Help with OP Amp circuit

Discussion in 'General Electronics Chat' started by pattom12, Sep 1, 2006.

  1. pattom12

    Thread Starter New Member

    Sep 1, 2006
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    0
    I am trying to build a simple voltage follower OP Amp circuit using an old AD542 a friend of mine gave me. I constructed a circuit as shown in the attachment but it does not work and I do not know why. Following are the measurement I get:

    I have a pH probe connected to the input and I know it is putting out a +mv signal.

    Measured output is 2.5 v with or without the probe connected,.
    Measured input is 1.25 v with or without the probe connected.

    I removed the 1 meg resistor from the output to see if that had any effect and it does not. Does anyone have any suggestions as to how I might make this circuit perform as a unity gain voltage follower?

    I am really new to this so please excuse me.

    Regards

    Pat
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Your circuit can't work with the 1K resistor on pin four. That is one of the power supply pins for the op amp. Just remove the resistor and tie the pin to ground. It should look more like a voltage follower.

    By the way, the 2M resistor to ground off the output is probably unnecessary.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    The minimum supply voltage for AD542 is +/-5V. It won't work with a single supply if your input voltage is near zero. The output of a pH probe is about 60mV per pH unit.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    76 mv per pH step.
     
  5. pattom12

    Thread Starter New Member

    Sep 1, 2006
    3
    0
    I removed both resistors and tied pin 4 to ground as suggested. What I now get is 1.9v out and .7v in regardless of the probe pH. Any ideas what I should try next to make this unity gain follower op-amp work properly?

    Thanks again

    Patrick Thomas
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Greetings pattom12,

    Though you have not labeled your diagram as a top view or bottom view, in looking at the ad542 datasheet the view in you diagram appears to be a top view.

    Is it possible that you have interpreted the pinout as a bottom view instead of a top view?

    hgmjr
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Read my lips: :)
    As I said above,
    You will need two power supplies, one at +5V (or more) and one at -5V (or more).
     
  8. pattom12

    Thread Starter New Member

    Sep 1, 2006
    3
    0
    I used an old computer power supply to get +12v and -12v which I hooked to pin 7 and pin 4 respectively. The op-amp then performed as it should. Thanks for the help.
     
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