Workaround to have NRSE and Diff channels in one AI task?

[jared.s]

According to the documentation:

NI-DAQmx Base is designed to complete one process/task per board functionality

at a time. You can have one task that does DIO and another task that does

AI on the same board at the same time. You cannot, however, run two AI tasks on

the same board at the same time.

So that means on my USB-6218 I can't have 20 channels of low speed (50hz) NRSE and 3 channels of high speed (2000hz) differential. Is that correct?

I can get around the speed difference by sampling all channels at 2khz and averaging the extra samples down to 50hz. I'm wondering if there is a way to set all channels to NRSE but wire up 6 NRSE channels such that in software, I can subtract/add a pair of NRSE channels to eliminate common mode noise. I'm thinking it might be possible as long as the pair of channels (e.g. AI0/AI8) are sampled simultaneously even when they are both configured as NRSE.

By the way, I'm doing continuous sampling. That brings up another idea. Could I have one task as 3 channel, high speed, differential set to continuous and have another task as 20 channel, low speed, NRSE but single sample mode?

[rolfk]

No to all! the NI-62xx series are all having one ADC only and also only one timing curcuit for the entire analog input curcuit. This makes it any of your proposed solutions impossible except the one where you sample all the channels with the highest required sample rate and average the slow channels down.

[jared.s]

Even this?

"I'm wondering if there is a way to set all channels to NRSE but wire up 6 NRSE channels such that in software, I can subtract/add a pair of NRSE channels to eliminate common mode noise. I'm thinking it might be possible as long as the pair of channels (e.g. AI0/AI8) are sampled simultaneously even when they are both configured as NRSE."

It has nothing to do with the one timing circuit limitation.

[NI-Bongo]

Hi Jared,

jared.s wrote:

Even this?

"I'm wondering if there is a way to set all channels to NRSE but wire up 6 NRSE channels such that in software, I can subtract/add a pair of NRSE channels to eliminate common mode noise. I'm thinking it might be possible as long as the pair of channels (e.g. AI0/AI8) are sampled simultaneously even when they are both configured as NRSE."

It has nothing to do with the one timing circuit limitation.

This is not because there is only one timing resource, but because there is only one ADC with a mux in front of it. 

Because of the mux, the channels are not sampled simultaneously.  However, the convert clock (that switches the mux) may be running fast enough relative to the frequency content of your signal that the slight dt between channels may not be critical to your application.  In DAQmx, the default convert clock period is 10 microseconds unless it must be faster for a higher sampling rate.  I don't know if there is a default rate for DAQmx Base.