Instrumentation amplifier design and applications

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Instrumentation amplifier is the front end component of every measuring instrument which improves the signal to noise ratio of the input electrical signal from the transducer. It uses the fact the noise is common to the both output terminals of a transducer across which the output is measured and sent to measuring instrument. The input signal flow to measuring instrument can be represented as

The source of the noise is the medium through the signal propagates from transducer to measuring instrument. Let the input signal to instrumentation amplifier be represented as

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Vi= Vd + Vn Then the output of the amplifier is of the form Vo= Ad * Vd + Ac * Vn

Where Ad is the differential gain and Ac is the common mode gain. Signal to noise ratio is defined as ratio of signal power to noise power and power is directly prop. At the input of instrumentation amplifier this is equal to Ps/Pn, at the output of amplifier this will increase by a factor of |Ad|²/|Ac|² as it is obvious from equation for Vo.

Design of instrumentation amplifier

Instrumentation amplifier circuit diagram

The design of instrumentation amplifier involves three Opamps with two of them used in non inverting amplifier configuration and the other in differential amplifier configuration. Also it involves seven resistors with 3 pairs of equal resistors R1, R2, R3 and R4. The circuit diagram of instrumentation amplifier is shown in below figure.

Instrumentation amplifier redrawn circuit diagram

Using the concept of virtual ground as the input stage is employing negative feedback; voltage at the inverting terminals is equal to the non-inverting terminal voltages which are V1 and V2 respectively.

Applying KCL at the inverting mode of 1st opamp, we will get

Due to infinite resistance provided by opamp

Iopamp = 0;

Similarly applying KCL at second node we will get

Va and Vb are inputs to an differential amplifier, output will be of the form Ad*(Va-Vb) where Ad is the differential gain and is equal to R3/R4.Therefore output will be

Substituting equations for Va and Vb in the equation for Vo we get output

Therefore the gain of instrumentation amplifier

From the equation for gain it is clear that the differential operation does not depend on matching of input resistors. Also the input resistance offered by the amplifier is very high due to high input resistance of opamp and no current is drawn from source. The gain of the amplifier can be varied by changing resistor R1 or resistor R2. When the feedback resistors of the input resistors are not matched then the expression for gain is given by

When same voltage is applied to input opamp stages the Voltage difference across resistor R1 will be zero no current flows in the circuit and output will be zero.

Single ended opamp

The Common Mode rejection Ratio of instrumentation amplifier depends on matching of resistors R3, R4 in both branches of differential amplifier individually that is R3 in inverting terminal should be matched with R3 in non inverting terminal. The instrumentation amplifier has superior CMMR compared to single opamp differential amplifier. This can be explained as follows

When differential voltage is applied the difference signal is amplified by a factor of (1+2*R2/R1) before it appears at the input of difference amplifier. Here we are assuming that the mismatch between resistors is less so that its effect on output difference signal is negligible. When the common signal is given to the two terminals of the instrumentation amplifier the same common voltage appears at the two input terminals of the difference amplifier as no current flows in input circuit containing R1 and R2 due to zero potential difference. The common mode signal gain depends only on mismatch of resistors in difference amplifier. So ration of differential gain to common mode gain in instrumentation amplifier is given by

CMMR is increased approximately by a factor of (1+2*R2/R1).

1. The gain of the instrumentation amplifier can be varied by just varying resistors in input circuit without affecting the resistors in difference amplifier circuit.
2. High CMMR.
3. High input resistance.

Applications of instrumentation amplifier

Instrumentation amplifiers are used where there is a need of high differential gain accuracy, stability must be maintained within a noisy environment, and where large common-mode signals (usually at the ac power line frequency) are present.

Some of the applications in which instrumentation amplifiers are used are:

1. In Data acquisition from low output transducers such as strain gauges, Thermocouples, Wheatstone bridge measurements e.t.c