shifted based on the waveform of negative amplitude, as follows:Figure 4. Series Negative ClipperThe negative amplitude waveform is cut, and the positive amplitude waveform is retained, as follows:Figure 5. Series Negative Clipper with Positive BiasThe negative amplitude waveform is cut, and the positive voltage is offset on the positive amplitude waveform, as follows:Figure 6. Series Negative Clipper with Negative BiasThe negative amplitude waveform is cut, and the negative voltage is offset on the positive amplitude waveform as follows: Shunt Clipper: Diode is in parallel with load resistance in circuit. 👇Figure 7. Shunt Positive ClipperFigure 8. Shunt Positive Clipper with Positive BiasFigure 9. Shunt Positive Clipper with Negative BiasFigure 10. Shunt Negative ClipperFigure 11. Shunt Negative Clipper with Positive BiasFigure 12. Shunt Negative Clipper with Negative Bias Dual (Combination) Clipper: It is desired to remove a small portion of both positive and negative half cycles. 👇Figure 13. Combination ClipperWhen the positive and negative waveforms must be limited, a combinational limiter circuit is required, as follows:Images Reference: Clipper Circuits - Series Clipper, Shunt Clipper, and Dual Clipper Frequently Asked Questions about Diode Limiter and Clipper Circuit1. What is Clipper and clamper?The major difference between clipper and clamper is that clipper is a limiting circuit which limits the output voltage while clamper is a circuit which shifts the DC level of output voltage. ... While clamper is used when we need multiples of the input voltage at the output terminal. 2. What is the function of clipper circuit?In electronics, a clipper is a circuit designed to prevent a signal from exceeding a predetermined reference voltage level. A clipper does not distort the remaining part of the applied waveform. 3. What is Clipper circuit and its types?A clipper is a device which limits, remove or prevents some portion of the wave form (input signal voltage) above or below a certain level, in other words, the circuit which limits positive or negative amplitude ,or both is called chipping circuit. The clipper circuits are of the following types. Series positive clipper. 4. What is the difference between a positive clipper and a negative Clipper?Positive Clipper and Negative Clipper. In a positive clipper, the positive half cycles of the input voltage will be removed. ... During the negative half cycle of the input, the diode is forward biased and so the negative half cycle appears across the output. 5. How does diode clipping work?The Diode Clipper, also known as a Diode Limiter, is a wave shaping circuit that takes an input waveform and clips or cuts off its top half, bottom half or both halves together. This clipping of the input signal produces an output waveform that resembles a flattened version of the input. 6. What is the main purpose of a diode limiter?The diode limiter also called Clipper as it is used to limit the input voltage. A basic diode limiter circuit is composed of a diode and a resistor. Depending upon the circuit configuration and bias, the circuit may clip or eliminate all or part of an input waveform. It limits the output voltage to a specific value. 7. What is the purpose of a clamping diode?The clamping circuit fixes the voltage lower limit to zero, that is, the start of the signal is 0 V. The positive clamping circuit blocks the input signal when the diode is forward biased. During the negative half cycle of an AC signal, the diode is forward biased and allows electric current through it. 8. What is a diode clamping circuit?A clamper circuit shifts the DC level or the reference level of the signal to the desired level without changing the shape of the waveform. The clamper circuit can be designed using the diode, resistor, and the capacitor.

IntroductionIn-memory computing (IMC), a technique of future computing, stores data in RAM to run calculations entirely in computer memory. With the rise of the big data era, faster data processing capabilities are required. Computer memory and storage space are also growing exponentially to adapt to large-capacity data collection and complex data analysis, which promotes the development of AI (artificial intelligence), and then derives emerging stuff, that is, in-memory computing.In-memory Computing (IMC) ExplainedCatalogIntroductionⅠ Memory Wall: Processor /Memory Performance GapⅡ Developing RequirementⅢ What Is In-memory Computing?3.1 In-memory Computing Definition3.2 Four Realization MethodsⅣ Driving Force of In-memory Computing and Market Prospects4.1 In-memory Computing for AI4.2 In-memory Computing Product Outlook4.3 In-memory Computing Market and ProspectⅤ ConclusionⅠ Memory Wall: Processor / Memory Performance GapThe von Neumann architecture has occupied the dominant position in computer system when the computer invented. This kind of calculation method is to store the data in the main memory first, and then fetch the instructions from the main memory to execute them in order when running. We all know that if the connecting speed of the memory cannot keep up with the performance of the CPU, the computing will be limited. This is a memory wall. At the same time, in terms of efficiency, the von Neumann architecture also has obvious shortcomings. It consumes more energy to read and write data than to calculate once time.Figure 1. Von Neumann Architecture DiagramThe performance of computer processors has developed rapidly based on Moore's Law, and has been directly improved with the invention of transistors. The main memory of the computer uses the DRAM. It is a high-density storage solution based on capacitor charging and discharging. Its performance (speed) depends on two aspects, namely the reading/writing speed of the capacitor charging and discharging in the memory and the interface bandwidth between the devices. The read/write speed of capacitor charging and discharging has increased with Moore’s Law, but the speed is not as fast as the processor. In addition, the interface between DRAM and the processor is a mixed-signal circuit, and its bandwidth increasing speed is mainly restricted by the signal integrity of the traces on the PCB. This has also caused the performance improvement of DRAM to be much slower than that of the processor. At present, the performance of DRAM has become an huge bottleneck of overall computer performance, the so-called "memory wall".  It blocks the computing performance improvement.Figure 2. Moore's Law Effect Ⅱ Developing RequirementIn the current AI technology, with the increasing amount of data and calculations, the original von Neumann architecture is facing more and more challenges. Rely on expanding CPU, the hardware architecture can’t have a large amount of calculation. Also the larger storage capacity is heavily rely on the past architecture, it is also very unsuitable for AI. When the memory capacity is large to a certain extent, it can only show that certain technologies need innovation. In order to solve the "memory wall" problem, future computers are not based on computing memory, but the in-memory computing, thereby reducing the cost of data access in the calculation process.Figure 3. Conventional Computing vs In-memory Computing Ⅲ What Is In-memory Computing?3.1 In-memory Computing DefinitionIn-memory computing (or in-memory computation) is a technique based on RAM data storage and indexing, which proposed by the MIT research group, and its main purpose is to accelerate the convolution calculation. We know that convolution calculations can be expanded into weighted accumulation calculations. From another perspective, it is actually a weighted average of multiple numbers. Therefore, the circuit realizes the weighted average of the charge domain. The weight (1-bit) is stored in SRAM, and the input data (7-bit digital signal) becomes an analog signal through the DAC. According to the corresponding weight in the SRAM, the output is multiplied by 1 or -1 in the analog domain, which averaged in the analog domain, and finally read out by the ADC as a digital signal. Specifically, since the weight of the multiplication is 1-bit (1 or -1), it can be controlled by using a switch and a differential line simply. If the weight is 1, the capacitor on the side of the differential line is charged to the required output value. Otherwise, let the other side of the differential line be charged to this value. As for average, connect several differential lines together in the charge domain.Of course, there is more than one circuit for in-memory calculation, and the calculation accuracy is not limited to 1-bit. However, we can see the above examples that the core idea of in-memory calculations is generally to convert calculations into weighted calculations. Store the weights in the memory unit, then modifications on the core circuit of the memory (such as the readout circuit) are made. So that the process of reading is like a process in which the input data and weights are multiplied in the analog domain, that is, convolution. Because convolution is a core part of AI and other calculations, in-memory computing can be widely used in such applications. In-memory computing uses analog circuits for calculations, which is the difference compared with traditional digital logic calculations.In more traditional architectures, there are some multiply-accumulate circuits (MAC) for tensor math, especially the matrix multiplication. These architectures attempt to arrange the MAC in a way that moves weights and activations to the appropriate location. Activations are calculated from the previous neural network layer. Multiplication usually involves activations and weights, both must be moved to the place where multiplies them. In-memory computing makes use of it. Therefore, if the weights are stored in memory, the memory can access through activations to obtain multiplication and accumulation. The only difference from the actual memory is that the in-memory computing concatenates all word lines at once, instead of decoding the input to get one word line only.Figure 4. In-memory Computing Diagram3.2 Four Realization MethodsThe attempt is to enter the analog domain and treat the storage unit as an analog unit instead of a digital unit to reduce consumption. We have already got a way to use simulation on the front end of the inference engine. That is in-memory computing. Therefore, we take digital data, using a DAC to convert it to an analog value, and then driving a memory with these analog content to obtain an analog bit-line output, finally using an ADC to convert the result back to a digital format. However, the in-memory computing is still in the exploratory stage, and there are many specific implementation methods to study, currently there are three types: RRAM, Flash, SRAM, and DRAM.Based on RRAMRRAM is the most common method of doing this, because it is easy to use by applying Ohm's law to a series of resistors, but it still has the problem of relying on RRAM. The relationship between programming and resistance is non-linear, which requires more work to be done to make viable calculation circuits in RRAM memory for market. So it is just an idea, and the specific plan is still under study. Based on FlashNOR Flash memory has a more traditional word-line/bit-line structure. It is both resistive and capacitive. Generally, the memory cell is a transistor that is turned on or off. However, if it is partially conductive, it can be used as a resistor. The resistance depends on the amount of charge on the floating gate of the memory cell (capacitor). When running all the time, the cell will conduct to its maximum capacity. During this process, it does not conduct at all, however, it can be partially programmed. There is a problem is that you cannot precisely control the number of electrons. Moreover, the response to any number will vary with the process and temperature and other variables.Two companies are studying this method. Microchip owns their memBrain array, thanks to their acquisition of SST, and Mythic is a start-up company dedicated to an inference engine that uses in-memory computing with flash memory. Both companies said that they are using extensive calibration techniques to deal with this change.Another