Unable to Set Correct Sample Rate on NI 9235

Updated May 31, 2018

Reported In

Hardware

  • NI-9235

Issue Details

I have a NI 9235 quarter-bridge strain gage module and am trying to set my sample rate in NI MAX or programmaticly in LabVIEW. When I do so, the actual sample rate is different than the value I set. Why?

Solution

Because the 9235 utilizes delta-sigma analog-to-digital converters (ADCs), the device has a minimum and maximum sample rate as well as a finite range of possible sample rates total. As per the NI 9235 datasheet, the following equation provides the available sample rates of the 9235 when using the internal master timebase of 12.8MHz:

 
Where fM is the 12.8MHz internal master timebase of the 9235, n is any integer from 2 to 63, and fs is the sample rate. When using the internal master timebase, the maximum sample rate is 10kS/s and the minimum is 794 S/s.

Additional Information

It is possible to utilize an external timebase with a frequency other than 12.8 MHz. When using an external timebase, the equation above still applies and yields different set of possible data rates.

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