Comparison of advantages and disadvantages between instrument amplifier and operational amplifier

What is an instrument amplifier

　　 This is a special differential amplifier , With ultra-high input impedance , Extremely good CMRR, Low input offset , Low output impedance , It can amplify those signals under common mode voltage .

　　 With the rapid development of Electronic Technology , Operational amplifier circuits have also been widely used . The instrument amplifier is a precision differential voltage amplifier , It comes from operational amplifier , And better than operational amplifier . The instrument amplifier integrates the key components in the amplifier , Its unique structure makes it have high common mode rejection ratio 、 High input impedance 、 Low noise 、 Low linearity error 、 Low offset drift, flexible gain setting and convenient use , Make it in data collection 、 Sensor signal amplification 、 High speed signal conditioning 、 Medical instruments and high-end audio equipment are favored . The instrument amplifier is a closed-loop gain module with differential input and single ended output at the relative reference end , Single ended output with differential input and relative reference . Unlike operational amplifiers, the closed-loop gain of operational amplifiers is determined by the external resistance connected between the inverting input and output , The instrument amplifier uses an internal feedback resistor network isolated from the input . Instrument amplifier 2 Apply an input signal to the differential input , Its gain can be preset internally , It can also be preset by the user through the internal setting of the pin or through the external gain resistance isolated from the input signal .

　　 What is an operational amplifier

　　 Operational amplifier （ abbreviation “ Operational release ”） It's a circuit unit with a very high magnification . In the actual circuit , Usually combined with the feedback network to form a functional module . It is an amplifier with special coupling circuit and feedback . Its output signal can be input signal plus 、 Subtraction or differentiation 、 The result of mathematical operations, such as integration .［1］ Because of its early application in analog computers , To carry out mathematical operations , Therefore, the name “ Operational amplifier ”. Operational amplifier is a circuit unit named from the perspective of function , It can be implemented by discrete devices , It can also be realized in semiconductor chips . With the development of semiconductor technology , Most operational amplifiers exist in the form of a single chip . There are many kinds of transporters , Widely used in the electronic industry .

　　 The operational amplifier has two inputs as shown in the figure a（ Invert input end ）,b（ In phase input ） And an output o. Also known as inverted input non inverted input and output . When the voltage U- Add to a End and common end （ The common terminal is the point where the voltage is zero , It's equivalent to a reference node in a circuit .） Between , And its actual direction is from a When the end is higher than the common end , Output voltage U The actual direction is from the common end o End , That is, the direction of the two is just the opposite . When the input voltage U+ Add to b Between the end and the common end ,U And U+ The actual direction of the two is exactly the same as that of the common end .

　　 Advantages and disadvantages of instrument amplifier

　　 advantage ： High precision . shortcoming ： your

　　 Advantages and disadvantages of instrument amplifier

　　 It is still difficult to manufacture inductive components in integrated circuit technology ; The manufacturing capacity is greater than 200pF The capacitance of is also difficult , And the performance is very unstable , Therefore, capacitors should be avoided in integrated circuits . And the operational amplifier adopts direct coupling between all levels , Basically, capacitive components are not used , Therefore, it is suitable for the requirements of integration .

　　 High speed operational amplifier , Exit time from saturation , Compare with the comparator , Still quite long .

　　 What is the difference between instrument amplifier and operational amplifier ？

　　 The instrument amplifier is a closed-loop gain unit with differential input and single ended output at the relative reference end . Most of the time , The impedance of the two input terminals of the instrument amplifier is balanced and the resistance value is very high , Typical values ≥109 Ω. Its input bias current should also be very low , Typical values for 1 nA to 50 nA. Same as op amp , Its output impedance is very low , In the low frequency band, it is usually only a few milliohms （mΩ）. The closed-loop gain of an operational amplifier is determined by the external resistance connected between its reverse input and output . Unlike amplifiers , The instrument amplifier uses an internal feedback resistor network , It is isolated from its signal input . Apply input signals to the two differential inputs of the instrument amplifier , Its gain can be preset internally , It can also be set by the user by connecting an internal or external gain resistor through pins , The gain resistor is also isolated from the signal input .

　　 Special instrument amplifiers are usually expensive , So we wonder whether we can use ordinary operational amplifiers to form instrument amplifiers ？ The answer is yes . An instrument amplifier can be formed by using three ordinary operational amplifiers . The circuit is shown in the figure below ：

　　

　　 The output voltage expression is shown in the figure .