issue, flash cells will lose electrons over time. Electrons will flow around, which brings up an interesting topic: on this type of memory array, data retention and durability will be like.From the application point of view, it depends on whether it is to be used in cloud computing or edge inference engine. At the edge, it may perform certain fixed reasoning functions throughout the life cycle of the device. Therefore, if there are enough arrays, then you will load the weights for the first time and don't need to program it anymore (unless you do a update), because the flash memory is non-volatile. Although you still need to move activations, there is no need to move the weights, which will be stored permanently in the array. This would indicate that data durability (number of times the device can be programmed before cumulative damage accelerates electron leakages to an unacceptable level) does not matter, it only need to program once.In contrast, in cloud applications, the device is likely to be shared as a general-purpose computing resource, so this requires reprogramming for each new application. This means that battery life becomes more important in the cloud. Mythic claims to have a 10K write cycle, and has observed that even if it is reprogrammed every day, it will last for more than 10 years.If set an analog value for it and use an analog value in the cell, then in theory, each electron is important. However, if there is enough electron migration, you need to refresh the storage unit, or compensate for electrons change in some way. Because the same analog input today will produce different results than a year ago. The calibration circuit can also deal with some aging problems. However, for data retention, Mythic said they do perform regular updates of the weight values stored in flash memory. This will make persistence the main wear-out mechanism rather than data retention. Microchip stated that its data retention time is TBD, but it is likely to reprogram the device quarterly or annually to restore the unit.So they need a large number of high-quality ADCs and DACs to keep the signal-to-noise ratio (SNR) within a scope of accurate reasoning, which is the focus of designing work. Mythic claims that they provide a novel ADC, so that Microchip can share it to reduce the number required. Although ADC does consume energy, it also greatly reduces overall system consumption. Based on SRAMThis idea came from a lecture at Hot Chips at Princeton University. By definition, SRAM is a bistable unit. Therefore, it cannot be in an intermediate state, how should this be handled? And the DACs and ADCs that need to be corrected more over than the array in terms of area and power consumption.The point of this problem boils down to the question of how to simulate. They explained that this method uses more than one-bit line for calculation. Since the unit is still a digital value, it takes several bit lines to perform a calculation. The bit line can be split, and different groups perform different multiplications. The following figure illustrates it.Figure 5. Bit LineWith 8 inputs at a time, so the input vector is sliced and several consecutive multiplications are carried out to obtain the final results. The bit line charge is deposited on the capacitor. When ready to read, the charge is read out and sent to the ADC for conversion back to the digital domain. Their basic unit structure is as follows:Figure 6. Bit CellThese capacitors may affect chip size issues, but they said that the metal above the cell can be used. Of course, one cell is now 80% larger than the standard 6T SRAM cell (even without capacitors), but they say that their overall circuit is still much smaller than a required circuit based on standard digital implementation. In addition, since their basic array operations are still in digital form, they are less sensitive to noise and changes, which means their ADCs can be simpler and consume less power.Figure 7. Chip SizeBased on DRAMThis idea refers to not using a lot of power to obtain DRAM content, and in some way incorporate calculations into the CPU or other computing structures and directly run it on the DRAM die, which is what UPMEM does. A simple processor is built on the DRAM die, also the architecture will not compete with Xeon chips, they call this set "processing in memory" or PIM.Figure 8. PIM ChipInstead of bringing data to calculations, they bring calculations to data. The runtime is performed by the CPU in DRAM chip. That is, there is no need to move the data to any location outside of the DRAM chip, just send the calculating result back to the host system. Also, since ML calculations usually involve a lot of reduction, less data required for calculations. Although this does require some minor changes to the DRAM, they did not change the manufacturing process. Under this case, a standard DRAM module will provide multiple opportunities for distributed computing. At the same time, it becomes complicated to use this function to write a program.They said that a server using PIM offload will consume twice as much power than a standard server connected to a DRAM module without PIM. However, with a throughput of 20 times, it still provides them with a 10 times energy efficiency advantage. In addition, this method can help defend against side-channel security attacks. Thus a group of computing threads originally contained in one or more CPUs flows to DRAM. Therefore, it is necessary to check all DRAMs and figure out where thread is in some way, but this will be a difficult task. Ⅳ Driving Force of In-memory Computing and Market Prospects4.1 In-memory Computing for AIPeople have recognized the problem of "memory wall" for a long time, but why is in-memory computing only raised in the past two years? So we have to analyze the boost behind its rise.The first motivation is the rise of AI based on neural networks, especially the hope that AI can be popularized in mobile and embedded devices. So that in-memory computing with a high energy efficiency ratio has attracted attention. In addition, neural networks have a high tolerance for errors in calculation accuracy. Therefore, errors introduced in simulation calculations of in-memory computing can often be accepted. That is to say in-memory computing and AI are good partners for each other.The second motivation is the new memory. For in-memory computing, the memory characteristics often determine the efficiency of in-memory computing. Therefore, new memories improvement will often drive the development of in-memory computing. For example, the recently popular ReRAM uses resistance modulation to store data, so the readout of each bit uses a current signal instead of a traditional charge signal. In this way, it is a very natural operation for current to accumulate (combining several currents directly to achieve the sum of currents, even without additional circuits). That is to say, ReRAM is very suitable for in-memory calculations. From the perspective of memory promotion, new memories are also willing to catch up with the AT trend. Therefore, new memory manufacturers are also happy to see in-memory computing based on their own memories to accelerate AI development, which will broaden the memory market. 4.2 In-memory Computing Product OutlookChip products for in-memory computing are expected to come in two forms. The first form is sold as a memory IP with computing functions. Such memory IP may be traditional SRAM, or new memory such as eFlash, ReRAM, MRAM, and PCM.The second form is to directly build AI acceleration chips based on in-memory calculations. For example, Mythic plans to make PCIe accelerator cards based on flash memory, that is, access data with the main CPU through the PCIe interface. The weight data is stored on the Mythic memory chip, so that when the data is sent to the Mythic IPU, the calculation can be directly read out. In this way, the action of reading the weights data is eliminated.Figure 9. Mythic is a Pcie Accelerator 4.3 In-memory Computing Market and ProspectWhat impact will in-memory computing have on the AI chip market? First of all, we see that in-memory computing uses analog calculations, so its accuracy will be affected by the low signal-to-noise ratio. Usually the upper limit of accuracy is about 8-bit, and it can only do fixed-point calculations not the floating-point calculations. So in-memory computing is not suitable for the AI training market that requires high calculation accuracy. In other words, the main battlefield of in-memory computing is the AI inference market. For example, it is more suitable for embedded artificial intelligence, which has high requirements for energy efficiency not the accuracy. In fact, in-memory computing is actually most suitable for occasions where large memory is needed. For instance, flash is inherently required in IoT and other scenarios, so if you can add the in-memory computing to flash, it is quite suitable. However, introducing in-memory computing in a large storage memory may not appropriate. Based on this analysis, we believe that in-memory computing may become an important part of embedded AI (such as smart IoT) in the future. Ⅴ ConclusionWith the rise of AI and new memories, in-memory computing has also become a new hot spot. Based on the unique characteristics of the memory, it combines with analog calculations in memory, thereby greatly reducing the memory read and write operations in AI. Although the accuracy of calculation in the memory is limited by analog calculation, it is also suitable for embedded AI applications that pursue energy efficiency most and can accept a certain loss of accuracy. Frequently Asked Questions about In-Memory Computing Technology1. Why do we need in memory computing?In-Memory Computing provides super-fast performance (thousands of times faster) and scale of never-ending quantities of data, and simplifies access to increasing numbers of data sources. 2. What does in memory mean?An in-memory database is a type of purpose-built database that relies primarily on memory for data storage, in contrast to databases that store data on disk or SSDs. ... Because all data is stored and managed exclusively in main memory, it is at risk of being lost upon a process or server failure. 3. How does in memory computing work?In-memory computing means using a type of middleware software that allows one to store data in RAM, across a cluster of computers, and process it in parallel. Consider operational datasets typically stored in a centralized database which you can now store in “connected” RAM across multiple computers. 4. What is in memory computing in SAP HANA?An In-Memory database means all the data from source system is stored in a RAM memory. In a conventional Database system, all data is stored in hard disk. It provides faster access of data to multicore CPUs for information processing and analysis. 5. How is data stored in memory?Normally memory is described as a storage facility where data can be stored and retrieved by the use of an address. This is accurate but incomplete. A computer memory is a mechanism whereby if you supply it with an address it delivers up for you the data that you previously stored using that address. 6. What is in memory data processing?In-memory processing is the practice of taking action on data entirely in computer memory (e.g., in RAM). ... Since the storage appears as one big, single allocation of RAM, large data sets can be processed all at once, versus processing data sets that only fit into the RAM of a single computer. 7. What is in memory database processing and what advantages does it provide?The major advantage of systems using in-memory databases vs traditional database systems is: its performance speed. ... Source data is loaded into the system memory in a compressed and format. Therefore, in-memory processing reduces disk seek time for accessing data and streamlining the work involved in processing queries. 8. What is big data computing?Big data computing is an emerging data science paradigm of multi dimensional information mining for scientific discovery and business analytics over large scale infrastructure. ... Big data is characterized by 5V's such as volume, velocity, variety, veracity, and value.

