ADC problem

[showtime]

I couldn't understand that opamp level shifter circuit. Please give me an example

[Charles Linquis]

You need to set up the op-amps as a differential amplifier, and I don't have time to go into the full design right now. If you aren't already familiar with differential amplifiers, I think that my suggestion of measuring the drain voltage and subtracting it from 20 is the easiest approach for you. If the 20V isn't stable, you could even use a second channel of the A/D to measure that voltage exactly. Four resistors (for two voltage dividers), two capacitors (for filtering) and a little code and your are done. Everything would be ground-referenced.

[showtime]

Thank you very much Charles Linquis. I will try to do this.