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AD620 Application: Small Signal Acquisition System Design

  • Contents

I. Desription

In the field of measurement and control, it is often encountered that the output signal of the monitored object is small, and it is difficult to directly collect it. Generally, it needs to be amplified before processing. This article introduces a method of implementing a small signal acquisition system. The smallest system is realized by using the single-chip STC25A60S2 with an A/D conversion function and the easy-to-use amplifier AD620 with a precise amplification function. The system design and implementation are discussed and the acquisition is introduced in detail. The process of small signal, and gives practical application examples, as well as the application of small-signal acquisition in related fields.

AD620

AD620

Catalog

I. Desription

II. AD620 Introduction

III. Introduction to STC12C5A60S2

IV. System Hardware Design

4.1 System Principle Block Diagram

4.2 Power Supply Circuit Design

4.3 Signal Conditioning Circuit

4.4 System Decoupling Circuit

4.5 Realization of A/D Conversion

4.6 Follow-up Work

V. Practical Application

VI. Conclusion

FAQ

Ordering & Quantity

II. AD620 Introduction

As a monolithic instrument amplifierAD620  has low power consumption, a chip with high gain through external resistors, and features low input drift and temperature drift. AD620 is developed from the traditional three operational amplifiers, but some of the main performance is better than the design of the instrument amplifier composed of three operational amplifiers, such as wide power supply range (±2. 3~±18 V), small design volume, and very power consumption Low (the maximum power supply current is only 1. 3 mA), so it is suitable for low-voltage, low-power applications.

Figure 1 Schematic diagram of AD620 principle

Figure 1 Schematic diagram of AD620 principle

 

The monolithic structure and laser crystal adjustment of AD620 allows circuit components to be closely matched and tracked, thereby ensuring the inherent high performance of the circuit. AD620  is a three-op-amp integrated instrumentation amplifier structure. In order to protect the high precision of gain control, the input transistor provides a simple differential bipolar input and uses the β process to obtain a lower input bias current. Through the input stage internal op-amp The feedback to keep the collector current of the input transistor constant, and the input voltage is added to the external gain control resistor RG. The two internal gain resistors of  AD620  are 24.7 k8, so the gain equation is:

 

G =49.4 kΩ/R G + 1 (1)

 

For the required gain, the external control resistance value is: R G =49.4/(G-1)kΩ (2)

 

III. Introduction to STC12C5A60S2

 

STC12C5A60S2 is a new generation 8051 single-chip microcomputer with an A/D conversion function. The instruction code is fully compatible with the traditional 8051, but the speed is 8-12 times faster. With 8 channels of high-speed 10-bit input A/D conversion (250k/s), it can be used for temperature detection, battery voltage detection, key scanning, spectrum detection, etc. The user can set any channel as A/D conversion, and the ports that do not need to be used as A/D can continue to be used as I/O ports. Its characteristics are as follows:

Figure 2 STC12C5A60S2

Figure 2 STC12C5A60S2

(1) On-chip integrated 1280 bytes RAM;

(2) With EEPROM function (STC12C5A62S2/AD/PWM without internal EEPROM);

(3) Enhanced 8051 CPU, 1T, single clock/machine cycle, instruction code is fully compatible with traditional 8051;

(4) Internal integrated MAX810 dedicated reset circuit (when the external crystal is below 12M, the reset pin can be directly connected to the ground with 1K resistance);

(5) User application space 8K /16K / 20K / 32K / 40K / 48K / 52K / 60K / 62K bytes;

(6) ISP (In-System Programmable)/IAP (In-Application Programmable), no need for a dedicated programmer, no need for a dedicated emulator, you can download the user program directly through the serial port (P3.0/P3.1), and one-piece can be completed in a few seconds ;

(7) STC12C5A60S2 series have dual serial ports, only those with the S2 logo suffix have dual serial ports, RxD2/P1.2 (can be set to P4.2 by the register), TxD2/P1.3 (can be set to P4.3 by the register) ;

(8) General I/O port  s (36/40/44), after reset, they are: quasi-bidirectional port/weak pull-up (normal 8051 traditional I/O port  ), which can be set to four modes: quasi-bidirectional port/weak Pull-up, push-pull/strong pull-up, only input/high impedance, open-drain, each I/O port  drive capacity can reach 20mA, but the whole chip should not exceed 120mA;

(9) A/D conversion, 10-bit precision  ADC, 8 channels in total, conversion speed up to 250K/S (250,000 times per second), universal full-duplex asynchronous serial port (UART), because the STC12 series is high-speed 8051, can reuse timer or PCA software to realize multiple serial ports.

