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ni 9215 multiple floating sources

I am using a NI 9215 BNC module in a CDAQ-9178 chassis to acquire measurements from multiple sensors.  


I had attached signals to each of the 4 channels and was monitoring ONLY the AI3 channel, a velocity transducer signal, when I noticed a strange waveform noise (FIG2).


Through disconnecting the other channel signals, I noticed the noise disappeared when the signal on channel AI2 was disconnected (FIG1).  


The velocity transducer on AI3 is powered by a 24VDC power supply with the AI3- being the 24VDC COM.  The other transducer, connected to AI2, is powered by a 5VDC power supply with AI2- being the 5VDC COM.  


As a check, I took the second transducer out of the equation and connected AI2 directly to the 5VDC power supply and saw the same results.


Is it not possible to connect multiple instruments to the same NI 9215 module if all the AI- are not tied together?  What would be suggested in this case?


Thank you for the advice.



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Message 1 of 7



Seems like something is not grounded correctly. I understood that you're using floating sources with diff configuration, did you use a resistor between AI+ and AI GND and another resistor between AI- and AI GND? Take a look to

Table 1 of this article.



Carmen C.



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Message 2 of 7



Thank you for your response.


You will have to forgive my ignorance. I am still quite new to LabVIEW and the NI hardware.


I believe you are correct when you point to this likely being a grounding issue.  I have not connected the bias resistors and frankly don't see where one would do so on the BNC module that I have.  This module does not provide a separate COM terminal that is connected to the module's isolated ground like the screw terminal variety does.


If you have a suggestion on where I should make the connections, that would be great.


Thanks again,



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Message 3 of 7

Hi Matt,


Sorry I missed the BNC part.


I think that it might be produced by Ghosting Effects.


Carmen C.

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Message 4 of 7



This module has a separate ADC for each channel, so I do not believe ghosting should be an issue.



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Message 5 of 7

How different are the voltages on AI2- and AI3- ?  While they don't need to be tied together, they can't be drastically different - the specifications for the 9215 require that all inputs "must remain within ±10.2 V of the average AI– inputs".


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Message 6 of 7

The signal on AI2 in this case is going to be between 0V and 5V while the signal on AI3 is between 0V and 10V.


The AI2- is from a transducer with the signal referenced to a 5VDC power supply common and the AI3- is from a transducer with signal referenced to a 24VDC power supply common.  There is no continuity between the two commons.


All AI- inputs on this module should be nominally 0V.



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