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Comparing Voltages

Discussion in 'Electronic Basics' started by Gareth, Jul 21, 2004.

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  1. Gareth

    Gareth Guest

    Yes, there is a chip called a comparator. It has two inputs: inverting
    input (usually marked "-") and non-inverting input (usually marked "+").
    If the inverting input is higher the output will be low, and if the
    non-inverting input is higher the output will be high.

    Maxim, and others, make these have a look at:

  2. KevinR

    KevinR Guest

    Use a pair of comparators,
    connect the inputs of one the other way around from the other
    comparator A + input on voltage 1 and - input on voltage 2
    comparator B + input on voltage 2 and - input on voltage 1
    Stick the outputs in to a two input OR gate

    Re-reading you post, I am thinking maybe you mean higher
    amplitude rather than higher voltage level, in which case, this won't
    be of any use at all. I guess you could rectify and smooth the signals
    and then do the above, but the response time would suffer
  3. andy

    andy Guest

    I've just built a circuit to do this with 2 transistors - comments from
    others on the group welcome, as i'm not sure if this is a good way to do

    12V ----------+------+-------------------
    | | |
    [] [] #/
    500k [] [] 22k +-#
    [] [] | #\
    | | 1M | |
    +------|--------[===]--+ |
    | | |
    | | | 3M +------ Vout
    Vin 1M | #/ [] |
    --[===]--+--# [] []
    #\ [] 100k [] 100k
    ----->[] []
    [] []
    [] []
    | |
    0V ------------------+------------------+

    When Vin is more than the voltage on the 100k pot plus 0.6v, Vout goes
    from 0 to 12V. The 3M resistor gives positive feedback to make some
    hysteresis - i.e. when the signal switches high, you have to go back down
    a short way before it will switch low again. Increasing the value makes
    the gap between switch points smaller - if you omit it, it's just a
    straight comparator.
  4. John Fields

    John Fields Guest


    Sounds like you could do it with a couple of fast comparators, but...

    1. If signal A = signal B anywhere in the range between 0V an 5V, you
    want the output to be low, otherwise you want the output to be high,

    2. How narrow a slice do you want to see? That is, if signals A and B
    are both perfect 12MHz sinusoids 180° out of phase, how wide, in
    degrees, would you expect the output to be high around the zero

    3. How narrow would you like the slice to be at other than the zero
  5. w_tom

    w_tom Guest

    The chip is almost as famous as the 555 timer chips. If I
    remember the number, it is the 339 Comparitor. Virtually
    every serious linear IC manufacturer sell a 339 Comparitor.
  6. Dave

    Dave Guest


    I want to compare 2 signals. If one signal is higher than the other then I
    want to output a logic zero, otherwise output a logic one. The signals will
    be at no more than 12MHz with a maximum voltage of 5V. Do any chips exist
    that will do this for me? If not, how else could I do it?

    Thanks for any help,
  7. w_tom

    w_tom Guest

    You have many questions about the 339. Most of those
    questions, you don't even know you have yet. Your first step
    is to get and read (many times) the 339 datasheet. For
    example, go to to find the datasheet for LMV339.
    Or go to to find datasheet for

    Your current question says you have not yet done basic and
    necessary research. And again, if you did not read that
    datasheet multiple times, then you have not yet done your
  8. Dave

    Dave Guest

    Thanks for all of the replies.

    The 339 comparator looks like it might be what I want. Could I power it
    from a standard 5V power supply?

    Thanks again,
  9. Bob Masta

    Bob Masta Guest

    The 339 is designed to be a single supply part, and 5V should be fine.
    However, it's not a very fast comparator. Response time is listed
    as 1.3 usec, or 300 nsec with large signal overdrive. I don't think
    this is going to work at 12 MHz!

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
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