Multiple AI/GND on USB 6001

[LonelyHunter]

Hello !
I am facing a new challenge with my DAQ card (USB 6001). 

I've been using my DAQ to read a force platform signal for a few years. The force platform is linked to my DAQ in RSE through AI0 (Fx), AI1 (Fy), AI2 (Fz), Ai3 (Mx), AI4 (My), and AI5 (Mz). It is also wired to a GND pin and has been working fine until now. 

I decided to add a potentiometer to my DAQ (RSE again). For the potentiometer, I've been using a 12v power supply (that actually only provides around 8v). V+ and V- on the potentiometer are linked to the power supply. The potentiometer middle pin (response) was then wired to the AI7 pin and V- to a second GND pin. When I try to read the voltage, I get high noise.

--> When the potentiometer is at the minimum, I get 0V with little to no noise.
--> When I put the potentiometer at maximum, I get values between 0-8.5V. When filtered, I get values around 8.5V with little noise.

--> When I put the potentiometer halfway, I get values between 0-4v. When filtered, I get values around 4V with little noise. 
It's as if noise intensity was dependent on the potentiometer position. 

Following the noise, I found a second problem. The moment I wire the potentiometer response to AI7, my force platform channels become crazy and the little normal noise that I normally get becomes too high. I also found that playing with the potentiometer changed the force platform voltage ... as if there was a connection. 
 

I did some additional troubleshooting and wired the potentiometer to the 5V pin on my DAQ card and used the GNDd also on the DAQ. The GNDd pin was linked to the V- (lower) pin on the potentiometer and the AI GND on the DAQ. The 5v pin was wired to the V+ pin on the potentiometer. The middle pin (response) was linked to AI1. The signal was perfect then. However, I don't know if it affected the force platform signal as I did this at home. 

Final information: I used a multimeter to assess the voltage at the DAQ level and found a sable voltage for the potentiometer. Therefore, I don't think that the problem is in my wiring but seems to be inside the DAQ. Is it possible that my multimeter has a lowpass filter inside?

Any thoughts on my problem would be much appreciated!

Thanks!

[AnalogKid2DigitalMan]

Multimeters are slow to respond, perhaps your 12V supply that only puts out 8,5V is the source of your noise.

-AK2DM

[LonelyHunter]

It might be the source of the noise, but can it explain why it is affecting the other channels?

I'm going to the lab to test a new power supply. 

 Here is the signal using the DAQ 5v pin. Not that the output is on AI6.

Here is the AI7 when using the output on AI6. There are some noise and voltage changes following motion on the potentiometer, but the values are really low. (gonna test it with the force platform later today).

Here is the potentiometer voltage (AI6 again) if my external power supply is used. Note the noise between 10 and 0 when not moving the potentiometer. 

And here is the effect of the external power supply on AI7. Note the higher peak noise. Which resembles what I saw when connected with a force platform. 

Am I to understand that my external power supply, which might be defective and noisy, has a larger effect on AI7, which results in higher cross-talk/ghosting?

======

Considering that my force platform and my potentiometer (and power supply) might have a different ground value, would it be better to put the potentiometer in differential and keep the force platform in RSE? If I move one connection from the force platform to another AI, I could use a differential pair of AI for my potentiometer. Otherwise, I could also use two DAQ, but this is less optimal.

Thanks! 

[Henrik_Volkers]

oh yes, ground is a .....

If you want to read out a potentiometer , have you tried the 5V provided from the 6001?   Still the 5v are noisy ... but a cap from the wiper to gnd can help.

[LonelyHunter]

Yess ! Managed to obtain something good with the 5v from the DAQ. Not really noisy... And we realized that all our external power meter were broken (not good). 

Now I'm playing with Crosstalk/ ghosting (not sure the difference between them). 

The potentiometer affect the other Channels. So when I turn the Pot, it stimulates the force plate channels. 

I managed to fix it by putting the potentiometer on ai7 and removing the channel on ai3 (adjacent to ai7) and putting it elsewhere... Then it created interference with other channels from the force plate.

I did some match and play to minimize the interference and I have something ok for the moment, but I know it's not optimal. 

Force plate Channel 1 :ai0

Force plate Channel 2 :ai1

Force plate Channel 3 :ai6

Force plate Channel 4 :ai5

Force plate Channel 5 :ai4

Potentiometer :ai7

Any idea how I could further optimize the reduction in ghosting/ Crosstalk?

Thanks !

[santo_13]

These articles will be very helpful

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P9mKSAS&l=en-US

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P84RSAS&l=en-US

[LonelyHunter]

Thanks for the info. Somehow google didn't directed me to these resources!

I will look at them.