A novel sensor cell architecture and sensing circuit scheme forcapacitive fingerprint sensors

摘要

Novel capacitive fingerprint sensor techniques are described. Wenpropose a novel sensor cell architecture to obtain high sensitivity,nwide output dynamic range, and contrast adjustment. For thenarchitecture, three circuit techniques were developed. A sensing circuitnwith a differential charge-transfer amplifier enhances sensitivity whilenit suppresses the influence of the parasitic capacitance of the sensornplate. A wide output dynamic range, which is needed for high-resolutionnanalog-to-digital (A/D) conversion, is achieved by transforming thensensed voltage to a time-variant signal. Finally, the sensing circuitnincludes an automatic contrast enhancement scheme that uses anvariable-threshold Schmitt trigger circuit to distinguish the ridges andnvalleys of a fingerprint well. The characteristics of a test chip usingnthe 0.5-Μm CMOS process show a high sensitivity to less than 80 fF asnthe detected signal, while the variation of the output signal isnsuppressed to less than 3% at ±20% variation of the parasiticncapacitance. The dynamic range of the time-variant signal is 70 Μs,nwhich is wide enough for A/D conversion. The automatic contrastnenhancement scheme widens the time-variant signal 100 Μs more. Ansingle-chip fingerprint sensor/identifier LSI using the proposed sensingncircuit scheme confirms the scheme's effectiveness