Op Amp Follower (Buffer?)

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Noone - 30 Nov 2004 14:55 GMT

I am very new to this newsgroup scene, so I am hoping that someone will
have some tips for me. The issue is that I am using a a PIC16F877 to do an
adc and I am getting floating inputs (The voltage is very stable, but the
PIC reading is very unstable). My circuit consists of a pressure sensor that
goes into a difference amp with a gain of 100. The output resistance from
all of that is very high (>10MOhm, my Multimeter cant read it, it is so
high). From googling around I learned that I need a voltage follower to
lower the resistance at the PIC analog input. The problem that I have is
that no matter what op amp I use, it doesn't lower the resistance. I have
tried several:
LM324N (Quad Op Amp)

LM741CN

LF347N (Quad Jfet Op Amp)

LM837N (Quad Op Amp)

All with the same result, my voltage is stable, but my PIC reading is
unstable. And my resistance at the pic input is still very high. So my
question is:

Are there only certain types of op amps that work as voltage followers
(buffers)??

And if so, what is a good (preferrably somewhat cheap) op amp to use?

TIA to anyone that can help me out. If anyone wants more information to help
me diagnose the problem I have a graphic of my circuit drawn and I can give
part numbers for my other components too.

Steve

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Rob Fell - 30 Nov 2004 15:11 GMT

> The output resistance from
> all of that is very high (>10MOhm, my Multimeter cant read it, it is so
> high).

An op-amp is an active device - you can't simply measure it's output
resistance with a multimeter.

> All with the same result, my voltage is stable, but my PIC reading is
> unstable. And my resistance at the pic input is still very high. So my
> question is:

I'd start with these key (IMO) questions;
Which voltage is stable? The supply to the pressure transducer or the
voltage to the A/D input pin?
How unstable are your PIC readings?
Have you ensured your PIC A/D has suitable reference voltages?

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Electronics Forum / Electronics / November 2004

Op Amp Follower (Buffer?)