At 01:20 PM 03/08/2000 -0500, you 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.
>

Your opAmp circuit needs resistors to properly amplify or
difference voltages. You cannot simply put signals onto the
+ and - terminals. You always need a feedback configuration for
proper operation (except with chips labelled "instrumentation amp*).

1 opAmp can do it.

           R1
V2 -------10K-----+--------+      opamp
                  |        |     +------+
              R2 100K      |     |      |
                  |        +-----|+     |
                 gnd             |      |---+---1K--- Vout
            R3                   |      |   |
V1 --------10K-------------+-----|-     |   |
                           |     |      |   |
                           |     +------+   |
                           +------100K------+
                                   R4

  Vout = [R2/(R1+R2)]*V2*[1+(R4/R3)] - V1*(R4/R3)

  Vout = [100/(10+100)]*V2*[1+(100/10)] - V1*(100/10)
       = 10*(V2 - V1)

the R's should be 1% tolerance, or the terms in the equation
won't cancel properly.

In general, the smaller R's (ie, 10K here) should be lots
bigger than the source resistances of the signals. If
necessary, make all resistors larger by the same factor,
ie, 10K/100K, 20K/200K, 50K/500K, etc. Larger R values are
ok with CMOS opAmps.

The source signals should not be greater than the voltages
powering the opAmp, since V+ = V2 (approx).

Be careful the voltage applied to the A/D is not outside the
range 0-5v - eg, by wrongly connecting the inputs. Not a problem,
if the opAmp is powered only by 5v, rather than +/-5v. The 1K
resistor in Vout lead helps protect the A/D in any case.

- Dan Michaels
Oricom Technologies
http://www.sni.net/~oricom
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