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Short note: Instrumentation amplifier

Short answer

An instrumentation amplifier is a precision differential amplifier with buffered high-impedance inputs, high CMRR, and gain set by a single resistor. The three-op-amp topology gives Vo = (R3/R2)*(1+2R1/RG)*(V2-V1); common-mode signals are passed with gain 1 then canceled. It is used for transducer signal conditioning such as ECG, strain gauges, and data acquisition.

Write a short note on instrumentation amplifier.

An instrumentation amplifier (IA) is a precision differential amplifier with buffered inputs. It amplifies small differential signals (mV or µV) from transducers such as strain gauges, thermocouples and ECG electrodes, while rejecting large common-mode noise.

Features:

  • Very high input impedance, so it does not load the source
  • High CMRR (100 dB or more)
  • Gain set accurately by one resistor RG, typically 1 to 1000
  • Low offset, drift and noise; low output impedance

Three op-amp IA:

 V1->(+)A1-Vo1-[R2]-+-[R3]-+
       |R1         (-)A3---+-> Vo
       RG          (+)
       |R1          |
 V2->(+)A2-Vo2-[R2]-+-[R3]-GND

A1 and A2 are input buffers with gain. A3 is a difference amplifier.

Vo = (R3/R2)(1 + 2R1/RG)(V2 − V1)

A common-mode input causes no current in RG, so it passes the first stage with gain 1 and is cancelled by A3.

Applications: bridge and strain-gauge amplifiers, temperature measurement, medical instruments (ECG, EEG), and data acquisition. Example ICs: AD620, INA128.

✓ Answer checked by Claude Opus 5.5 (high effort)

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