Instrumentation Amplifier Circuit Principle, Composition and Circuit Design (Part 2)

　　Solution 4 is implemented by a monolithic integrated chip AD620, 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. In 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 is mainly simulated and tested from the aspects of 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 Tables 1 and 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).

　　Note: (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 problem, the simulation of Scheme 3 using Multisim failed in the experiment. The failure data is indicated by “-” in Table 1.

　　(4) Schemes 1 to 4 in the table represent instrumentation amplifier circuits based on LM741, OP07, LM324 and AD620 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 structure and device parameters of the circuit 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 function and performance of the circuit.

　　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 advantages of simple circuit, superior performance, and saving design space, except for the relatively small maximum gain. High cost is the biggest disadvantage of Scheme 4. There is not much 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.

4. Characteristics of instrumentation amplifier:

　　● High common mode rejection ratio

　　The common-mode rejection ratio (CMRR) is the ratio of the differential-mode gain (Ad) to the common-mode gain (Ac), that is: CMRR = 20lg | Ad/Ac | dB; the instrumentation amplifier has a very high common-mode rejection ratio, and the typical CMRR value is above 70 to 100 dB.

　　● High input impedance

　　The instrumentation amplifier must have extremely high input impedance. The impedance of the instrumentation amplifier's non-inverting and inverting input terminals are both very high and well balanced with each other, with typical values ​​of 109 to 1012Ω.

　　● Low noise

　　Since the instrumentation amplifier must be able to handle very low input voltages, the instrumentation amplifier cannot add its own noise to the signal. At 1 kHz, the input noise referred to the input is required to be less than 10 nV/Hz.

　　● Low linearity error

　　Input offset and proportional coefficient errors can be corrected by external adjustment, but linearity error is an inherent defect of the device and cannot be eliminated by external adjustment. A high-quality instrumentation amplifier has a typical linearity error of 0.01%, and some are even lower than 0.0001%.

　　● Low offset voltage and offset voltage drift

　　The offset drift of the instrumentation amplifier is also composed of two parts: input and output. The typical values ​​of input and output offset voltages are 100 μV and 2 mV respectively.

　　● Low input bias current and offset current errors

　　The base current of a bipolar input operational amplifier and the gate current of a FET input operational amplifier will produce an offset error when this bias current flows through an unbalanced signal source resistance. The typical bias current of a bipolar input instrumentation amplifier is 1 nA to 50 pA; while the typical bias current of a FET input instrumentation amplifier at room temperature is 50 pA.

　　● Ample bandwidth

　　Instrumentation amplifiers provide sufficient bandwidth for specific applications, with typical unity-gain small-signal bandwidths ranging from 500 kHz to 4 MHz.

　　● Has a “detection” end and a “reference” end

　　The instrumentation amplifier is also unique in that it has a "sense" terminal and a "reference" terminal, which allows the output voltage to be sensed remotely while minimizing the effects of internal resistor voltage drops and ground line voltage drops (IR).