Assuming that you have followed the general rules (capacitor across every chip etc.)
In order of importance - as I think:

1) make all analog signals meet at ONE ground point (especially input return to ADC)

2) use a separate Vcc decoupling for the analog part (coil or ferrite and capacitor).

3) make a slit in your ground plane (if you do not have a ground plane, that is a problem) so that you have an analog ground 'island' that is only connected to the rest at one point (power supply return).

4) if you have a switching power supply on the board, shield the inductor.

5) put ferrite beads in series with the wires between tha ADC and uC.

If this does not do it there are some very exotic tweaks, but you do not want to have to do those.

Have fun,

Erik