Using just a TVS is a brute force method and will not make the inputs safe for steady state overvoltages that comes from a power supply with.... say a 5A capability.The key is to use the low current required by the Arduino inputs to your advantage.As they say there is more than one way to skin a cat....My suggestion is below. It will withstand 15V input for an unlimited time, as well as input spikes and reverse voltage. The resistor values are not critical and the Zener doesn't have to be a TVS, it can be just a regular zener diode.
The circuit here is an example, just ignore the "zener or schottky" labels, definitely only schottky diodes
I respectfully disagree with the deletion of the of the bottom zener. I agree the top diode might be needed for some application.In the OP's case, the application of 15V to the circuit you suggest could easily overvoltage the board (depending on other loads on the 3.3V rail).In my proposal, the series resistor to the board will deal with voltage applied to the input when the board is not powered.All our automotive designs were required to be capable of 18V on any pin for an unlimited period of time. We found the zener works best.
The standard clamp circuit is series resistor into schottky diodes that bolster/take over from theinternal diodes. This will protect whether or not the Arduino is powered, the zener circuitcannot protect at all if powered down and doesn't protect against undervoltage either.The circuit here is an example, just ignore the "zener or schottky" labels, definitely only schottky diodeswork.https://electronics.stackexchange.com/questions/234308/how-to-make-clamping-circuit-to-clamp-exactly-at-3-3-volts
Not clear , but if this is digital input an opto isolator is a simple and reliable option or even a relay .
When the Z-diode starts to clamp, say a 4.90V may read as only 4.85 volts, depending on the knee curve of the Z diode.
Irrelevant for a digital input. Anything over 2.7V or so (have to check data sheet for the exact number) reads as HIGH.
Schottky diodes are right at the pin, resistor between pin and input. So current from the input gets limited by that resistor, and then any over (or under) voltage is syphoned off by the Schottky diodes.That 5.2V zener in #1 is simply redundant and serves no function, the 0.1µF cap is also better removed as it slows the reaction of the pin (with a 5k cap the reaction time is down to about 0.5 ms) which may be an issue for fast changing signals.If higher voltage spikes are possible (>100V) an optocoupler may be the proper solution.
Yeah, this thread is about analog input.
You mentioned your design will protect even if the MCU is off? Can you elaborate how? IF 2.5V is applied wouldn't it still run into the MCU? I know the resistor can limit current, is it that the current protection alone and not necessarily a voltage protection would be sufficient? Or is it that the current would be limited enough so the internal diodes won't be over burdened?
With that resistor can it accurately read a 0-5V input?
The ADC is optimized for analog signals with an output impedance of approximately 10 k or less. If such asource is used, the sampling time will be negligible. If a source with higher impedance is used, the samplingtime will depend on how long time the source needs to charge the S/H capacitor, with can vary widely. The useris recommended to only use low impedance sources with slowly varying signals, since this minimizes therequired charge transfer to the S/H capacitor.
The capacitor of course is not for static voltages but spikes and noise, in general it is a good practice to add it to your circuit.