Hello,
A closer inspection of Fig. 22 within the datasheet shows that Radin represents the series resistance associated with the channel select multiplexer. This series resistance will interact with the shunt capacitance Cadin.
However, the external series resistance (Ras) will have an upper limit mainly determined by pin leakage, and the conversion accuracy required. For a 12-bit conversion, the upper limit may be as little as 2k ohm, possibly requiring the use of a buffer amplifier. The reference manual for the device gives the following formula:
Ras < Vddad / (2^N * leakage_current)
Regards,
Mac
