The user is recommended to only use low impedancesources with slowly varying signals, since this minimizes the required charge transfer to the S/Hcapacitor.
28.8 ADC Characteristics
23.6.1 Analog Input Circuitry
The typical value of the pin capacitance CPIN is about 5 pF. The other parameters are very dependent ona manufacturer. The serial resistance can vary in the range from 1 kOhm to 100 kOhm in an Atmel microcontroller .The value of sampling capacitance CADC is 120 pF in a PIC microcontroller  or it is 14 pF in an Atmelmicrocontroller . This capacitance is only connected to the input pin when a SAR ADC is sampling