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

CMRR measures an amplifier's ability to reject signals common to both inputs while amplifying the differential signal. Expressed in dB: CMRR = 20 * log(differential gain / common-mode gain). High CMRR (80-120 dB typical) is critical for instrumentation amplifiers measuring small differential signals in presence of common-mode noise (e.g., thermocouple signal with ground noise). A 100 dB CMRR means common-mode signals are attenuated by 100,000:1. Essential for accurate measurement in noisy industrial environments.