Hi Owen,

Ohhh, the plot thickens! Thank you. If it’s bouncing a square wave between -5V and 5V, then the zero crossing detection should work, but this gives me a different theory, and I think it explains why you see better results as you raise the sampling frequency: A square wave has component frequencies much higher than the fundamental (in the case of the DMMCheck plus the fundamental is 100Hz). Since the Mooshimeter is only sampling at 8000/4000/2000 Hz, it drops all frequency information above Nyquist, resulting in lower RMS reading.

If your sampling theory is a little rusty, the intuitive way to think about this would be to see that when the meter is sampling at 1000Hz, every sample represents a smearing together of a 1ms window. If the signal crosses from 5V to -5V within that time, the meter will read some value in between 5 and -5 for that sample. When it does the RMS calculation later, the sample buffer now has a low readings at every transition of the square wave, resulting in the RMS calculation being too low. The effect gets less pronounced as you raise the sampling frequency because every transition of the square wave affects a smaller ratio of samples.

Also I have a DMMCheck (not a DMMCheckPlus) on my desk, big fan of these products :)

Please let me know if this explanation makes sense to you! Best
~James