Hello,
I’d like to
measure the time delay between three analog signals. This delay might be rather
large (say a few hundred milliseconds) and I’d like to measure it accurately,
the more accurate the better (with reasonable resolution down to a few microseconds).
For that, I have a NI USB-6259 (M series), LabVIEW 8.2.1, NI-DAQmx 8.7 drivers
and a Core2Duo PC.
Before
using real signals, I’d like to test the vi using AO on the DAQ device. This
leads me to the included vi acquisitionContinue.vi, which should do synchronized
AI and AO, AO using /Dev1/ai/SampleClock and /Dev1/ai/ StartTrigger, this is heavily
inspired by LV’s included examples (Multi-Function-Synch AI-AO.vi). I use three
analog outputs for which I create signals using a custom made vi
genereSignal.vi. AO0, AO1 and AO2 being
respectively wired to AI0, AI1 and AI2 on the 6259 device.
The
analysis is simple enough that it does not prevent continuous acquisition while
done “in place” (in the acquisition while loop) and this vi can run for hours
without any problem.
However,
I’m a little bit disappointed since the signals on AI seem to be time shifted from
the signals I send to AO, moreover, this phase shift is different for each
signal and not constant in time. I believe that there might be something
hardware-related but I just can’t get it. What seems even more strange is that
either AI signals are in phase advance of a very few samples or they are
shifted for a very long time (around one period of a typical 10 kHz signal).
I thus
would like to get your opinions about this. Am I missing something? Is the
synchronization done well? Where does this phase shift come from and might it
be avoided or predicted?
Thank you
for your attention and time.
