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Lowest possible latency to digital and analog outputs

latency DAC DO

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#1 fmcastro

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Posted 19 April 2015 - 02:51 PM



I'm considering the U3 for a project but I need to know the lowest possible latency from the write command to voltage raise/drop on both digital and analog outputs. 


As I could infer from other posts and the documentation, there's a minimum 0.6 ms round trip time for commands that don't read analog inputs, but:


1) For digital outputs, can I assume that the write time is half that time (0.3 ms) being the other half the time to reply the outcome to the host?


2) For analog outputs, there's an old post relating the DAC conversion time to the timer clock. I don't know if that still holds but I'm assuming in the best case I could use a 48MHz clock and an 8 bit DAC to take that time down to ~5 usec. This would be on top of the same 0.3 ms above resulting in 0,303 ms total write time.


3) What about the actual voltage raise/drop? Do you have the specs for the time it takes to bring the voltages from 0 to 5V and vice-versa?



Thanks a lot,






#2 LabJack Support

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Posted 20 April 2015 - 08:38 AM

  • Your assumptions about the round-trip communication times are going to be approximately correct.  We haven't measured exactly when the instruction that sets the DIO channel gets executed in relation to the USB packet being received but your intuition is correct.
  • The DAC outputs on the U3 are still generated via PWM signals, so this old post still applies.  See the U3's data sheet: 2.7 - DAC.  We do have functions that allow you to set the device's DAC output via non-binary numbers via the eDAC function or the AddRequest/ePut functions.  If you don't alter the U3's core clock to use the U3's timer & counter features response times should be in-line with other speeds command response speeds (0.6ms), this won't be true if you need to decrease the core clock frequency like the person was doing in your mentioned post.
  • The slew rate for the DAC outputs is documented on the U3's appendix-a page and is 0.4V/ms.

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