How Do I Change the Acquisition Frequency of my 9239?

Updated Nov 30, 2018

Reported In

Hardware

  • NI-9239

Issue Details

I am using an NI-9239 module and I want to change the acquisition frequency. It does not work as I expect and I do not know why. What can I do to change the frequency?

Solution

In the case of the NI-9239, the ADC being used is referred to as a DSA ADC. This ADC has discrete sampling frequencies it can achieve, and these discrete frequencies are called out in the data rates section of the NI-9239 Datasheet. When setting the sampling rate for the module in software the value will be coerced to the closest frequency that the module can actually achieve. This is why in certain cases changing the requested sampling rate in software may not actually change the sample rate that the module is using.

NI C Series modules use multiple ADC technologies throughout the platform. The user must be aware of what type of ADC is used in their particular module to fully understand how their acquisition is being performed. Reference Various ADC Types Used in NI DAQ Devices for more information on common ADC's used in NI's products.

 

Additional Information

Even when using SAR ADC's not all sample rates will be possible because of the timing resolution of the Reference Clock used. Refer to your chassis documentation to determine the timing resolution available to you.

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