I'm surprised that you can reverse bias it that much. I'm not sure how I can confirm this easily without risking exploding Unos?For a -15V input I would need a >15K resistor. Will the ADC readings still be accurate from such a high impedance source? My sense is that the approach would be to clamp /clip somehow..?
You can use a pair of 1N4148 diodes as clamps. They have a max current rating of 200mA. You could also use Shottky diodes as they have a lower forward junction voltage.
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