Historically we have been using PXI-4070 DMMs in our test systems. PXI-4070s are no longer available so we have a mixture of PXI-4070s and PXIe-4080s in our systems.
We see differences between the two devices when we acquire a waveform at 1.8e6 samples per second (the max rate). With the PXI-4070s, we normally see a peak-to-peak voltage of about 300 mV. When we take the same measurement with the PXIe-4080, we see a peak-to-peak voltage of about 20 mV.
I have read through the DMM documentation and the only difference I came across was that the 4070s have an Auto-Zero feature and the 4080s do not, however, we have the Auto-Zero feature disabled on the 4070s.
Is there a difference in configuration between the 4070s vs 4080s that I am overlooking? Or does anyone have any ideas why this peak-to-peak value would be so much lower with the PXIe-4080 DMM?
I observe that the 4070 has a 300kHz bandwidth and the 4080 is 240kHz. Based on your signal, there could be some attenuation.
I was not able to find the DC accuracy specifications for Digitizer mode on the 4070 but available on the 4080. If possible, please verify the actual signal using an oscilloscope.
I did overlook that bandwidth difference, thank you for bringing that up.
My colleague found this NI waveform acquisition page which, combined with my signal data shown below, leads me to think the signal is just too fast to measure accurately. It does not explain why the 4070 has such a higher amplitude signal though.
Here is an oscilloscope measurement:
Here is the data taken by the 4070:
Here is the data taken by the 4080:
To get this data, I used the same DUT. I also used the same wiring. I physically switched the DMMs in the chassis, everything else remained the same.
It looks like the 4080's waveform more closely matches the o-scope. I'm not sure why the 4070's signal would have such a higher amplitude.