Number of samples read is not equal to number of AI Sample Clock pulses - why?

Question

Dear Community,
I am using several PXI-4472s for acquisition and at the same time I am routing the AI Sample Clock signal to a PXI-6608 counter/timer card so that I can time stamp each sample.
Unfortunately I am finding that I always get more AI Sample Clock pulses than samples, so I don't know which time stamp corresponds to which sample. Why is that?
For acquisition I am using something very much like the following DAQmx Sync and Stream example:
Sync and Stream.vi
After stopping the tasks, I use DAQmx Read vi again to read whatever samples might be left in the buffer, but that request always comes back empty. I can't figure out if there are AI Sample Clock pulses at the beginning of the task that don't actually acquire any data, or if I have data left over in the buffer after I stop the task that I am not reading properly.
Any help would be greatly appreciated!
Cas

Answer 1

Cas,
DSA boards (like the 4472) differ from standard DAQ devices. While most DAQ devices use a relatively common type of ADC known as a sequential approximation ADC, a DSA device uses a more specialized ADC known as a Delta-Sigma ADC for its measurements. Delta-Sigma ADCs allow for very precise measurements, and this precision is accomplished by oversampling (frequently 128x or 256x the sampling rate that is being used).
The above is a very brief explanation of the DSA board's inner workings. It is discussed in more detail in the document linked below. If after reading that tutorial, you still have questions, please post back so that this forum can assist you.
Synchronization with Dynamic Signal Acquistion (DSA) Products with DAQmx
Best of luck with your application.
Regards,
Jed R.
Applications Engineer
National Instruments