A 200 atto farad capacitance sensing with a differential signaling method for a mutual capacitive finger-print sensors

This paper describes an atto farad capacitive sensing system for a finger-print sensor. The architecture of mutual capacitive finger-print sensor is obtained by investigating the capacitance change by the finger-print based on the Q3D Extractor model. An electrostatic simulation shows that the 200aF difference caused by the valley and the ridge in the sensor. In order to sense the atto farad capacitance in finger-print sensor, a continuous-type fully differential architecture with a differential transmitted signals is proposed to amplify a capacitance difference only with a reduced the parasitic effect caused by a large number of electrodes. The prototype IC with 0.18um CMOS achieves 300mV/200aF.