Noise reduced capacitive image sensor and method operating the same

A capacitive image sensor, comprising:

• a plurality of capacitive sensing units forming an array, each capacitive sensing unit for transforming a distance between a portion of a surface of an approaching finger and a top surface thereof into an output electric potential, wherein a value of the output electric potential is changed by a driving signal applied to the capacitive sensing unit;
• at least one sample-and-hold circuit for capturing and retaining different output electric potentials;
• at least one signal conditioning circuit, each comprising at least one differential amplifier for amplifying a difference between two electric potentials retained by the sample-and-hold circuit;
• and
• a driving source, for providing the driving signal to the capacitive sensing units,
• wherein the driving signal is a potential change or potential changes, caused by a positive waveform and/or a negative waveform provided by the driving source;
• the electric potential at each part of the capacitive sensing units is set to a constant value during reset stages;
• the difference between two output electric potentials retained in the sample-and-hold circuit is a noise-reduced value representing a distance between the capacitive sensing unit and the portion surface of the finger above the capacitive sensing unit;
• the sample-and-hold circuit retains at least a first and a second output electric potentials under a corresponding positive waveform or negative waveform;
• the capacitive image sensor sequentially collects the noise-reduced values under the corresponding positive waveform and negative waveform for each pixel, and maps the noise-reduced values to corresponding locations of the capacitive sensing units to obtain a noise-reduced image of the finger,
• wherein the capacitive sensing unit further comprises:
• a sensing electrode;
• a voltage follower, wherein an input node of the voltage follower is connected to the sensing electrode, and an output node of the voltage follower is connected to the sample-and-hold circuit;
• 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, and
• wherein the comparative capacitor, a portion of the bias voltage supply circuit, and the voltage follower are MOS devices formed in an isolated well.