PCI 6225 multiplexer

[WildEnte]

Hi folks,

I have a PCI 6225 with 80 AI channels that are (which AFAIK) multiplexed to one ADC reading at 250kS/s (= it does not support simultaneous sampling).

I want to do quasi-simultaneous samling by reading N channels at some few kHz in an interleaved mode (i.e., on a time axis I get Ch1[0];Ch2[0],...,ChN[0],Ch1[1],Ch2[1]...ChN[1],Ch1[2],...).

I have set up a task in MAX where I read 2 channels (ACQ mode N samples, 5kS to read at 25kHz), and I have plugged the same signal to both channels. It does seem like it's doing what I want: reading t0's from the returned waveforms gives me a difference in start times of only 14usec and the two recorded waveforms look very much the same apart from a bit of jitter.

I'd still like to be sure that it does indeed read the channels in an interleaved mode... I really don't have any room for silly errors here. So if someone could confirm that how I do things is correct, that'd be great!

Thanks!

[TCPlomp]

I think you are correct, the manual of your card should support that.
It looks like you use this to get higher sampling frequencies?
In such a case I would just get a better card (6255).
The interchannel delay is not specified if I'm correct.
For instance:
You use a sampling rate of 1 Hz with 10 channels.
What is the interchannel delay?
Could 0.1 s ([1 s/S]/10) or could be 0.004 ms (1/250 kS/s) with a rest between channel 10 and 1.

I believe it's the latter but I don't think it is officially specified anywhere.

Ton

[WildEnte]

"It looks like you use this to get higher sampling frequencies?"

No, I need to sample a couple of channels (almost) simultaneously to be able to compare their waveforms. A couple of usec time delay doesn't hurt me, but I wouldn't want it to be more than that (i.e., reading all samples of channel 1 and THEN starting with channel 2 is no good).

Playing with the card a bit more and re-reading the manula has corroborated my suspicion that it multiplexes the way I want it.

[J_Thomas]

Hi WildEnte,

You are correct in your suspicion about how the DAQ card multiplexes through the different channels in your scan list. By default, DAQ cards are configured to do “Interval Scanning.” Interval scanning collects a single sample from each channel in the scan list, and then repeats the scan list until all of the desired samples are gathered. Interval scanning uses the minimum interchannel delay for your hardware, so that you sample from each channel as fast as possible during a scan.

The following link describes the differences between interval scanning and round robin (another sampling method).  The document is helpful in that it includes a picture of how internal sampling is performed.

What Is the Difference Between Interval Scanning and Round Robin Scanning?

[WildEnte]

thanks a lot to both of you!