use an opamp which goes up to the positive rail with its output. short r3 and
r6 and take r5 out. set r2=r7 and r4=r8.

Ao=r8/r7=r4/r2

Vo= i1 * r1 * Ao

you can make the opamp to output current, but i thought you want to measure
current and output voltage...

ge


At 21:25 01/07/99 +0000, Peter L. Peres wrote:
>Hello,
>
>  I have a small question. It might be stupid, but...
>
>  I need to measure a DC current using a probe resistor in a circuit. The
>resistor is in the +-branch and at the highest voltage available in the
>system. This current needs to be transformed into a ground-referenced
>voltage for ADC purposes. I'll try an ASCII schematic:

                   R1
U1, I1 >-----*---/\/\/\---*-----> U1 - R1*I1, I1
             |            |
          R2 /            |
             \            |
             /            |
             \            |
             |            |                  |\
             *------------ ------------------+ \
             |            |    R7            |  >----*----O Vo
             |            *---/\/\/\----*----- /     |
             |                          |    |/      |
             /                          |     R8     |
          R4 \                          *---/\/\/\---+
             /                          ^
             \                          A
             |
            ===
            ///
            GND


>  Assuming R2 == R4 and the differential amplifier gain is Ao = R8/R7, the
>expression for the output voltage assumes the form:
>
>
>Vo = Ao*U1*I1*R1*R5/(R3 + R5)
>
>
>this is very nice, but I'd like to get rid of U1, for obvious reasons. U1
>is not fixed, obviously. The most direct way is, of course, to measure U1
>at the same time, and divide Vo by something proportional to U1 (in
>software). (U1 IS measured at the same time by other parts of the
>circuit).
>
>Question: Is there a cunning way to avoid this, using *simple* analog
>circuits to inject a current proportional to U1 into the '-' input of the
>op-amp at point A ? I know how to do this with 2 current mirrors and a R.
>Any other ideas ? An op-amp voltage controlled current source referenced
>to U1 ? Pros and cons ?
>
>Note that the expressions come off a bit of paper, and that it' 9PM here.
>There may be errors.
>
>tia,
>
>        Peter