Touch signal readout IC for capacitive touch-screen

Abstract

A novel capacitance measuring circuit based on a constant current charging method for a capacitive touch-screen application is proposed in this thesis. Two major limitations of the current capacitive touch-screen topology are parasitic capacitance cancellation and external and substrate noise rejection. These problems are solved by employing a varying threshold voltage comparator at the interface between panel and digital circuitry. This provides higher sensitivity to capacitance variation even in the large parasitic capacitance environment and fully differential topology by making similar impedance at both input nodes of comparator provides higher external and substrate noise immunity. In addition, time domain sub-ranging/two-step architecture allows faster conversion and breaks exponential relationship between a bit resolution and the conversion time required. 8bit architecture is implemented with 0.13μm CMOS technology. Post-simulation result shows 200kHz/sample conversion time with 10 MHz clock frequency. Total average current consumption per conversion is 56~80 μA depending on the code with 1.2V supply voltage. Core area is 21,266 $μm^2$ without pads and output buffer