Q:

Reducing electrical noise induced from a small DC electrical fan

What's a canonical method for reducing / eliminating electrical noise induced in ADC from a small brushless DC fan? For reference below, I'm also using this wall-wart and this 5V linear regulator.
My baseline system looks like this:

I have some options in my head and I'm wondering what the best one might be.
My first idea is this [OPTION A]:

My second idea is this [OPTION B]:

My third idea is this [OPTION C]:

There are probably other options that I haven't thought of, as well as permutations and combinations of these options. What are the merits and drawbacks of these options? What option would you choose and why?

A:

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.

A:

Usually fan noise is pretty low frequency so the ferrite bead in option B won't work very well. I tend to use a PTC fuse element with a largish capacitor (100uF+) to ground on the fan side and a ceramic capacitor on the other. The PTC is nice because it's a resistor and it will also provide short current protection, but sometimes you have to use a regular resistor to get enough filtering. You can optimize the values by looking at the power rail with a scope.

