DAQ voltage change when adding additional inputs

[ByrneLab]

Hi All, 

Hi all - relatively new to data acquisition here. 

I have a thermocouple that I have wired to a AD8495. The output from the board is wired into a USB-6002. I also have 2 pressure sensors from Millar with their outputs from their control units into the USB-6002. I wanted to calibrate the thermocouple but I have a big program that acquires and sends data out from multiple DAQs. So instead of running that big program I wrote a very simple program that uses a differential configuration to collect data from the thermocouple continuously at a rate of 2 Hz. I used the simple program to collect voltage output from the thermocouple and correlated it with temperature data taken from a control thermometer to create a calibration curve. However, when I plugged the calibration numbers into the big program the numbers were way off - even though it works in the simple program. I found that if i remove the 2 Millar pressure sensors from the DAQ assistant in the big program (big_program_pressure_removed.png) the voltage value from the thermocouple matches that of the simple program (simple_program.png). But if I add the pressure sensor signals back in - there is an offset introduced (big_program_pressure_removed.png). 

Is this normal or is there some kind of cross talk happening between my channels that shouldn't be there? 

I cannot share my "big program" file. 

[CDuck]

USB6002 doesn't have channel isolation.  Have you tried increasing the physical distance between channels?  What about using just 1 pressure sensor instead of 2? 

Are you using common ground between all sensors?  If not, you may be dealing with a ground loop causing that offset.

[CDuck]

Great video that may help grow intuition for your problem

https://youtu.be/Xc8dP0PdC_4?si=AC8_eDzrhL8emtmg

[mcduff]

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA03q000000YHy6CAG&l=en-US

[ByrneLab]

I tried increasing the channel delay and put the temperature reading first in the channel list, but didn't see an improvement in the ghosting (~ 0.01V). The signals from the pressure sensors and the temperature sensors are relatively close in value (1.28 for temp vs 1.25 for pressure) so I didn't know if a dummy channel would help. However, I found a different issue that might be the problem. When I did the calibration I had the DAQ assistant set to differential for the channel, but had the wires in an RSE configuration (channel and 3 ground) not a differential configuration (AI3+ and AI3-). The jpeg of the wiring is attached. 

I cut a chunk out of the "big program" so that I could share it here. 

I really appreciate your help - could you advise on if the wiring could be the issue? I'm not entirely convinced as I don't change the wiring when switching between VIs, but I see a difference between the two voltages. However, I know there is alot going on here that I don't fully understand. 

[ByrneLab]

Could you check out my other reply? 

[LVNinja]

The issue seems to be most likely connected to ground wiring. 

[ByrneLab]

Could you elaborate?