IntroductionAs we all know, the most basic passive linear components are resistors (R), capacitors (C) and inductive components (L). These components can be used to form 4 different circuits: RC circuit, RL circuit, LC circuit and RLC circuit. They have some important properties for analog electronics, and can be used as passive filters. In practice, capacitors (and RC circuits) are usually used instead of inductors to form filter circuits. This is because capacitors are easier to manufacture with smaller size. This article mainly introduces the RC Circuit in series and parallel state.RC circuit (resistor–capacitor circuit), also called RC filter or RC network, has a resistor and a capacitor in series connection. When connected to a DC voltage source, the capacitor charges exponentially in time. That is, a capacitor can store energy, and when a resistor placed in series with it will control the rate at which it charges or discharges. This produces a characteristic time dependence that turns out to be exponential.RC Circuits Basic ExplainedCatalogIntroductionⅠ RC Circuit Basics1.1 What is RC Circuit?1.2 RC Circuit CharacteristicsⅡ How to Calculate RC Circuit?Ⅲ RC Circuits Classification3.1 Series and Parallel Circuits3.2 Example: RC Low Pass FilterⅣ Visualizing Filter Response4.1 Frequency Response4.2 Low Pass Filter Phase Shift4.3 Second-order Low-pass FilterⅤ ConclusionⅠ RC Circuit Basics1.1 What is RC Circuit?For a RC circuit (resistor-capacitor circuit), the primary composes of a resistor and a capacitor. According to the arrangement of resistors and capacitors, it can be divided into a RC series circuit and a RC parallel circuit. In addition, simple RC parallel circuits cannot resonate, because resistor does not store energy. However, LC parallel circuits can resonate. RC circuits are widely used in analog circuits and pulse digital circuits. If a RC parallel circuit connected in series in the circuit, it can attenuate low-frequency signals, and if it connected in parallel in the circuit, it can attenuate high-frequency signals. That is filtering.RC circuit is common element in electronic devices. It also play an important role in the transmission of electrical signals in nerve cells. A capacitor can store energy and a resistor placed in series with it will control the rate at which it charges or discharges.Figure 1. Passive Low-pass RC Circuit1.2 RC Circuit CharacteristicsIn the analog circuit, the passive RC filter circuit can be divided into a low-pass filter circuit and a high-pass filter circuit according to the connection and size of the capacitor.The low-pass filter circuit is somewhat equal to the integrator circuit (capacitor C is in parallel at the output.), but both circuits are applied to different requirements. The integrator circuit mainly uses the integration effect of the capacitor C when it is charged. In the case of square wave input, periodic sawtooth wave (triangular wave) will generate, so the capacitor C and resistor R are selected according to the square wave. While the low-pass filter circuit bypasses the higher frequency signal (because XC=1/( 2πfC), when f is larger, XC is smaller, which is equivalent to a short circuit), so the value of capacitor C is determined by referring to the value of the low frequency. For the filter circuit of the power supply, theoretically the larger the value of C, the better.Figure 2. Low Pass Filter CircuitThe high-pass filter circuit has the same form as the differential circuit or the coupling circuit. In the pulse digital circuit, due to the different relationship between RC and pulse width, it is divided into a differential circuit and a coupling circuit. In an analog circuit, choosing an appropriate capacitance C value can pass higher frequency signals selectively, even block DC and low-frequency signals. For example, a capacitor connected in series with a tweeter, is to prevent the low pitch from entering the tweeter to avoid burnout. What’s more, in the multi-stage AC amplifier circuit, the high-pass filter circuit is also a coupling circuit.Figure 3. High Pass Filter CircuitⅡ How to Calculate RC Circuit?From a mathematical point of view, suppose that the RC circuit has been connected to a DC power supply with a voltage value of U0. The voltage on the capacitor is equal to the power supply’s, and at a certain moment t0 the left end S of the resistor is grounded, then the capacitor discharges. In the theoretical analysis, the time t0 is taken as the zero point of time.According to KVL's law, establish the circuit equation: The initial condition is .This is a first-order homogeneous differential equation, and its general solution is .After substituting into the original equation: The characteristic equation is .The characteristic root is .According to , get .Therefore, the required initial value of the differential equation is It can be seen that the voltage attenuation speed on the capacitor depends on the , and its size only depends on the circuit structure and component parameters.When the unit of resistance is Ω and the unit of capacitance is F, the unit of product RC is seconds (s), which is represented by τ, then the capacitor voltage can be written as .tτ2τ3τ4τ5τ...∞uc(t)Uo0.368Uo0.135Uo0.05Uo0.018Uo0.0067Uo...∞0The τ time constant is the time it takes for the capacitor voltage to drop to 1/e=36.8% of the initial value. Specifically, it is the time required to charge the capacitor, through the resistor, from an initial charge voltage of zero to approximately 63.2% of the value of an applied DC voltage, or to discharge the capacitor through the same resistor to approximately 36.8% of its initial charge voltage. When t=4t, the capacitor voltage is very small, and it is generally considered that the circuit enters a steady state, which is also called the zero input response of the RC first-order circuit. Ⅲ RC Circuits Classification3.1 Series and Parallel CircuitsRC Series CircuitIn circuit, the capacitor cannot flow DC current, and R & C have an obstructive effect on the current. So the total impedance is determined by the resistance and capacitive reactance, and it changes with frequency. RC series circuit has a turning frequency: f0=1/2πR1C1. When the input signal frequency is greater than f0, the total impedance is basically unchanged, and it is equal to R1.RC Parallel CircuitThe RC parallel circuit can pass both DC and AC signals. It has the same turning frequency as the RC series circuit: f0=1/2πR1C1. On the one hand, when the input signal frequency is less than f0, the total impedance of the circuit is equal to R1, on the other hand, when the input signal frequency is greater than f0, the capacitive reactance of C1 is relatively small, and the total impedance is the sum of resistance and capacitance. In addition, when the frequency is high to a certain level, the total impedance is zero.Introduction to Parallel RC CircuitWhat’s more, as frequency increases, the capacitor will act like a short circuit to high frequency current in its path. At low frequencies, the capacitor tends to block current flow.3.2 Example: RC Low Pass FilterCircuit AnalysisTo create a passive low-pass filter, we need to combine the resistor elements with the reactance elements. That is a circuit consisting of a resistor and a capacitor or an inductor. Theoretically speaking, the RL low-pass topology is equivalent to the RC low-pass topology in terms of filtering ability. However, in practice, RC circuits are more common.Figure 4. RC Low-pass FilterAs shown in the figure, connecting a resistor in series with the signal path and a capacitor in parallel with the load, an RC low-pass response can be generated. In the figure, the load is a single part, but in actual circuits, it may be more complicated, such as the input stage of an analog-to-digital converter, amplifier, or oscilloscope to measure the response of the filter.If a resistor and a capacitor form a frequency-dependent voltage divider circuit, we can intuitively analyze the filtering function of the RC low-pass circuit.Figure 5. Change RC Low-pass Filter into a Voltage DividerWhen the frequency of the input signal is low, the impedance of the capacitor is high than the resistor. Therefore, most of the input voltage will drop on the capacitor (and both ends of the load, which is in parallel with the capacitor). When the input frequency is higher, the impedance of the capacitor is lower than the impedance of the resistor, which means that the resistor voltage decreases and less voltage is transferred to the load. Therefore, low frequencies pass and high frequencies are blocked.Cutoff FrequencyWhere the filter does not cause significant attenuation for a frequency range is called the passband, and the opposite is called the stopband. Analog filters, such as RC low-pass filters, always gradually transit from the passband to the stopband. This means that it cannot be recognized that the filter stops passing the signal and starts blocking one frequency of the signal. This is why the cutoff frequency concept introduced.When checking the frequency response graph of the RC filter, the signal spectrum is "cut" into two halves of the image, one of which is retained and one is discarded. Because as the frequency moves from below the cutoff point to above the cutoff value, the attenuation gradually increases.The cut-off frequency of the RC low-pass filter is actually the frequency at which the input signal amplitude is reduced by 3dB (this value is chosen because a 3dB reduction is equal to a 50% reduction in power). Therefore, the cutoff frequency is also called -3dB frequency. The term bandwidth refers to the width of the passband of the filter. For a low-pass filter, its bandwidth is equal to the -3dB frequency (as shown in the figure below).Figure 6. Cutoff Frequency -3dBFilter Response CalculationWe can discuss the theoretical behavior of the low-pass filter by a typical voltage divider. The output of the resistor divider is expressed as following:The RC filter uses an equivalent structure, using a capacitor XC replace R2. Then we need to calculate the total impedance and place it in the denominator, so there is The reactance of a capacitor represents the opposite amount of current, but unlike resistance, the opposite amount depends on the frequency of the signal passing through the capacitor. Therefore, we must calculate the reactance at a specific frequency. The equation we use for this as follows: In the above design example: R≈160Ω and C=10nF. We assume that the magnitude of VIN is 1V, so we can simply remove VIN from the calculation. First, let's calculate the amplitude of VOUT with a sine wave frequency: While suppressing noise, the amplitude of the sine wave is basically unchanged. Because the cutoff frequency (100kHz) we chose is much higher than the sine wave frequency (5kHz).Let’s see how the filter successfully attenuates the noise component.The noise amplitude is only about 20% of its original value. Ⅳ Visualizing Filter Response4.1 Frequency ResponseThe most convenient way to assess the effect of a filter on a signal is to examine the frequency response graph. That is Bode plot, which has amplitude (in decibels) on the vertical axis and frequency on the horizontal axis; the horizontal axis usually has a logarithmic scale so that the physical distance between 1Hz and 10Hz is the same as 10Hz to 100Hz and 100Hz to 1kHz. This configuration allows us to quickly and accurately evaluate the behavior of the filter over a large frequency range.Figure 7. Bode PlotEach point on the curve represents the amplitude that the output signal is 1V and the frequency is equal to the corresponding value on the horizontal axis. For example, when the input frequency is 1MHz, the output amplitude (assuming the input amplitude is 1V) will be 0.1V (because -20dB corresponds to a tenfold reduction factor).The curve in the passband is almost completely flat, and then as the input frequency approaches the cutoff frequency, it starts to drop faster. Finally, the rate of change of attenuation becomes stable, that is, for every ten times the input frequency increases, the amplitude of the output signal decreases by 20dB. 4.2 Low Pass Filter Phase ShiftThe way in which the filter modifies the amplitude of various frequency components in the signal has been discussed above. However, in addition to amplitude effects, reactive circuit elements always involve phase shifts.The concept of phase refers to the value of the periodic signal at a specific moment in the cycle. Therefore, when we say that a circuit causes a phase shift, we mean that it creates a misalignment between the input signal and the output signal. That is the input and output signals no longer start and end their periods at the same time. The phase shift value, such as 45° or 90°, indicates how much misalignment has been created.Each reactance element in the circuit introduces a 90° phase shift, but this phase shift does not occur at the same time. The phase of the output signal is the same as the amplitude of the output signal, and it changes gradually as the input frequency increases. In the RC low-pass filter, we have a reactive element (capacitor), so the circuit will eventually introduce a 90° phase shift.As with the amplitude response, the phase response can be most easily evaluated by examining the graph on the horizontal axis which represents the logarithmic frequency. The following description is the general pattern.The phase shift is initially 0°, and it gradually increases until it reaches 45° at the cutoff frequency. During this part of the response, the rate of change is increasing. With time, the phase shift continues to increase, but the rate of change is decreasing. As the phase shift approaches 90°, the change of rate becomes very small.Figure 8. Phase Shift4.3 Second-order Low-pass FilterAs above mentioned, we have assumed that the RC low-pass filter consists of a resistor and a capacitor. This configuration is a first-order filter. The "order" of passive filters is determined by the number of reactive components (ie capacitors or inductors) in the circuit. Higher-order filters have more reactive components, which lead to more phase shift and steeper roll-off.Second-order filters are usually built a resonant circuit consisting of inductors and capacitors (this topology is called "RLC", or resistor-inductor-capacitor circuit). However, it is also possible to create a second-order RC filter. As shown in the figure below, all we need to do is to cascade two first-order RC filters.Figure 9. Second-order Filter CircuitAlthough this topology can produce a second-order response, it is not widely used. Because its frequency response is usually not as good as a second-order active filter or a second-order RLC filter.Frequency ResponseWe can try to create a second-order RC low-pass filter by designing a first-order filter based on the required cutoff frequency, that is connecting two first-order stages in series. This set has a similar overall frequency response, with a maximum roll-off of 40dB/decade instead of 20dB/decade.However, we cannot simply connect these two stages together and analyze the circuit as a second-order low-pass filter. In addition, even if we insert a buffer between the two stages so that the first RC stage and the second RC stage can be used as independent filters, the attenuation at the original cut-off frequency will be 6dB instead of 3dB. Because the two stages work independently.Figure 10. Frequency Response of RC-RC FilterA limitation of the second-order RC low-pass filter is that the designer cannot tune the conversion from passband to stopband by adjusting the Q factor (this parameter indicates the degree of damping of the frequency response.) of the filter. If two identical RC low-pass filters are cascaded, the overall transfer function corresponds to the second-order response, but the Q factor is always 0.5. When Q = 0.5, the filter is at the boundary of over-damping, which results in a "sag" frequency response in the transition region. While second-order active filters and second-order resonant filters do not have this limitation, designers can control the frequency response of the transition region. Ⅴ ConclusionAll electrical signals contain a mixture of requiring frequency and unwanted ones. Undesirable frequency components are usually caused by noise and interference, and in some cases they have a negative impact on the performance of the system.Filters are circuits that react to different parts of the signal spectrum in different ways. The low-pass filter is designed to pass low frequency components and block high frequency components. The output voltage of an RC low-pass filter can be calculated by considering the circuit as a voltage divider (frequency-independent) composed of resistance and reactance.The graph of amplitude (in dB, on the vertical axis) vs. log frequency (in Hz, on the horizontal axis) is a convenient and effective way to check the theoretical behavior of the filter. You can also use phase and log frequency graph determines the amount of phase shift that will be applied to the input signal.The second-order filter provides a steeper roll-off, and its response is useful when the signal cannot provide broadband separation between the desired frequency and the unwanted one. You can make a second-order RC low-pass filter by connecting two identical first-order RC low-pass filters, but the overall -3 dB frequency will be lower than expected.In RC filtering circuit, the capacitor can store energy, and the resistor placed in series with it can control the charge-discharge rate. And this produces a characteristic time dependence that turns out to be exponential. Frequently Asked Questions about RC Filter Circuit1. What does an RC filter do?RC circuits can be used to filter a signal by blocking certain frequencies and passing others. The two most common RC filters are the high-pass filters and low-pass filters; band-pass filters and band-stop filters usually require RLC filters, though crude ones can be made with RC filters. 2. What is RC filter in electronics?A resistor–capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors. ... A first order RC circuit is composed of one resistor and one capacitor and is the simplest type of RC circuit. 3. How do you calculate RC circuit?The (real value) impedance is the real part of the complex impedance Z. For a series RC circuit, we get Z=√R2+(1ωC)2 Z = R 2 + ( 1 ω C ) 2 . We see that the amplitude of the current will be V/Z=V√R2+(1ωC)2 V / Z = V R 2 + ( 1 ω C ) 2. 4. What is RC circuit used for?The RC circuit has thousands of uses and is a very important circuit to study. Not only can it be used to time circuits, it can also be used to filter out unwanted frequencies in a circuit and used in power supplies, like the one for your computer, to help turn ac voltage to dc voltage. 5. What is RC series circuit?A circuit that contains pure resistance R ohms connected in series with a pure capacitor of capacitance C farads is known as RC Series Circuit. A sinusoidal voltage is applied and current I flows through the resistance (R) and the capacitance (C) of the circuit.

