Quote Originally Posted by Dave View Post
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In other word it is acting as a limiter....
Yes but not like a switch! It is like a normal diode in characteristics slightly steeper. But has a knee. That means if voltage is increasing above about 4,3Volts the zener might start to conduct and spoil the readings of the ADC.

I may have not stated correctly in engineering terms. If anyone with native english understood, please help here.

Ioannis