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question about op-amps

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

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  1. russ

    russ Guest


    I am trying to read external data into the serial port of a computer.
    Basically if 'something' is at a few millivolts, then to input a '1'
    or +10 V with respect to ground into the computer.

    This means I want to amplify my input. I tried a standard '741'
    op-amp wired up with resistors etc to make an inverting input with a
    gain of a couple of hundred times. I had a lot of trouble getting it
    to work consistently though, what with bad soldering, no decent signal
    generator or oscilloscope and probably interference due to the high

    What I wanted to know was, is there an easier way of going about this?
    Is there what I want sold as a unit in an electrical shop?

    Thanksd in advance

  2. You have confused me. A serial port accepts binary data that is
    either +3 to +12 volts ( which is a logic zero) or -3 to -12 volts
    (which is a logic one). Normally, these states make sense only in
    they are patterned into a sequence that represents characters.

    So how does your millivolt signal fit into a serial port, regardless
    of how much you amplify it?
  3. russ

    russ Guest

    Basically, my signal of interest is either zero or some small signal
    (mV). I wanted a circuit that will put +3 to +12 V on one of the pins
    when the signal, is zero and -3 to -12 V on the pin when my signal is
    non-zero. ie amplify the signal such that it is either max voltage or
    min voltage when the signal is above or below some threshold.
  4. Bob Masta

    Bob Masta Guest

    What you are looking for is known as a comparator. It's just
    an op-amp run without negative feedback, so the voltage
    difference between the + and - inputs is multiplied by the
    open-loop gain of the op-amp (a very big number). The
    way you use it is to apply the threshold voltage to the -
    input and the signal to the + input. Whenever the signal
    goes above the threshold, the output will be high, otherwise
    it will be low.

    If you use a conventional op-amp for this, "high" and "low" of
    the output will be near the supply rails. In your case, use
    +/-12 for the supplies. (I guess this is for some sort of
    RS232 application?) For conventional computer inputs,
    you are probably better off using a part with an open collector
    output. (LM311, LM339 quad single-supply, etc) The output
    will switch between ground and whatever you connect the
    collector pull-up resistor to.

    Note that simple comparators can have lots of problems
    with chatter due to noise on the signal near transitions.
    This is often easily solved with by applying a little positive
    feedback to give some hysteresis. Plenty of examples in
    data sheets for comparators.

    Hope this helps!

    Bob Masta

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

    Zorknob Guest

    an >inverting input with a gain of a couple of hundred times.

    You're better off using a comparitor like an LM311 than a general
    purpose op-amp. Then you need to drive the comparitor output into a
    serial port buffer like a DS90LV011 to convert the signal to the
    voltage levels required for serial port communication. Then, of
    course, there is the issue of protocol...
  6. andy

    andy Guest

    you might be better using the parallel port - serial ports expect to
    receive data in blocks of 8 bits (give or take), so you would have to add
    a serial port driver chip to your circuit. You can read individual lines
    of the parallel port in software, by reading the whole port and then just
    masking out the bits you're not using.
  7. When dealing with such high gain, it is often helpful to include some
    low pass filtering to reduce the noise added to the signal. How fast
    do you need to respond to changes of state of your millivolt digital
    signal? Think about a gain block that is also a low pass filter,
    followed by the decision block that digitizes the level, perhaps with
    some hysterisis.
  8. John Jardine

    John Jardine Guest

    Use an LM393. It'll compare voltages down to ground (and even a few 100mv's
    below). It's output is an unconnected transistor collector so you can set
    the voltage swing you want.
  9. Zorknob

    Zorknob Guest

    The only problem here is that the comparitor output can only swing
    from some V+ to ground. A serial port signal swings above and below
    ground (+[3 to 12V] to -[3 to -12V]) , so you still need a serial port
    driver for voltage translation.

    The advantage of using the serial port drivers over dual supply
    comparitors is that they'll create their own +/- voltages for driving
    the output from a single supply, and they're pretty cheap.
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