Hi,

Darrel and myself once tried to profile the leakage and breakdown of the zener diodes on the analog pin. For faster acquisitions of the s/h cap a low impedance path is always preferred to the ADC which can kill the PIC due to shootups or downs (negative).

I had been using zener without any problem cause I almost always use a temperature compensated reference for Vref mostly 2.5V. So the 5.1V zener did not come into picture. But Darrel did an actual test with a modulating input and found that the zener mis-behaves (turns leaky) near its threshold. I found ambient temperature variations could create large error in the measurement if you are using VDD (+5) as your reference.

So the bottom line is if you could lower your Vref much lower than Vdd then zeners of Vdd ratings can be a safe bet.