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Op Amp Gain error

Discussion in 'Electronic Basics' started by Mike Blankenship, Feb 16, 2004.

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  1. Hi,

    I have a 2 stage Op Amp Curcuit. The first stage works good as a voltage
    follower
    and out puts 100mv. The second stage is what I am having problems with. The
    second
    stage uses a MAX400 with a gain of 10. However the out put always rails to 5
    volts.
    The input does receive 100mv. Here is my setup :

    Stage 1 >>>>>>> 100mv

    Stage 2 (MAX400)

    Pin 1 ... Vos TRIM
    Pin 2 ... -IN
    Pin 3 ... +IN
    Pin 4 ... V-
    Pin 5 ... No Connection
    Pin 6 ... Vout
    Pin 7 ... V+
    Pin 8 ... Vos TRIM

    The output of My Stage 1 Op Amp goes directly into Pin3 Max 400
    Pin 6 is connected to Pin 2 with a 100K resistor. And Pin 2 is connected
    to ground with a 10K resistor. This should give me a Gain of 10. I have
    tried
    2 different OP Amps with no sucess. The 100mv signal is good with very
    little noise ~5mv

    Any help would be appreciated.

    Thanks

    mike
     
  2. Jamie

    Jamie Guest

    you should check the voltage on Pin 2,to do what you are claming it
    has to be lower than Pin 3.
    also there is another possibility
    i don't know your full circuit but if you are running a
    +/- supply then the + input has to be near 0 volts.
    or at least less than the - input..
    if you do have a well balanced supply then i think the
    - input is getting 0 volts center and you are injecting 100 mv's in the
    + side hence the full rail to what i assume 5 volts to be your Vcc.
    in other words its acting as a voltage comparator.
    using an OP with very low offset makes it a very sensitive device.
     
  3. opps Newby error .... The Common mode voltage for this part has to be at
    least
    a 1 volt input. It not rail to rail inputs. The Max400 can go +5 -5 then I
    could measure
    the 100mv input.

    Now I need to generate a -5V power supply. Thanks for the info.

    mike
     
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