I have looked at this sort of thing in the past. Two main issues - offset temperature drift and linearity. IIRC the circuits that offered minimal offset drift were more prone to non-linearity & etc. This was using a feedback type circuit with 2 optos (Actually in the same package to equalise temperature). We did require very low offset drift however. It all got too complicated and I figured the best way was to do as already suggested - run a ADC on the isolated side and transmit the data across the isolation barrier digitally. Wouldn't take much more than a PIC, a regulator and maybe an opamp. Use the UART and you only need one opto. We eventually used a non-isolated version anyway. I think I did come up with a simple sigma-delta type ADC that put the ADC sort of across the barrier with only digital switching - slow and might have worked using mostly simple components, but it didn't even get tried. RP On 13 March 2013 10:02, peter green wrote: > Neil wrote: > > Hi all, > > > > Looking for a way to measure a voltage of 200VDC or a bit more with a > > PIC circuit. I don't need the full range all the way to zero -- only > > about 100V to 200V. The PIC circuit already has its own power supply, > > so I only need the circuit to drop the 200V to something under 5V, or > > some varying resistance perhaps. And it needs to be cost-effective > > (under say $5 in 100-pc quantities). > > > > A resistor divider is not doable because the sections are isolated. I > > did find some special opto-isolators for analog signals, but they're > > very pricey. But I understand that some common (ie: typically used for > > on/off signals) opto-isolators can work for linear signals. Anyone her= e > > know if this is true, or have another way to do this > One possibility may be to use a SPI ADC on the "hot side" and then > opto-isolate > the digital signals between the ADC and PIC. > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .