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 capacitive sensing unit 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 the other 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 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.