Multiple ATI Load Cells to PCI-6221

[woeileong1104]

I am trying to connect two ATI load cells to one PCI-6221 using NRSE. First load cell (Ch0-Ch5) works fine, but voltage readings from second load cell (Ch8-Ch13) keeps jumping in square wave. Both load cells had their AI0- connected to AI Sense, AI1- to AI5- are not connected to anything. Both load cells share the same AI GND and DGND. Any feedback on this will be highly appreciated. Thanks.

[Nathan-M]

A couple questions to help debug:

1. What happens if you swap the load cells? (i.e. does the 'square wave jump' follow the load cell or the DAQ channels)
2. How are you doing these readings? (your custom application, DAQmx shipping examples, NI MAX Test Panel, etc.)
3. When you read from these channels, are you reading from all at the same time, or individually?
4. What is your channel configuration? (min voltage, max voltage, etc.)
5. What values (in volts) is your signal 'jumping' by? 

Also, it seems like these are just 6-axis sensors, right? There's no other wiring involved? If you have a wiring diagram that could be helpful, too. 

You also shouldn't need to connect AIGND to DGND. They're (loosely) connected internally and should stay separated. It's a fair troubleshooting step, but you generally want to keep the high frequencies associated with digital signals (a square wave is the sum of higher and higher frequencies of sine waves) separate from the ground plane of your analog side. The noise could interfere with your ADC conversions.

I suspect that you're reaching the quantization limit of your ADC but will need to know more about the system to be sure.