how can I acquire 2 signals simultaneously with SCXI-1122

[julesjay]

Hi,

I would like to acquire 2 simultaneous signals. My config is : SCXI 1001 chassis with severals SCXI-1122. DAQ M 6251 connected onto it.

I 've created a single voltage input (slot1/ai0) with MAX and configured it to acquire 100 samples at a rate of 1kHz => Everything goes well

Then, I have put a second voltage input (slot1/ai1) , and I can only  go at a rate of 50Hz  which is normal according to the SCXI-1122 specification (100 S/s max).

So I've tried to put the second input on another slot (slot3/ai0) => I can again only go up to 50Hz!! And I don't understand why??

Is there a limitation due to the SCXI chassis?

thanks for help

julesjay

[julesjay]

Hi

I still have this problem...

Nobody can help me?

thanks

[Brad_K]

Hi julesjay,

The SCXI-1122's analog front end has a mechanical relay multiplexer and a shared lowpass filter, so the available sample rates when scanning more than one channel at a time are very limited. The knowledgebase Incorrect Measurements with SCXI-1122 when Scanning Two or More Channels has more information about this issue and possible workarounds.

Also note that with multiplexed SCXI (and with M Series), the maximum sample rate must be divided by the number of channels you are scanning.

Brad


Message Edited by Brad K on 02-18-2008 03:44 PM

[julesjay]

thanks for your answer.

I 'm ok with your explaination for the case where i've got 2 channels on the same SCXI-1122 card.

But in the case of 2 channels with 2 SCXI-1122 cards (so 1 channel per card) I don't understand why I'm also limited to 50HZ.  I thought that in this case I should be limited by the the M series (1.25MS/s) / 2 which is far more than what I need!!

How do you explain that?

[Brad_K]

Hi julesjay,

A multiplexed SCXI acquisition with two channels on separate modules must multiplex the outputs of both modules onto the same analog bus. The chassis "slot 0" scanning controller controls when each module drives the analog bus. Some settling time is required to account for this, but it appears that DAQmx is using the same settling time calculations whether the channels are on different modules or the same module. So it accounts for the time needed for the relay multiplexer and filter to settle, even with one channel per module, when there are multiple modules in the task.

Could you describe your application? It sounds like you are trying to acquire multiple waveforms simultaneously using a separate SCXI-1122 for each signal. If simultaneous acquisition is not actually necessary for your application, scanning one channel at a time (using a separate DAQmx task for each channel) will still let you acquire a waveform or average out many samples before moving to the next channel. How much slower it is than the ideal depends on how many signals you are trying to measure.

Is this a new application or are you porting an existing application from Traditional NI-DAQ (Legacy) to M Series and NI-DAQmx?

Brad

[julesjay]

Hi Brad,

Simultaneous acquisition is required for my application. I've to check that I've got two pulse at the same time (28V - during 10ms).

I try to acquire both waveform (1000 Samples at  50000S /sec  on each channel) with a trigger on the first one (pretrig 100).

It's a new application under CVI 8.5 with DAQmx and a PCI-6251 M Card.

julesjay

[Brad_K]

Hi Julesjay,

It sounds like you might also run into a couple of other issues.

First, the SCXI-1122 has a selectable lowpass filter cutoff frequency of 4 Hz or 4 kHz. You can't turn off the filter. When you acquire waveforms at 50 kS/s with the SCXI-1122, all frequencies above 4 kHz will be severely attenuated. Depending on the width of the pulse you are trying to measure, this might be exactly what you want, or it might be a showstopper.

Second, it sounds like you want to use an analog reference trigger on the first channel of your task so that you can acquire 100 pre-trigger samples. M Series and E Series multifunction DAQ boards only support analog reference trigger from a channel in the task when there is one channel in the task. If there were multiple channels in the task, the switching of the input multiplexer would cause the reference trigger to happen early. This doesn't happen for start triggered tasks that acquire post-trigger samples.

To work around this, you could connect your trigger signal to the APFI0 pin on the M Series using an SCXI-1180 feedthrough panel or the connector on the side of the SCXI-1349. However, the signal being wired to APFI0 would not go through the SCXI-1122, and would have to be attenuated externally. Digital reference trigger sources also are not subject to this restriction.

If you already are planning to use multiple SCXI-1122s, you might have more success by putting them in separate chassis and connecting them to separate M Series multifunction DAQ boards, synchronized over a RTSI cable, so that you could acquire from them simultaneously, but that would complicate your application code and require doubling up on more hardware. A different SCXI module might work: for example, the SCXI-1104C has an input range of +/ 60 VDC and has a 10 kHz lowpass filter cutoff frequency, but you would still be limited to one channel for analog reference triggering, if that is indeed a requirement. An S Series simultaneously sampling board or an isolated counter-timer board like the PCI-6624 might also be more appropriate, depending on the specifics of the pulses you are trying to measure. If you haven't already done so, I would recommend having our sales representatives (http://www.ni.com/contact) help you find the best product for your application.

Brad