But I do not see an input protection circuitry. Only the 250Ω resistor.

I think better to use a 240Ω resistor, then a series resistor with diodes to Vdd and ground for the least protection.

Better use these:

1. A ferrite ring (say Murata BLM21, 1K@100MHz) in the input wire. Then a small capacitor of 47-100pf to ground. This will give a protection to very short transient input voltages.

2. Then 1K in series and 100pf to ground for a strong low pass filter so a 500V pulse will give a 5V/ns slew rate on the 100pf.

3. Two LND150 with a common resistor of 1K will give a two way current limit of 10mA so the diodes mentioned above can handle the excess current to power supply.

Ioannis