Noise present on the input signal path of a DMM results in offset on the ACrms function. It is not advisable to reduce this error by doing a subtraction operation because the value displayed is the rms sum of the inputs.

As an example, assume a 0.5 mV offset error on the 5 V range exists. If an input of 50 mVrms is applied, the reading is:

Reading = √(S2 + N2)

Reading = √[(50 mV)2 + (0.5 mV)2]

Reading = 50.0025 mVrms

What appeared to be a huge offset error now accounts for only a 0.005% error in the measurement because of the rms conversion.

50 mV—0.5 mV = 49.5 mV

This result represents a 1% error.