Patent ID: 7525322

Claim:
A capacitive sensor for detecting a physical quantity, the sensor comprising: a sensor element including a movable electrode displaceable in accordance with a physical quantity, and a fixed electrode faced to the movable electrode, wherein a periodic carrier wave voltage is applied to the fixed electrode; and a detecting circuit including a C-V conversion circuit for outputting a voltage signal corresponding to a capacitance between the movable and fixed electrodes, wherein the C-V conversion circuit includes a diagnostic switch, a differential amplification circuit for differentially amplifying the voltage signal from the C-V conversion circuit and outputting the amplified signal as a sensor output in a normal measuring mode, and a high-voltage generating circuit for applying a diagnostic voltage to an input side of the C-V conversion circuit in a diagnostic mode so as to diagnose a sticking of a foreign object between the movable electrode and the fixed electrode, wherein the periodic carrier wave voltage in the diagnostic mode is larger than the periodic carrier wave voltage in the normal mode, the diagnostic switch is provided between the C-V conversion circuit and the differential amplification circuit, and closed in the normal mode so as to input the voltage signal from the C-V conversion circuit into the differential amplification circuit, and the diagnostic switch is opened in the diagnostic mode so as to disconnect the C-V conversion circuit and the differential amplification circuit.