Analog Electronics

Difference Amplifier and CMRR

A difference amplifier amplifies the DIFFERENCE between two inputs: with four resistors, out = (R2/R1)(V2 − V1). Whatever is common to both inputs (common mode) is ideally cancelled.

The common-mode rejection ratio (CMRR) depends on resistor matching: with 1%-tolerance resistors you get ~40 dB; for precision measurements, instrumentation amplifiers with matched resistors are used.

This block underlies current measurement across a shunt, thermocouple reading, and receiving differential lines in noisy environments.

Formulas

Vout = (R2/R1)·(V2 − V1)

CMRR = Adiff / Acm

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Test Yourself

If R2/R1 = 5, V2 = 3 V, V1 = 2.6 V, what is the output?

Answer: 2 V — out = 5·(3 − 2.6) = 2 V.

What does CMRR measure?

Answer: The ratio of differential gain to common-mode gain — High CMRR = good rejection of noise common to both inputs.

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