What does common mode rejection refer to in amplifiers?

Common mode rejection refers to the ability of an amplifier to reject or ignore signals that are present equally on both input terminals, particularly external interference or noise that can affect these inputs. This feature is crucial in ensuring that the amplifier primarily amplifies the differential signal, which is the actual signal of interest, while effectively minimizing the impact of unwanted common noise or interference that could distort or obscure that signal.

In many applications, such as in EEG systems and other medical instrumentation, the rejection of external interference is essential for accurate and reliable data acquisition. If an amplifier did not possess this common mode rejection capability, it would amplify not only the desired signal but also any noise that is simultaneously picked up by both inputs, leading to inaccurate readings. Hence, common mode rejection becomes a critical parameter in the design and evaluation of amplifiers used in sensitive measurements, such as those found in EEG technology.