Capacitance measuring apparatus for LCD touch screen

摘要

An apparatus 20 for measuring capacitance comprises a capacitor network 31 having a plurality of states presenting different capacitances. A sense amplifier 30 compares a capacitance to be measured with the capacitance of the network 31 and a comparator 32 supplies an output indicating whether the capacitance to be measured is larger or smaller than the capacitance of the network 31. A control circuit 33 causes the network 31 to switch through its states and monitors the output of the comparator 32 to select the state of the network 31 presenting a capacitance adjacent the capacitance to be measured. The digital measurement corresponding to the capacitance presented by the network 31 is supplied to an output 34 and provides a measure of the capacitance to be measures. The apparatus may be used to measure variations in pixel capacitance of an active matrix liquid crystal display (AMLCD) to provide a touch sensitive screen. The apparatus may also be used in other active matrix devices such as fingerprint sensors.