On Thu, 7 Jan 1999 21:25:55 +0000 "Peter L. Peres" 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. ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]