Fingerprint Sensing Circuit Noise Reduction via Digital Shutdown
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Solution Overview
Problem
Fingerprint sensing circuits, particularly capacitive sensors, are susceptible to noise and parasitic capacitive coupling, which degrade the quality of acquired fingerprint images.
Innovation Solution
The solution involves shutting down the digital device or disabling its clock signal during sampling of the analog response signal to allow noise to settle, and using a high-quality receiver with a larger number of transmitting elements and a smaller number of receiving elements to reduce noise and improve dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If capacitive sensing elements are used in fingerprint sensors, then fingerprint images can be generated, but noise and parasitic capacitive coupling degrade image quality
Solution Approach 1:
The patent applies preliminary action by shutting down the digital device or disabling its clock signal before the analog response signal is sampled. This preliminary shutdown allows noise to settle before the critical sampling moment, preventing noise from degrading the fingerprint image quality. The timing is carefully controlled so the digital device is operational before sampling and restored afterward, but inactive during the vulnerable sampling window.
2Reliability
If the digital device is shut down during sampling, then noise is reduced, but device operation is interrupted
Solution Approach 1:
The patent implements periodic action by cyclically shutting down and restoring the digital device in synchronization with the sampling process. The digital device operates normally, then shuts down periodically during sampling intervals, and restores operation afterward. This periodic on-off cycle allows noise reduction during sampling while maintaining overall device functionality and productivity outside the sampling windows.
3Measurement precision
If a high-quality receiver with more transmitting elements is used, then dynamic range is improved, but device complexity increases
Solution Approach 1:
The patent applies asymmetry by using an unequal number of transmitting and receiving elements, specifically more transmitting elements than receiving elements. This asymmetric configuration optimizes the dynamic range and signal quality by having multiple transmitting elements that can be selectively activated, while using a smaller number of high-quality receiving elements. This approach achieves improved measurement precision without proportionally increasing overall device complexity.
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 effectively reduces noise in fingerprint sensing circuits, leading to improved image quality and reliability, and allows for the use of fingerprint sensors in compact, cost-effective designs.
Implementation Method 1
Passive capacitive technology typically utilizes an array of plates to apply an electrical current to the finger. The voltage discharge is then measured through the finger.
Implementation Method 2
capacitive sensors may be particularly susceptible to noise and parasitic capacitive coupling, which may degrade the quality of the acquired fingerprint image
Data Source
AI summary
An apparatus for reducing noise in fingerprint sensing circuits is disclosed in one embodiment of the invention as including a fingerprint sensing area onto which a user can apply a fingerprint. An analog front end is coupled to the fingerprint sensing area and is configured to generate an analog response signal. An analog-to-digital converter (ADC) samples the analog response signal and converts the sample to a digital value, which may be received by a digital device such as a processor or CPU. To reduce the amount of the noise that is present in the analog response signal and therefore reflected in the digital value, the digital device may be shut down while the ADC is sampling the analog response signal.