Common-Mode Rejection Ratio

Formula

Description

Common-mode rejection ratio quantifies how well a differential amplifier or instrumentation amplifier rejects signals that appear equally on both inputs (common-mode signals) while amplifying the difference signal. Higher CMRR means better rejection of noise, power supply variations, and ground potential differences. Ideal op-amps have infinite CMRR, but real devices range from 60-120 dB. CMRR degrades with frequency due to asymmetric parasitic capacitances and finite gain-bandwidth product. External resistor matching in discrete difference amplifiers often limits CMRR more than the op-amp itself.

Variables

• CMRR — Common-mode rejection ratio (dB)
• Ad — Differential gain (V/V, linear)
• Acm — Common-mode gain (V/V, linear)

Practical Notes

Typical CMRR: general purpose op-amp 80-100 dB, precision op-amp 100-120 dB, instrumentation amplifier 80-120 dB. A CMRR of 80 dB means a 1V common-mode signal produces only 100 µV of error at the output. For Wheatstone bridge measurements with remote sensors, high CMRR is essential to reject noise picked up on the long cable connecting the bridge to the amplifier.