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Normal-mode rejection ratio (NMRR) describes the ability of the DMM to reject an AC normal-mode signal, usually at powerline frequencies. NMRR is given by the following formula:
NMRR = 20*log(Vin/Verror)
where Verror is the value returned by the DMM for an applied AC normal-mode voltage Vin.
NMRR is useful for measurement systems that can eliminate signals at a given frequency or over a range of frequencies. NMRR, which is often used to indicate the capability of the instrument to reject powerline noise of 50 or 60 Hz, is valid only at the specified frequency and is useful when making DC measurements.