Common-Mode Signals

Definition

Common-mode signals are identical signal components on both the + and - inputs of a differential amplifier or instrumentation amplifier.

A common example is in a balanced pair, where a noise voltage is induced in both conductors. Another example is where a DC component is added (e.g. due to a difference in ground between the signal source and the receiver).

In an ideal differential amp, the common-mode element is cancelled out, since the differential (+ and -) inputs should subtract out the identical components. A measurement of the actual ability to do this is called the Common Mode Rejection Ratio, or CMRR.

Synonyms