IntroductionIn September of this year, Apple's new generation of watch Apple watch series 6 launched the blood oxygen measurement function. In the following month or two, smart watches with blood oxygen measurement function were released together, vivo WATCH, Huawei Watch GT2, Honor Watch GS Pro, Hua Mi Amazfit GTR 2 and GTS 2 flagship smart watches almost all include this feature. In October, Huami Technology released the Amazfit Pop, which is priced at only 299RMB, and is also equipped with blood oxygen measurement. The blood oxygen sensor has begun to penetrate into low-end and mid-range computers, which may set off a greater wave.So, what exactly is the blood oxygen measurement function on the smart watch? Is it reliable? After you finish reading this article, you will get the answer.CatalogIntroductionCatalogI Blood Oxygen & Blood Oxygen Saturation 1.1 What is Normal Blood Oxygen Level? 1.2 How Important is Blood Oxygen Saturation?II How Does a Smart Watch Measure Blood Oxygen?III What is the use of Measuring Blood Oxygen Saturation?IV Is it Reliable to Measure Blood Oxygen with a Smart Watch?V Comparison with Pulse Oximeter 5.1 What is a Pulse Oximeter? 5.2 How Does a Pulse Oximeter Work? 5.3 Main Components 5.4 Can a Smart Watch be Used as a Pulse Oximeter?VI Is it Necessary to Have an Oximeter at Home During Covid-19? 6.1 Can a Pulse Oximeter Diagnose Covid-19? 6.2 Is it Necessary to Buy a Pulse Oximeter?VII FAQI Blood Oxygen & Blood Oxygen Saturation1.1 What is Normal Blood Oxygen Level?Blood oxygen refers to the oxygen in the blood, and the human body's normal blood oxygen saturation is above 95%. The higher the oxygen content in the blood, the better the human metabolism. Of course, high blood oxygen content is not a good phenomenon. The blood oxygen in the human body has a certain degree of saturation. Too low will cause insufficient oxygen supply in the body, and too high will cause cell aging in the body.Figure1. Blood Oxygen1.2 How Important is Blood Oxygen Saturation?The cells in the human body rely on oxygen to survive. The oxygen that enters the human body through the respiratory system will be combined with hemoglobin and transported to various organs. Blood oxygen saturation is a measure of the percentage of hemoglobin combined with oxygen, so blood oxygen saturation is a key indicator to measure the health of the body. This parameter can be used to understand the oxygen content in the human blood. Under normal circumstances, the normal blood oxygen saturation is between 95% and 100%. If the blood oxygen saturation is less than 90%, it can be considered as hypoxemia. When blood oxygen saturation is too low, it means that there will be hypoxia, which will affect the central nervous system, liver, kidney and other important organs. Therefore, blood oxygen saturation is an important indicator, and the measured data can assist in judging the current physical health.Figure2. How Important is Blood Oxygen Saturation?II How Does a Smart Watch Measure Blood Oxygen?The blood oxygen measurement function of the smart watch actually judges health by measuring the human arterial blood oxygen saturation. The blood oxygen saturation specifically refers to the percentage of hemoglobin combined with oxygen in the blood, that is, the concentration of blood oxygen in the blood. Generally speaking, if the blood oxygen saturation is below 94%, it will be regarded as insufficient oxygen supply. Many clinical diseases will cause insufficient oxygen supply, which directly affects the normal metabolism of cells, and in severe cases, it can be life-threatening. Therefore, blood oxygen testing is very important for clinical medicine. The most primitive method of tracing blood oxygen measurement requires blood sampling first, and then electrochemical analysis by a blood gas analyzer to obtain the blood oxygen saturation. This method has complicated steps and cannot achieve continuous detection. However, with the development of clinical medicine, non-invasive blood oxygen measurement is now widely used. As long as a finger pressure photoelectric sensor is worn for the patient, continuous blood oxygen detection can be realized. The essence is to use red light with a wavelength of 660nm and near-infrared light with a wavelength of 940nm as the intake light source, measure the light transmission intensity through the tissue bed, calculate the blood oxygen concentration and blood oxygen saturation, and display the results by the instrument. The principle of measuring blood oxygen with a smart watch is similar to acupressure measurement, but the difference is that the part illuminated by the light source of the watch is the wrist, which is not as "transparent" as the finger. Visible light and infrared light cannot penetrate, so it is more challenging. However, as a wearable device with a very high frequency of use, the stimulation brought by the development space of smart watches is far greater than the challenges they need to face.Figure3. How Does a Smart Watch Measure Blood Oxygen?III What is the use of Measuring Blood Oxygen Saturation?Because the blood oxygen saturation can be detected anytime and anywhere through smart wearable devices, the application scenarios of blood oxygen detection are very wide. (1) Assist in judging the state of sleep breathingIntermittent apnea may occur during sleep, which may cause insufficient oxygen supply. Through continuous blood oxygen detection, the blood oxygen saturation data during sleep can be recorded, and the data can be used to analyze whether there is hypoxia during sleep, so as to determine the sleep health status. In addition, like some people who often work overtime, they can also use the blood oxygen saturation detection function to judge the current state. If the blood oxygen saturation is low, you may need to take a quick rest, and that is friendly to your health.Figure4. Judging the State of Sleep Breathing by Blood Oxygen Saturation (2) Monitor the physical state during exerciseThe blood oxygen saturation detection function not only allows you to know your physical condition at any time in your daily life, but also can play a better health support role in some special scenes. For example, in outdoor extreme mountaineering and other sports scenes, you can know your physical condition at any time through blood oxygen saturation, determine whether you need to rest or adjust the exercise intensity, so as to better cope with various sports scenes.Figure5. Monitor the Physical State During Exercise (3) Monitor the health of parentsGenerally, the APP on the mobile phone can synchronize the health data of the smart wearable device, so even if you are away from home, as long as you equip your parents with a smart wearable device, you can learn about your parents’ blood oxygen data and some other health data through remotely synchronized data. , adding more details for your concern.IV Is it Reliable to Measure Blood Oxygen with a Smart Watch?At present, the blood oxygen measurement function on a smart watch has not been certified by NMPA or FDA, that is, it has reached the level of medical diagnosis. The blood oxygen monitoring module of the smart watch consists of three parts: an optical sensor, a front-end signal acquisition system, and an algorithm. The principle is based on the different absorption rates of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) in the blood to red light and infrared light, the red light and infrared light are irradiated to the skin through an optical sensor, and then the blood vessels under the skin are obtained. After the reflection of the red light and infrared light, the blood oxygen is calculated by the algorithm. The optical sensor is the core of the blood oxygen detection module, which is usually composed of several LED lights and diodes. Apple's official website shows that the blood oxygen sensor equipped with Apple Watch 6 consists of four groups of LED light clusters and four photodiodes, and is integrated in the crystal glass back. If a smart watch is required to measure both heart rate and blood oxygen, it can be achieved through the same optical sensor. Ams has launched an optical sensor that integrates blood oxygen measurement and heart rate measurement. Li Minghao, field application engineer manager at ams, said, “Heart rate measurement usually uses green light, and blood oxygen measurement uses red light and infrared light. Compared with optical sensors that can only measure heart rate, optical sensors that can simultaneously perform two functions The composition usually adds several red and infrared LEDs.Figure6. Samsung Galaxy Series Smart WatchAs the core of blood oxygen measurement, the accuracy of optical sensors is very important to the accuracy of measurement results. For example, deviations in the wavelength of the red light produced by the diode may cause inaccurate measurement results. "Smartwatch measuring blood oxygen is a signal obtained by measuring capillaries. The signal is very weak. Usually this signal is submerged on it, and the feedback signal may be only about 1% useful. Therefore, the design of optics and filtering is very important." Li Minghao said.Currently, companies in the industry are taking various measures to improve accuracy. The senior technical staff of Huami Technology said, "Any sensor has various non-ideal errors. In terms of control errors, Huami conducts strict testing and evaluation of all key parameters when selecting optoelectronic devices. Each product of Mi has undergone multiple measurements on the production line to ensure that the accuracy of the factory product parameters is within the design range. In addition, at the beginning of the design, select the relatively flat waveband of the human skin spectral absorption curve to limit the wavelength error to the blood oxygen The impact is within an acceptable range." Increasing the number of sensors is also one of the ways to improve accuracy. The vivo watch is equipped with a self-developed 5-core optical heart rate blood oxygen sensor to achieve blood oxygen measurement. "5-core refers to a multi-sensor system composed of 5 sensors, in which the circuit and optical design are the results of vivo's independent research. Compared with the single sensor or multi-sensor design in the industry, it has a larger signal receiving area and receiving capacity. Stronger, can achieve more accurate monitoring effects." said a senior technical staff of vivo. However, judging from the actual effect of the current smart watch for blood oxygen measurement, although there is an improvement in accuracy, there is still a certain gap between achieving medical-level blood oxygen monitoring. When the news that Apple Watch 6's blood oxygen measurement function was inaccurate, Apple came out to clarify that this function is only for health reference, not as a medical diagnosis standard. The accuracy requirements of consumer and medical products are different. Taking the heart rate as an example, there is no problem with a few beats on the wrist, but in medical treatment, there can only be a difference of 1-2 beats.V Comparison with Pulse Oximeter5.1 What is a Pulse Oximeter?Pulse oximeter has been a common medical device since the 1970s. It is most commonly used for people with respiratory diseases and sometimes for athletes and pilots who must monitor blood oxygen levels. They are mainly used for clinical testing and monitoring, but for certain groups of people, they may also be used at home.Figure7. Pulse Oximeter5.2 How Does a Pulse Oximeter Work?Based on the change in light absorption during arterial pulsation. Two light sources located in the visible red spectrum (660 nanometers) and infrared spectrum (940 nanometers) alternately illuminate the tested area (usually fingertips or earlobes). The amount of light absorbed during these pulses is related to the oxygen content in the blood. The microprocessor calculates the ratio of the two spectra absorbed, and compares the result with the saturation value table stored in the memory to obtain the blood oxygen saturation. A typical oximeter sensor has a pair of LEDs that face a photodiode through a translucent part of the patient's body (usually a fingertip or earlobe). One of the LEDs is red light with a wavelength of 660nm; the other is infrared light with a wavelength of 940nm. The percentage of blood oxygen is calculated by measuring the two wavelengths of light with different absorption rates after passing through the body. 5.3 Main ComponentsA microprocessor, memory (EPROM and RAM), two digital-to-analog converters that control LEDs, a device that filters and amplifies the signal received by the photodiode, and an analog-to-digital converter that digitizes the received signal to provide the microprocessor . The LED and photodiode are placed in a small probe that is in contact with the patient's fingertip or earlobe. Pulse oximeters generally also include small liquid crystal displays. 