getsample & getdata discrepancies

Kern — Wed, 17 Dec 2008 20:43:02 -0500

I'm using Windows Vista, Matlab R2007a (Student Edition) with dataacquisition Toolbox 2.1 (R2007a).

I have an MCC USB2527 16 bit data acquisition module and configured the inputs as differential (low channels tied to ground with 10k resistors). Range set to +/-10V. At 16 bits, the measurement accuracy is 0.30518mV.

If I pick a channel, connect it to a 9V (slightly depleted) battery and make measurements with getsample and getdata, I obtain different results.

Using a Fluke 35 DVM, the battery voltage is measured as 8.948V

Using getsample, I obtain on average 8.947V consistently (ranges 8.9468 to 8.9483).

Using getdata and setting the SamplesPerTrigger to 1000, the average value is 8.942V. However, if you only look at the first measurement in the set, you'll see it's around 8.948 +/-0.001. All other measurements in that set range from 8.9404 to 8.9432.

Why do these two methods not produce the same result on average? 1000 readings from getsample still produces the above (more correct) result. I need to use getdata, but I would like to have it give me the correct reading.

Incidentally, the test/configuration program, InstaCal, that comes from Measurement Computing produces the same results as getsample.

cheers

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getsample & getdata discrepancies