common mode rejection ratio (CMRR)

The common mode rejection ratio (CMRR) is a characteristic value of differential and operational amplifiers that characterizes the rejection of common mode signals. It is a measure of quality, usually expressed in decibels( dB) but also in percent, and is formed from the quotient of differential gain and common-mode gain.

When common-mode signals, i.e. completely identical voltages, are applied to the inputs of a differential amplifier, the common-mode voltages compensate and the output voltage is zero. In this case, the common-mode rejection goes to infinity. However, if a voltage difference occurs, the common-mode rejection decreases.

Relationship between common-mode rejection, differential gain and common-mode gain

The signal ratios of differential signal and common mode signal can be derived from the common mode rejection ratio. In practice, the values for operational amplifiers range from 60 dB to 120 dB. At 120 dB, the ratio between the two voltages would be 1:1,000,000.