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Actual Record Length Changes When I Change the Requested Sample Rate with NI-SCOPE



Primary Software: Driver Software>>NI-SCOPE
Primary Software Version: 2.6.1
Primary Software Fixed Version: N/A
Secondary Software: N/A

Problem:
I noticed in my application that when I change the requested sample rate of my acquisition, the actual record length of my acquisition changes as well. This happens even if the actual sample rate for the acquisition stays constant. Why does the actual record length depend on the requested sample rate? Shouldn't it be independent or at least related to the actual sample rate?

Solution:
The way in which the NI-SCOPE driver calculates the actual record length is defined by the IVI standard. IVI specifies that the time per record of an acquisition must remain constant. Since NI-SCOPE is IVI compliant, it also tries to keep the time per record constant.

Therefore, if you request 1000 samples at 24 MS/s, the total record time is 0.0416 ms. Since the NI 5122 can't sample at exactly 24 MS/s, it coerces the requested sampling rate to 25 MS/s which is now the actual sample rate. Since IVI specifies that the time per record must remain constant, NI-SCOPE calculates the new record length to be (0.0416 ms) * (25 MS/s) which is 1042 samples.

More information can be found in the NI High-Speed Digitizer Help document which is installed with the NI-SCOPE driver and available for download from the link below.


Related Links:
NI High-Speed Digitizer Help
Specifications Explained: NI Oscilloscopes and Digitizers

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Report Date: 05/03/2004
Last Updated: 10/04/2017
Document ID: 392EC1NQ

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