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Operational Amplifier (Voltage Follower)

  1. Oct 17, 2005 #1
    Hi Guys,

    I'm trying to amplify a millivolt signal from an electrode that has a high input resistance (~ 200 MOhm). To avoid losses over the series resistance i'm using a high input impedance Op Amp TL071CP as a voltage follower. The problem i'm having though is that for some reason when the op amp is powered with +-12V (or any +- votlage for that matter) I am getting -10.2V at the non-inverting input and consequently seeing that voltage at the output, even when the probe is disconnected. Ideally I would have 0V at the output when nothing is connected. Does anyone have any suggestions as to why this may be happening?! It's been bugging me for hours.

    Cheers.
     
  2. jcsd
  3. Oct 17, 2005 #2

    berkeman

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    I don't think that opamp is meant to be powered by +/-12V. The datasheet says +/-8V max. Maybe that chip is blown now. Try using a fresh chip and only +/-5V or so. Also, what is the equivalent circuit of your sensor? Does it just look like 200MegOhm to ground?
     
  4. Oct 17, 2005 #3

    berkeman

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    Also maybe check that you have the pinout correct. If you accidentally connected the output to the + input, you might get a similar behavior.
     
  5. Oct 17, 2005 #4
    The datasheet says +-18V. Hmm, in the past i've never had an issue with powering the same IC with +-15V. I don't see why +-12V would affect operation?

    The circuit is very simple, i'll upload an image for you to see for yourself.

    Thanks :)
     

    Attached Files:

  6. Oct 17, 2005 #5

    berkeman

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    My bad, I was looking at the TLC071 datasheet and saw the +/-8V max. You're correct that the TL071 has the wider supply range, so that's not the problem. I'm not sure what to suggest, guy. A follower is a follower. Hook a power supply up to the + input and make sure that it follows that low-impedance voltage input across its input Vicm range. Then take a closer look at the other input sources that you are presenting to it. And try a few different TL071 chips -- maybe you've got a bad one.
     
  7. Oct 17, 2005 #6

    Ouabache

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    I see what you're saying about +/- 18V supply voltages. ref
    I would go with berkeman's 2nd suggestion and recheck your pin connections.
     
  8. Oct 18, 2005 #7

    dlgoff

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    Are you using the offset null inputs?
     
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