ADC protection best practice
simulate this circuit &ndash; Schematic created using CircuitLab

If you have need a resistive divider, it can limit current instead of an explicit current limiting resistor.

The diode rail clamps can only clamp to the rails if there is voltage on the rails. They won't offer protection if the ADC is unpowered. But zeners or TVSs which breakdown in reverse will provide protection when not powered (as well as powered, but they might not clamp to as constant voltage as the diode rail clamps.