"Thomas C. Sefranek" wrote:
>
> Jim Dossey wrote:
>
> > I'm a programmer, not an EE, so I'm hoping someone here can point me in
> > the right direction.  I'm currently working with a circuit that someone
> > else designed, and now I'm writing software for it.  I can supply a
> > schematic for this circuit, but I don't have it here in front of me.
> > Basically, we have 2 DC signals and we need to take the difference
> > between the 2, and amplify that difference by a factor of 10.  That
> > input is then feed into one of the A/D converters on a 16C74A (thus the
> > reason I am on this list).  The larger of the 2 signals is fed into the
> > + input on an LMC6482, and the other is fed into the - input.  The
> > problem is that when I graph the output signal, and a calculated value
> > for S1-S2, they don't match.
>
> What do you mean by "they don't match."?
>
> > So my questions are these:
> > 1. Is this a valid use of an op-amp?  Maybe we should have a multi-stage
> > op-amp circuit instead of just using one.
>
> I am not familiar with the LMC6482, but you have not specified the range of
> voltage of the signals either.
>
> > 2. Will an op-amp subtract 2 signals and then amplify that difference?
>
> An Op-Amp can amplify the difference between two signals,
>
> >
> > 3. Is there a mailing list, web page, or book somewhere that will help
> > me with op-amp design?
> >
> > Thanks,
> > Jim Dossey

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.

Thanks again to all who replied.  I think I am starting to understand
the problem.