5.4 Can a Smart Watch Replace a Pulse Oximeter?The answer is no. Fitness bracelets (including some Garmin devices) and smart watches with the function of detecting blood oxygen levels cannot be used as medical equipment. Garmin said that blood oxygen saturation can help you understand your body's adaptation to high altitude (especially for mountain sports and adventure), and it can also be a reminder for symptoms of sleep apnea or overwork during exercise. But it also clearly states that these data cannot be used for medical purposes, nor can it diagnose, treat, cure or prevent any disease or condition. The location where the device is worn may affect accuracy. The fitness tracker is worn on the wrist instead of the fingertips, it is easier to move when worn on the wrist, and the blood concentration on the skin surface is lower than that on the fingertips, so the obtained blood oxygen measurement may have a large error. However, although fitness trackers and smart watches are not medical devices, if they do detect abnormalities and alert you, then you should pay attention.Figure8. Apple WatchVI Is it Necessary to Have an Oximeter at Home During Covid-19?One of the powerful aspects of COVID-19 is that some patients with very low blood oxygen levels do not feel it themselves and are not aware of the severity of their illness. This has led some doctors (especially in the United States) to recommend pulse oximeters at home. Sounds reasonable, doesn't it? However, you must know that although they are commonly used in hospitals, their value for healthy people at home is limited. Equipment problems or incorrect use may cause inaccurate readings, so it is not wise to rely solely on oximeters without comprehensive diagnostic support from medical staff. 6.1 Can a Pulse Oximeter Diagnose Covid-19?The pulse oximeter may indicate a problem with the blood oxygen level, which may be related to the coronavirus, but it is only part of a comprehensive diagnosis. Blood oxygen saturation can help clinical decision-making, but it cannot replace clinical evaluation, nor can it be diagnosed alone. Some doctors suggest that for patients with suspected symptoms of the new crown but not serious enough to be hospitalized, they can consider using a pulse oximeter at home for monitoring.Figure9. Covid Prevention6.2 Is it Necessary to Buy a Pulse Oximeter?If you don’t have any breathing problems and have never used it before, then you don’t need to buy a pulse oximeter. It is usually only used if recommended by a doctor. Professor Xu, a clinical assistant professor at the University of Hong Kong and honorary consultant of respiratory and intensive care medicine at the Royal Free Hospital in London, said that pulse oximeters are still useful as early home self-monitoring equipment. Those who are weak, prone to respiratory failure, suffer from chronic respiratory diseases or need oxygen therapy at home can prepare one. Some people want to buy one for a self-test to find out their "normal" oxygen level, just in case. In fact, if you are not a suspected patient and have no symptoms, there is no need to buy one. Without the help of medical personnel, the measured readings are not very useful, and if you have not used them before, misuse or misinterpretation of the numbers is easy to happen. Dr. Andy Whittamore, the clinical director of the British Asthma Association and the British Lung Foundation, suggested: "Any surveillance at home should be part of the diagnosis, but not a substitute for clinical advice. All people who are concerned about their symptoms should see a doctor. In addition, like many coronavirus-related products, such as masks, due to the promotion of pulse oximeters, demand has also begun to increase, so beware of unscrupulous sellers selling substandard products at high prices.  VII FAQ1. Can a smartwatch measure blood oxygen?Thankfully, tech companies like Apple, Samsung, Realme, and Garmin have been experimenting with Sp02 sensors on their wearables to allow users to monitor not just blood oxygen saturation but other health data, on the go. 2. How do smartwatches measure oxygen levels?Both medical and wearable oximeters use light to do so. Typically, a pulse oximeter features two LEDs with different light wavelengths — one red and one infrared. ... Most recent watches and fitness trackers usually have sensors that provide pulse ox sleep tracking too. 3. How is oxygen level in blood measured?A pulse oximeter measures how much light is absorbed by your blood. This tells us how much oxygen your blood contains. The pulse oximeter shines 2 lights through your fingertip or earlobe: one red light and one infrared light.  

IntroductionOperational amplifier (op amp for short) is basically a voltage amplifying device designed to be used with components like capacitors and resistors, between its in/out terminals, or is simply a linear Integrated Circuit (IC) having multiple-terminals. In electronics, the open-loop voltage gain of the actual operational amplifier is very large, which can be seen a differential amplifier with infinite open loop gain, infinite input resistance and zero output resistance. In addition, it has positive and negative inputs which allow circuits that use feedback to achieve a wide range of functions. And meanwhile, it can be further simplified into an ideal op amp model, referred to as an ideal op amp (also called ideal OPAMP).CatalogIntroductionⅠ Ideal Op Amp Characteristics1.1 Infinite Input Resistance1.2 Zero Output Impedance1.3 Infinite Open-loop Gain1.4 Infinite Common-mode Rejection Ratio1.5 Infinite BandwidthⅡ Assumptions of Ideal Op AmpⅢ Working Characteristics of Ideal Operational Amplifiers3.1 Work in Linear Region3.2 Work in Nonlinear RegionⅣ Analysis of Ideal Operational Amplifier CharacteristicsⅤ Balanced Resistance Presets5.1 The Role of Balanced Resistance5.2 Input Balancing Resistor ExplanationⅥ Ideal Op Amp EquationsⅦ Several Common Op Amp CircuitsⅧ Difference Between Ideal Op-amp and Practical Op-ampⅠ Ideal Op Amp CharacteristicsWhen analyzing various application circuits of operational amplifiers, the integrated operational amplifier is often regarded as an ideal operational amplifier. The so-called ideal op amp is to idealize various technical indicators of op amps, and it must have the following characteristics.Characteristics of An Ideal Op-Amp1.1 Infinite Input ResistanceThe input terminal of an ideal operational amplifier does not have any current to flow in. In electronics, op amps are voltage gain devices. They amplify a voltage fed into the op amp and give out the same signal as output with a much larger gain. In order for an op amp to receive the voltage signal as its input, the voltage signal must be dropped across the op amp. If you know the concept of a voltage divider, voltage drops primarily across components with high impedances, proportionally according to ohm’s law by the formula V=IR. So the greater the resistance (or impedance) of a device, the greater the voltage drop across that device is. To make sure that the voltage signal drops fully on the op amp, it must have a very high input impedance, so that the voltage drops fully across it. If it had a low input impedance, the voltage may not drop across it and it would not receive the signal. This is why op amps must have high-input impedances.It’s also easy to make the input impedance lower (put a resistor in parallel) or the source impedance higher (put a resistor in series).Figure 1. Ideal Op Amp Symbol and Transfer Characteristic Curve 1.2 Zero Output ImpedanceThe output of an ideal op amp is a perfect voltage source, no matter how the current flowing to the amplifier load changes, the output voltage of the amplifier is always a certain value, that is, the output impedance is zero. In practice, zero output impedance is actually a distinct property from infinite input impedance, but for a very long time infinite input impedance was approached only with compromises in offset voltage and noise. 1.3 Infinite Open-loop GainIn an open-loop state, the differential signal at the input has an infinite voltage gain. This feature makes the operational amplifier very suitable for practical applications with upper negative feedback configuration. 1.4 Infinite Common-mode Rejection RatioAn ideal operational amplifier can only respond to the difference between the voltages at both ends of V+ and V-. In addition, the same part of the two input signals (ie common mode signal) will be completely ignored. What’s more, a high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong electromagnetic interference (EMI). An example is audio transmission over balanced line in sound reinforcement or recording. 1.5 Infinite BandwidthThe ideal operational amplifier will amplify the input signal of any frequency with the same differential gain, which will not change with the change of signal frequency.Ⅱ Assumptions of Ideal Op AmpThe op amp can be considered a voltage controlled current source, or it is an integrated circuit that can amplify weak electric signals. Based on it, for an ideal OPAMP, what is the relationship between it and these electrical signals?First, assume that the current flowing into the input of the op amp is zero. This assumption is almost completely correct for FET op amps, because the input current for FET op amps is below 1pA. But for dual high-speed op amps, this assumption is not always correct, because the input current of it can sometimes reach tens of microamperes.Second, assume that the gain of the op amp is infinite, so the op amp can swing the output voltage to any value to meet the input requirements. It means that the output voltage of the op amp can reach any value. In fact, when the output voltage is close to the power supply voltage, the op amp will saturate. Maybe this hypothesis does exit, but needs a limit in practical. For example, at higher frequencies, the internal junction capacitors of transistor come into play, thus reducing the output and therefore the gain of amplifier. The capacitor reactance decreases with increase in frequency bypassing the majority of output. The opamp is in saturation state.Figure 2. Op Amp SaturationFor example, as per datasheet of LM741, large signal voltage gain is 200V/mv. It means an open loop gain of 200,000. If you operate an op-amp in open-loop condition(i.e. without negative feedback) ,even microvolts of input voltage (input offset voltage of LM741 is 3mv) will drive the output to saturation.In most of the amplifier circuits op-amp is configured to use negative feedback which greatly reduces the voltage gain (i.e. closed loop gain). In oscillators and schmit triggers, Op-amp is configured to use positive feedback. Comparator circuit is an example of the circuit which utilizes open-loop gain of op-amp. Its output will be always at saturation either positive or negative saturation. In an integrator circuit, the DC gain should be limited by adding a feed back resistor in parallel with capacitor ;else the output will get saturated .Even in amplifier circuits, the amplitude of the input signal and the voltage gain of the circuit should be balanced so that the output voltage does not exceed power supply voltage . For example for a non-inverting amplifier with a voltage gain of 100, the maximum permissible input voltage will be 150 mv if the VCC is 15 Volts. If you apply a signal of 200 mv ,the op-amp output will goto saturation as the required output will be 20 volts which exceeds the VCC of 15 Volts.Third, the assumption of infinite gain also means that the input signal must be zero. The gain of the op amp will drive the output voltage until the voltage (error voltage) between the two input terminals is zero. The voltage between