how can i avoid phase delay in annalog input data acquisition of multi channels

[qinhk]

    I am using a PCI-6251 M Series board and wish to acquire two analog signals and output one analog signal simultaneously.  I have a question about multi-function -synch ai-ao.vi .With this Vi I work out annalog  input of one channel  and   data output of one channel synchronously.I take some  change in the vi which I make the sampling rate and sampling number of input and output channel be equal. But when i increase the input channel from one to two,a problem come that is the data acquired always shift towards one direct which depends on the sampling rate(for example data shift turn left when 250kS/s,right when 200kS/s).  Does the phenomena mean phase delay or lag during accquring data? or mean data loss? how it happen ?
     I wonder if the acquisition on both channels is perfectly simultaneous.I  hope the start of data  acquisition each time be the same all time.
          I would appriciate it  if any comment and suggestion.

            Attachments: modified VI from the Multi-Function -synch ai-ao.vi
            I  delete some part about data processing  in while cycle.vi in additonal file

[johnsold]

The specifications for the PCI-6251 state that it is not a simultaneous sampling board. This means that the board has one A/D converter which is multiplexed between channels. How much time delay is between channels is determined by how the board timing works, but it cannot be less than the reciprocal of the maximum sampling rate. For that board this is 1/1.25 MS/s or 800 ns. The delay could be as much as the reciprocal of your actual sampling rate. You need to look at the detailed specification for the device to determine the intersample delay. Some boards require addditional delay when switching channels to allow the multiplexer and amplifier to settle.

Perhaps you can sample at a rate that is high enough that the delays produce acceptably small phase shifts. If your signals a deterministic enough, you may be able to calculate the shift and compensate.

If you need true simultaneous sampling, you will need to check the specifications to be sure that the device you select offers that capability. I took a quick look at NI's web site DAQ search page and that is not a top level search option, but as you start to specify channels and sampling rates it becomse available.

Lynn