 

IV. System Hardware Design

4.1 System Principle Block Diagram

 

Generally speaking, before the signal is used, it needs to be filtered and then amplified, or amplified and then filtered, and then obtained/perceived by means such as A/D. For small signals, the signal amplitude is only a few millivolts or even smaller. If filtered first, useful signals may be filtered out. Therefore, in this case, you need to amplify first, then filter, and then perform A/ D conversion or other processing. According to the characteristics of this system, the interference in the system can be ignored, so the signal filtering link is not considered. Therefore, the system is mainly realized through three important links: signal extraction, signal amplification, and A/D acquisition. The data generated in the third link can guide people's work or display relevant information. The block diagram of the entire system is shown in Figure 3.

Figure 3 System block diagram

Figure 3 System block diagram

4.2 Power Supply Circuit Design

 

AD620 amplifier can use single power supply or dual power supply, but when using dual power supply, its performance is better than single power supply. In integrated circuit design, single power supply is easy to implement, but considering the working performance of the chip, dual power supply is used in this system. Use the ICL7660S chip to convert an external single power supply into a dual power supply. ICL7660S is a voltage conversion chip that can realize the function of converting a positive voltage to a negative voltage, and its peripheral circuit is relatively simple. The specific circuit is shown in Figure 4.

Figure 4 Power supply implementation schematic diagram

Figure 4 Power supply implementation schematic diagram

 

The other chips in the system are powered by a single 5V power supply, and the connected 5V power supply can be used without any processing, which is not described here.

 

4.3 Signal Conditioning Circuit

 

The actual weak signal is generally mV level or even smaller. Before processing, it needs to be amplified and then A/D collected. According to the A/D function of STC12C5A60S2, it is necessary to accurately amplify the signal to reach the V level, so the AD620 amplifier is used. AD620 has a good amplification effect on 2 input differential signals. In practical applications, the signals are generally generated by electric bridges. In order to realize signal amplification, AD620 needs an external resistor, which determines the amplification factor together with the internal resistor. Suppose the magnification is G, then there is the following formula.

G=(RG/R1)+1(1)

can also be written as the following formula: G=49.4kΩ/RG+1(2)

 

1) In the formula, RG is the internal resistance of AD620.  and R1 is the external resistance. It can be seen from formulas (1) and (2) that the size of RG in formula (1) is 49.4kΩ.

 

The conditioned signal is output through the 6-pin of AD620,  At this time, it can be directly connected to the A/D conversion chip to realize data collection. It can be reduced by the corresponding multiple when using it. The principle of signal conditioning is shown in Figure 5.

Figure 5 Signal conditioning circuit

Figure 5 Signal conditioning circuit

 

4.4 System Decoupling Circuit

 

Since the system mainly realizes small signal amplification and A/D conversion after amplification, the chip that completes the A/D function of this system, namely STC12C5A60S2, uses its own working power supply as the reference voltage. In order to ensure the consistency of the conversion results, it is necessary to ensure the power supply voltage stability. To filter out the interference in the power supply, it can be filtered by multiple capacitors in parallel. After the capacitors are connected in parallel, the capacitance value increases, but the equivalent resistance inside the capacitor is reduced due to the parallel connection, which is beneficial to reduce the loss. Therefore, many capacitors are used in parallel, and the realization principle is shown in Figure 6.

Figure 6 Power decoupling circuit

Figure 6 Power decoupling circuit

 

4.5 Realization of A/D Conversion

 

As mentioned earlier, STC12C5A60S2 is a single-chip microcomputer with an A/D conversion function.  which is convenient, simple, and multi-functional. Its A/D conversion only requires 90 clock cycles (related to its operating frequency) at the fastest. This system Uses it to achieve A/D conversion.

STC12C5A60S2 uses port P1 as an 8-channel A/D conversion input interface. When using it, you only need to set it as an analog interface. By setting the corresponding register, the A/D conversion can be completed. The unused pins can still be used as ordinary tubes. This system realizes the A/D conversion of one input signal, so it only needs to set one. In this system, P1.0 port is used as the signal input port. This system realizes the principle of A/D conversion as shown in Figure 7.

Figure 7 A/D acquisition circuit

Figure 7 A/D acquisition circuit

 

4.6 Follow-up Work

 

After the A/D conversion is completed, data analysis is required. Generally, it can be sent to the upper computer through the communication port (usually serial port), and the data is processed by the upper computer. According to the different characteristics of the specific system, the data processing methods are also different, so it's no need to  discuss them in detail here.

Table 1 AD conversion data and actual data

Table 1 AD conversion data and actual data

After the system performs A/D conversion on signals of different sizes, a series of actual data and theoretical data are obtained, as shown in Table 1. Draw the curve of A/D data through Excel, and found that the system A/D converter has good linearity. As shown in Figure 8.

Figure 8 Linearity of A/D converter

Figure 8 Linearity of A/D converter

 

V. Practical Application

 

The small signal conditioning, A/D conversion, and processing methods are discussed in more detail above. The specific application is introduced below through examples.

