I'm designing an input circuit for a mega328 ADC input to read a variety of external position sensors that use a 360 degree pot (pots range from 1k to 20k depending on vendor). They are usually configured to generate a 0-5v voltage from the pot's wiper that is read by the ADC. I'd like to just feed that sensor voltage through a current limit resistor to the ADC pin with 2 Schottky signal diode clamps to +5 and ground (already eliminated zeners and standard silicon diodes).
Since some of these sensors have a deadband where the resistance goes infinite for a few rotational degrees there usually is a high resistance up to 1M ohm pullup or pulldown on the pot wiper to keep the sensor voltage defined during the deadband (some vendors build the pullup into the sensor).
Problem is that the Schottky diodes I've tried have enough leakage current (~0.5uA max at 25C) that at elevated temps the typical pullup of 470k-1M ohm causes a large sensor voltage error. At room temp all is fine but this controller has to work at higher T's also. And then there's the AVR's own leakage current which is spec'ed at 1uA max.
So I see 2 choices: either use much lower leakage Schottky diodes (or equivalent) or resort to adding a rail-rail x1 gain op amp buffer assuming it comes with better intrinsic input protection. Would appreciate a few seasoned veteran suggestions.