why do I get huge number of samples even with small data rate and execution time

[odessy27]

I am using Signal express as my data acqusition software. NI9239 Daq is data acquisition system is my data acquisition Hardware. Problem I am having is, irrespective of the data rate and number of samples i set I get huge amount of samples. e.g. I set Data rate to 10 samples per second and number of samples to 600 (in continuous mode) i get more than 64000 samples where I should only be getting 600 samples total.  I tried setting data rate to 10Hz and number of sasmples to 10, it gives me an error saying data buffer overrun.Does anybody know any way around it?

any answers are highly appreciated.

[Wayne.C]

Change to 'Finite Samples' mode.

[Brad_K]

Hi odessy27,

Wayne.C's suggestion of using "Finite Samples" mode is correct, but the data will be sampled at a faster rate than you expect. The minimum sample rate for the NI 9239 is 1.613 kS/s (see the NI 9229/9239 Operating Instructions and Specifications). When you specify a sample rate less than 1.613 kS/s, DAQmx coerces the sample rate to 1.613 kS/s.

If you were programming the NI 9239 through the DAQmx API you could query the DAQmx Timing property SampClk.Rate to find out what the rate was coerced to, but I don't think you can do this in SignalExpress.

Brad

[odessy27]

Thanks Wayne.C and Brad K for your responses.

It makes sense now. But I need to be able to collect as low as 6000-10000 samples over 10 minutes. Would it even be possible with current hardware and setup? Reason I need lower number of samples is I need to do processing on it using excel which can not take more than 64000 samples in a file. 

I was wondering if there is any processing I can use from signal express menu which would reduce number of acquired signals. Like running average but running average does not reduce the number of samples.

e.g. if I acquire signals @ 1.7kS/s (which is minimum sampling rate for my hardware) for 10 minutes, i ll end up with 1020000 samples. Is there any way I can reduce these samples to 6000-10000 samples by using any kinda signal processing in signal express so that I can export it in Excel.

Thanks! 

[Brad_K · Accepted solution]

Hi odessy27,

Perhaps you could use the Processing >> Analog Signals >> Subset and Resample step to resample the data to a lower sample rate.

Brad

[odessy27]

Worked like a charm. Thank you very much Brad K!