Real-Time Measurement and Control

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Noise on cRIO 24V input affecting AI measurements


My system is configured as follows:

  • NI-9144 Ethercat cRIO chassis
    • NI-9211 in slot 2, all data is on the scan engine
      • AI0 is 24V power (same power as cRIO power) (+24V connected to AI0, -24V connected to COM)
      • AI1 is a tachometer (+ connected to AI1, - and shield connected to COM)

I am using the other channels of the RIO as FPGA control for a 3-phase motor drive (analog control).

So I have noise on my tachometer signal when my motor drive is enabled.  I believe the motor drive is connected correctly and the shield is done per the manufacturers specifications, this portion isn't my specific questions, just a note for you.  


At first I thought the noise was on my 24V AI (unshielded wire to AI0) but when I disconnect the 24V from AI0 (and COM) the noise is still present.  Then, I disconnected the tach and reconnected the 24V and still saw noise on my tach.  Further, I shorted AI1 to COM and STILL saw noise on my tach (AI1).  If I remove both 24V (AI0) and the tach (AI1) the noise disappears.

To make this even more confusing, the tach wiring is completely isolated from the rest of the system (the 24V comes from some terminal blocks that feed other things).

Is the noise being introduced through the cRIO's 24V supplied power?  Is it possible to get noise onto the AI signals in that manner?

Let me know if I need to clarify anything and thanks.

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The 9211 is a thermocouple module. I'm guessing you have a 9221. The spec says -75db of crosstalk so I'd expect on that order of magnitude to get through.

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