Re : Re: Re : Incorrect reading with NI 9205 in differential mode. Role of COM not clear.

[blind1]

Role of COM is still not clear... 

I have a earth grounded source. I  want to measure it differential with a NI9205 installed in a cRIO.

Should i connect as  illustrated  in  A or B ?

A.

B.

[Pcorcs]

For your configuration, you will be using Figure B. Note, the your source is already referenced to earth ground; such that, your physical wiring will jump the AI- to the COM on the NI-9205, there will not need to be any additional earth ground. The COM terminal provides a reference to prevent the 9205 ADC from floating, as the ADC is isolated from the module input topology to help protect the conversion circuits from external noise, or over-voltage. 

I have illustrated the physical system below. Please note, where I have drawn the isolation barrier, it may not be in that exact location in the C-Series topology. It is drawn to demonstrate the existence of such a barrier, and the importance of providing a COM path through the barrier to properly reference the ADC. This barrier also does not mean that the module is fully "isolated," it is a means of regulating the analog input information to reduce measurement noise. It is not functioning as an industrial isolated system, for information on the system input requirements, please refer to the 9205 specifications. 

Hope this helps explain the function of the COM input terminal. Please post back any additional questions.

Thanks,  

[Vakpo]

Hi everyone,

just to say that I solved my porblem by simply changing the calibration mode of the 9205. In fact when updating my program from labview 8.6 to labview 2009, I used the new example vi that comes with LabView 2009 for the 9205 module. This example vi works if the calibration mode of the 9205 is set to "calibrated" instead of "Raw". So pay attention to that point and check the calibration mode in the properties window of your 9205 module. I used the configuration B illustrated by blind1.

Vakpo

[John_P1]

I just wanted to correct Pcorcs, the 9205 is isolated (250 Vrms CAT II Continuous;  2300 Vrms Withstand).  The specs can be found here. 

I think what was meant was that the 9205 is not channel to channel isolated, rather it is bank isolated, meaning that all of the input lines share the same reference (COM).  Since the working voltage is specced at 10.4V, all channels must remain within 10.4 V of COM.  For this reason, you should connect COM to a reference point that is 1) stable and 2) within 10.4 V of your input signals (even in differential mode).

In your case, since the sensors are ground referenced, a common solution would be to simply connect COM to Ground.  The following knowledgebase gives information about how to connect the 9205 in different measurement types:

Noisy Readings on the 9205

Best Regards,