6009 voltage reading jumping around

[mattlikesit]

I am using an NI-6009 to read and send to a Hastings mass flow controller.

I have the output for the MFC going to ao0 and the input from the MFC going to ai2.  Here is the problem:

I am getting voltages that jump like this:

-1.434

-1.045

-1.678

-.991

But if I take the ground and signal from the MFC and hook them up to a voltmeter, I get voltages like this:

2.313

2.309

2.316

2.308

These voltages are very stable when read with the voltmeter, but when I put the exact same inputs to the usb 6009 I see jumping.  What is going on??

Thanks

[crossrulz]

Voltmeters have a tendency of performing averages or measure very slowly.  You should try probing with an oscilloscope to see what noise might be on the line.  It is also sounding like you might have a configuration and/or wiring issue.

[mattlikesit]

Thank you.  I might have stumbled upon a clue.

For some reason I tried to put the signal common, not into the grnd of the 6009, but into the twin of ai2 (ai6) and now things are settled and in the right range!

Can you explain why this happened?  I thought the signal common went to the ground.

[crossrulz]

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@mattlikesit wrote:

For some reason I tried to put the signal common, not into the grnd of the 6009, but into the twin of ai2 (ai6) and now things are settled and in the right range!

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That sounds like you have your channels set up as Differential when you originally wired it as Reference Single Ended.