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Agilent 3458A DMM waveform capture problem

Question asked by CODER on May 29, 2013
Latest reply on Jun 7, 2013 by lhornburg
I’m trying to use the 3458A to do a real-time capture of the data comprising a periodic ramp that goes from 0 to 4v. I want to do this in REAL TIME capturing all data on the FIRST ramp after the trigger.  I have a function generator connected to the 3458 delivering this waveform. I want to capture and store the voltage values into an array for post processing. I’m using the Labview Application Read that comes with the HP3458A Labview driver. Using the default “number of samples” and “sampling rate” I get the expected data display (see goodwaveform.jpg) . 100 samples at the sampling rate of 50Hz (50 samples/sec) captures 2 seconds of the waveform. The waveform is a 500mS ramp with 50% duty cycle. This is good.  However I want to gather data ONLY on a 500mS segment of the waveform, and it seems logical that I should be able to set the sampling rate to 1000 samples/sec (1ms per sample) and read 500 samples to view a 500ms section of the waveform. Obviously I’m wrong because the second screenshot shows the results of those settings. It appears the sampling rate is closer to ~72 samples/sec instead of the desired 1000 samples/sec. What am I doing wrong? VI attached.