If you are running at 16 bit resolution, and a Vref of Vdd then each
lsb is ~ 76 uV. Use a DSO, look at Vdd, set it to infinite persistence,
and you will probably see ~ 200 mV to 400 mV of noise. Translate
that to lsbs =~ 2800 lsbs of noise.
So bypassing is critical, see references below. Use Polymer Tants
as they are ~ 10 x better f vs Z performance.
Also plan on A/D pin input not next physically to pins switching a lot
of digital I/O.
http://www.cypress.com/?rID=39677 AN57821 - PSoC® 3, PSoC 4, and PSoC 5LP Mixed Signal Circuit Board Layout Considerations
http://www.cypress.com/?rID=40247 AN58827 - PSoC® 3 and PSoC 5LP Internal Analog Routing Considerations
http://www.cypress.com/?rID=39974 AN58304 - PSoC® 3 and PSoC 5LP – Pin Selection for Analog Designs
Another thing you can do is averaging N samples, for uncorrelated noise you can
get significant "filtering" from this, acts as a LPF.