Effective Common-Mode Rejection Ratio (ECMRR)
Effective common-mode rejection ratio (ECMRR) is the sum of CMRR and NMRR at a given frequency and is only valid for DC measurements. It is the effective rejection on a given noise signal that is applied to both input leads because it is rejected first by the CMRR capability of the instrument and then again by its NMRR capability. This specification is useful at powerline frequencies, particularly for laboratory and manufacturing floor environments. An equivalent equation to represent ECMRR is as follows:
ECMRR = 20*log10(VCM/Verror)
where Verror is the value returned by the DMM in response to an applied common mode voltage VCM.
For example, if you are measuring 1 mVDC with a DMM that specifies an ECMRR of 130 dB at 60 Hz, and you have a common-mode interference (noise) of 100 mVrms, then the resulting measurement error is:
Measurement Error = 10(-130/20) X 100 mV = 316 nV
which is 0.03% of your measured signal instead of the 10,000% error that the 100 mV interference would otherwise imply.