a cap between 0V output and AC(N). Common mode noise will go right through an SMPS transformer
Common mode noise is by definition present on both hot and neutral conductors. If that noise gets through the transformer then connecting a cap between output and high side neutral would be connecting back to the source of the noise, and if anything make it worse.
So there needs to be a path for that to get to neutral.
Some offline switcher designs have a capacitor between output ground and earth or primary side reference if using a non-earthed power supply design.
Typically around 1nF (1000pF) safety rated capacitor is what you see in commercial designs, although that is usually based on passing EMI testing, and may not be optimal for low noise designs.
But it needs to be pretty big. You could try your 2200uF
I had to go walk around for a little bit and get
two cups of coffee before I could calm down enough to use more polite language than I would use to your face for that suggestion.
For the sake of readers who may be inexperienced or uneducated enough to follow that suggestion I will walk through the basics.
Capacitive reactance is inversely proportional to frequency and capacitance, with the magnitude given by the formula:
|Xc| = 1/(2*pi*f*c)
Where |Xc| is the magnitude of the reactance (i.e. drop the phase information), pi is the standard geometry constant, f is the frequency you are concerned with, and c is the capacitance in Farads.
Plugging in North American power line frequency and 2200uF that gives:
Xc=1/(2*3.14*60*2200^e-6) = 1/0.83 = 1.2
So you have just suggested that someone connect an equivalent of
1.2 Ohms between output side ground and high voltage side.
Besides injecting all of the noise on the neutral line caused by I*R drop of power currents directly into the output ground, it creates a
potentially fatal connection between power voltage and output ground in the case of a line/neutral swap (which is a relatively common mistake in wiring, and should be benign in properly built equipment).
To quote a
recent post "Impedance is the key to understanding E" so I am more than a little surprised to see such an irresponsible, potentially deadly suggestion.