Measure a DC voltage

[lvrichard]

Hi All,

a simple one,

I am using a USB6216 DAQ to measure a 3V dc signal. I am inputting the signal to a analog input.

Attached it the signal being read from the DAQ. Becasue of this the signal could be anywhere between 0-3 volts, which is casuing incorrect results.

How would i go about properly reading this voltage ?

Thanks,

Richard.

[GerdW]

Hi Richard,

usually you follow the manual as it explains how to measure input signals…

How does your wiring look like? How does the signal source look like?

[LocalDSP]

Hi lvrichard,

     Can you clarify a few things?

• Your signal is not a DC signal but rather a more or less square wave, is this expected look?
• Were you expecting a more or less constant level (in your case approx. 2,9 V)?
• If the square look is expected, what value are you trying to measure, the high level (approx. 2.9 V on your screenshot)? 

[lvrichard]

Hi,

The input (confirmed with an oscilloscope) is a constant 2.9volts and not a square wave. I feel the square wave is a result of the sampling rate for some reason.

Thanks

[LocalDSP]

Pure guessing...

Your disturbing pattern is exactly at 50 Hz and I can see your DAQ step is configured in differential mode, so I would suspect you are having serious (main power) common mode issues that may overload your DAQ device. Have you connected your ground correctly? Have you tried single-ended mode and if so... does that fix your issue?

[Bob_Schor]

Hmm.  You appear to have a signal that "goes to ground" at very close to 50 Hz.  Are you, perhaps, at a location that has 50Hz A/C power?  Do you know about how to wire a DAQ device for analog voltage, including proper grounding?

While you show no code nor wiring diagram, the nature of the signal you show suggests a "wiring" issue as my "best guess".

Bob Schor

[lvrichard]

Hi Bob,

I am using a differntial set up. Do I still need to connect the negative terminal to AIGND ?

This seems to solve the issue ?

Thanks

[crossrulz]

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@lvrichard wrote:

I am using a differntial set up. Do I still need to connect the negative terminal to AIGND ?

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Even differential signals need a ground reference.  So you need to make sure everybody is using the same ground.  I will just defer to a white paper from here: Field Wiring and Noise Considerations for Analog Signals