How To Select An Operational Amplifier?



Introduction

What is an operational amplifier?

This episode of Electronic Basics will explain the three golden rules when working with OpAmps and how to use them in order to understand and build all kind of OpAmp circuits. The main focus will be the noninverting and inverting amplifier and the comparator circuit.

Catalog

Introduction

Definition

Device Principle

Out Ends

Inverting Op Amp

Non-inverting Op Amp

Classification

Seven Types

Design Consideration

Basics

Operational Environment

Buying

Other Points

Examples Expressions

SAR ADC

In an actual circuit, operational amplifier is a circuit unit with high magnification, usually combines the feedback network to form a certain functional module. Since it was used in analogue computer to realize mathematical operation, it is named "operational amplifier", more commonly known as op amps. Operational amplifier is a circuit unit based on its function, which can be implemented by discrete devices or semiconductor chips. With the development of semiconductor technology, the vast majority of operational amplifiers are in the form of single chip. Nowadays, there are many kinds of operational amplifiers, which are widely used in almost all industries.

Op Amp Diagram

Internal Structure Of Op Amp

Fig. 1 Op Amp Diagram

Device Principle

The operational amplifier has two input terminals a (inverse input), b (in-phase input) and one output. There are also referred to as backward input end, non-backward input end, and output end respectively. When the voltage U- is applied to the a terminal and the public end (the common end is a point where the voltage is zero, it is equivalent to the reference node in the circuit.), and meanwhile a terminal of the actual direction of the output voltage U is higher than that of the common terminal, the actual direction of the output voltage U is from the common end to the o terminal, that is, the direction of the two terminals is opposite. When the input voltage U+ is added between the b terminal and the common terminal, the actual direction of U and U+ is exactly the same as that of the common terminal. For the distinction, end a and end b are divided “-”and “+”, in addition, don't mistake them for the positivity and negativity of the voltage reference direction. The positivity and negativity of a voltage should be marked separately or as an arrow.

Op Amp Symbols and Terminals

Fig. 2 Op Amp Symbols and Terminals

Inverting amplifiers and non-inverting amplifiers are shown below:

Inverting Op Amp

Fig. 3 Inverting Op Amp

Non-inverting Op Amp

Fig. 4 Non-inverting Op Amp

Generally, the operational amplifier can be simply regarded as a high gain direct coupling voltage amplifier unit with a signal output port (Out) and two in-phase, and inverse high impedance input terminals. Therefore, an operational amplifier can be used to fabricate in phase, inverse and differential amplifiers.

Operational amplifier power-supply mode can be divided into two types: dual power supply and single power supply. For dual power-supply operational amplifier, the output can be changed on both sides of the zero voltage, and the output can also be set zero at the differential input voltage of zero. As for the single power supply, an operational amplifier that uses a single power supply, a range of input variations is between the power supply and the ground.

The input potential of operational amplifier is usually higher than a certain value of negative power supply, but lower than a value of positive power supply. Specially designed operational amplifiers can allow input potentials to vary throughout the range from negative to positive power, even slightly higher than positive power supply or slightly lower than the negative source. This operational amplifier is called a rail-to-rail input operational amplifier.

The output signal of the operational amplifier is proportional to the voltage difference between the two inputs. In the audio band, the output voltage = A0 (E1-E2), where A0 is the low frequency open-loop gain of the operational amplifier, E1 is the input signal voltage at the in-phase and E2 is the input signal voltage at the inverse phase.


Classification

General type: Its performance parameters are suitable for general use (low frequency and slow signal change), such asμ741A, LM358 (double OP Amp), LM324 and LF356  with FET as input stage. They are the most widely used integrated operational amplifiers.

High-Z type: The characteristic of this kind of amplifier is that the input impedance of differential mode is very high and the input bias current is very small, general rid > 1GΩ~1TΩ, IB is several to dozens of picoamps. The main measure to achieve these targets is to make use of the high input impedance of FET, using FET as input stage not only has high input impedance and low input bias current, but also has the advantages of high speed, wide band and low noise, but the input offset voltage of this kind of operational amplifier is larger. Such operational amplifier have LF356, LF355, LF347, CA3130, CA3140, etc.

Low-temperature drift type: In precision instruments, weak signal detection and other automatic control instruments, the bias voltage of operational amplifier is small and does not change with the temperature. The low temperature drift operation amplifier is designed for this purpose. At present, the commonly used operational amplifier has OP07, OP27, OP37, AD508 and ICL7650 composed of MOSFET device and so on.

High slew-rate type: In fast A/D converter, D/A inverter and video amplifiers, the conversion rate of the operational amplifier must be high, and the BWG of the unit gain bandwidth must be large enough. Common operational amplifier has LM318, 175A and so on, while the SR=50~70V/us, BWG>20MHz..