　　 After seeing this, you may ask how the above expression is derived ？ Why the above circuit can realize the instrument amplifier ？ Now we will discuss these problems . Before that , Let's first look at the following differential circuit we are very familiar with ：

　　

　　 If R1 = R3,R2 = R4, be VOUT = （VIN2—VIN1）（R2/R1）

　　 This circuit provides the function of the instrument amplifier , That is, amplifying the differential signal while suppressing the common mode signal , But it also has some drawbacks . First , The impedance of in-phase input and in-phase input is quite low and unequal . In this case VIN1 The inverting input impedance is equal to 100 kΩ, and VIN2 The in-phase input impedance is equal to twice the inverse input impedance , namely 200 kΩ. therefore , When voltage is applied to one input and the other is grounded , The differential current will flow according to the applied voltage received at the input .（ This source impedance imbalance will reduce the circuit performance CMRR.）

　　 in addition , This circuit requires resistance pairs R1 /R2 and R3 /R4 The ratio of is matched very precisely , otherwise , The gain at each input will vary , Directly affect common mode rejection . for example , When the gain equals 1 when , All resistance values must be equal , Only one of these resistors has electricity Resistance value Yes 0.1% loss with , Its CMR then Next drop To 66 dB（2000:1）. Again , If the source impedance has 100 Ω The imbalance of will make CMR falling 6 dB.

　　 To solve the above problems , We add a voltage follower to both the positive and negative input terminals of the op amp to improve the input impedance . As shown in the figure below ：

　　

　　 The two preamplifiers above are used as voltage followers , We now change to an in-phase amplifier , The circuit is as follows ：

　　

　　 The output voltage expression is shown in the figure above . The circuit shown above increases the gain （A1 and A2） when , It adds the same gain to the differential signal , The same gain is added to the common mode signal . in other words , Compared with the original circuit, the common mode rejection ratio of the above circuit is not increased .

　　 below , It's time for the most ingenious changes ！ Look at the circuit first ：

　　

　　 This standard three op amp instrument amplifier circuit is a clever improvement on the buffered subtracter circuit . Like the previous circuit , Above picture A1 and A2 Op amp buffer input voltage . However , In this structure , Single gain resistor RG Connected between the summing points of the two input buffers , It replaces the circuit with buffer subtracter R6 and R7. Since the voltage at the summation point of each amplifier is equal to the voltage applied to its positive input , therefore , The entire differential input voltage is now present at RG Both ends . Because the input voltage is amplified （ stay A1 and A2 The output of the ） The differential voltage of is shown in R5,RG and R6 On these three resistors , So the differential gain can be changed by just RG Adjustment .

　　 This connection has another advantage ： Once the gain of this subtracter circuit is set with a ratio matched resistor , There is no longer any requirement for resistance matching when changing the gain . If R5 = R6,R1= R3 and R2 = R4, be VOUT = （VIN2-VIN1）（1+2R5/RG）（R2/R1） because RG The voltage at both ends is equal to VIN, So flow through RG The current of is equal to VIN/RG, Therefore, the input signal will pass through A1 and A2 Gain and amplification . However, it should be noted that the common mode voltage applied to the input of the amplifier is RG Both ends have the same potential , So it won't be in RG Generate current on the . Because there is no current flowing RG（ No current flows R5 and R6）, amplifier A1 and A2 Will operate as a unit gain follower . therefore , The common mode signal will pass through the input buffer at unity gain , And the differential voltage will press 〔1+（2 RF/RG）〕 The gain factor of is amplified . This means that the common mode rejection ratio of this circuit is larger than that of the original differential circuit 〔1+（2 RF/RG）〕 times ！

　　 Show in theory , The user can get the required front-end gain （ from RG To decide ）, Without increasing common mode gain and error , That is, the differential signal will increase in proportion to the gain , Common mode error is not , So the ratio 〔 gain （ Differential input voltage ）/（ Common mode error voltage ）〕 Will increase . therefore CMR Theoretically, it increases directly in proportion to the gain , This is a very useful feature .

　　 Last , Due to structural symmetry , Common mode error of input amplifier , If they track , Will be eliminated by the subtracter of the output stage . This includes errors such as common mode rejection with frequency transformation . These characteristics are the explanation for the wide application of this three op amp structure .

　　 Come here , We derive the of this classical circuit ; context ： Differential amplifier --》 Pre voltage follower --》 The voltage follower becomes an in-phase amplifier --》 An instrument amplifier composed of three operational amplifiers .