US 6,049,620 A · Capacitive fingerprint sensor with adjustable gain

Abstract

The present invention relates to a fingerprint sensing device comprising a planar array of closely-spaced capacitive sense elements. When a finger is placed in close proximity to the sensing device, the capacitive sense elements measure a capacitance between the finger and a single capacitor plate in each sense element. This is accomplished by precharging each capacitor plate, and then using a known current source to remove a fixed amount of charge from each capacitor plate. The measured capacitance varies as a function of the distance between the capacitor plate and the finger surface. Thus, a capacitance measurement allows the distance between the capacitor plate and the finger surface to be determined. Distance measurements across the array of sense elements are combined to produce a representation of the pattern of ridges on the finger surface which comprise a fingerprint. After a fingerprint is acquired, the quality of the fingerprint is evaluated, and if necessary, a gain parameter for the sense elements is iteratively adjusted until a satisfactory fingerprint is acquired.

Claims14

Dependent claims

A method for detecting a fingerprint from a finger surface, the method using a plurality of sense elements, the method comprising the steps of:

placing the finger surface over the plurality of sense elements;
measuring electrical signals from the plurality of sense elements, the electrical signals indicative of an interaction between the plurality of sense elements and the finger surface;
forming the electrical signals into a representation of the fingerprint from the finger surface;
evaluating the quality of the representation of the fingerprint;
and
if the representation is of poor quality due to a quality of the finger surface, adjusting a gain of the plurality of sense elements, and measuring the electrical signals again to form another representation of the fingerprint,
wherein adjusting the gain includes adjusting a current feeding into each of the plurality of sense elements or adjusting a time interval before a measurement is made.

4 more elements

An apparatus for detecting a fingerprint on a finger surface, comprising:

an array of sense elements, sense elements in the array including, an electrode plate which forms a capacitor with the finger surface, and an amplifier, which amplifies a voltage on the electrode plate;
an insulating and receiving surface disposed over the array of sense elements;
driving circuitry coupled to the sense elements in the array of sense elements for driving a voltage onto the electrode plates in the sense elements;
sensing circuitry coupled to the sense elements for sensing a voltage on the electrode plates in the sense elements, the sensing circuitry producing a first representation of the fingerprint;
means for evaluating the quality of the first representation;
and
gain adjusting circuitry coupled to the sense elements for adjusting the gain of the sense elements if the first representation is of poor quality due to a quality of the finger surface, the gain adjusting circuitry including a circuit to adjust a current flowing into each of the sense elements or a circuit to adjust timing of a sampling interval for the sense elements,
wherein if the first representation is of poor quality, the sensing circuitry producing a second representation of the fingerprint once the gain adjusting circuitry has adjusted the gain of the sense elements.

5 more elements

Dependencies are inferred from the claim text.

Capacitive fingerprint sensor with adjustable gain

Life of this patent

Capacitive fingerprint sensor with adjustable gain

Life of this patent

Abstract

Claims14