Operational Amplifier (Voltage Follower)

[splitendz]

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.

 

[berkeman]

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?

 

[berkeman]

Also maybe check that you have the pinout correct. If you accidentally connected the output to the + input, you might get a similar behavior.

 

[splitendz]

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 :)

 

[berkeman]

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 :)

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.

 

[Ouabache]

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

 

[dlgoff]

Are you using the offset null inputs?