Instrumentation Amplifier Circuit Design

Instrumentation Amplifier Circuit Design

0 Introduction

The signals input by smart instruments through sensors generally have the characteristics of "small" signals: the signal amplitude is very small (millivolt or even microvolt level), and is often accompanied by large noise. For such signals, the first step of circuit processing is usually to use an instrument amplifier to amplify the small signal. The main purpose of amplification is not gain, but to improve the signal-to-noise ratio of the circuit; at the same time, the smaller the input signal that the instrument amplifier circuit can distinguish, the better, and the wider the dynamic range, the better. The performance of the instrument amplifier circuit directly affects the input signal range that the smart instrument can detect. Starting from the structure and principle of the instrument amplifier circuit, this article designs four instrument amplifier circuit implementation schemes. Through analysis and comparison, the characteristics of each circuit scheme are given, providing a certain reference for circuit design enthusiasts and students to conduct electronic circuit experiments.

1 Composition and principle of instrument amplifier circuit

The typical structure of the instrumentation amplifier circuit is shown in Figure 1. It is mainly composed of two-stage differential amplifier circuits. Among them, op amps A1 and A2 are in-phase differential input mode. In-phase input can greatly increase the input impedance of the circuit and reduce the attenuation of the circuit to weak input signals; differential input can make the circuit only amplify differential mode signals, and only follow the common mode input signal, so that the amplitude ratio of the differential mode signal to the common mode signal sent to the next stage (i.e., the common mode rejection ratio CMRR) is improved. In this way, in the differential amplifier circuit composed of op amp A3 as the core component, the precision matching requirements for resistors R3 and R4, RF and R5 can be significantly reduced under the condition that the CMRR requirements remain unchanged, so that the instrumentation amplifier circuit has better common mode rejection capability than the simple differential amplifier circuit. Under the conditions of R1=R2, R3=R4, and Rf=R5, the gain of the circuit in Figure 1 is: G=(1+2R1/Rg)(Rf/R3). It can be seen from the formula that the adjustment of the circuit gain can be achieved by changing the resistance value of Rg.

2 Instrumentation Amplifier Circuit Design

2.1 Instrumentation Amplifier Circuit Implementation

At present, the implementation methods of instrumentation amplifier circuits are mainly divided into two categories: the first category is composed of discrete components; the other category is directly implemented by a monolithic integrated chip. According to the existing components, this paper designs four instrumentation amplifier circuit schemes with single op amps LM741 and OP07, integrated quad op amp LM324 and monolithic integrated chip AD620 as the core.

Solution 1: A three-op-amp instrumentation amplifier circuit is formed by three general-purpose op-amps LM741, supplemented by related resistor peripheral circuits, plus a bridge signal input circuit at the in-phase input terminals of A1 and A2, as shown in Figure 2.

A1 to A3 in Figure 2 can be replaced by LM741. The working principle of the circuit is exactly the same as that of a typical instrumentation amplifier circuit. Solution 2 consists of three precision op amps OP07, and the circuit structure and principle are the same as Figure 2 (three OP07s are used to replace A1 to A3 in Figure 2).

Solution 3 is implemented with a four-op-amp integrated circuit LM324 as the core, as shown in Figure 3. Its feature is that four independent op-amps are integrated into the same integrated chip, which can greatly reduce the performance differences of each op-amp due to different manufacturing processes; the use of a unified power supply is conducive to reducing power supply noise and improving circuit performance indicators, and the basic working principle of the circuit remains unchanged. Solution 4 is implemented by a monolithic integrated chip A13620, as shown in Figure 4. Its feature is a simple circuit structure: an AD620, a gain setting resistor Rg, and an external working power supply can make the circuit work, so the design efficiency is the highest. The circuit gain calculation formula in Figure 4 is: G=49.4K/Rg+1.

