Noise problem using zener protection on precision analog inputs?
Low voltage zener diodes (&lt;5v) work by a different breakdown mechanism to those above 5v, and are very much softer, conducting significant nA at quite low voltages, which could interfere with your readings. However, you say that you will use 15v signals, and zeners to handle these voltages are much sharper. Whether they are sharp enough in your application, you will have to measure.

The input clamping ESD technique I always use is to use silicon diodes to voltage sources, ideally the rails for convenience, but if it has to be lower voltages, then a biassed zener diode in parallel with a huge electrolytic to quiten any noise. The reverse bias leakage in a good silicon diode should be much less than the sub-threshold leakage in a zener, so interfere less with the input.