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concerto comparator output delay

rs
Intellectual 891 points
Other Parts Discussed in Thread: F28M35H52C

TEK00000.TIF

 

I'm noticing a delay of ~200ns from the time when the comparator input changes state and when the comparator output changes state. Seems long... Is this typical? I can't find an input to output delay spec in the data sheet.

I'm using the F28M35H52C eval card. For the non-inverting input to the comparator I have an ePWM pin connected and I'm using the dac at mid rail for the inverting input (DACVAL = 512). I'm using the async version of comparator output.

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update: here's what it looks like on a scope after taking off the 3300pF shunt caps on the comparator input pins. The delay is ~30 ns.

TEK00001.TIF

 

  • 0
    TI__Mastermind 24095 points
    Closest is Comparator to PWM trip delay specified in Table 5-40. Electrical Characteristics of the Comparator/DAC. 200ns seems high. Can you double check filter setting and make sue async path is on?

    -Bharathi.
  • 0 in reply to Subrahmanya
    Intellectual 891 points

    Thanks for replying. Yeah, that's all I could find as well but wasn't exactly sure how to interpret the wording . By filter settings do you mean the sync qualification? These are my comparator register settings:

    Comp2Regs.COMPCTL.bit.COMPDACEN = 1; // Enable COMP2

    Comp2Regs.COMPCTL.bit.COMPSOURCE = 0; // Use DAC1 as minus
    // input to comparator

    Comp2Regs.DACVAL.bit.DACVAL = 512; // Set DAC2 to 1.65

    Comp2Regs.COMPCTL.bit.CMPINV = 0; // Do not invert
    // comparator output
    // If input A > input
    // B output high

    Comp2Regs.COMPCTL.bit.SYNCSEL = 0; // Asynchronous output


    Debugger gives the following register values, which are consistent with my code:

    COMPCTL 0x0001
    DACVAL 0x0200

  • 0 in reply to rs
    Intellectual 891 points

    By the way, I have tried this with two different eval cards and am getting the same results. Does it have to do with the eval card and not the concerto itself? This delay is critical in my design.

  • 0 in reply to rs
    TI__Mastermind 24095 points
    Register settings look fine. How are you monitoring the comparator output?
  • 0 in reply to Subrahmanya
    Intellectual 891 points
    I'm using an oscilloscope. I attached an image on the original post.

    Magenta is the comparator input and cyan is the comparator output. You can see the edge measurement on the right side. The rise time of the input signal is a little slow because of the 3300 pF caps on the adc pins on the eval board.

    I'm thinking it might have to do with those caps. I was originally using two comparators both with the same input (so there were two 3300 pF caps in parallel at the input) and was noticing ~400 ns delay. I'll see if I can remove the cap.
  • 0 in reply to rs
    Intellectual 891 points
    I took the 3300pF shunt cap off of the adc pin and the delay is ~30 ns now. I'll attach an image to the original post. Big relief. Thanks Bharathi.
  • 0 in reply to rs
    TI__Mastermind 24095 points
    Hi, thanks for the update - good luck !!