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I am considering to use Zener diode for over-voltage protection for a digital input pin.

The absolute maximum rating for the typical IC pin is given as from (GND-0.3V) to (VDD+0.3).

I can use Zener diode to limit voltage below VDD. However, the forward voltage of typical zener diode is 0.7V. which fails to limit low voltage to -0.3V.

My signal is 1MHz signal, and will It be safe to ignore since it is marginal difference and the duration is short?

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You can limit the negative excursion with a shottky diode in parallel with your zener. Your trick will be finding a zener that's guaranteed to turn on in the 0.3 V window between Vdd and your IC's absolute maximum value. More importantly, what kind of source are you connecting to that you're worried about it violating the input voltage limits? – The Photon Mar 2 at 2:51
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You can also use a Schottky from your signal (anode) to VCC (cathode) to clamp the signal to VCC+0.3V. – John D Mar 2 at 3:02
    
What are your signal max/min voltages? – PDuarte Mar 2 at 9:13
    
@PDuarte Signal will have Maximum AC amplitude of 15V with steady DC bias of 2.5V – Steve Mar 2 at 10:10
up vote 2 down vote accepted

For over-voltage protection of signal lines a zener diode is generally a poor choice. You haven't given enough details of your application for me to give many specific reasons, but you should consider the following:

  1. A BZX84C 3.3V zener has up to 450pF capacitance. You have a 1MHz signal, which will probably be attenuated due to a RC filter effect in your circuit.

  2. Zeners do not have a sharply defined conduction voltage. They start to conduct at a much lower voltage than the zener voltage Vz. This becomes a problem when your signal has a high source impedance, or if you have introduced some series resistance to try to limit the effects of over-voltage. To counter this you may need to use a higher voltage zener, but this then defeats purpose of including a zener.

  3. If you are concerned about high energy over-voltage transients then a zener is of no use at all. It reacts too slowly, and cannot absorb much energy.

Generally, for signal lines, a Transient voltage suppressor is a much better part.

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You could consider a Transient Voltage Suppressor (TVS). They are basically diode arrangements that are intended to protect circuits like yours from over/under voltages. Since they are designed to work with microcontrollers and other devices for this purpose, you can choose the voltage range you're using when you buy them (e.g. designed for 3.3V and goes into a full clamp at 3.5V).

Operational voltage ranges and tolerances for these devices are typically better than straight-up zener diodes.

Negative protection is included in the package as well. They work very well for ESD protection. There are a lot of them out there, but here is one example:

http://www.semtech.com/images/datasheet/uclamp3301d.pdf

TVS Diode Schematic

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