Design of highly noise-immune controllers for capacitive touch-screen panels

Abstract

Touch-Screen Panels (TSPs) have recently come into wide use due to their intuitive user interface and demands for high area-usability of displays. Several methods such as resistive, capacitive, and infrared TSPs are primarily used. Among them, capacitive TSPs are applied to most mobile devices and tablet PCs owing to their multi-touch function and superior visibility. However, the Signal-to-Noise Ratio (SNR) of the analog-front-end is deteriorated by various noise sources leading to wrong decisions by the TSP controller. This thesis focuses on the novel read-out methods to resolve critical noise issues in the capacitive touch-screen panels.

In Chapter 1, Delta-Integration, a novel readout method, is introduced in an effort to solve the noise and speed issues impeding the performance of in-cell type TSPs. In the proposed method, a differential sens-ing scheme effectively cancels common noise components and locally occurring noise is spatially low-pass filtered. Due to the consequently enhanced SNR, the proposed scheme is simply implemented by a comparator and a counter only in place of a complex ADC. The comparator is designed to have a ‘dead-zone’ for noise-immune characteristics. In addition, a new global charge amplifier that has much wider bandwidth and higher current-driving capability than the conventional one is proposed for high speed operation. The prototype chip consumes a static power of 1.15mW and the total chip area without panel loads is $2.5mm^2$.

In Chapter 2, Delta-Integration method for add-on (or on-cell) type TSPs is introduced to improve the SNR of the readout circuit. The architecture shows better performance in spite of requiring less power consumption and using an integration capacitor that is roughly half in size compared with a previous work. It averages noises by repeated integration and substantially mitigates the effect of display noise through a differential sensing technique. Furthermore, it does not require additional calibrati...