the two input terminals is zero. The zero voltage between two input terminals means that if one input terminal is connected to a hard voltage source like ground, the other input terminal will also be at the same potential. In addition, since the current flowing into the input terminal is zero, the input impedance of the op amp is infinite.Fourth, of course, the output resistance of an ideal op amp is zero. An ideal op amp can drive any load without any voltage drop due to its output impedance. At low currents, the output impedance of most op amps is in the range of a few tenths an ohm, so this assumption is true in most cases. Ⅲ Working Characteristics of Ideal Operational Amplifiers3.1 Work in Linear RegionWhen the ideal op amp works in the linear region, the output and the input voltage show a linear relationship. Where u0 is the output voltage of the integrated op amp; u+ and u- are the voltages at the non-inverting input terminal and the inverting input terminal, respectively. Auo is the open loop differential voltage magnification. According to the characteristics of the ideal op amp, two important characteristics of the ideal op amp in the linear region.1) Zero differential input voltageSince the open-loop differential voltage magnification of an ideal op amp is equal to infinity, and the output voltage is a certain value, the voltage values at the non-inverting input terminal and the inverting input terminal are approximately equal. Just like short circuit between input and output, but it is fake. Because it is an equivalent short circuit, not a real short circuit, so this phenomenon is called "virtual short".2) Zero input currentSince the open-loop input resistance of an ideal op amp is infinite, no current flows into the op amp at either input. At this time, the current at the non-inverting input terminal and the inverting input terminal are both equal to zero. Like an disconnection, but an equivalent disconnection, so this phenomenon is called "virtual break". Virtual short and virtual break are two important concepts for analyzing the ideal op amp working in the linear region.In fact, the ideal operational amplifier has the characteristics of "virtual short" and "virtual break". These two characteristics are very useful for analyzing linear amplifier circuits. The necessary condition for virtual short is negative feedback. When negative feedback is introduced, at this time, if the forward terminal voltage is slightly higher than the reverse terminal voltage, the output terminal will output a high voltage equivalent to the power supply voltage after the amplification of the op amp. In fact, the op amp has a respond time changing from the original output state to the high-level state (the golden rule of analyzing analog circuits: the change of the signal is a continuous change process). Due to the feedback resistance of the reverse end change will inevitably affect its voltage, when the reverse end voltage infinitely close to the forward end voltage, the circuit reaches a balanced state. The output voltage does not change anymore, that is, the voltage at the forward end and the reverse end is always close. (Note: The analysis method is the same when the voltage decreases.) 3.2 Work in Nonlinear RegionWhen the op-amp operates in the nonlinear region, the output voltage no longer increases linearly with the input voltage, but saturates. The ideal op amp also has two important characteristics when operating in the nonlinear region.1) When u+ ≠ u-, the output voltage of the ideal op amp reaches the saturation value.When u+ > u-, the op-amp operates works in positive saturation region with a positive output voltage.When u+ < u-, the op-amp operates works in negative saturation region with a negative output voltage.Ideal op amp operates in the nonlinear region, u+ ≠ u-, there is no “virtual short”.2) The input current is equal to zero.Although the input voltage u+ ≠ u- above, the input current is considered to be zero. Ⅳ Analysis of Ideal Operational Amplifier CharacteristicsAs for Op-amp, there's probably a description like this: three-terminal element (circuit structure with double-ended input, single-ended output), ideal transistor, high-gain DC amplifier.(1) High input resistanceUnder this situation, the current flowing into the input terminal is close to 0, almost no signal source current is used, which is close to the voltage control characteristic. And virtual break is derived from this.(2) Lower output resistanceIt has the characteristics of adapting to any load. And the impedance of the subsequent load circuit will not affect the output voltage.(3) Infinite voltage amplification(4) Under a certain supply voltage condition, the amplifier can only work in closed-loop (negative feedback) mode, and the actual amplification is limited. Because op-amps themselves don't have a 0V connection but their design assumes the typical signals will be more towards the center of their positive and negative supplies. Thus, if your input voltage is right at one extreme or forces the output toward one supply, chances are it won't work properly. Working in open-loop mode is the like a comparator, and the output is high level orlow level.In the closed-loop (limited amplification) state, the amplifier is randomly compare the potentials of the two input terminals. The output stage makes immediate adjustments when they are not equal. So the final purpose of amplification is to make the potentials of the two input terminals equal. And virtual short is derived from this. Ⅴ Balanced Resistance Presets5.1 The Role of Balanced Resistance1) A suitable resistance is generally required to ensure that the input impedance is matched.2) In order to reduce the input current imbalance, the in-phase resistor should be equal to the parallel value of the two resistors at the reverse end in theory. In practice, as a result of the closed loop, especially in deep negative feedback conditions, the misalignment is not obvious at the output. And there is no need of in-phase grounding resistor when the misalignment is not the main problem. Because a balanced resistor is the starting point for an ideal op amp. In-phase grounding resistance is useful for bipolar op amps, and has no meanings for MOS-type op amps.3) Ground input termination resistance: it is necessary for impedance matching and high frequency setting.4) Bias current and offset current.For operational amplifiers with bias current greater than offset current, input resistance matching can be reduced, and precision circuits can compensate bias current to a minimum. If the bias current and offset current are similar, the matching resistance will increase the error.5) Set for the bias current at the input, the purpose of which is to equalize the impedance of the invertingand non-inverting inputs, so that two inputs with equal bias currents are assumed to have equal voltage drops, thereby counteraction can be made. 5.2 Input Balancing Resistor ExplanationA op-amp is connected to an inverting amplifier:Set the input resistance for R1, feedback resistance for Rfi,Assume that the non-inverting end is not connected to a balanced resistor, but grounded directly.Set the input bias current for the op-amp IB (same voltage in inverting and non-inverting end).The current flows through R1 and Rf are represented by I1 and If.Inverting voltage is V-, The op-amp gain is A.Use KCL in the inverting end (set the input signal to 0).Where (0-V-)/R1- (A+1)V- /Rf=IBFrom the above equation, it follows that V-=-(IB×R1×Rf/(Rf+(A+1)R1))At this time, the output voltage of the op-amp is Vo=A×(IB×R1×Rf/(Rf+(A+1)R1))The above formula can be approximated as Vo=IB×((A×R1)/Rf)If the in-phase terminal passes through a resistor R2 to ground and R2=R1/Rf, then the voltage at the in-phase terminal is V+=-IB×R2KCL is applied to the inverted terminal, where (0-V-)/R1+(A×(V+-V-)-V-)/Rf=IBAt this time the output voltage of the op-amp is Vo=0. Ⅵ Ideal Op Amp EquationsUnderstanding the basic conditions of an ideal op amp, and combining it with the Kirchhoff's current law (KCL) node voltage method and the superposition theorem of the node, is an effective method to analyze the ideal op amp circuit.As shown below, find the output voltage uoFigure 3. OPAMP Circuit1) Equation based on KCLFrom the concept of virtual break, i+=i-=0, then i1=i2, i3=i4, so (a)Based on virtual break, u+=u-, then (b)2) Node voltage methodList the node voltage equations for node 1 and node 2, and get (c)Note: Because the output current of the op amp is unknown at 1) and 2), it is not possible to list the KCL equation or node voltage equation at the output of the op amp. In addition, the op amp output uo in 2) should be treated as an independent voltage source. 3) Superposition theoremWhen there are multiple signal inputs, choosing the superposition theorem to solve can simplify the analysis and calculation process. The size of the output signal uo can be regarded as the superposition of the output signal obtained by the independent action of u1 and u2. When u1 acts alone, the u2 terminal is grounded, and the op amp output is: (d)Therefore, the final output of the operational amplifier is:   (e) Ⅶ Several Common Op Amp CircuitsNon-inverting Amplifier CircuitA non-inverting amplifier is an op-amp circuit configuration which produces an amplified output signal. It provides a high input impedance along with all the advantages gained from using an operational amplifier. Inverting Amplifier CircuitAn inverting amplifier (also known as an inverting operational amplifier or an inverting op-amp) is a type of operational amplifier circuit which produces an output which is out of phase with respect to its input by 180 degrees out of phase with respect to input signal. In the following figure, two external resistors to create feedback circuit and make a closed loop circuit across the amplifier. Op-amp as AdderAn adder circuit can be made by connecting more inputs to the inverting op amp. The circuit diagram of a summing amplifier is as shown in the following figure. Differential AmplifierDifferential amplifier is an analog circuit with two inputs and and one output in which the output is ideally proportional to the difference between the two voltages. It is a very useful op-amp circuit and by adding more resistors in parallel with the input resistors as shown in the following. Composite AmplifierThe composite amplifier is termed as a combination of multiple operational amplifiers that are cascaded together with a negative-feedback loop around the entire network. The resistance in the circuit is generally selected at the K ohm level, the ratio of the resistance affects the gain and bias, in addition, the supply current, frequency response and capacitive load driving capability of the op amp determine their specific values in circuits. If it is used in a high-frequency circuit, the resistance needs to be reduced to obtain a better high-frequency response, but it will increase the input bias current, thereby increasing the current of the power supply. Ⅷ Difference Between Ideal Op-amp and Practical Op-ampIdeal op amps use no power, have infinite input impedance, unlimited gain-bandwidth and slew rate, no input bias current, and no input offset. They have unlimited voltage compliance.Practical op amps consume some power, have very high input impedance have limited gain-bandwidth and limited slew rate, have some input bias current and input offset voltage. Voltage compliance is limited by the power supply rail, or frequently even less.Still practical op amps are very useful because most of the limitations listed above are way better than what your circuit needs.For an ideal amplifier, it does not draw any current at all from its input. Assuming a two input amplifier the signal current in both input probes is zero. In other words the input impedance must be infinite. The output, should operate as the output of an ideal voltage source. This means that the potential between the output and the ground must be A(v2−v1), no matter how much current would a load connected to the output would draw. In other words the output impedance must be zero.For a real amplifier, the input impedance must be as large as possible while the output impedance must be as low as possible.In fact, An op-amp in real life, however, cannot operate with zero current flow. Frequently Asked Questions about Ideal Op Amp1. What is characteristic of ideal opamp?Ideal op amps will have infinite voltage gain, infinitely high impedance, zero output impedance, its gain is independent of input frequency, it has zero voltage offset, its output can swing positive or negative to the same voltages as the supply rails, and its output swings instantly to the correct value. 2. How does an ideal op amp work?An operational amplifier, or op amp, generally comprises a differential-input stage with high input impedance, an intermediate-gain stage, and a push-pull output stage with a low output impedance (no greater than 100 Ω). ... Open-loop voltage gain runs very high, on the order of 1 million. 3. Why are op amps not ideal?Op-amps with FET inputs have an Ibias that is so small that this method becomes less practical. Instead of measuring the voltage drop across a resistor, one can monitor the change in voltage across a capacitor as it is charged by the bias current. 4. How are real op amps different from ideal op amps?In real op amps, the amplified signal will not fully reach the DC supply rails. They will fall short of it. In an ideal op amp, the output will swing instantly to the amplified voltage value. There will be no time delay between the time the voltage is input into the op amp till the time it is output. 5. What are the four main ideal characteristics of an open-loop op amp?An ideal op amp is usually considered to have the following characteristics:Infinite open-loop gain G = vout / vInfinite input impedance Rin, and so zero input currentZero input offset voltageInfinite output voltage rangeInfinite bandwidth with zero phase shift and infinite slew rateZero output impedance R

