Common-mode rejection ratio ($\mathrm{CMRR}$)

The common-mode rejection ratio compares a receiver's differential gain $A_d$ with its gain $A_c$ for common-mode voltage. In a linear small-signal model,A high value means that equal disturbances on the two inputs have relatively little effect on the output. It does not specify the maximum common input range before saturation.^[ref]

For $A_d=100$ and CMRR $=80$ dB, the ratio is $10000$ and $|A_c|=0.01$. A 1 V common disturbance then creates 10 mV of output error, equivalent to 0.1 mV at the differential input. A measurement applies equal input changes while keeping their difference fixed, subtracts the initial output, and compares the measured common gain with a separate differential-gain measurement. Both gains depend on frequency, so a DC specification alone does not describe rejection of a rapidly changing disturbance.