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3.3

Instrumentation amplifier

3.3.1 Difference amplifier

Note: The negative input to the op-amp is not a virtual earth (0 V) in this circuit. The internal input resistance of the op-amp is MΩ but because the input bias currents are nA, V_s= 0 and so the voltage at negative input is equal to I₂R₂.

3.3.2 CMRR

The common mode rejection ratio (CMRR) is the ratio of the differential gain to the common mode gain. The common mode gain is that obtained when V₁ = V₂

The more general expression for difference gain is:

With a common mode signal, ...

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