circuit-protectionindustrialinputsurge-protectiontvs
Take for example a Siemens S7 24V digital input.
I read a couple of articles and also questions on SE and it seems there should be (maybe integrated, maybe as a discrete part):
Is this correct?
How is an industrial 24V SPS input protected?
What protection and in which order would this input have?
Does somebody have an layout of such an input to study?
Your solution provides protection for -4.2 volts and 6.7 volts. This exceeds the maximum ratings of the ADC and is not safe. Also Zener diodes often have higher resistance than normal diodes, making them less ideal for protection.
It is of course possible to protect you ADC this way, by finding a zener that is a perfect match, but I would recommend doing it another way.
A simple resistor will in most cases do to limit currents, But if the ADC draws much current it is already broken. Usually the protection for high input currents would be in the signal conditioning part.
Two diodes connected from the input, as shown in the schematic, will protect your input from voltages under -Vf and over VCC+Vf, so if you choose diodes that have a forvard voltage of Less than 0.3 volts, you will be safe.
Diode type is at your choosing, but you might want some schottky diodes in there for the forward voltage to be right. (1n4148 is standard in the schematic editor) Diodes with Vf under 1 volt exists.
If your input is designed to be floating, I think you should add a high value resistor between 100k to 4Meg from the input to ground to have it acting less like an antenna and provide a path for offset currents. (this will of course affect high frequency behavior)
simulate this circuit – Schematic created using CircuitLab
The problem of inrush currents into massive capacitive loads has been solved in industry by use of thermistor elements (negative or positive temperature coefficient thermistors), see "PTC Thermistors For Inrush Current Limiting" Without them every PC would blow household breaker circuits every time you plug a PC into AC outlet.
Surge protection will be also improved/solved after installing NTC or PTC inrush limiters.
Best Answer
From the image below taken from the Siemens S7-200 Manual you can see that there is some protection, they do not fully define the items. But it looks like the opto is there, and resistors for surge suppression and a diode for reverse polarity.
Digikey also has a nice article here.
How much protection you need is really up to you and your particular situation. Including but not limited to Cost / Board Space / Time / etc. For example maybe you won't need to have reverse polarity protection because of the way that the system is installed / used.