The usual way to do this would be two diodes from the signal pin to the converters VCC and GND.
Leakage depends on the diode type, will be large if you use schottky diodes, low with normal silicon diodes and extra low with the B-C junction of a 2N3904 type transistor or a JFET used as a diode. Most common are 1N4148 type silicon diodes here.
This limits the voltage to one diode drop above VCC and below GND, that is OK for most ADC inputs, but not in general because there will be still current flowing into the ADC pin (through its protection network to VCC or GND). Often the datasheet allows a maximum of 0.3V below GND / above VCC, if you exceed this, the ADC won't die instantly, but current starts flowing into the pin. You'll have to limit this current to less than 10mA (this is a quite common value here) to prevent the input pin from beeing damaged. 10mA flowing into a pin most probably will leak into other pins, disturbing other ADC inputs, usually not a big issue with digital inputs.
The next better but still simple solution would be to use the diodes, but place a resistor between the ADC input and the diodes, this limits the current into the ADC - the resistor value needed depends on the ADC specs, often it isn't specified how much current into its protection diodes is allowed without disturbing other ADC inputs. In many cases you'll be on the safe side if you limit the current into the ADC pin to 0.5 ... 1mA, sometimes 5mA are still OK. Other ADC may insist on no current flowing, then you'll need another approach.