You can't measure anything till you get rid of AC. Simple RC is the easiest way. Resistor limiting current below 1 mA , and you o'k with internal diodes build in AVR chips.
Only 1M too much, analog reading requirement < 50 k. Even with 20 k in voltage divider it save to overvoltage input up to : V = R * I + 5V = 20k * 1 mA + 5V = 25V