large variation on analog input reading

[HeatherRalph]

I am reading a voltage signal and see large fluctuations in the reading.  I am using a NI PXIe-1082 chasis with a NI PXIe 6341 card and a SCB-68A breakout board.  Using a handheld multimeter to measure the voltage at the pins of the SCB-68A, I see a voltage varying from 1.415 to 1.381V.  Using both MAX and DAQmx read, I see readings between 0.8 and 1.65,  I have the maximum voltage for that task and channel set to 3V and the min set to 0 V in both MAX and using DAQmx create channel.  Is my hardware acting incorrectly?

[johnsold]

Most multimeters do some averaging, often over a period of a hundred milliseconds to a second. A variation from 1.415 to 1.381 V suggests that the signal you are measuring may have a significant amount of some high frequency component. (In this context "high frequency" is anything faster than the reciprocal of the multimeter averaging time.) Power line frequency interference is a probable suspect.

Your VI takes one instantaneous reading every 200 ms. My guess is that the fluctuations you see are the normal response to the differences in the instruments. Try setting your sample rate to 1000 samples per second. Then read 200 samples on each iteration. Let the DAQ device set the timing, meaning remove the Wait. Calculate the mean of the 200 samples and display that as data. I suspect the fluctuations will be much smaller.

Even better, plot the array of 200 points to see what the signal looks like. You may be able to see periodic behavior which will help you to identify the source of the variation. Spectrum analysis can also be useful.

Lynn

[Henrik_Volkers]

Small addition to johnsold:

If you use 300ms (or 300 samples at 1kS/s) you fit multiples of 50Hz and 60Hz line frequency 😉

[HeatherRalph]

Thank you!  noise was the issue and averaging seems to take care of the problem.  It might also be an issue with how it is grounded.

-Heather