The first thing to say is that zener diodes aren’t perfect. Some current starts to flow below the rated voltage, and you have to go well above the rated voltage for the zener to pass high current. How that affects the output of your op-amp is up to you to check. You probably have a series resistor on the op-amp output to limit current when the zener starts to clamp, and that needs to be chosen carefully.
From memory, this is particularly true of zeners below 5V which actually rely on different physics from a higher-voltage zener diodes.
The next thing to think about is what might go wrong if you put in too-high a voltage onto a pin of the stm32. ST generally say you shouldn’t exceed 0.3 V above Vdd. One reason is that the isolation between different parts of the chip are often made by diode junctions. We all know that diodes turn on at 0.6 V, but they actually start to leak current nearer 0.3 V. This leakage first starts to affect sensitive parts of the stm32 such as the ADC, where you might get the wrong reading - perhaps only slightly wrong, or crazy if there’s a higher voltage hence higher leakage.
And I might add that although things might be worse for pins that can be inputs for the ADC, over-voltage on any pin can lead to misbehaviour.
The next thing to say is that these effects can happen at lower levels of over-voltage than those that lead to permanent damage to the chip - letting out the “magic smoke�? or simply killing a single pin or port.
Modern ICs like stm32 are much better than things were (say) 30 years ago, but it’s an analog world out there and we do need to design with care.
Hope this helps,
Danish
