Voltage referenced to zero

[cmorale4]

Hello all! 

I am working with a Wheatston bridge circuit so I have a differential voltage measurement in my DAQassistant VI. Everything was working as expected but now, from the sudden, it seems like the measurement is always referenced to zero as you can see in the following plot. I checked the output of my circuit with an external digital voltmeter and the signal shown is as expected, around 0.60mV with electronic noise of 60Hz. However, the signal output from the DAQ is completely different. I tried using a different DAQ to checked if the problem was that my DAQ was no longer working but I had the same response with a different one. I was wondering if someone here knows what might have changed in the DAQassistant configuration that is making the signal to be plotted like this. 

Thanks!

[DianeS]

Could your acquisition be configured for RSE instead of differential?  I never use the DAQ Assistant, but you may have your channel configuration set to "default".  That means that the DAQ Assistant will choose the channel configuration based on the board.  Make sure your configuration is set explicitly to "differential".

Give that a try.  If it doesn't work, post your code so we can take a look.

[cmorale4]

Hi!

I do have the DAQ assistant configured as differential. As I mentioned before, it was working just fine and then from the sudden it is not working anymore. I have a very simple code. All I have is the DAQ assistant to acquire the signal from my DAQ device. Please find bellow the configuration of my DAQassistant. Any suggestions are greatly appreciate it.

Thanks!

[DianeS]

Would you mind posting the actual code?  The image is great but doesn't let me see all your settings.

Also, exactly what changed between when it was working and when it wasn't?  Did you get a new computer?  New board?  Different circuit?  Software doesn't just up and decide to work differently one day, so something clearly changed somewhere.  Pinning down what it was, even if it seems unrelated, will probably point you towards the problem.

[Ben]

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@DianeS wrote:

Would you mind posting the actual code?  The image is great but doesn't let me see all your settings.

Also, exactly what changed between when it was working and when it wasn't?  Did you get a new computer?  New board?  Different circuit?  Software doesn't just up and decide to work differently one day, so something clearly changed somewhere.  Pinning down what it was, even if it seems unrelated, will probably point you towards the problem.

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pssssttt...

CMR will come and go if there is no reference betwen the circuit under test and the ground ref of the DAQ widget.

When it drifts into range all is well. When the CM voltage exceeds the limit all bets are off.

Ben

[crossrulz]

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@Ben wrote:

CMR will come and go if there is no reference betwen the circuit under test and the ground ref of the DAQ widget.

When it drifts into range all is well. When the CM voltage exceeds the limit all bets are off.

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Ben has a good point.  Make sure the circuit ground and AI GND of the DAQ board are connected.