9207 incorrect read out - possible wiring problem

[Hymstra]

Connecting both voltage and current  sensors to my NI 9207 result in faulty readouts in Distributed system manager.

Situation:

NI cRIO9075 with module NI9207 powered by a NI PS16.

To this I have connected a floating voltage channel, a grounded voltage channel and a self powered current channel. (see schematic png)

For the grounded voltage channel I measure the same values in Distributed system manager as with my Fluke 83 Multimeter.

For the floating channel I measure a correct  value as well.

But for the self powered current channel I see a slightly lower value for NI compared to my actual current output.

(i.e. output = 18mA, NI=15mA  or   output = 0.12mA NI = 0.10 mA etc.)

I suspect there is a problem with the grounding or reference point of the current measuring module. Since I had similar problems with my floating

voltage channel before I grounded it correctly using the 1M resistor.

Is this the correct way to connect the module? Or is something amiss?

[crossrulz]

I assume you meant AI8 for the current, not AI18.  But you do have your wiring messed up for the current measurement.  If you look on the side of the 9207, you will see how it is supposed to be connected.  The current readings are done with a shunt resistor to ground.  So your return for your circuit should go to the AI8 channel.

[Hymstra]

Indeed, I ment AI8 instead of A18. Thanks for pointing this out.

However I assumed the shunt was included in the module, since it is drawn within the dashed module border.

As I also assume the COM is connected to the NI chassis ground internally.

Is this not the case? 

[crossrulz]

I'm not sure COM is connected to the chassis.  But you should still wire up the COM to your ground.  The shut resistor is part of the module.  But you need to connect the return of your circuit to the analog input and the shut resistor will ground your return.

[Hymstra]

I see there is an error in my picture. The arrow of the current source is in the inverse direction. So in the actual situation, I have the return of my current circuit connected to AI8 and the second (ground/reference) pin of my circuit is connected to COM. (see corrected sketch) I tried wiring the COM to my ground as well but this does not help.