Instrumentation Amplifier

A three-op-amp circuit with very high input impedance and common-mode rejection, gain set by one resistor.

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Two buffered non-inverting stages feed a difference amplifier:

$A_v = \left(1 + \frac{2R}{R_G}\right)\frac{R_3}{R_2}$

Single-chip parts (INA128, AD620, INA333) set gain with one resistor $R_G$ . They amplify tiny differential signals — strain gauges, load cells, thermocouples, ECG electrodes — while rejecting large common-mode noise. The key spec is CMRR (often > 100 dB).

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