I'm using an NI 4072 to measure a voltage which I expect to be around 0V. There's some noise, so I am taking 50 samples and arriving at a mean to return to a higher level function.
I was surprised to see that there were some NaNs being returned a the mean value. After all this is a 300V capable DMM and I'm measuring about 1.0V pk-pk AC noise on 0V DC. Taking a closer look, I added an indicator to show what range had been selected by the DMM (it's set in Autorange) and I noticed that it was 1V. So, I could see in my raw sample array a lot of values which were very close to 1V and given that there is the occasional NaN, I am obviusly 'sampling' voltages in excess of 1.05 (105% is the NaN overrange limit).
OK, so for my application, I could select 300V range and still get 1mV resolution which is fine for me, but it begs the question, how does the auto-range function work? Of course, I understand the objective of trying to maximise 'dynamic range' of the instrument, but I would question the sampling/filtering regime in place for the auto-range selection. If the peak value is *so* close to 105% of the range selected, doesn't that risk returning (unhelpful) NaNs?
So I would alert users to the potential for the auto-range to deliver a range setting whose maximum is very very close to the value being measured - such that if a variation in the value measured occurs within a sampling period it will return NaNs...at the same time wondering what have I missed about using this instrument that I have fallen 'victim' to this behaviour?
TIA