Why is there a delay in my DAC?

Discussion in 'General Electronics Chat' started by phantomzz, Aug 10, 2014.

  1. phantomzz

    Thread Starter New Member

    Sep 26, 2013
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    I created a simple Digital to Analog Converter(DAC). I am attaching the circuit diagram that I made in MultiSim.

    While it works fine with increments in output on each count increment, my issue is this:

    I have used a 1 Hz clock in a synchronous counter using T F/F . So it's natural to expect an increment in the output of the op-amp every 1 second right? But instead the increment occurs every 3-4 seconds roughly. I can't understand why! Is it because of the propagation delays? I don't believe that to be the case. I thought propagation delays were of the order of nano seconds and a T F/F contains 2 gate levels....

    Initially I made a ripple counter and thought perhaps the ripple was causing the delay, but the same effect occurs for the synchronous counter too. Could anyone provide an explanation?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. phantomzz

    Thread Starter New Member

    Sep 26, 2013
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    0
    My query is not answered by that page. In fact I understand the working of a counter but my doubt lies rather on the mismatch between the frequency of the counter and the frequency of the voltage variation at the output.

    Ok an update on this issue, I tested the output using an oscilloscope and the voltage increment is in sync with the clock freq.

    So my guess is there is a limitation on the voltmeter/multimeter to record changes instantaneously? I increased the clock freq to 8Hz and the voltmeter was skipping increments. The general increment is 300mV per count but at 8 Hz the increment is 600mV per count. I wonder why? Is it because the multimeter contains an ADC in it? Is this a limitation of multisim?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When the counter is clocked with 1 Hz, the first output of the counter will change with 0.5 Hz AKA every 2 seconds.

    I have no experience with multisim, but is there a scope available?

    Bertus
     
  5. phantomzz

    Thread Starter New Member

    Sep 26, 2013
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    0
    I believe that with a 1Hz clock, positive edges occur every 1 second( time period) ; the output increments every 1 second.

    Yes there is an oscilloscope. It displays the output correctly on the scope.

    The issue lies somewhere within the voltmeter.
     
  6. tshuck

    Well-Known Member

    Oct 18, 2012
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    You will likely need to add a voltage follower stage in between the amplifier stage and the R-2R ladder. DACs made with a resistor ladder are easily loaded...

    Edit: the voltmeter may give an average/RMS over your simulation time...
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,054
    3,245
    Yes, just like a real voltmeter the virtual voltmeter in Multisim has an averaging circuit (low-pass filter) so it does not display changes in voltages instantly. To observe fast transitions you must use the oscilloscope or slow down the speed of the transitions.
     
    Last edited: Aug 11, 2014
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