Noise on ADC input is rectified by ESD diodes and makes an offset voltage.
The 1uF filter cap also has an ESL (1nH shown) that forms a high impedance notch for a small range of RF noise to get through.
If I use another cap ... say .1uF it will also have a notch, I can only hope they don't line up with each other, anyway that is one of the problems with long wires and ADC inputs.
