Anti-Aliasing Filters on a NI-9234 in Scan Mode

Updated Jun 23, 2018

Reported In

Hardware

  • NI-9234
  • CompactRIO Chassis
  • CompactRIO Controller

Issue Details

I am using an NI-9234 C Series Sound and Vibration Input Module in a cRIO chassis in scan mode. For my acoustic measurements I need to leverage the anti-aliasing filters that the 9234 provides so that I get a clean frequency domain reading. Can I leverage these anti-aliasing filters in scan mode on a cRIO?

Solution

You will not get accurate frequency domain readings when using an NI 9234 C Series Sound and Vibration Input Module in a cRIO chassis in scan mode. Either switch to using FPGA mode with the LabVIEW FPGA Module, or switch to using a chassis that supports NI-DAQmx.

Additional Information

In scan mode on a cRIO the 9234 card will acquire samples at the maximum possible sample rate of 51.2 kHz and store them on a register. The cRIO will then re-sample the data based on the scan rate that you have specified. Due to this re-sampling, you will not get accurate frequency domain readings. Also, the anti-aliasing filter will have a cutoff frequency that is based on the sample rate of the 9234 card, so you will see aliased signals after the slower re-sampling is done from scan mode. 

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