2.2 Performance Testing and Analysis

Among the four schemes for implementing the instrumentation amplifier circuit, four resistors are used to form a bridge circuit to convert the double-ended differential input into a single-ended signal source input. The performance test mainly simulates and tests the actual circuit performance from the maximum input and minimum input of the signal source Vs, the maximum gain of the circuit and the common mode rejection ratio. The test data are shown in Table 1 and Table 2 respectively. Among them, the maximum (minimum) input of Vs refers to the maximum (minimum) input of the signal source when the circuit output is not distorted under given test conditions; the maximum gain refers to the maximum gain value of the circuit that can be achieved when the output is not distorted under given test conditions. The common mode rejection ratio is calculated by the formula KCMRR=20|g|AVd／AVC|(dB).

illustrate:

(1) f is the frequency of the Vs input signal;

(2) All voltage measurement data in the table are expressed in peak-to-peak values;

(3) Due to the simulation device, the simulation of Scheme 3 using Multisim failed in the experiment, and “-” is used to represent the failed data in Table 1;

(4) Schemes 1 to 4 in the table represent instrumentation amplifier circuits with LM741, OP07, LM324 and AD620 as the core respectively.

As can be seen from Table 1 and Table 2, the simulation performance is significantly better than the actual test performance. This is because the performance of the simulation circuit is basically determined by the performance of the simulation device and the structural form of the circuit. There are no external interference factors, and it is a test under ideal conditions; while the actual test circuit is limited by environmental interference factors (such as ambient temperature, space electromagnetic interference, etc.), human operation factors, and the precision, accuracy and range of the actual test instrument, so the test conditions are not ideal and the measurement results have certain errors. In the actual circuit design process, simulation and actual testing have their own strengths. Generally, the circuit structure and device parameters are preliminarily determined through simulation testing, and then the specific performance indicators and parameter settings are improved through actual circuit testing. In this way, the efficiency of circuit design is greatly improved while ensuring the circuit function and performance.

It can be seen from the measured data in Table 2 that Scheme 2 has the best performance in terms of signal input range (i.e., maximum and minimum input of Vs), circuit gain, common mode rejection ratio, etc. In terms of price, it is a little more expensive than Scheme 1 and Scheme 3, but much cheaper than Scheme 4. Therefore, among the four schemes, Scheme 2 has the highest cost performance. Scheme 4 has the highest performance after Scheme 2 except for its relatively small maximum gain. It has the advantages of simple circuit, superior performance, and saving design space. High cost is the biggest disadvantage of Scheme 4. There is little difference in performance between Scheme 1 and Scheme 3. Scheme 3 is slightly better than Scheme 1, and they both have absolute price advantages, but their performance is not as good as Scheme 2 and Scheme 4.

Based on the above analysis, Scheme 2 and Scheme 4 are suitable for occasions with high performance requirements for instrumentation amplifier circuits. Scheme 2 has the highest cost-effectiveness, while Scheme 4 is simple and efficient, but the cost is high. Schemes 1 and 3 are suitable for occasions where performance requirements are not high and cost savings are required. According to specific circuit design requirements, different schemes are selected to achieve optimal resource utilization. After the circuit design scheme is determined, the following aspects should be paid attention to in the specific circuit design process:

(1) Pay attention to the selection of key components. For example, for the circuit shown in Figure 2, make sure that the characteristics of op amps A1 and A2 are as consistent as possible. When selecting resistors, use resistors with low temperature coefficients to obtain the lowest possible drift. R3, R4, R5, and R6 should be matched as closely as possible.

(2) Pay attention to adding various anti-interference measures in the circuit, such as adding power decoupling capacitors at the power input end, adding RC low-pass filtering at the signal input end, or adding high-frequency noise reduction capacitors in the feedback loop of op amps A1 and A2, carefully layout and reasonably wire in PCB design, correctly handle ground wires, etc., to improve the circuit's anti-interference ability and maximize the circuit's performance.

3 Conclusion

Based on the detailed discussion of the structure and principle of instrumentation amplifier circuits, four instrumentation amplifier circuits are designed. Through simulation and actual performance testing, the characteristics of the four schemes are analyzed and summarized, providing some ideas for reference for designers of instrumentation amplifier circuits.