You mean that you want the switching noise on the supply made by the DCDC converter (buck regulator) and the digital circuits not to reach the sensitive analog circuits?
Then I would personally ignore EMI filters!
Well, OK, I would only think about EMI filter after at least implementing the suggestions I give below.
Because these are only reasonably effective at high frequencies, think above 10 MHz or so. Also, the higher frequencies do not tend to travel so much through the wires as through the air!. A wire is about 1 nH (nano Henry) per mm. So a long wire is already an obstacle for any RF signal.
The DCDC converter will switch at a much lower frequency (usually between 50 kHz to 2 MHz) so EMI filters will only suppress part of the noise. For the digital the same is true, it can generate many frequencies also below the range where EMI filters have much effect.
Here are some tips that are more effective:
Choose a ground point and implement a star grounding scheme keeping the digital and analog grounds connected but minimize them influencing each other. If you share the same ground than the return currents cause local, small voltages in the ground. Your sensitive analog IC might not like this. The solution is separate ground wires /connections so that a circuit only "sees" the ground voltages that it has created itself and not the voltages from ground currents from other circuits.
Bypass and decouple your supplies as close as possible to the source of the noise. This keeps the (current) loop for high-frequency signals short and minimizes the chance of noise "escaping".
Use multiple capacitors of different values in parallel for bypass and decoupling. Capacitors are not ideal, to make more ideal ones use multiple, see EEVBlog Dave's video on the subject.
For the analog circuit, consider using a dedicated LDO (non-switching voltage regulator) to make the supply for that circuit only.
What you do is put 0 ohm resistors here and there in the supply lines, then if later it turns out that there is a high-frequency issue you have the chance to add a series resistor or EMI filter (or just an inductor) to solve the issue. Also add (the option) to add a capacitor to ground on both sides of that zero-ohm resistor. No need to add a capacitor just yet but when you do need it, adding one will be easy.