issues for 6363 AI and AO at the same time

[taoxd]

Hi,

  My task is to generate waveform from AO0 and AO2 and measure the waveform from AO0 using AI7. P0.1 and P0.2 is also used for output of pulse. I made three events and all those event is trigered by P0.4. The sampling rate is 522.0Ksamples/s for all the channels.I use VC++ and windows XP.

  I find that the data from the AI7 is different from the output of AO0. The AI speed is faster than the AO speed.  I lost some of data from AI comparing with the AO0. If I slow down the speed of AI　by 0.995. Than everything is fine.

  Dose anyone know what's problems with this card?

Thanks

[John_P1]

Hi taoxd,

The 6363 generates the internal clocks by dividing down one of the internal timebases (generally it uses the 100 MHz timebase).  Your specified rate of 522 kHz is not an integer divide-down of 100 MHz.  The closest integer divisors are 191 and 192:

    100 MHz / 191 = ~523.56 kHz

    100 MHz / 192 = ~520.83 kHz

The discrepancy you are seeing comes from the way rates are coerced in DAQmx.  Analog Inputs are always coerced up, whereas Analog Outputs are coerced to the closest available rate.  So, your analog input is actually running at 523.56 kHz, while your AO is running at 520.83 kHz. 

If you specify 519.39 kHz (522 kHz * .995) for analog input, this will coerce up to 520.83 kHz.  The next lowest sample rate available is ~518.13 kHz (100 MHz / 193).

You can verify the coerced rates by reading back the DAQmx timing property.

Best Regards,