Noise reduced capacitive fingerprint sensor and capacitive sensing unit included therein
First Claim
1. A capacitive sensing unit for a capacitive fingerprint sensor, comprising:
- a sensing electrode;
a first switch, for providing a constant bias voltage to all components connected to the sensing electrode while being switched on;
a voltage follower, comprising;
an adjustable current source, for providing at least two distinct current levels; and
a MOS transistor, comprising;
a source node, connected to ground via the adjustable current source and serves as an output node of the voltage follower;
a gate node, connected to the sensing electrode and serves as an input node of the voltage follower;
a drain node, connected to a power source, for providing power to the voltage follower; and
a bulk node, connected to the source node; and
a reference capacitor, formed between the output node of the voltage follower and the sensing electrode.
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Abstract
A capacitive fingerprint sensor which includes capacitive sensing units is disclosed. Each of the capacitive sensing unit includes a sensing electrode; a first switch; a voltage follower; and a reference capacitor. The voltage follower includes an adjustable current source, for providing at least two distinct current levels; and a MOS transistor. The MOS transistor includes a source node, connected to ground via the adjustable current source and serves as an output node of the voltage follower; a gate node, connected to the sensing electrode and serves as an input node of the voltage follower; a drain node, connected to a power source, for providing power to the voltage follower; and a bulk node, connected to the source node.
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Citations
14 Claims
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1. A capacitive sensing unit for a capacitive fingerprint sensor, comprising:
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a sensing electrode; a first switch, for providing a constant bias voltage to all components connected to the sensing electrode while being switched on; a voltage follower, comprising; an adjustable current source, for providing at least two distinct current levels; and a MOS transistor, comprising; a source node, connected to ground via the adjustable current source and serves as an output node of the voltage follower; a gate node, connected to the sensing electrode and serves as an input node of the voltage follower; a drain node, connected to a power source, for providing power to the voltage follower; and a bulk node, connected to the source node; and a reference capacitor, formed between the output node of the voltage follower and the sensing electrode. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A sensing circuit, for measuring a capacitance formed between an object and the sensing circuit, comprising:
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a sensing electrode, wherein the capacitance is formed between the object and the sensing electrode; an output node, wherein an output voltage is measured at the output node; a reference capacitor, wherein one node of the reference capacitor is connected to the sensing electrode and the other node is connected to the output node; a MOS transistor, comprising; a source node; a gate node, connected to the sensing electrode; a drain node, connected to a power source for providing power to the transistor; and a bulk node, connected to the source node; and an adjustable current source, formed between the source node of the MOS transistor and ground, for providing at least two distinct current levels to the MOS transistor, wherein the MOS transistor is operated in the saturation region; wherein the MOS transistor and the adjustable current source together behaves as a voltage follower; and wherein the capacitance has one node connected to ground, and another node connected to the sensing electrode. - View Dependent Claims (14)
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Specification