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NI-USB 6351 AI signal continuously increase even when disconnected

Dear all,

I'm experiencing a big issue with my NI-USB 6351. The board was working properly up to the end of the next week when suddenly I noticed that the acquired signal  ( a ramplike voltage signal from an amplified photodetector acquired thorugh AI0 port) had an offset continuously increasing: the signal than continuously grow and rapidly reach the saturation value of 10V.

I tested the board  using the test panel from NI-Max disconnecting every input and every output and closing the input on 50 ohm load. The test  continuously show me that even when there is no signal connected the acquired signal is monotonically increasing up to 10 V, value at which the saturation is reached. If I stop the test and I restart it, the acquisition start from the value reached in the last test ( so if it is saturated , it remains in saturation no matter how many times I stop and start the test) . The only way to "reset" the acquisition is switching off the board . 

I tried to make a self test and a self calibration other than a reset from Ni MAX but this tests didn't have any effect.

I'm using Labview 2017 on windows 10.

There is someone that has any idea to fix this problem?

Thank you in advance



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I have the same problem, with nothing connected to the USB-6351, I see a rising voltage from 0 to 10VDC every time. See attached pic.

Did you ever fix USB-6351 or find root cause? Did NI provide any help?




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Message 2 of 4

You should not typically leave AI disconnected since the inputs are high impedance, the charges near the traces quickly builds up causing this behaviour.


As long as you don't see this issue with a signal connected, this is not a problem at all, just a misunderstanding of behaviour.


refer this article for more information,


Soliton Technologies

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Message 3 of 4

Santo_13, thanks. Yes, it appears you're correct. With a signal connected or small impedance it does disappear.

It can be misleading in the beginning of code development. Good to know and thank you for your reply.

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