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op-amp comparator

Discussion in 'Electronic Basics' started by nag, Aug 8, 2007.

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  1. nag

    nag Guest

    Hi,

    I was doing the sine to square converter with op-amp comparator LM339.
    During the course i was stuck with a problem. The configuration of the
    circuit is the sine wave (+/- 10V) was given to the inv. i/p and the
    reference voltage of around 0 V was given to non-inv, i/p. To bring
    the hysterisis option i have added feed back resistor to the non.inv
    input. The op-amp was powered with +10V and Ground.

    When i was checking the input signals at inv and non.inv i/p through
    CRO, the observation was my reference signal was also follows the
    input sine wave during the negative signal period of the input sine
    wave. Also my -10V sine wave was clamped to around -0.5V
    automatically. Also i have got the high and low transition two times
    per cycle of the input sine wave meaning that my square wave output
    frequency was also double that of the input sine wave frequency.

    Here my doubts are 1. Why was the clamping at the input sine wave had
    happened during -ve period? 2. Why was my ref. input followed the
    input sine wave during the -ve period alone. 3. Also why was the
    double the time frequency have come?

    I hope this is general basics of op-amp itself. Could any one please
    clarify my basics.

    Thanks in advance.

    Regards,
    M.Nagarajan.
     
  2. Have you looked at the data sheet for the comparator?
    http://www.onsemi.com/pub/Collateral/LM339-D.PDF

    The absolute maximum input common mode range is -.3 volts
    with respect to the negative supply rail (zero volts, in
    this case) to the positive supply. The functional common
    mode range is 0 to Vcc-1.5 volts.

    There is an internal diode junction between the inputs and
    the negative supply pin that turns on and conducts if the
    inputs are driven negative. Look at the internal schematic
    and see the base to collector junctions of the input
    transistors.

    When this diode turns on, lots of internal functions go to
    hell, including the high impedance between the two inputs.

    To make your system work, you will have to add resistance
    between your +- 10 volt signal and the input and clamp that
    input with a diode that conducts before the voltage reaches
    -.3 volts. This would take a germanium or Schottky diode.

    Of you change the negative supply rail to -10 volts or more
    negative.
     
  3. DJ Delorie

    DJ Delorie Guest

    Or use a resistor network to change the input signal to be in the
    right range. Perhaps a 10k between signal and input, and another 10k
    between input and +10v, giving you a 0-10v sine centered at 5v? You'd
    need to adjust the IN- reference too.
     
  4. John Larkin

    John Larkin Guest

    If you drive either input of the 339 below its ground voltage,
    internal substrate diodes conduct, spray charge everywhere, and all
    sorts of very strange things happen.

    John
     
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