I think it would depend on how you've set up the AO channel (which we can't see because you didn't post it). It will also depend, to some extent, on the properties of the DAQ device, and what you mean by "the exact time of analog output update".
DataFlow can help. If I had a DAQ AO device that did "On Demand" output, I could conceivably get a TimeStamp (with the Error Line running through it) and then do the DAQmx Write, or do the Write followed by the TimeStamp. Depending on how much time the AO process took (should be negligible in most cases) and whether or not the Write returns immediately or waits for the data to be written. This last, of course, is much more important if you are doing a multi-point Buffered Write (assuming your device supports that).
Bob Schor
