Analog world is too much complicated by itself, you don't need to
complicate it more by not giving detailed information... :)
If what you are trying to do is what I am thinking, for example reading
a sensor in a form of a bridge or something like that, your inputs S1
and S2 can not receive feedback from the output, if you do, your sensor
voltage output will be affected by this "not expected" signal,
impedances will change, signals will travel alone, etc. When reading
sensors, most balanced or sensible ones, you MUST use an instrumentation
amplifier OR build your cheap instrumentation amplifier with 3 op-amps.
Most designs for cheap Inst.Op. use 4 op-amps, but you can do with only
3.

It can be done using a multiple amplifier chip, as for example (but also
not recommended LM324) with 4 amp ops inside, you will use only 3 for
this purpose.  With this fashion, your S1 and S2 will go directly to the
setup inputs without any interference from the feedback.

The zero (offset) adjust is almost always necessary in op-amps, since
perfect zero offset is just theory. In the suggested cheap Inst.Op. this
adjust has nothing to do with the inputs, they still free to receive
"just" the sensor signal.

If you are interested in such design email me directly.

Wagner.

"Robert A. LaBudde" wrote:
>
> At 02:11 PM 3/8/00 -0500, Jim wrote:
> >I'm working with a 5V circuit.  The 2 inputs range from 1V to 4V, with
> >the difference between the 2 ranging from 0 - 500 mV.  S1 > S2.  Here is
> >a simplified version of the circuit:  (NOTE: There is also a variable 1K
> >resistor on S1 that is used as a zero adjust)
> >
> >             10K      |\  LMC6482
> >   S1 >----vvvvvv-----|+\        1K
> >             10K      |  >-+---vvvvvv-----> output
> >   S2 >----vvvvvv--+--|-/  |
> >                   |  |/   |
> >                   | 100K  |
> >                   L-vvvv--|
> >
> >The output is not what I expect because it seems to be influenced more
> >by S2 than it is by S1.  If both S1 and S2 are increasing (over a 1
> >minute period), then the output also increases.  If S1 increases and S2
> >decreases, the output decreases.  If S1 decreases and S2 increases, the
> >output increases.