How to reduce ADC noise through PCB routing?
Separate the PCB into a quiet section and a noisy section.  Imagine that you've drawn out a quiet section on the PCB (or do it literally on a user layer, if your PCB layout tool will allow it).  Keep all of the digital and other noisy signals out of the quiet section.  Let the ADC straddle that dividing line, with it's analog side on the quiet side, and its digital signals &amp; power on the noisy side.

You really want to be careful with grounds, too -- all the grounds running to parts inside the quiet section should have just one entry point to the noisy section, possibly at the ADC's "AGND" pin.  If you dump current from a noisy thing onto your nice quiet analog ground, it'll make, well, noise.