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LM741, OP07, LM324, AD620 Instrumentation Amplifier

  • Contents

I Description

The instrumentation amplifier circuit has the following features:

  1. High Input Impedance;
  2. High Common-mode Rejection Ratio;
  3. Low Drift;
  4. ...

The above features make it widely used in fields of small signal amplification of sensor output. This blog will introduce 4 implementation options of instrumentation amplifier circuits. These 4 options are designed based on different electronic components. And they are also on the basis of explaining the circuit structure and principle of the instrument amplifier. The electronic components discussed in this blog include: LM741, OP07, LM324, AD620. We will summarize features of the 4 circuit through testing, analysis and comparison. I believe this blog can provide a certain reference for circuit design beginners.

What Are Instrumentation Ampilfiers?

Catalog

I Description

II Introduction

2.1 Instrumentation Amplifier Overview

2.2 Instrumentation Amplifier Stucture and Principle

III Instrumentation Amplifier Circuit Design

3.1 LM741 Circuit Option

3.2 OP07 Circuit Option

3.3 LM324 Circuit Option

3.4 AD620 Circuit Option

IV Performance Test and Analysis

FAQ

Ordering & Quantity

II Introduction

2.1 Instrumentation Amplifier Overview

The signals input by smart meters through sensors generally have the characteristics of "small" signals:

  1. The signal amplitude is very small (millivolt or even microvolt magnitude);
  2. Often accompanied by loud noise.

For such signals, the first step of circuit processing is usually to amplify small signals with an instrumentation amplifier. The main purpose of amplification is not to gain, but to improve the signal-to-noise ratio of the circuit. At the same time, for the instrumentation amplifier circuit, the smaller the input signal that can be resolved, the better; the wider the dynamic range, the better. Therefore, the performance of the instrumentation amplifier circuit directly affects the range of the input signal that the smart instrument can detect.

2.2 Instrumentation Amplifier Stucture and Principle

The typical structure of the instrument amplifier circuit is shown as in Fig. 1. It is mainly composed of two-stage differential amplifier circuit.

instrumentation amplifier

Figure 1. Structure of Instrumentation Amplifier

 

Among them, the operational amplifier A1, A2 are in-phase differential input modes. Non-inverting input can greatly increase the input impedance of the circuit. At the same time, it can also reduce the attenuation of weak input signals by the circuit; Differential input can make the circuit only amplify the differential mode signal, and only follow the common mode input signal. In this way, the ratio of the amplitude of the differential mode signal to the common mode signal (ie, the common mode rejection ratio CMRR) sent to the subsequent stage is improved.

In this way, in the differential amplifier circuit composed of operational amplifier A3 as the core component, under the condition that the CMRR requirements remain unchanged, the accuracy matching requirements for resistors R3 and R4, Rf and R5 can be significantly reduced. As a result, the instrumentation amplifier circuit has better common mode rejection capability than a simple differential amplifier circuit. Under the conditions of R1=R2, R3=R4, 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 Rg resistance.  

III Instrumentation Amplifier Circuit Design

At present, the implementation methods of instrumentation amplifier circuits are mainly divided into two categories:

  1. The first category is composed of discrete components;
  2. The second category is directly implemented by a single integrated chip.

In the blog, with single op amp LM741 and OP07, integrated four op amp LM324 and monolithic integrated chip AD620 as the main electronic components, 4 kinds of instrumentation amplifier circuit options are designed.

3.1 LM741 Circuit Option

Consists of three general-purpose operational amplifiers LM741 to form a three operational amplifier instrument amplifier circuit form. And supplemented by related resistor peripheral circuits. At the same time, add the bridge signal input circuit of the non-inverting input terminals of A1 and A2, as shown in Figure 2.

instrumentation amplifier

Figure 2. Single Op Amp Instrumentation Amplifier

 

A1~A3 in Figure 2 can be replaced with LM741 respectively. The working principle of the circuit is exactly the same as that of a typical instrumentation amplifier circuit.

3.2 OP07 Circuit Option

Composed of 3 precision operational amplifiers OP07, the circuit structure and principle are the same as in Fig. 2 (3 OP07s are used to replace A1~A3 in Fig. 2 respectively).

