Common-Mode Rejection Ratio (CMRR)
Common-mode rejection ratio (CMRR) describes the ability of the DMM to reject a common-mode signal and is often specified with a 1 kΩ resistance in the LO input lead, as shown in the following figure.
CMRR is important because it indicates how much of the common-mode signal affects your measurement. The CMRR is defined by the following equation:
CMRR = 20*log10(Differential Gain/Common-Mode Gain)
For example, if you are measuring a thermocouple in a noisy environment, the noise from the environment appears on both input leads. Therefore, this noise is a common-mode signal that is rejected by the CMRR of the instrument.