IntroductionSemiconductor diode, also known as crystal diode, has obvious unidirectional conductivity. It is a kind of electronic components widely used in electrical equipment for protection, rectification, switching, and many other applications. So it is pretty common to see diodes in daily electronic circuits, such as Zener diodes, light-emitting diodes, photodiodes, etc. Therefore, it is necessary to know how to test whether a diode is properly working or not.How to Test a Diode Using a MultimeterCatalogIntroductionⅠ Diode Basics1.1 To Figure Out Diode Anode and Cathode1.2 What Would Cause a Diode to Fail?1.3 Common Diode Failures AnalysisⅡ How to Test Diode with a Multimeter?2.1 Digital Multimeter and Analog Multimeter2.2 Common Diodes Testing Rules2.3 Testing Methods of Types of DiodesⅢ Example Analysis3.1 Test a Diode in Circuit3.2 Power-off and Power-on Testing Methods3.3 ConclusionⅠ Diode Basics1.1 To Figure Out Diode Anode and CathodeThe anode and cathode of diode can be distinguished by screen printing on PCB board, which are as shown in the following:1) The notched end is the cathode of diode.2) The end with a horizontal bar is the cathode.3) The end with white parallel bars is the cathode.4) One end of the triangle arrow is the cathode.5) The small end of the plug-in diode is the cathode, and another big end is the anode.1.2 What Would Cause a Diode to Fail?The common reasons for a diode failure are open circuit, short circuit and unstable voltage regulation. Among these three types of failures, there may be signs. For example, the power supply voltage rises, the supply voltage drops to zero or the output is unstable. Therefore, specific problems must be analyzed in detail for the diode test.The common measurement tool for diode is a multimeter, including on-circuit measurement (the diode is on the circuit board) and off-circuit measurement (the diode is not on the circuit board). As for the basic principle of diode measurement, the forward resistance and the reverse resistance of the PN junction are measured, and the basic judgment is based on the values of them. Therefore, to do a good job in diode test, it is necessary to understand the basic structure and working principle of diodes, and then to understand the main fault characteristics of the diode. 1.3 Common Diode Failures Analysis1) open circuitThis means that the positive and negative electrodes of the diode have been disconnected, and the forward and reverse resistance of the diode have become infinite. After the diode is open, the circuit is in an open state.2) voltage breakdownThis means that there is a path between the positive and negative electrodes of the diode, and the forward and reverse resistance are as large as or close to each other(but not infinite). After a diode breaks down, the action between the positive and negative electrodes may always exit, because there are different manifestations in different circuits.3) forward voltageIf the forward resistance of the diode is too large, the voltage drop of the signal on the diode will increase, which will cause the output signal to decrease, and the diode will be damaged due to the heat. After the forward resistance becomes larger, the unidirectional conductivity of the diode will become poor.4) reverse voltageThe reverse resistance of the diode becomes smaller, which means the unidirectional conductivity of the diode be effected.5) performance degradationUnder this circumstance, the diode does not have obvious failures such as open circuit or breakdown. However, when the situation is getting worse, the stability of the circuit will deteriorate or the output signal voltage of the circuit will drop. Ⅱ How to Test Diode with a Multimeter?2.1 Digital Multimeter and Analog MultimeterWhen using a digital multimeter to test a diode, the red probe connects with the anode and the black probe connects with the cathode. At this time, the measured resistance is the forward conduction resistance of the diode, which is just the opposite of the test result of an analog multimeter. 2.2 Common Diodes Testing Rules(1) The forward resistance of the low-power germanium diode is 300Ω～500Ω, and the silicon diode is lkΩ or more. The former reverse resistance is tens of thousand ohms, and the latter is above 500kΩ (the value of high-power diode is smaller).(2) The polarity of the diode can be judged according to the resistance values (small forward resistance and large reverse resistance). Set the multimeter to the ohm block (Usually use R×100 or R×1k block, do not use R×1 block or R×10k block. The R×1 block is in a large current, it is easy to burn the tube, while using R×10k block may cause the tube broken down with high voltage). Connect the two polarities of the diode with the test probes respectively, and measure the two resistance values. When the measured resistance value is smaller, the end connected to the black lead is the anode. In the same way, when the measured resistance value is larger, the end connected to the black probe is the cathode. If the measured reverse resistance is small, it means that the diode is short-circuited, on the contrary, if the forward resistance is large, it means that the tube is open. In both cases, the diode can’t be work normally.(3) Silicon diodes generally have a forward voltage drop of 0.6V～0.7V, and the forward voltage drop of a germanium diode is 0.IV～0.3V. By measuring the forward voltage of the diode, it can be judged that the tested diode is a silicon tube or a germanium tube. This method is to connect a resistor (lkΩ) behind the power supply, and then connect with the diode according to the polarity characteristic to make the diode forward conducting. At this time, use a multimeter to measure the tube voltage drop. In addition, it is more convenient if it is used in energized dynamic measurement. 2.3 Testing Methods of Types of DiodesZener DiodesHow to test a Zener diode? The following here is to give some ideas.(1) Generally use the low-resistance block to test the Zener diode with a multimeter. Since the battery in the meter is 1.5V, this voltage is not enough to make the Zener diode reverse breakdown. So the forward and reverse resistance should be the same as a normal diode.(2) Measurement of the voltage stabilization value Vz of the Zener diode. When measuring diode, the power supply voltage must be greater than the stable voltage of the tube under test. In this way, the high-resistance block of the multimeter (R×10k) must be used. At this time, the battery in the meter has a higher voltage. When the multimeter's range is set to high barrier, measure diode reverse resistance. If the measured resistance is Rx, the voltage regulation value of the Zener diode is:In the formula, n is the override of the gear used. For example, if the highest electrical barrier ofR0 is the central resistance of the multimeter.E0 is the highest battery voltage value of the multimeter used.Example: Use an MF50 multimeter to measure a 2CW14 diode.R0=10Ω, the highest electrical barrier is R×10k.E0=15V, the measured reverse resistance is 75kΩ, then its voltage regulation value is:If the measured resistance is very large (close to infinite), it means that the voltage Vz under test is greater than E0, therefore, tube will not break down. If the measured resistance is very small (0 or only a few ohms), it means that the test probes are connected reversely, and then just swap the test probes. Light-emitting Diodes (LED)A light-emitting diode is a semiconductor device that converts electrical energy into light energy. It has the characteristics of small size, low working voltage and low working current.(1) There is a PN junction inside the light-emitting diode, so LED has the same characteristic of unidirectional conductivity. Its detection is similar to the measurement of ordinary diodes.(2) Use the R×1k or R×10k gear, and the forward and reverse resistance values are measured. Generally, the forward resistance is less than 50kΩ, and the reverse resistance is greater than 200kΩ.(3) The working current of the light-emitting diode is an important parameter. If the working current is too small, the light-emitting diode will not light up, and it is too large, the light-emitting diode will be easily damaged.(4) The forward turn-on voltage of the light-emitting diode is 1.2V ~ 2.5V, and the reverse breakdown voltage is about 5V. PhotodiodesPhotodiode is a semiconductor device that can convert light intensity into electrical signals.(1) There is a window on the top of the photodiode that can inject light, and the light irradiates the die through it. Under the excitation of the light, a large number of photoelectric particles are generated in the photodiode, which greatly enhances its conductivity and reduces internal resistance.(2) The photodiode is similar to the Zener diode. It also works in the reverse state, with reverse voltage.(3) The forward resistance of the photodiode does not change with the light. Its reverse resistance is larger when there is no light, and becomes smaller when it is exposed to light. That is, the stronger the light, the smaller the reverse resistance. Without light, the reverse resistance will return to the original value.