Comparator CIrcuit

Discussion in 'The Projects Forum' started by philm01, Aug 30, 2014.

  1. philm01

    Thread Starter Member

    Mar 22, 2014
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    Hey everyone!

    So, I have been taking a break from my endeavour of working on a buck converter controlled by a microcontroller and switched gears to another aspect f my project.

    First, I have a DDS (AD983BRUZ) and the output of this is going to a non-inverting amplifier to bring the voltage up to the 5 V scale (min: 250 mV - max: 5 V). Then, this is going to a comparator, the AD790JNZ. I am incorporating hysteresis and I would like to get something less then 50 mV hysteresis. Which, I have gotten pretty close on the breadboard. Bandwidth wise, I need to be able to go up to at least 1 MHz.

    Now, I am running into a snag because when I adjust the reference voltage and it gets to a certain point, the duty cycle jumps very quickly. This is when it gets close to 0%. Actually, probably closer to 10% when it does this. Very interesting. I as wondering if anyone else had that problem with this chip and if there was a way around it?

    Alternativly, I am open to changing chips! If you feel that there is a better method for doing the same thing (or a better chip), please post here! I am attaching a copy of the circuit that I am building.
     
  2. philm01

    Thread Starter Member

    Mar 22, 2014
    33
    0
    Does anyone have any idea? Or should I expound a little more?
     
  3. philm01

    Thread Starter Member

    Mar 22, 2014
    33
    0
    So did I place this question in the wrong spot or something? Because I am not getting any hits. Is there something wrong with my post?
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    Do you have 0.1uF filter capacitors on the power pins?
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    Show the waveforms at the comparator inputs.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,804
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    Does the comparator need a pull-up resistor on the output?
     
  7. philm01

    Thread Starter Member

    Mar 22, 2014
    33
    0
    Here are screenshots of the waveform. The top waveform is the input, the bottom waveform is the output. Sadly, I do not know how to make a gif or I would completely make a little "video" about what is happening so you are going to have to take my word on this. I am adjusting the reference voltage via a single turn pot (If you look at my original circuit, this pot represents the DAC).

    Now between the two pictures, I literally turned the pot only 1/64 of a turn if even that. And I am not yet even maxed out the pot. I still have more to go. Probably 1/8 of turn left.

    @ Alec_t

    Good thought but I call analog and they said that it is a push-pull output rather then an open collector
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You don't show the voltage on the other input (I assume the DC from the pot). Without seeing that I can't determine what the problem might be.
     
  9. philm01

    Thread Starter Member

    Mar 22, 2014
    33
    0
    Here we go, the first picture is with a measurement (shot 3) and the next picture (shot 4) is with the cursors turned off. This is right at the point when the output voltage (Bottom) is low. The top measurement is the input to the inverting pin and the bottom is the output of the circuit.
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,033
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    Well, we're getting closer. :) What I would like to see is the two inputs shown simultaneously on the screen with the same zero volt position and vertical sensitivity for both, for each of your first two conditions (just before and just after the output pulse quits).

    Also could you reduce the size (resolution) of the image before posting? I can only see about 1/8th of the image at a time on my screen.
     
  11. philm01

    Thread Starter Member

    Mar 22, 2014
    33
    0
    Here we go, now 50% smaller
     
  12. philm01

    Thread Starter Member

    Mar 22, 2014
    33
    0
    Darn it, give me one sec to post the other one
     
  13. crutschow

    Expert

    Mar 14, 2008
    13,033
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    I believe I see the problem. Look at the data sheet for the comparator Input Characteristics common-mode voltage. The maximum is 2V below the plus supply voltage. It's not an input rail-rail type comparator. So the triangle wave must be no greater than 3V peak for proper operation.
     
  14. philm01

    Thread Starter Member

    Mar 22, 2014
    33
    0
    Darn, that sucks!

    Well what chip do you recommend for my application? I am going to be using a DAC to control the duty cycle and I need the frequency to go up to at least 1.5 MHz.
     
  15. philm01

    Thread Starter Member

    Mar 22, 2014
    33
    0
    Well wait, let me try the function generator at a peak of 3 V
     
  16. philm01

    Thread Starter Member

    Mar 22, 2014
    33
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    Hey look at that, it works!
     
  17. crutschow

    Expert

    Mar 14, 2008
    13,033
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    Good news. It's amazing how often things work better when the data sheet is followed. ;)
     
  18. philm01

    Thread Starter Member

    Mar 22, 2014
    33
    0
    Indeed, must have skipped that one
     
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