On Thu, 7 Jan 1999 21:25:55 +0000 "Peter L. Peres" <plp@ACTCOM.CO.IL>
writes:

>  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.


Here's a simple circuit commonly used for that purpose:

(+supply)-------Rs-------(load)
           |         |
          R1        R3
           |         |
           |---   ---|
          R2   |  | R4
           |   +  -  |
          GND   O----x---- Out

The +,- and O at the bottom is an op-amp.  It is necessary of course that
R1/R2 = R3/R4 in order to reject changes in the supply voltage.  Voltage
dropped across Rs is amplified by a gain of R4/R3.  If the supply voltage
is rather high, it is hard to keep the op-amp inputs within their range
and also have good gain.  This circuit is not optimal in that case.

I'm not sure what your circuit is supposed to do but it looks
unnecesarily complicated.  Of course the conventional 2 and 3-amplifier
"instrumentation amplifier" circuits can also be used, offering somewhat
higher performance.

It is sometimes possible to rearrange the circuit so your ADC's ground is
one end of the resistor.  This simplifies the current measurement a lot.


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