Arduino PIC

The Arduino/PIC input WILL draw current if you pull it beyond the power supply rails, because the internal protection diodes will conduct. But as long as you limit this current, the device won't be damaged. The resistance of the voltage divider will inherently limit this current; you just need to choose resistor values high enough so that the device's maximum pin current won't be exceeded. This current will be listed in the data sheet. Actually I would limit the current to a lot less than the stated figure - a few mA at most.

But if you're concerned about losing the ground on the voltage divider, you could just put the bottom resistor of the voltage divider right next to the PIC so the ground can't be lost.

You may also need a capacitor from the input pin to 0V because the ADC generates pulses of current during conversion that will disturb the pin voltage if it is not held fairly stable. I normally use a 0.1 µF metal film capacitor (don't use a ceramic capacitor) for this.

 

I think so, but I strongly advise you to limit the current to a lot less than that - say 5 mA. Remember that that figure is in the absolute maximum limits section of the data sheet and data sheets normally say that the device is not guaranteed to work properly at those limits and that exposing the device to those limits can compromise reliability.

Yes. That would be good.

 

I thought so too, but that's not how the specifications are worded. Where is the specification for the maximum protection circuit current?

Yes, you're probably right. Current flowing into the protection circuit could cause VDD to rise and this would throw off all of the measurements if VDD is being used as VREF.

For best accuracy, don't clamp the input voltage using a zener, because of the leakage. I got good results using "clamping rail" consisting of a zener biased from VDD with a zener voltage around 0.4V lower than VDD, and a 1N914/4148 diode from each input pin (anode side) to the clamping rail. This ensures that the clamping current doesn't flow into VDD.