Patent ID: 8164352

Claim:
A capacitance detecting apparatus which gives a detected signal indicating a change in capacitance of a measuring object, comprising: a signal generator to generate a probe signal of a predetermined frequency and supply said probe signal to one end of said measuring object; a differential amplifying circuit to amplify differentially a response signal obtained from the other end of said measuring object and said probe signal; a detecting unit to produce said detected signal based on the amplified signal from said differential amplifying circuit; a computing unit to reproduce information recorded in a ferroelectric medium that constitutes said measuring object based on the change in said capacitance detected by said detecting unit; a sensor unit to read out said response signal from said ferroelectric medium; wherein said ferroelectric medium comprises: an electrode layer made of conductive material; and a record layer made of a ferroelectric and placed on said electrode layer, wherein said sensor unit comprises: a probe placed such that its tip faces a surface of said record layer; an annular probe placed around said probe and apart from the surface of said record layer; a connection terminal to give said probe signal to said electrode layer through; and an output terminal to output a signal developed on said probe corresponding to said probe signal as said response signal; and a connection terminal to apply a constant reference potential to said annular probe through; wherein said differential amplifying circuit has an inverting input terminal to which said response signal is inputted and a non-inverting input terminal to which said probe signal is inputted.