3.3 LM324 Circuit Option

Take a four operational amplifier integrated circuit LM324 as the main component, as shown in Figure 3. Its characteristic is to integrate 4 functionally independent operational amplifiers into the same integrated chip. What are the advantages of using LM324? That is, it is possible to greatly reduce the difference in device performance of each op amp due to different manufacturing processes. In addition, the use of a unified power supply is conducive to the reduction of power supply noise and the improvement of circuit performance indicators. And the basic working principle of the circuit remains unchanged.

instrumentation amplifier

Figure 3. LM324 Instrumentation Amplifier

3.4 AD620 Circuit Option

The circuit consists of a monolithic integrated chip AD620 as the main electronic components, as shown in Figure 4. It is characterized by a simple circuit structure: an AD620, a gain setting resistor Rg, and a working power supply. Therefore, the design efficiency is very high. The circuit gain calculation formula in Fig. 4 is: G=49.4K/Rg+1.

  

instrumentation amplifier
Figure 4. AD620 Instrumentation Amplifier

IV Performance Test and Analysis

The four options of the instrumentation amplifier circuit all adopt the form of a bridge circuit composed of 4 resistors, which changes the double-ended differential input into a single-ended signal source input. The performance test is mainly to carry out simulation and actual circuit performance test from the following aspects:

  1. The maximum input of the signal source Vs;

  2. Vs minimum input of signal source Vs;

  3. The maximum gain of the circuit;

  4. Common mode rejection ratio.

The test data are shown in Table 1 and Table 2. Among them, the maximum (small) input of Vs refers to the maximum (small) 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 the given test conditions. The common mode rejection ratio is calculated by the formula KCMRR=20|g | AVd/AVC|(dB).

Note:

  1. f is the frequency of Vs input signal;
  2. The voltage measurement data in the table are all expressed by peak-to-peak value;
  3. Due to the simulation device, the simulation of option 3 with Multisim failed in the experiment, and "-" in Table 1 indicates the failure data;
  4. Options 1 to 4 in the table respectively represent the instrumentation amplifier circuit composed of LM741, OP07, LM324 and AD620 respectively.

From the measured data in Table 2, we can see from it:

  • For option 2, it has the best performance in terms of signal input range (that is, the maximum and minimum input of Vs), circuit gain, and common-mode rejection ratio. In terms of component price, it is a little higher than the cost of the LM741 option 1 and the LM324 option 3, but it is much cheaper than the AD620 option 4. Therefore, among the four options, option 2 of OP07 has the highest cost performance.
  • For option 4, in addition to its relatively small maximum gain, its other performance is second only to option 2. option 4 has the advantages of simple circuit, superior performance, and saving design space. However, the high cost is its biggest disadvantage.
  • For option 1 and option 3, there is little difference in their performance. option 3 is slightly better than option 1, and they also have absolute price advantages, but their performance is not as good as option 2 and option 4.

Based on the above analysis, option 2 and option 4 are suitable for occasions with higher performance requirements for instrument amplifier circuits. Among them:

  • Option 2 of OP07 is the most cost-effective
  • Option 4 of AD620 is simple and efficient, but the cost is high.
  • Option 1 of LM741 and Option 3 of LM324 are suitable for occasions where performance requirements are not high and cost savings are needed.

According to specific circuit design requirements, different options are selected to achieve optimal resource utilization.

In addition, after the circuit design plan 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, there are a few things to pay attention to:

  • Make the characteristics of op amp A1 and A2 as consistent as possible;
  • When selecting resistors, resistors with a low temperature coefficient should be used to obtain the lowest possible drift;
  • The selection of R3, R4, R5 and R6 should match as much as possible.

2. Pay attention to adding various anti-interference measures in the circuit. such as:

  • The power supply decoupling capacitor should be added at the lead-in end of the power supply;
  • RC low-pass filtering should be added to the signal input terminal or high-frequency noise canceling capacitors should be added to the feedback loop of the operational amplifier A1 and A2;
  • The PCB design should be carefully laid out and routed reasonably, and ground wires should be handled correctly.

FAQ

  • What is lm324?

LM324 is a Quad op-amp IC integrated with four op-amps powered by a common power supply. The differential input voltage range can be equal to that of power supply voltage. ... Generally, op-amps can perform mathematical operations.

  • Which is the difference between lm324 and lm339?

The LM324 has a complementary output while the LM339 is open collector. In the complementary output, current can flow in either direction as required (either source or sink) while the open collector output can only sink current.

  • What is op amp use for?

Operational amplifiers are linear devices that have all the properties required for nearly ideal DC amplification and are therefore used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation.

  • How does an op amp work?

  • What is lm324 used for?

LM324 IC Applications

The applications of IC LM324 include the following. By using this IC, the conventional op-amp applications can be implemented very simply. This IC can be used as oscillators, rectifiers, amplifiers, comparators etc.

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