Why can't I have more than one continuous task running for a DAQ card with per-channel ADC's?

[SeanDonner]

The PXIe-4300 DAQ card states as having "8 simultaneously sampled analog input channels; 250 kS/s/ch sampling rate; ADC per channel".  If each channel has its own dedicated ADC, then why is it that I cannot have two seperate continous tasks, each with mutually exclusive channels but on the same card?  When I try to do this I get a resource conflict error.

For Example:

MyVoltageTask_1 : Continuous Samples, 1 kHz Rate

|-PXI1Slot1/ai0

|-PXI1Slot1/ai1

|-PXI1Slot1/ai2

|-PXI1Slot1/ai3

MyVoltageTask_2 : Continuous Samples, 100 Hz Rate

|-PXI1Slot1/ai4

|-PXI1Slot1/ai5

|-PXI1Slot1/ai6

|-PXI1Slot1/ai7

I would have thought that one of the major advantages of having an ADC per-channel would be the ability to continuously sample channels at different rates simultaneously.  What's going on here to prevent me from doing this?

[SimonH]

Sean,

I'm not an authority on the subject but I suspect the board only has one timing engine. You can simultaneously sample all 8 channels at once due to the parallel ADCs but they have to be at the same rate.

If I'm right the easiest fix is probably to sample all the channels at the higher rate and either discard 9/10 samples on ai:4-7 or average the 10 samples together (this will have the side effect of giving you a more accurate measurement).

-Simon

[SeanDonner]

This timing engine theory seems very plausible.  To the best of your knowledge, are there any DAQ cards that have multiple timing engines?

[Patrick_NI]

There aren't any Multifunction DAQ cards with multiple timing engines.  However, RIO FPGA devices can use different timing on different channels, and cDAQ chasses have three timing engines.  For more info, please see the following link:

http://digital.ni.com/public.nsf/allkb/C2C0A8A609F0E1C88625782B00496089?OpenDocument