Offset Errors

Offset Errors

Noise present on the input signal path of a DMM results in offset on the AC_rms function. It is not advisable to reduce this error by doing a subtraction operation (or null in the DMM Soft Front Panel) because the value displayed is the rms sum of the inputs.

As an example, assume a 100-count offset error on the 5 V range exists. This represents 0.5 mV. If an input of 50 mV_rms is applied, the reading is:

Reading = √(S² + N²)

Reading = √[(50 mV)² + (0.5 mV)²]

Reading = 50.0025 mV_rms

What appeared to be a huge offset error now accounts for only a 0.005% error in the measurement because of the rms conversion. If instead you use the DMM Soft Front Panel (DMM SFP) Null feature, the result is:

50 mV—0.5 mV = 49.5 mV

This result represents a 1% error.