How to protect an input from ESD?
What other solutions do I have ?


Given that the data sheet says this: -

Maximum Current into Any Pin.........................................±50mA

It gives you some leeway to add a resistor in series with the input pin.

But you also need to know how big a resistor can be used and there's a clue on page 14: -



So, I estimate that the inputs are at least 50 MΩ and so a 1 kΩ in series with each input isn't going to have much of a performance implication.


  How to protect an input from ESD?


So, at a maximum current of 50 mA, the voltage drop across 1 kΩ is 50 volts i.e. plenty of headroom to add a TVS of 10 volts or 20 volts on the non-chip side of the input resistor. I wouldn't use a zener - use a TVS or TVS array suitably specified for protecting against ESD.

And, as @Justme points out, the device has the following level of ESD protection built in: -



Note 10 says: ESD test performed with 1kΩ series resistor designed to withstand 8kV surge voltage.

So, I guess 1 kΩ (as I derived above) is perfectly sensible. However, given that I have suggested using an external TVS in front of the 1 kΩ that resistor need not be rated at 8 kV: -