Patent ID: 7755369

Claim:
A capacitive fingerprint sensor, comprising: a sensor cell comprising: a fingerprint capacitor having a capacitance that is either a valley capacitance C FV or a ridge capacitance C FR ; a reference capacitor C S , with one end connected to the fingerprint capacitor, and the other end connected to a trigger signal, wherein the trigger signal is asserted only during a precharge phase, and C FV <C S <C FR ; a first transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by a scan line, the input terminal is connected to the fingerprint capacitor, and the output terminal is connected to a readout line; a control circuit comprising: a second transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by a first reset line and the input terminal is connected to the readout line; an operational amplifier, with one input end of the operational amplifier connected to the output terminal of the second transistor; a third transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by a second reset line, and the input and output terminals are connected to the input and output ends of the operational amplifier; and a multiplexer connected to another input end of the operational amplifier, wherein inputs of the multiplexer are connected to a first voltage VA and a threshold voltage Vth.