Noise on SCXI 1300 but not 1303

[BME_genuis]

I have been having an intermittent noise issue isolated to my SCXI 1300 and specifically to one input signal (CO2 Analyzer).  The erractic behavior goes away when I connect the signals to the SCXI 1303. 

The signal that is giving me a problem on the 1300 board is ground referenced 0-7.5 V dc.  No matter what channel I use on the 1300 for that input I see the erratic behavior on all channels.  Other channels suffer due to fact it is in multiplexed mode.  I am using differential mode.  I am seeing no issues with the module SCXI-1100.  And, when I try other 1300 terminal blocks I am seeing the same exact behavior. 

I think I have an issue with ground loops.  The grounding in my system is undesirable.  My theory is that the 1303 has pull up and bias resistors that prevent the ground loop.  This makes the amplifier input into a single ended configuration which works.  Furthermore, I have a datalogger that is also single ended and I connected the output to that device and signal looks fine.

What do yall think?

[SunainaK]

Hi BME_genius,

Is the CO2 analyzer a floating source or a grounded source? The following document shows how to wire the sensor for the different types of signals:

http://www.ni.com/white-paper/3344/en

[BME_genuis]

It is a grounded source.  So you would think acquiring in differential mode with the 1300 would work.  However, only when I use the 1303 so it is essentially single-ended does the noise go away.  If on the 1300 I tie the negative terminal to ground through a 10 kohm resistor the problem goes away (like how the 1303 is configured).  However, since it is not a floating source you would not think you would need to do that.

[BenRobinson]

Hi BME_genius,

It looks like this issue is being addressed by our engineers in the below linked forum.

http://forums.ni.com/t5/Signal-Conditioning/1303-current-measurement-configuration/m-p/2188336#M6929

We will collect our combined troubleshooting knowledge in this forum post.