(4) According to the related principle, use a multimeter to measure the reverse resistance of the photodiode. Change the light intensity when measuring, and observe the change of the reverse resistance of the photodiode. If there is no change or less change of the reverse resistance when light changes, it indicates that the tube has failed. High-speed Switching DiodesThe method of detecting high-speed silicon switching diodes is the same as that of ordinary diodes. The difference is that the forward resistance of this tube is relatively large. Measuring with Rxlk block, the forward resistance value is 5k ~ 10k in general, and the reverse resistance value is infinite. Fast Recovery Diodes / Ultrafast Recovery DiodesDetecting fast recovery and ultra-fast recovery diodes with a multimeter is basically the same as that of detecting plastic-encapsulated silicon rectifier diodes. That is, first use the Rxlk block to test its unidirectional conductivity. Generally, the size of forward resistance is about 4 ~ 5k, and the reverse resistance is infinite. And then use the Rxl block to repeat the test, at this time, the forward resistance is several ohms, and the reverse resistance is still infinite. DIAC (Diode for Alternating Current) DiodesUse the Rxlk block, and measure the forward and reverse resistance values of diac, which should be infinite. If the test probes are exchanged to measure, the pointer swings to the right, which indicates that the test tube has a leakage fault. Another method is placing the multimeter in the DC voltage block. During the test, shake the megohmmeter, and the voltage value indicated by the multimeter is the VBO value of the tube. Then change the two pins of the tested tube, and measure the VBR value in the same way. Finally, compare VBO and VBR. The smaller the difference between the absolute values of the two, the better the symmetry of the diac diode. TVS DiodesFor the dual TVS, resistance values between the two pins should be infinite when the red and black test probes of multimeter are exchanged at random. Otherwise, the tube has poor performance or has been damaged. High-frequency Varistor Diodesa. Identify Diode PolarityThe difference between high-frequency varistor diodes and ordinary diodes is that their color code is different. It is generally black of ordinary diodes, while high-frequency varistor diodes’ is light. Its polarity rule is similar to that of ordinary diodes. That is, the end with the green ring is the cathode, otherwise it is the anode.b. Measure Forward and Reverse ResistanceThe specific method is the same as the method of measuring ordinary diodes. Using the Rxlk block of a AM-500 multimeter, the forward resistance is 5k～55k, and the reverse resistance is infinite. Varactor DiodesUsing Rx10k block, no matter how the red and black test leads are exchanged for measurement, the resistance between the two pins of the varactor diode should be infinite. During the measurement, if the multimeter swings slightly to the right or the resistance value is zero, it means that the varactor diode under test has a leakage fault or has been broken down. No matter the loss of varactor diode capacity or internal open-circuit fault, it is impossible to detect them with a multimeter. When necessary, the replacement method can be used for inspection to make judgment. Infrared Light Emitting Diodes (IRED)Put the multimeter in the Rxlk block and measure the forward and reverse resistance of the IRED diode. Generally, the forward resistance should be about 30k, and the reverse resistance should be above 500k. It means the tube can work normally. The larger the reverse resistance, the better. IR Receiver Diodesa. Appearance Identification: Diode Cathode / Anode(1) Common infrared receiving diodes are black in appearance. In addition, there is a small oblique plane at the top of the tube body of the infrared receiving diode. Usually, the pin with one end of the oblique plane is the negative pole and the other end is the positive pole.(2) Use the Rxlk block to test the resistances between two pins. When a diode works normally, the resistance value of two pins are different. And exchange the test leads several times to get some pairs of values. According to the smaller resistance value, the pin connected to the red probe is the cathode, and the pin connected to the black probe is the anode.b. Performance DetectionUse a multimeter to measure the forward and reverse resistance of the infrared receiving diode. According to the resistance values, whether the diode is damaged can be judged preliminarily. Laser DiodesUse the Rxlk block of multimeter, and determine the order of the pins of the laser diode according to the method of detecting ordinary diodes. Because the forward voltage drop of the laser diode is larger than that of the ordinary diode, when detecting the forward resistance, the pointer of the multimeter is slightly deflected to the right, and the reverse resistance is infinite. Unijunction Transistor (UJT)a. Discrimination of ElectrodesBased on the R×1k block, use two meter pens to measure the forward and reverse resistance between any two of the three electrodes ( base B1 and base B2, and emitter E) of the ujt diode. The measured resistance values between the two electrodes are both 2~10kΩ, in addition, B1 and B2 will be different.b. Performance JudgmentThe performance of an ujt diode can be judged by measuring whether the resistance between its pins is normal. Use the R×1k barrier, the black test lead connect to the emitter E, and the red test lead connect to the two base electrodes in turn. Normally, a resistance value should be several thousand to ten thousand ohms. On the contrary, the red test lead connects to the emitter E, and the black test lead connects to the two base electrodes in turn, and the resistance should be infinite under normal conditions. The forward and reverse resistance values between the two bases are both in the range of 2~10kΩ. If they differ greatly from the normal value, the diode is damaged. Ⅲ Example Analysis3.1 Test a Diode in Circuita. Diode Test UsingAnalog MultimeterThe following measurements are all based on silicon diodes. If it is a germanium diode, the forward and reverse resistance of the diode will decrease.1) Measure forward resistance FRThe following figure is a wiring schematic diagram for measuring the forward resistance of a diode with an analog multimeter:Give the result as follows:IndicatorDescriptionUse the R×1k block to measure the diode, the forward resistance is several thousand ohms, and the pointer indicates stability. If the pointer swings slightly, it indicates that the thermal stability of the diode is poor.If the pointer indicates hundreds of kiloohms when measuring the forward resistance, it means that the diode is open.If the pointer indicates tens of kiloohms, it indicates that the diode has a large forward resistance and poor diode performance. Description of measurement of forward resistance:Forward Resistance (FR)DescriptionThousands of ohmsNormalZero or much less than a few thousand ohmsBreakdownHundreds of kilosLarge FR, the diode is openDozens of kilohmsLarge FR, bad forward characteristicsThe pointer is unstablePoor stability 2) Measure reverse resistance RRThe following figure is a wiring schematic diagram for measuring the reverse resistance of a diode with an analog multimeter:Give the result as follows:IndicatorDescriptionWhen measuring the reverse resistance, the value should be several hundred kiloohms. The larger the resistance value is, the better the indicator should be stable.If the reverse resistance is only a few thousand ohms, it means that the diode has broken down and has lost its unidirectional conductivity. Description of measurement of reverse resistanceReverse ResistanceDescriptionHundreds of kilosNormalZeroBreakdownMuch less than a few hundred thousand ohmsDiode’s reverse characteristic is not good.Pointer does not moveThe diode is open. Note: The reverse resistance of some diodes is very large, at this time, it is not certain that the diode is open, so that its forward resistance should be measured. If the value is normal, it means that the diode is not open.Pointer is unstableThe pointer cannot be stabilized at a certain resistance value during measurement, indicating that the diode has poor stability. 3.2 Power-off and Power-on Testing MethodsDiode in-circuit measurement is divided into two situations: Power-off and Power-on statea. Power-off MeasurementThere are something should be noted the method of this test.The influence of the external circuit on the test result is the same as the resistance and capacitance measured of internal circuit. And the influence of the measured forward resistance by the external circuit is lower than the reverse resistance.If there is any doubt about the measuring result, the diode should be removed from the circuit and measured separately. b. Power- on MeasurementWhen the circuit board is powered on, the test point is the tube voltage drop. Because the diode has a very important characteristic: when it is turned on, the tube voltage drop is basically unchanged. So the voltage drop is normal after being turned on, that is to say, the diode is normal.Measurement method: The diagram below shows the connection diagram of the tube voltage drop after the diode in the DC circuit. Setting multimeter in DC voltage 1 V block, the red probe is connected to the cathode of the diode, and the voltage indicated is the forward voltage drop of the diode. Diode forward voltage drop measurement results are analyzed as follows:DiodeDescriptionSilicon diode0.6VThe diode is normal and in a forward conducting state.> 0.6VThe diode is not in the conducting state.Close to 0The diode is in a breakdown state, the current in the loop will increase.Germanium diode0.2VThe diode is normal and in a forward conducting state.> 0.2VThe diode is off or is faulty.Close to 0In the breakdown state, the loop current increases significantly, without unidirectional conductivity. 3.3 ConclusionThe following points should be noted when measuring diodes:1) The diode in AC is in the cut-off state, because the diode is in the reverse state, and the reverse voltage at both ends is very large. The average voltage across the diode measured by the DC block is negative at this time.2) Use different blocks of the same multimeter to measure positive and negative resistance of one diode, their values will different. The forward and reverse resistances of the same diode measured with different multimeters are also different.3) When measuring the forward resistance of a diode, if the pointer cannot stop at a certain resistance value and constantly swings, it indicates that the thermal stability of the diode is not good.4) Some multimeters will provide a “diode check” function that displays the actual forward voltage of the diode when its conducting current. Such meters typically indicate a slightly lower forward voltage than what is “nominal state” for a diode, due to the very small amount of current used during the measurement. Frequently Asked Questions about Diode Test1. What is a diode test?A diode is best tested by measuring the voltage drop across the diode when it is forward-biased. ... A multimeter's Diode Test mode produces a small voltage between test leads. The multimeter then displays the voltage drop when the test leads are connected across a diode when forward-biased. 2. How do you test a rectifier diode?Touch the red (positive) probe of the multimeter to the positive terminal of the diode closet to the welder case interior. Touch the black (negative) probe of the multimeter to the negative terminal of the same diode. The multimeter should read a resistance between 0 and 1 ohm, or the diode is faulty. 3. How can you tell if a diode is positive or negative?Sometimes it's easiest to just use a multimeter to test for polarity. Turn the multimeter to the diode setting (usually indicated by a diode symbol), and touch each probe to one of the LED terminals. If the LED lights up, the positive probe is touching the anode, and the negative probe is touching the cathode. 4. How do you test a Schottky diode?Connect the red positive test lead to the anode of the Schottky diode and the black common test lead to the cathode of the diode. Listen for a “beep” or a “buzz” from the multimeter. If the Schottky diode responds as expected, the multimeter will sound a tone. 5. Can I test a diode in circuit?A diode is best tested by measuring the voltage drop across the diode when it is forward-biased. A forward-biased diode acts as a closed switch, permitting current to flow. A multimeter's Diode Test mode produces a small voltage between test leads. ... Voltage may be present in the circuit due to charged capacitors. 6. How do you check a diode?Diode PolarityThe polarity of both diodes is indicated with a stripe on one end of the body. The stripe corresponds to the line in the schematic symbol, indicating the cathode. The other end (no stripe) is the anode, indicated by the triangle in the schematic symbol. 7. What happens when a diode fails?However, a failed diode can short out too. In this case, the diode will exhibit a small resistance in both directions. The common reasons for a diode failure are excessive forward current and a large reverse voltage. Usually, large reverse voltage leads to a shorted diode while overcurrent makes it fail open. 8. How can you tell if a diode is blown?Turn the dial to “diode test” mode.This level of current is high enough to produce a reading, yet not so high that the diode will fail. It may also be labeled as “diode check” on your multimeter and is usually indicated by a small diode symbol. The diode symbol will look like a triangle pointing towards a line.