Noise handling algorithm in 8 wire touch screen

[hanhao]

Noise handling algorithm in 8 wire touch screen
here's how a touch screen work http://focus.ti.com/lit/an/slaa298/slaa298.pdf


Problem:
when the touch pen is stationary on a point and reading ADC values from a touch screen using the micro controller, the ADC values are never the same
eg, 1st reading might be 500, 2nd reading is 510, 3rd is 505. This causes the the cursor to drift about the touch point, not something nice for the user . caps are already in place. it's a standard schematic so hardware shouldnt be a problem.

Proposed solution:
Now what i did was to create a boundary circle around the point, effectively locking all other bad readings into the circle.




but the problem is that
- for slow moving touch pen speed movements, due to this boundary circle, the cursor jumps from 1 circle to another. very ugly and it disallows slow movement.
- the size of the boundary circle has to be expanded when the touch screen wears out because of more noise

does anyone have a better solution?
thanks in advance
- hanhao

[Ldanielrosa]

How often can you poll the screen for the touch coordinates?  This sounds similar to several other things that I've pondered.

I would consider getting enough data to establish a probable center for the "boundary circle", which you probably already did.  Then keep sampling input data and averaging it with the previous position.  You may be able to poll the position often enough that the jitter will be less than one pixel width.