CMRR mode in 6211

[DKFF]

Hello all,

I am building a data acquisition system that measures very low voltage (human electrodes) using NI USB-6211. I am using NRSE mode, using AI +, AI Ground and AI Sense (reference) channels. This part seems working OK but I also need to measure single-ended (or differential) analog input on the other AI channel. I created a task with 2 physical channels (each with different mode) but when I measure both channels, channel with NRSE mode also reads what the other channel is reading. I can't figure out how to measure these 2 AI channels independently from each other. Can someone help where I'm doing wrong? Much appreciated.

[Bob_Schor]

For low-level signals, you really want an intermediate amplifier to be hooked up close to the subject in differential mode and maybe give you a "factor of 10" (or higher) buffering (and impedance-matching).  I would also recommend either doing everything NRSE, differential, or single-ended (I think I would start with all-differential, if possible).  Note -- I'm not an engineer ... but do have experience measuring bio-signals.

Bob Schor

[DKFF]

Thank you for the reply. The problem I have is, 2 signals are coming from different source - one is measuring human electrodes reference to human ground (I'm trying to build Electroretinogram program) and the other channel is stimulus signal from Light source - I'd like to plot both stimulus and response in the same plot in the same time scale. When I set both channels to NRSE, sharing the same ground (AI0, AISense and AIGround to the subject and AI1 to stimulus source), I read the stimulus signal on both channels...Is there a way to separate these 2 channels completely? Also, is CMRR implemented in all modes, RSE/Differential/NRSE in 6211?

Thank you again.