What is Common Mode Rejection Ratio (CMRR) and why is it important?

CMRR is the ratio of differential-mode gain to common-mode gain in a differential amplifier, expressed in dB. It measures the ability to reject signals common to both inputs while amplifying the difference. High CMRR (typically 80-120 dB for op-amps) is crucial for rejecting noise and interference that appears equally on both input lines, essential in instrumentation and sensor applications.