NI9239 external master timebase with the cDAQ-9172

[hibux]

Hi

I'm trying to sample at 1.2 kS/s with the 9239 AI module, but the module only supports sampling below 1.613 kS/s if using an "external master timebase" according to the operating instructions. I'm using the cDAQ-9172 chassis and configuring the setup in MAX. Under advanced timing I've set the module sample clock type to external and tried several clock sources (which most of them are invalid giving error messages), but still not able to sample below 1.613 kS/s. The cDAQ-9172 chassis is supposed to have a timing engine that can be used as an external source for the 9239 module.

Am I totally wrong in my assumptions and what I'm trying to achieve here? And could someone be kind and guide me in the right direction?

Thanks.

[hibux]

Bump

[TomAD]

Hi!

Check out this KB:

http://digital.ni.com/public.nsf/allkb/EEB574335BA0B4EB862572060055E9DD

Tom

[ZachHindes]

Hi hibux,

The reason the 9239 is lmiited to 1.613 kS/s is that is what we refer to as a Delta-Sigma module.  The module has an internal oscillator running at 12.8MHz.  That timebase is implicitly divided by 256 and can be further divided by any value from 1 to 31.  So the lowest rate you can acquire at would be 12,800,00 / 256 / 31 = 1.613 kS/s.

You read that you can sample at different rates by using an "external master timebase".  That is different from using an external sample clock, whichi isn't supported by these modules.  That would explain the errors you were receiving.  To get this behavior you would set the SampClk.Timebase.Source and SampClk.Timebase.Rate on the DAQmx Timing Property node.  You could choose a timebase of 9,523,200 Hz so that 9,523,200 / 256 / 31 = 1.2 kS/s.  However this is not officially supported by NI-DAQmx and cDAQ.  Depending on your version of NI-DAQmx you may or may not be able to get this to work.

If you can't get the above to work, it may be worth acquiring a faster rate and decimating the data or figuring out a way to acquire at some faster, supported, rate.