What is Common Mode Rejection Ratio (CMRR)

Unwanted signals (noise) appearing with same polarity on the both i/p lines are essentially cancelled by OP-Amp (Operational Amplifier) and don not appear on the o/p.
The measure of an amplifier ability to reject common signals is a parameter called CMRR.

Numerically, CMRR= A_ol/A_cm or 20 log (A_ol/A_cm)(dB)

Here, ol=Open loop gate & cm= Common loop gate

Example: Aol = 100000 , Acm = 0.2

CMRR= 100000/0.2 =500000 = 114 (dB)

Physical significance of CMRR: CMRR=10000 means the desire i/p signal is amplified 1000 times than the unwanted noise.Thus noise or interfenence has been essentially eliminated.the higher the CMRR the better.