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Issue with Comparator circuit

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by alok1982, Sep 17, 2017.

  1. alok1982

    alok1982 Member

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    Hi,


    I made a comparator circuit in which according to different input resistance I am giving indication using LED.


    While testing I am facing one problem;

    Whenever I bring my hand near to IC LT1707 the LED automatically starts to glow.

    Even when I touch multimeter probe to pin 2 and pin 5, the LED becomes bright otherwise it glows less.


    How to solve this issue.

    Attached circuit is for your reference.


    Thanks!
     

    Attached Files:

  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    With such high value resistors on the opamp inputs the circuit is sensitive to interference. Your body acts as an antenna for picking up interference which can be coupled capacitively to the opamp inputs.
    Is the circuit inside a screened enclosure? If not, it should be.
     
  3. alok1982

    alok1982 Member

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    Hi Alec,
    Thanks for reply!
    What type of enclosure I can use?

    Can any grounding issue is there or can we modify ground to solve this?

    Thanks!
     
  4. dave

    Dave New Member

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  5. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Die-cast screening boxes are one example. They're available in various (but commonly small-ish) sizes.
     
  6. atferrari

    atferrari Well-Known Member

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    Are they justified?
     
  7. alok1982

    alok1982 Member

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    What would be reason for, " by touching multimeter probe to pin 2 and pin 5, the LED becomes bright otherwise it glows less"?

    I am still thinking any grounding issue.
     
  8. ci139

    ci139 Active Member

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    the comparator is !basically! an open loop op amp make your schematic first with these (inexpensive ones) see how it acts
    the schematic to work fast takes biasing of near minimum allowed resistance values also minimum not to "distort" your input (by dc bias e.g. if your comparator input biasing/compensation network is 100 x MΩ range but your source is 1 x GΩ and the ΔVin is small it might have no NRG to drive the detector or to drive it properly)

    if your input is high impedance you likely want to buffer(filter/alter) it for the same reason about impedance converters https://en.wikipedia.org/wiki/Buffer_amplifier to match your preferred biasing/compensation for comparator -- the drawback is you introduce an additional time delay and error-function to the input chain
    ◄ this is when you use op amp for comparator or to get the hint what's going on in biasing/feed-back network (as the difference of referencing your multimeter's COM to variety of potentials avail around your test circuit or modifying it's probes with series resistors or j-Fet input translator/buffer)

    notice the "Problem 6.3 - Follower" + "Problem 6.7 - Even More Virtues of Feedback - I" & "Problem 6.8 - Even More Virtues of Feedback - II" in http://instrumentationlab.berkeley.edu/Lab6
     
    Last edited: Sep 21, 2017

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