Low -consumption type: Due to the wide application of portable instruments, low power supply and low power consumption must be used. Commonly used low-power operational amplifier has TL-022C,TL-160C and so on. The operating voltage is ±2V~±18V, and the current consumption is 50 ~ 250μA. At present, the power consumption of some products has reached μW level, for example, the power supply of ICL7600 is 1.5V and the power consumption is 10mW, which can be supplied by single battery.

High voltage and power type: The output voltage of operational amplifier is mainly limited by power supply. In ordinary operational amplifier, the maximum output voltage is only dozens of volts and the output current is only dozens of Ma. In order to increase the output voltage and current, the auxiliary circuit must be added to the external circuit of the operational amplifier. High-voltage and high-power operational amplifier can output high voltage and high current without any additional circuit. For example, the power supply voltage of D41 integrated operational amplifier can reach ±150 V, and the output current of μA791 integrated operational amplifier can reach 1A.

Programmable control type: In the usage of instruments will be involved in the measurement range problem. In order to get the fixed voltage output, we must change the magnification of the operational amplifier. For example, there is an operational amplifier with a magnification of 10 times, where the input signal is 1mv, the output voltage is 10mv, when the input voltage is 0.1mv , the output is just 1mv. In order to obtain 10mv, the magnification must be changed to 100. Programmable control operation amplifier is to solve this problem. For example, the PGA103A, changes the magnification by controlling the level of pin 1 and pin 2.


Design Consideration

After knowing some basic types op amps, there are some basic questions you should ask before looking for a suitable op amp.

Basics:

What is the input signal going to look like?

Current-input or voltage-input?

What is the expected operating frequency range? Maximum range?

What amplitude is needed? (Typical and maximum values.)

What’s the impedance of the circuit it’s going into?

What is an acceptable output signal going to look like?

What is the expected range of frequencies the output signal might cover?

What is the expected amplitude range?

Will the op amp be driving another device? If so, how much power will be needed?

How accurate or precise does the op amp need to be?

The operating environment:

What supply voltage(s) are available?

Is there a physical size limitation? You may need to make a list of packages of an acceptable size.

What is your operating temperature range? Figure out a Max, Min, and Typical. 

Look at how the temperature affects your most critical parameters using the graphs in the datasheet. If the information you need is missing, you can contact the company or set it aside and move on to another spec that is more thorough.

Buying:

Are you restricted to certain manufacturers that your company deals with?

Will you need to second source the op amp?

What is the lifecycle of the op amp? Do not select any op amp that is Not Recommended for New Design (NRND), End of Life (EOL), or otherwise a special factory order (this might mean that it’s about to go EOL).

Price might be a specification of a sort, but this should be one of the last parameters you look at when you are deciding between otherwise identical op amps.

Other points:

When selecting parameters, it's good to allow a margin of error on the specifications. Not every op amp will be precisely the values as listed, and op amp values change with temperature, age, and stress.

Make sure the finalists in your part selection are actually for sale. “Vapor-ware” is when a manufacturer announces a part to be released in the near future, but some parts have been known as “about to release” for a year or more, depending upon the manufacturer. That’s why you second source your product, and why you confirm the product's lifecycle prior to finalizing.


Example Expression

The ADC architecture, resolution, signal bandwidth, and other specific application details are at work when understanding the various types of operational amplifiers that determine the best way to choose the best amplifiers. We consider these issues in the context of driving SAR ADC in this article.

SAR ADC is the mainstay of the A-D converter world. In general, this kind of ADC is located between high resolution, low speed incremental ADC and high speed, low resolution pipeline ADC. By virtue of its delay-free feature, SAR ADC is often a better choice than ΔΣ ADC and pipeline ADC in applications with multiplexed signals, or applications that need to implement accurate first-time conversions after an arbitrary idle cycle (such as ATE), what's more, applications where ADC is located in a loop that requires quick feedback.

In most cases, the sensor output cannot be directly connected to the SAR ADC input. An amplifier is needed to obtain the optimal SNR and distortion. SAR ADC to sample the input to the internal capacitor and to compare the input voltage with the reference voltage with a successive binary weighted sequence. When the switch to the sampling capacitor is open, the charge is injected into the input node due to the voltage mismatch from the sampling capacitor to the input node. A simple monopole RC filter is arranged between the amplifier and ADC. It not only used to filtering high-frequency noise and aliasing components, but also to absorb this injected charge. Care must be taken when selecting cutoff frequencies for such filters. In addition, the cutoff frequency should be set at a low frequency enough, which can effectively absorb the injected charge and filter the noise, but the frequency should be high enough so that the amplifier can achieve stability within the sampling time of the data converter. Since this filter can't limit noise alone, it is generally included at the amplifier input end, and a filter with a lower cut-off frequency is also installed simultaneously.

