Apparatus and method

A capacitive sensor used e.g. as a fingerprint sensor comprises: three thin film transistors (TFTs) 20, 22, 24 in a pixel 12 and a readout circuit 200. The gate of the sense TFT 22 is connected to a sensing electrode 14 and also a reference capacitor 18. The voltage on the source of the sense TFT fo...

Ausführliche Beschreibung

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Bibliographische Detailangaben
Hauptverfasser:	Henricus Derckx, Wilhelmus Van Lier
Format:	Patent
Sprache:	eng
Schlagworte:	ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVEREDIN A SINGLE OTHER SUBCLASS CALCULATING COMPUTING COUNTING ELECTRIC DIGITAL DATA PROCESSING MEASURING MEASURING ELECTRIC VARIABLES MEASURING MAGNETIC VARIABLES MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR PHYSICS TARIFF METERING APPARATUS TESTING
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Beschreibung
Zusammenfassung:	A capacitive sensor used e.g. as a fingerprint sensor comprises: three thin film transistors (TFTs) 20, 22, 24 in a pixel 12 and a readout circuit 200. The gate of the sense TFT 22 is connected to a sensing electrode 14 and also a reference capacitor 18. The voltage on the source of the sense TFT follows the gate voltage and is output 30 to an output line 32. A reference or bias current Ibias flows through the sense TFT 22 from current source 202. TFT 20 acts as a select switch to activate the pixel. TFT 24 is a reset TFT connected to a reference voltage Vbias. The readout circuit 200 includes a differential amplifier 206 in an integrator 204. The amplifier inputs are the voltage from the sense TFT source and a reference sample voltage Vcap. The capacitance at sense electrode 14 may be determined based on the difference between the reference sample voltage 14 and the source voltage of TFT 22 when the reference current Ibias flows through it.