There are several aspects which determine the choice of input filter for adc. It depends on whether you have SHA in the ADC or not, your bondwire inductance, whether you are driving it with a amp or through a transformer, how linear is the ON time of SHA switch, and of course on the sampling capacitor. I 'm also trying to design one such filter.  
  If you have SHA then the kick on the filter cap will be linear, otherwise the kick will be non-linear. The ON time of the sampling switch, (if it is a boosted switch) will also limit the linearity of the SHA's kick. If the series resistance that you put in the filter is very high then your transient settling might get affected. If the resistance is too small then circuit may start ringing because of bondwire inductance.