AC RMS Noise
Any noise added to the signal being measured can increase the apparent rms value of a signal. This applies both to external noise sources and to noise originating inside the NI 4070/4071/4072 itself. Due to the nature of the rms computation, added noise increases the reading in a nonlinear fashion. Specifically, if S is the rms value of the signal and N is the rms value of the noise, the total reading is T = √(S2 + N2). In the case of noise originating in the NI 4070/4071/4072, factory calibration and self-calibration measure the internal noise, and the DSP subtracts the measured noise from the result to improve accuracy. Auto Zero can be used for the same purpose. The subtraction is not linear, but takes into account the nonlinear way in which noise increases measured readings.
The NI 4070/4071/4072 may sometimes appear to return noisier AC readings than other DMMs. In fact, it is quieter than most, but it appears noisy because it responds so quickly to changes in signal amplitude. It is very difficult to find an AC voltage source quiet enough to characterize the noise performance of the NI 4070/4071/4072. If readings appear too noisy, increase the measurement aperture.