Reducing electrical noise induced from a small DC electrical fan
You need to decide how the interference is getting to your ADC input. The best way of dealing with the problem is at the source of the problem. The problem could be: -


radiated EM emissions from the fan - try moving it closer or further away
radiated EM emissions from the wiring to the fan - try moving the wires closer or further away
conducted emissions from the fan/wiring causing power supply problems such as upsetting the ADCs voltage reference
Bad practice on laying out the circuit board such as ground and power currents to the fan also sharing copper with sensitive inputs.


These are just a few and until you resolve what is wrong and where it is wrong, asking for canonical solutions is premature.

There may be several solutions, all of which will work with any of the pictures in your question and these might involve: -


Redoing the PCB layout to avoid shared copper for power and signal
Adding decoupling caps to various places
Using balanced measurement techniques on the ADC
Improving wiring and adding shields
Ferrite beads and clamps.


Work out what is causing the problem first and why.