Easy way to protect ADC input

[KKK]

I think I will choose the first solution by also adding a TVS diode maybe 30V. But I have a doubt, can the TVS diode capacitance, when the trip threshold has not been reached, affect the analog reading considerably?

[Kleinstein]

The capacitance of TVS diode can vary quite a bit. There are low capacitance ones for something like USB and high power ones with rather high capacitance. Depending on the resistors the capacitance (especially as it tends to be voltage dependent) can effect the linearity a little.
A 30 V TVS would not provide much protection. one would want more like a rather low voltage one, like 6 V or 8 V and than still 2 resistors (one at the ADC and one at the very input).

[radiolistener]

Usual practice is to install two anti-parallel diodes. They clamp input voltage to about 0.5 V which is usual working range for ADC input. Using high frequency diodes with low capacitance allows to minimize effect from added diodes.