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My DAQ is sampling at a different rate then what I have it set at in the DAQ assistant

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I am having an issue that when I enter a sampling rate into my NI-9234 DAQ using the DAQ assistant it does not match what I get when I calculate it in Matlab. I have checked my code in Matlab with other colleagues and confirmed the Matlab code is correct. So that leaves me to believe the DAQ is just not sampling at the rate I am setting it as in the DAQ assistant. I also made sure that the sampling rate was larger then the minimum sampling rate of the card and it still differs by 50 hz or more depending on the set sampling rate. The image below shows the block diagram of the system being used to test why the sampling rate is different then what is set by the DAQ assistant.

RLRI_0-1658942106829.png

Thank you

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That image gives no useful information.  What sample rate are you trying to set?  How many channels?


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Sorry about that, I am currently using 4 channels. 3 of the channels are connected to an accelerometer for x,y,z and the fourth is for a impact hammer. I have it set at 1652 Hz because the minimum sampling rate of the card seems to be 1651.6 Hz. I found that because when I enter any sampling rates below 1651.6 Hz it gives back out 1651.6 Hz. With it set at 1652 Hz the sampling rate for the data I received came out to 1706.7 Hz.

 

Thank you.

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Accepted by topic author RLRI

Hi RLRI,

 

You're seeing expected behavior. The 9234 has a delta-sigma ADC with a modulator running at 13.1072MHz. The specifications list the formula for "data rates" that are integer divided multiples of this frequency. DAQmx looks to be rounding the user input rate to the next highest multiple. Even with a simulated 9234 and LabVIEW DAQ example I get an actual rate of 1706.67 for an set 1652Hz sample rate. 

algarcia_1-1658957525485.png

 

 

Alex
Hardware Engineer
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