comparator input

Discussion in 'Homework Help' started by buildcircuit, Jun 11, 2009.

  1. buildcircuit

    Thread Starter New Member

    Jun 11, 2009
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    Hi, does anyone know if there is a limit in the frequency of the analog input signal that i inject into a comparator?

    The compartor works fine for a few hz up to 10s of hz. However, if i increase my input to 100+khz, the digitized waveform starts to distort.

    I am using the comparator as a analog to digital converter. Tks
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    There are many comparators out there. Can you include a link to the datasheet for the one you are using? Also, can you post a schematic of how you use the comparator, including both inputs and outputs of the comparator?

    Another question: What kind of oscilloscope are you using to view the signal?
     
  3. buildcircuit

    Thread Starter New Member

    Jun 11, 2009
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  4. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
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    A few things to try...

    Increase the value of the pull-up resistor connected to the output of the comparator. Try around 5kohms or 10kohms.

    Check the power supply voltages near the input pin of the comparator and see if you see any distortion in the supply. Add a 10 uF electrolytic capacitor, or higher, near the comparator.

    What is the bandwidth of the oscilloscope?
     
    Last edited: Jun 15, 2009
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    This circuit, correct?
    http://www.seas.upenn.edu/~ese206/labs/adc206/ADC-Encoding.pdf

    I'll suggest StayatHomeElectronics is going the wrong direction; the pull-up resistors on the output of the comparators should be decreased in value to between 850 Ohms and 1.5k Ohms; the latter should look pretty good.

    The comparator AND the 74LS148 should each have a 0.1uF (100nF) bypass capacitor across their Vcc/ground pins, with the leads as short as possible. Also, use a larger electrolytic capacitor (10uF or larger) in the near vicinity.

    If you have any unused comparators, connect one input to Vcc and the other to GND to prevent them from floating and randomly causing glitches.
     
  6. buildcircuit

    Thread Starter New Member

    Jun 11, 2009
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    StayatHomeElectronics and SgtWookie, Thanks for your valuable suggestions :) I will try out and feedback on which configuration works. Tks
     
  7. buildcircuit

    Thread Starter New Member

    Jun 11, 2009
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    Hi, i have tested the circuits with both higher and lower values of Resistor. It seems that the higher the R value, the faster the distortion. However, for lower R value, it does not seem to work as well. For eg, i tried down to about 470ohm and the digitized output still start to distort after 400kHz (although an improvment of the previous 100kHz). I would like to achieve an input signal in the MHz range, probably between 1Mhz-8Mhz. Is there a technique to choose the R value?
     
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