What is the common-mode gain?

The common-mode gain is part of the common-mode rejection ratio formula that measures the output voltage of a differential amplifier based on two inputs. The common-mode gain is typically a much smaller number in the formula compared to the differential gain that can be measured from the same formula.

A common-mode signal, which is measured to determine whether or not there is common-mode gain, is usually static or interference that is picked up in the output of a differential amplifier. The common-mode gain, as part of the common-mode rejection ratio, is used to divide differential gain and yields results that can be measured in positive decibels.