Grounding unused/floating AI channels on PCI-6225

[Matt_D]

Hi, I'm currently running two DAQ cards on the same computer, PCI-6225 using 80 AI's and PCI-6723 using 32 AO's, that stimulate and record to/from a test system. This test system is a little unusual where sometimes both AI and AO channels can float and there may be no current passing to some AI channels. When reading single-ended AI's, if some AI’s float, I see a coupling between unused channels and used channels, i.e. if channels AI 1 to 4 are in use, and only AI 1 & 2 receive a signal (say 3V from AO), channels AI 3 & 4 take on the value of 1 &/or 2 (not 0V). This seems like a ground-loop problem, but after trying different ground configurations and source types using the “Signal source type” diagram in http://www.ni.com/white-paper/3344/en/, I’m just getting confused. 

If I want the floating channels to go to ground should the unused channels have pull-down resistors to the AI's GND? Which configuration in the diagram should I use? 

[crossrulz]

What you are seeing is actually "ghosting" due to the MUX inside of the DAQ.  A resistor to GND would help, but you are technically draining a capacitor.  So you may have to play around a little to find a resistor value that works well for you.

[Matt_D]

Thanks for the reply. After reading about ghosting here (http://digital.ni.com/public.nsf/allkb/73CB0FB296814E2286256FFD00028DDF), this makes some sense.

For the above example, I tried to lower the sample rate but the ghosting problem still persists -- maybe this is because of the high impedance/floating channels.

I'm going to look at using a Unity Gain Buffer (Voltage Follower) for each channel, as I would like to keep the sample rate high, and I can't really lower the input source impedance.