wrote:
> There is an article in the October issue of Circuit Cellar on driving a
> STN display. The article mentions that since the STN the image is
> constantly refreshed there is a large amount of noise that is
> capacitively coupled. I am using a touch screen controller from TI (
> ADS7843E) and when the CSTN was on my values for the touch screen were
> all over the place! Even when the touch screen was not being touched
> you could see the cursor bouncing around! 
Now for my TFT project
> things don't seem as bad but I am seeing some slight jitter in the
> values. The ripple on the reference voltage isn't bad and running in
> differential mode to minimize effects of ripple of the power supply.
>
> Do you guys have any thoughts on how much noise I should be expecting
> from a TFT screen and what I can do about it? Just a simple RC filter?
> Any type of shielding that might be helpful?
>
> Thanks in advance for your thoughts!
>
> -DM
Poor layout will kill touchscreen controllers. In a typical setup,
there are 3 considerations:
1. The touchscreen controller. Usually they have two supplies - Digital
and Analog, for both power and grounds. Proper layout is an absolute
requirement.
2. The display and it's controller. As you have seen, a lot of noise
emanates from these devices (it's the biggest headache for portable
devices at EMC). Proper decoupling and shielding is required [it's
possible to shield other units *from* it, but that doesn't help EMC].
PSTN and TFT screens have a shield around them; **ground it properly**.
That way the noise goes to ground via the screen, rather than the power
supply return (which is in close proximity to other wiring, of course).
3. The backlight and circuitry. Even parallel only LED backlights can
generate a lot of noise if the supply is from a SMPS. In an existing
design I am using a CCFL backlight, and the touchscreen controller is
within 3mm of the wires with 700Vrms across them - and they _do_
radiate. I'm moving to LED backlights for a new design, but they still
have noise from the SMPS to deal with.
As every layout is unique, it's difficult to give specific advice, but
the datasheet gives the usual warnings of interference (all SAR
converters suffer from this sort of sensitivity) and it's up to the
designer to take it all into account.
Certainly putting caps at the X and Y (both sides) inputs to the
converter can help (I do so), but much depends onm the converter, and
caps will slow down the maximum sample rate.
With proper care the touchscreen can have apparently zero noise (there
are some things one can do in software). Mine is used for signature
capture amongst other things (the usual, such as pressing buttons) and
high resolution / low noise was always a design requirement, so I
engineered it that way.
If you have a snapshot of the layout, either post it to a.b.s.e or link
to a website with it - that's the only way we'll be fully able to help.
Cheers
PeteS
Linear Mode
