Fingerprint Sensing Circuit with Alternating Reference Potential
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Solution Overview
Problem
Existing fingerprint sensing systems face challenges in acquiring high-quality fingerprint images, particularly for 'difficult' fingers like dry fingers, as they require changing the finger potential, which is not always feasible, and there is a need for improved performance through thicker dielectric structures.
Innovation Solution
A fingerprint sensing system with a device connection interface, sensing structures covered by a thick dielectric structure, and read-out circuitry that alternates the sensing reference potential between two levels, using capacitors connected in parallel and series to enhance the potential difference between the finger and sensing structure potentials, thereby improving image quality without requiring finger potential changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a thick dielectric structure is used to improve sensing performance, then the capacitance coupling between sensing structures and finger decreases, but using conventional sensing methods requires changing finger potential which is not always feasible
Solution Approach 1:
Instead of changing the finger potential to achieve sufficient potential difference for sensing, the patent inverts the approach by keeping the finger potential constant and instead alternating the sensing structure potential between two levels. This is accomplished by connecting sensing structures to a sensing reference potential that switches between first and second levels, enabling effective capacitance measurement without requiring any change in finger potential.
Solution Approach 2:
The patent changes the parameter being controlled from finger potential to sensing structure potential. By alternating the sensing reference potential between two distinct levels and measuring the resulting charge changes on sensing structures, the system achieves effective fingerprint sensing with thick dielectric structures without needing to modify the finger's electrical state.
2Measurement precision
If voltage boosting components are added to increase potential difference, then sensing performance improves, but device complexity and size increase
Solution Approach 1:
The patent implements a self-service mechanism where the sensing reference potential providing circuitry generates the alternating potential levels internally without requiring external voltage boosting components. The circuit alternates the sensing reference potential between first and second levels and directly applies it to the sensing structures, enabling the system to create the necessary potential difference through its own internal circuitry rather than relying on additional bulky voltage boosting equipment.
3Measurement precision
If finger potential is changed to achieve high-quality images, then image quality improves, but energy consumption increases
Solution Approach 1:
The patent inverts the conventional approach by keeping the finger potential constant and alternating the sensing structure potential instead. This eliminates the need to continuously charge or modify the finger's electrical state, thereby reducing energy consumption while still achieving high-quality fingerprint images through the alternating potential applied to the sensing structures.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances fingerprint sensing performance by increasing the potential difference between the finger and sensing structure, achieving high-quality images efficiently while maintaining low energy consumption and eliminating the need for bulky voltage boosting components.
Implementation Method 1
a plurality of sensing structures, each being covered by a dielectric structure and being arranged to capacitively couple to the finger when the finger is placed on the dielectric structure
Implementation Method 2
a first state in which the first capacitor and the second capacitor are connected in parallel to the charging circuitry to allow the charging circuitry to charge the first capacitor and the second capacitor
Implementation Method 3
a second state in which the first capacitor and the second capacitor are disconnected from the charging circuitry and connected in series between the device reference potential input and the sensing arrangement
Data Source
AI summary
A fingerprint sensing system comprising a device connection interface including a device reference potential input, a sensing arrangement, and sensing reference potential providing circuitry. The sensing arrangement includes multiple sensing structures and read-out circuitry connected to each of the sensing structures. The sensing reference potential providing circuitry provides a sensing reference potential to the sensing arrangement in the form of a sensing reference signal alternating between a first sensing reference potential and a second sensing reference potential, and comprises a first capacitor; a second capacitor; charging circuitry; and switching circuitry for alternatingly switching the sensing reference potential providing circuitry between a first state in which the first capacitor and the second capacitor are connected in parallel to the charging circuitry; and a second state in which the first capacitor and the second capacitor, when charged, are connected in series between the device reference potential input and the sensing arrangement.