Hello. I don't know your application here of course, but it may not meet your needs simply to divide the incoming line voltage and give it to an ADC, since that would connect your logic ground to the AC neutral at the ADC, and this might not be a good plan. Using the "ground" reference from elsewhere in your system could introduce ground noise at the ADC, and throw off your readings.
If this point is valid, and you need to isolate the AC line being measured from the ADC, you might use a voltage controlled oscillator, and an optoisolator. Provide a very small off-line power supply for the VCO, and use rectified line voltage, (suitably filtered, and divided down with a precision resistor network, and overvoltage protection), as the control voltage for the VCO. Then the output of the VCO can drive the LED of the optoisolator, and the transistor of the opto will switch at a frequency proportional to the rectified line voltage. This approach would also eliminate the need for an ADC, since the frequency can be digitally measured. You could also use an optoisolator in linear mode, and use the transistor's output voltage as an analogue of the line voltage, (eliminating the VCO), and use the ADC again on that signal. However optoisolators often have a current transfer function that changes over time, and the system may need recalibrating later. Be slightly cautious not to draw significant current from a half-wave rectifier (single diode), since the harmonics introduced by half-wave power supplies have led to such supplies being disallowed by the European Community, I believe. Jonathan Malton S-S Technologies, Inc. Kitchener, Ontario, Canada. "I'm not so arrogant as to suggest that my opinions are corporate policy"
