Is arduino equiped with AC protection?
If you combine 5V DC and 20V AC you will have an AC waveform with the 0V point at 5V. It basically just shifts the waveform up.
I have an input coming from a car. It goes from 0V to roughly 18V at max. I was hoping to build a simple voltage divider using about 500k and 2M (arduino seeing the voltage across the 500k which is about 3.7V at max which would be plenty for analog reading). But there is one problem. There is roughly 25V of AC riding on that DC and I don't know what frequency. All I have is a basic multimeter. What should I do? Should I block the AC or use a diode somehow or does the arduino have something built in that can help me with this?