First you must ensure the Zener will not interfere with your signal.
Zeners have capacitance and current leakage. No problem if your signal
is slow and of a reasonably low impedance.

I always use some kind of protection to MCU pins that have off-board
connections.

One circuit I use is comprised of a series resistor from the connector
pin to the Zener's cathode (Zener anode to ground, obviously) then a
second resistor from the Zener's cathode to the MCU pin. If I need a
pull-up I connect it to the Zener's cathode also.

If the resistor's values are correctly chosen you can communicate
bi-directionally (e.g.  I2C).

The first resistor is to limit current into the Zener in case of an
accidental connection to +12V.
The Zener besides protecting against ESD will protect against continued
connection to battery voltage.
The second resistor will protect the MCU's internal parasitic diodes to
VCC in the case where the MCU is not powered and the external connection
is being supplied a voltage large enough to the point that some of the
Zener current is diverted to the MCU's pin and damage it.


Best regards,

Isaac


Em 19/05/2013 08:17, Electron escreveu:
> Hello!
>
> An ECU uses some PIC analog pins to connect to external sensors
> that return 0 .. 3.3V analog signal, like e.g. a TPS.
>
> 0603 10uF 6.3V capacitor in parallel to the PIC input, to smooth
> things out.
>
> Now I wonder if a zener is necessary too, to protect from ESD.
>
> OK, they won't harm. But are they really necessary in your opinion?
>
> PCB's are already made and space is little, more hand work is non
> desiderable etc.. so if they are really useful OK, else why add them..
>
> Thanks,
> Mario
>

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