LTC2379 18-bit 1.8Msps  Differential Input SAR ADC

Fig. 5 LTC2379 18-bit 1.8Msps  Differential Input SAR ADC

SINGLE-ENDED-INPUT SIGNAL

SINGLE-ENDED-TO-DIFFERENTIAL DRIVER 

SAR ADC  Drive Differential Input SAR ADC

Many of the sound performance SAR ADC use differential input to maximize the dynamic range of low power supply voltage. One such example is the LTC2379-18 shown in Fig, which operates with a 2.5V power supply and a reference of up to 5V to achieve a peak-to-peak differential input range of 10V. If the input signal is differential, all that is needed to buffer the signal and drive the ADC, or may be a low-noise, fast and stable dual-channel operational amplifier such as LT6203. These amplifiers are configured which as unit gain buffers for the input signal provides a high impedance input.

In many cases, however, the input is single-ended and must be converted to a differential signal. This task can be easily accomplished with amplifiers such as LT6350. This type of amplifier has two stages: the first stage generates a buffered non-invert input signal and the second stage generates an inverted output. If the input signal matches the input range of the ADC, the amplifier can be used to provide a high impedance buffer for the signal, as shown in the upper part of Fig. 6(a). If the signal needs to be expanded and shifted to match the input range of the ADC, it can be done as shown in Fig. 6(b) below. In In this example, a single-ended ±10V signal is converted into a differential signal from 0 to 5V (R2 and R3 are used to shift the signal, and RIN and R1 are used to expand the signal). What is often overlooked in accurate analog circuits is the need for a high match between gain setting and level shift resistors. If a discrete resistor with 0.1% accuracy is used, the mismatch will vary with time, temperature and common-mode voltage range, which makes it possible that it will be the main source of fault circuit. Using precisely matched resistors such as LT5400 will help improve the this situation.

The amplifier needs space between the supply voltage and the output voltage. In order to maintain optimal accuracy and linearity, depending on the amplifier, the output must generally be within 0.5V or more of the power rail. This means that the amplifier must be provided with a power supply voltage range wider than the ADC input range, or that the ADC must accept a limited input range from the amplifier. ADC such as LTC2379-18 includes a "digital gain compression" function. The function sets the full scale of the ADC from the inside and the difference between the ground voltage and the reference voltage is 0.5V. This allows the use of a single 5V amplifier matches the full scale of the ADC.

Single-to-difference conversion using LT6350

Fig. 6(a): Single-to-difference conversion using LT6350

Single-to-difference conversion using LT6350

Fig. 6(b): Single-to-difference conversion using LT6350

ADC  Driving Pseudo Differential ADC

Another way is, when converting a single-ended analog signal to a digital signal, skipping the differential conversion completely and using a new pseudo-differential ADC, such as LTC2369-18. The shortcoming is that the noise-signal ratio of SNR which up to 6dB is lost due to a smaller input range. In addition, differential structures are inherently easier to eliminate even harmonics. However, using it also has some important advantages. The drive circuit is simpler: it can be as simple as using a low-noise, fast, stable operational amplifier, such as LT6202, while another operational amplifier and resistor are not required to establish the inverted input. Apart from using fewer groups, the power and noise of the circuit are generally low. Because a lower noise anti-aliasing filter behind the amplifier, it can have a higher cut-off frequency.

This makes it easier for the amplifier to achieve stability within the ADC conversion time, making it a good choice in applications where successive conversions are likely to change throughout the scale, as is the case with multiplexed signals. 

It is necessary to emphasize again that the space of the amplifier must be considered-the supply voltage must be far enough away from the output of the amplifier, which can drive the signal without distortion. In other words, this means that the amplifier must be provided with a negative orbit. One way to solve this problem is to use products such as LTC6360. This new amplifier (Fig. 7) is optimized to drive the SAR ADC with an integrated ultra-low noise charging pump that generates its own internal negative voltage rail. Although it has a single positive source, this allows the output to swing to the ground, even slightly lower than ground. The LTC6360 maintains excellent accuracy (250V misalignment, 2.3nV/Hz noise) and is fast and stable (16-bit, 150ns).

when using a single power source, the LTC6360 wobbles to 0V

Fig. 7: When using a single power source, the LTC6360 wobbles to 0V

Complement

Several amplifier topologies can be used to drive SAR ADC. The best choice depends on the input signal, ADC input architecture and application details, such as whether the input signal is multiplexed. Factors to be weighed include power, complexity, performance and speed (conversion rate and stabilization time).

Conclusion

Choosing an op-amp requires matching your requirement to the op-amp datasheet. Blindly assuming that any op-amp will work in any circuit is only going to result in frustration and disappointment. What's more, using the right op-amp can allow you to do things you never thought were possible.

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