 

As a kind of sensing element, resistance strain gauge is commonly used to monitor the deformation of the object. Generally, the strain gauge is attached to the side point of the component. After the component is stressed, due to the strain of the measuring point, the resistance changes, resulting in a weak voltage change. The voltage change can be calculated to obtain the deformation degree of the component, so as to achieve the purpose of monitoring the condition of the component and guide the relevant engineering personnel to deal with it.

 

This system can be applied to the voltage generated by the electric bridge. A schematic diagram of the electric bridge is shown in Figure 9. In the figure, R4, R3, R1, and R2 are the four arms of the bridge, R4 and R3 are fixed resistances of impedance, and R1 and R2 are in One is a resistor whose resistance value changes after being stressed. R4 and R3 have the same resistance value, and R1 and R2 have the same resistance value when they are not stressed. In the case of no force, the two points of the bridge 3 and 4 are equipotential, that is, the potential difference is 0.

 

If it is input as AD620,  the input signal is considered to be 0, and the bridge is said to be balanced at this time. When the force of R1 or R2 changes, the result of the change is reflected in its resistance value, which can be obtained by Ohm’s law. The potential at points 3 and 4 is different, that is, there is a potential difference. At this time, the bridge is out of balance, but at this time The signal is very weak and cannot be collected directly. Therefore, the signal is amplified through the signal conditioning circuit mentioned in the article, that is, points 3 and 4 in the bridge are connected to pins 2 and 3 of AD620.  After amplification, the A/D acquisition is performed.

Figure 9 Schematic diagram of electric bridge

Figure 9 Schematic diagram of electric bridge

This system uses a simple electric bridge built by itself during simulation, as shown in Figure 10.

Figure 10 Simple electric bridge

Figure 10 Simple electric bridge

 

By adjusting R2 in the figure, different weak signals are generated, and the simple bridge 1 and 2 are connected to the signal conditioning circuit, and then after A/D conversion, weak signal acquisition can be realized. The 1, 2 ends of the simple electric bridge correspond to the 3 and 4 ends in the figure.

 

In this simulation, adjust R2 to generate about 5.35mV at both ends of 1, 2 and adjust the external resistance in the signal conditioning circuit to 160.7Ω. The calculated magnification is about 308.4 times, and the A/D reference voltage is 4.256V By measuring the output of AD620, it can be obtained that the voltage is 1.645V, and the magnification factor G=1.647V/5.35mV≈308 can be calculated. It can be seen that the magnification effect is good (after removing the magnification effect, the error is only nV level). Through multiple A/D conversions, the returned results are all around 0x018B, which proves that the system has high credibility (it has been used in actual systems).

 

VI. Conclusion

 

This article describes in detail the design and implementation of small-signal acquisition systems from chip selection, circuit design, etc., such as 8-bit single-chip STC12C5A60S2 as the controller and A/D converter; AD620 as the main chip of the signal conditioning circuit; electric bridge principle, etc. Through the test, the circuit has realized its function well, and it has accomplished the expected goal excellently in the actual system, which has certain practical value.


FAQ

  • What is AD620?

AD620 is a low-cost, high-precision instrumentation amplifier. It only requires an external resistor to set the gain. The gain range is 1 to 10,000.

  • Can I change AD620 to AD623 when making MCU products?

Both AD620 and AD623 are single instrumentation amplifiers, and the pin arrangement is exactly the same.

The main difference is: AD620 must use positive and negative power supplies, AD623 can be a positive and negative power supply or a single power supply.

If the original board is AD620, you can replace it with 623; if the original board is AD623, you may not be able to replace it with 620 (it depends on whether the power supply of the original board circuit is dual power supply or single power supply).

After replacing AD620 and AD623 in single-chip products, the program can work normally without modification.

  • What is the difference between AD620BR and AD620AN?

Their packages are different.

  • What is the output resistance of AD620? How to adjust it?

AD620 is a kind of low power consumption instrument amplifier, its output resistance is about 10K, this is the inherent characteristic of this chip, generally it is difficult to adjust.

If you have requirements for output resistance, you can generally use an external circuit to solve it.

  • Is AD620 a positive phase amplification or a reverse phase amplification?

AD620 is an instrument amplifier, the output voltage is [(Vin+)-(Vin-)]*gain.

If the desired signal is (Vin+)-(Vin-), the gain is positive, which is equivalent to positive amplification.

Conversely, if the desired signal is (Vin-)-(Vin+), the gain is equivalent to negative, which is equivalent to reverse amplification.

  • What is an instrumentation amplifier?

Instrumentation amplifier, an improvement of the differential amplifier, has an input buffer, does not require input impedance matching, so that the amplifier is suitable for measurement and electronic instruments.

 

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