If a datasheet for a touch screen adc says: "As indicated previously, noise can be a major source of
error in touch screen applications (e.g., applications that
require a backlit LCD panel). This EMI noise can be
coupled through the LCD panel to the touch screen and
cause flickering of the converted data. Several things can
be done to reduce this error, such as using a touch screen
with a bottom-side metal layer connected to ground to
shunt the majority of noise to ground. Additionally, filtering
capacitors from Y+, Y–, X+, and X− pins to ground can also
help. Caution should be observed under these
circumstances for settling time of the touch screen,
especially operating in the single-ended mode and at high
data rates" can you recommend some values for those filtering capacitors?