I share Bob's questions about the filter -- many of us long-time LabVIEW'ers tend to avoid express vi's because they're more *opaque*. That is, their behavior isn't fully specified by the inputs visible on the diagram, there are also other "hidden" settings that are changed with a gui interface when you double-click them.
I'll add another guess. You didn't identify your DAQ device, but it may not support an exact 1.5 MHz sample rate. You may actually be getting a different sample rate that's as close as device can manage. What's the "dt" value on the direct waveform output from DAQmx Read? It will indicate the actual sample rate, which is sometimes different than what you asked for.
For example, consider an old E-series device with a max timebase of 20 MHz. When you ask for 1.5 MHz sampling, the device needs to try to achieve that using an integer divisor of 20 MHz. The *accurate* divisor is 13.3333333. However, the device will *need* to use a neighboring integer, either 13 or 14. A divisor of 13 would yield the slightly higher sample rate 1.538 MHz. (I don't know the algorithm for how DAQmx chooses whether to round up or down, but it *will* behave consistently for a given requested freq).
It turns out that 15000 samples at 1.538 MHz takes -- TA DA! -- 0.975 seconds.
-Kevin P
ALERT! LabVIEW's subscription-only policy came to an end (finally!). Unfortunately, pricing favors the captured and committed over new adopters -- so tread carefully.
