NI-9205 Input Reading Variation

[ZachMay]

I'm having issues with some of the readings I'm getting from my NI-9205 analog input card. Due to limited availability of NI hardware, I had to change from a NI-9209 to a NI-9205 in my test and measurement system, and now my readings seem to have a lot more variation than I've ever seen with the NI-9209. While the variation is concerning, now I have two analog inputs that appear to float anywhere between 0.7V and -0.1V while the output of the transducers (same type of transducer) feeding those analog inputs are extremely stable.

I have connected the COM of all of my transducers tied together, the 9205 COM tied to them, and all of it connected back to earth ground. I've also tried connecting directly to the common of my 24 VDC supply instead of ground, and it made no impact to the measurement. Also worth noting, my 24 VDC power supply is a switching power supply and it is floating referenced to ground.

Not sure if it's related or not, but I have another strange issue where LabVIEW is reading some of my analog inputs as ~ 0V (which they are) while the NI Distributed System Manager is reading the same inputs as ~ +5V at the same time. I've verified that the outputs of my transducers are at 0V, but nothing I have tried changes the reading of the Distributed System manager.

This is my first experience with the NI-9205 so I'm just not sure if there's something I'm not understand or considering about the card. I have tried replacing it with a spare card that I have, but it didn't seem to have any impact on any of my issues. Does anyone have any suggestions?

[Tom_Powers]

Looking at the data sheet.  That device can be 16 channel differential or 32 channels single ended. 

When you set it up as in create channel subVI?  Often times the default is differential.

[ZachMay]

I just went through the LabVIEW project and verified that every output is set as +/- 10 V and RSE connection type, so that shouldn't be the issue!

[Tom_Powers]

Is there a reason that one side of your DC power supply is not tied to (signal) ground.  It is normal for DC power supply modules to have both terminals floating with respect to ground.  The normal approach is to ground the negative terminal when using it as a positive supply.  You may need to provide some more information regarding your the specific transducer and signal conditioning chain so that the problem can be solved.

[ZachMay]

We have a few different power supplies inside of our cabinet, but our main 24 VDC power supply has to be floating for a few of our devices to work properly. I haven't tried powering the modules/chassis with the common tied to ground though, so I'll test it and see if that has any impact.

The transducer is an Allen Bradley 931N-X221 signal conditioner. It's powered from the same 24 VDC power supply as the 9205 and I have confirmed that the output on it is steady with a DMM. The output goes directly to the 9205 with no other signal conditioning between them.