Like David said. You might consider using a Schottky diode (like a BAT54 or equivalent) instead of a normal silicon diode. Its lower forward voltage guarantees that the PIC's ESD protection diode won't conduct even a little bit.

The cathode of the Schottky goes to Vdd, and the anode goes to the ADC pin. A large-ish resistor (~5k) goes from the ADC pin to the top of the 250 ohm sense resistor. So when the voltage at the ADC pin tries to exceed Vdd + 0.3 V, the diode conducts and clamps the voltage, and the 5k resistor limits the current, and nothing blows up.

A few tens of nanofarads from the I/O pin wouldn't hurt either, decrease high frequency noise and maybe get things a bit more robust against ESD (static electricity).

Thanks for helping, I had not used schottky in the past
but I will now.
Regards
David

It is good practice to tie unused I/O pins high or low, either in the circuit (by connecting them to Vdd or ground) or, better, by driving them as outputs in the program.

But if you don't, then nothing very bad happens. The biggest consequence is that the input stages might draw some class A current (where both the n- and p-FETs conduct, and current flows from Vdd to ground). That increases the total power consumed, but will not lead to damage.

A big silicon diode will have a lower forward voltage (at a given current) than the PIC's internal protection diode, so even with a non-Schottky diode, very little current flows into the PIC's protection diode. And a small amount of current into that protection diode is unlikely to cause real problems. You could omit the extra diode entirely (but keep the resistor), and everything would seem to work fine, although it's not good practice. But the Schottky turns on well before the PIC's protection diode will, so it's the nicest solution.

But LDmicro will automatically set them (unused I/Os) as outputs when compiling, I purpose?

Sorry, I meant: "I SUPPOSE!"

Can somebody make schematics for this
Thanks!

Thanks Jonathan

The 1N5819 will work, but its reverse leakage current may contribute non-negligible error, especially at high temperature. Consider Vishay's datasheet:

http://www.vishay.com/docs/94614/1n5819.pdf

That leakage current is rated at 6 mA max, at Vr = 40 V and 100 degrees C. Of course, we should never in practice have more than (5 V - (250 ohms)*(4 mA)) = 4 V across the diode, which Fig 2 would suggest adds less than a microamp typical at room temperature, though a ~milliamp at 125 degrees C.

The 1N5819 also adds lots of capacitance, a couple hundred pF. The circuit above already has 100 nF to ground at the input, though, so that's negligible.