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Open circuit behavior of analog inputs USB 6210?

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USB 6210, using NI-DAQmx Python API

I have a signal generator at 100 Hz +/-5v square wave connected to a1 which is set up as NRSE input with AISN connected to the signal generator GND. The graph below is the captured data (sample rate 30 KHz). Th first section is with a1 not connected to the signal, second section connected (this as expected), third section a1 is open circuit again, fourth a1 connected to signal generator GND (this as expected), then finally open circuit again.




This is scenario is just a mock up of the phenomena that I will be investigating. The actual signals will not be as clean as those coming out of the signal generator and there will be periods when the signal will be absent. My concern here is that tests I will be carrying out will have periods where I will be getting these reading from a none connected state & they will be difficult to differentiate from the connected state.

Is this kind of this to be expected from this equipment? What exactly is a1 measuring here when it isn't actually connected to anything?

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Accepted by Dr_Nic

In short, yes it is expected, instruments with high-impedance inputs should not be left open. If you leave the inputs open, it is expected that the reading will not be zero.


Read more here -


If you would like the signal to go zero when nothing is connected, add a pull-down resistor on the analog inputs, thus the residual charges are discharged from the line. Choose an appropriate resistor value that will not load your signal significantly and at the same time can discharge the residual reading fast enough.

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Do you have any other signals connected?


You also might want to give this article a good read: Field Wiring and Noise Considerations for Analog Signals 

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