Interference between two sensor measurements

[LucyXF]

Hi,

I'm using USB-6211 DAQ device and I have multiple analog and counter inputs and outputs. Recently, I added an encoder input to the device and noticed that there's huge interference on the torque sensor that I had installed before.

To be specific, my torque sensor needs an digital excitation which is connected to PFI4. It's also connected to the +5v power and AO GND for power. It's output is collected through AI7 channel. The added encoder needs a +5v power too, and its output is collected through counter PFI0. I connected both of them to the same +5v and AO GND(The AO GND is also connected to a motor drive). I don't know if this is causing the problem. 

Please let me know how I can solve this problem. Thank you very much. 

-Lucy 

[Chris_W.]

Hi Lucy,

All of the ground channels are connected to eachother at one point, namely, AI GND, AO GND and Dig. GND. The Digital GND is the one that is actually connected to the ground of the bus and computer. So, this could be the cause of your problem with so many things connected to this AO GND (especially the motor drive). The +5V pin and the digital input and outputs are actually reference specifically to the D GND, shown on page 91 of the user manual for the USB-6211. That means that your encoder and sensor should be connected to that Digital GND and not the AO GND.

Also, what type of connection are you using with your torque sensor? Is it a differential connection or RSE? Differential helps to reject noise from that measurement. Page 43 and 57-60 of the user manual address good field wiring practice and considerations. There is also a good Developer Zone article that addresses field wiring considerations if you go to ni.com and search "rdfwn3". One other thing I wanted to mention was that the +5V pins and all the digital outputs have a total output current limit of 50mA, so you'll want to make sure you don't exceed that.

One thing you can do to troubleshoot the source of the noise is to see if you can measure the frequency of it to see if that matches up with a 60Hz noise or some other frequency that you know you are using from another device. The motor drive seems like it may have something to do with this issue, how is that powered?

Here is a link to the user manual. There is a lot of good information here about connections and considerations.

http://digital.ni.com/manuals.nsf/websearch/937C8FEC72331772862574030069E2B6

Chris N White

High Speed Products Group