Noise-reduced capacitive image sensor and method operating the same

A capacitive image sensor, for sensing a fingerprint pattern of a finger, comprising a semiconductor substrate, at least one driving source and an array of capacitive sensing units formed on said semiconductor substrate, characterized in that each of said array of capacitive sensing units comprises:

• a protective layer to be touched by said finger;
• a sensing electrode arranged underneath said protective layer;
• a voltage follower, wherein an input node of the voltage follower is connected to the sensing electrode;
• a comparative capacitor, wherein one node of the comparative capacitor is electrically connected to the voltage follower, and another node thereof is electrically connected to the driving source;
• and
• a bias voltage supply circuit, for providing different bias voltages to the sensing electrode,
• wherein the comparative capacitor, a portion of the bias voltage supply circuit, and the voltage follower are formed in an isolated well;
• the isolated well is configured in such a way that current is able to be prevented from flowing across an interface in the isolated well and surrounding structures;
• the driving source is connected to a bulk node of the isolated well such that well potential of the isolated well equals to the output electric potential of the driving source;
• a change of output electric potential of the driving source is caused by a positive and/or negative waveform.