A high-resolution capacitive fingerprint sensing scheme with charge-transfer technique and automatic contrast emphasis

A novel capacitive fingerprint sensing scheme obtains an accurate fingerprint image through direct finger surface contact with a pixel array that includes sensor cells. A sensing circuit with a differential charge-transfer amplifier suppresses the influence of the parasitic capacitance. For high-resolution A/D conversion, the output dynamic range is widened by transforming the sensed voltage to a time axis. Moreover, the sensing circuit includes an automatic contrast emphasis scheme. The characteristics were evaluated by a test chip using the 0.5-/spl mu/m CMOS process, and a single-chip fingerprint sensor/identifier LSI using this scheme is demonstrated.