Here is an app note from Microchip describing how to drive analog inputs of Successive Approximation Register analog to digital converters.
Driving any A/D Converter (ADC) can be challenging if all issues and trade-offs are not well understood from the beginning. With Successive Approximation Register (SAR) ADCs, the sampling speed and source impedance should be taken into consideration if the device is to be fully utilized. In this application note we will delve into the issues surrounding the SAR Converter’s input and conversion nuances to insure that the converter is handled properly from the beginning of the design phase.