On Wed, 27 May 1998 10:30:54 +0100 Keith Howell writes: > When I couldn't get it going in a hurry I fell back to >the plan of opto-isolating the digital signals to/from the serial ADC >(clock, select, data) and this worked fine. To stay on topic, consider a PIC with ADC on the isolated side. It could measure several voltages (not isolated from each other) and pass the results back through one cheap optocoupler. It would be quite simple to use an asynchronous serial format. Passing analog signals, especially where DC counts, through optocouplers is fraught with problems. A circuit using two optos where one provides a reference feedback is essential. Even then, the two must be well-matched. I had considered isolating through a pulse transformer. The input side chops the DC voltage to pulses of proportionate amplitude. The output would rectify the voltage. An identical rectifier on the input side with a third winding could be used for linearizing feedback. If some connection between the two sides is permissible, a differential amplifier would likely be overall best. _____________________________________________________________________ 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 Or call Juno at (800) 654-JUNO [654-5866]