Synchronizing different DAQ cards

[ehrlich]

I have a PXI-4472B and a PXI-6289 card and would like to acquire synchronized data from the two. The examples for synchronizing e.g. the analog input of multiple DAQ cards which I found all assume that one uses the same type of card but here I am mixing M series and DSA.

So before diving into this: Is that possible at all or are there any limitations here?

[Kevin_Price]

This can be tricky due to the filter delay in your DSA board and the restrictions on its sampling timebase, etc.  You definitely aren't going to want the 6289 to be the timing master in the arrangement.  If you use the DSA board as the timing master you might have to export the sample clock explicitly for use by the 6289.

Even then, you need to verify things carefully.  If, as I *think* I might recall, the DSA sample clock asserts at the end of its filter delay, then the 1st sample on the DSA board is from something that happened a little in the past (1 filter delay worth of time) while the 1st sample on the 6289 is from something happening right now.

I'd plan to simply discard the first N samples I read from the DSA device, where N compensates for the filter delay.  Here's why:  if you wire the same waveform into both the DSA and the 6289, share the DSA's exported sample clock signal, and discard the first N samples from the DSA, then the 1st samples you keep from the two boards refer to what was happening at the same actual point in time.

There are more complicated ways to set up sync via timing signals but I'd avoid them if possible.  When I had to deal with that in a past project (AO-based, so there was no luxury of ignoring or discarding signals), things got pretty hairy.  Granted, I had a lot more boards and some were older legacy types with less flexibility for routing their timing signals, but still.

-Kevin P

[Christoph_D · Accepted solution]

Hi,

Kevin is right in his assumptions. There is always a sampling delay with delta-sigma ADCs (depends on the sampling rate). That's specified in the specifications either as a time delay or as a delay in samples. See the section 'ADC Filter Delay' in the PXI-4472B specs:

NI 447x Specifications
http://www.ni.com/pdf/manuals/373861d.pdf

To account for the delay you normally pre-read the calculated number of samples from the delta-sigma devices.

Why Is My Data Delayed When Using DSA Devices? - National Instruments
http://digital.ni.com/public.nsf/allkb/F989B25FF6CA55C386256CD20056E27D?OpenDocument

To synch your cards you can either use the Channel Expansion (which does NOT account for the delay, so you might remove the samples afterwards) or you follow the approach from the 'Why Is My Data Delayed When Using DSA Devices' article and synch your cards manually. The article 'Synchronizing Analog Input C Series Modules with NI-DAQmx' describes what channel expansion will automatically do in the background (which you could do manually):

SAR/Slow Sampled and a DSA - When a SAR or Slow Sampled module and a Delta Sigma module are placed in the same hardware timed AI task, the DAQmx driver configures the Delta Sigma Module to export its internal sample clock to be used by the slave module. The internal sample clock is equivalent to the data rate configured for the Delta Sigma Module fs=fm/256/n. Where fs is the data rate, fm is the internal Master Timebase specified in each modules specification sheet, and n is a divisor. The driver will return duplicate samples for the Slow Sampled module in order for the Delta Sigma module to run at a faster rate.

(http://www.ni.com/tutorial/5376/en/)

Channel Expansion Explained - National Instruments
http://www.ni.com/white-paper/52634/en/

Also please read the following additional information that apply to your PXI-4472B:

Synchronizing Legacy Dynamic Signal Acquisition (DSA) Products with NI-DAQmx
http://digital.ni.com/public.nsf/allkb/A133ED27DF9BCC5986256F2E004BA342

Best regards,

Christoph

[ehrlich]

Thanks for the explanations and pointing out the parts to pay attention to. I am happy to hear that in principle it should be possible (my main fear was that this doesn't work for some hardware reason).

I'll look into the links you provided.

Thanks again!