Capacitive Fingerprint Sensor Integrator Circuit
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Solution Overview
Problem
Capacitive fingerprint sensors face issues with non-linearity in distance-to-capacitance conversion and noise interference due to increased dielectric layer thickness and manufacturing tolerances, leading to inconsistent output and sensitivity problems.
Innovation Solution
A capacitive fingerprint sensor design with a sensing array, integrator, and comparison circuit that eliminates the influence of bus parasitic capacitors and background capacitors, ensuring linear charge discharge rates and improved noise resistance through a controlled charge and discharge process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the dielectric layer thickness is increased from 10 um to 100 um, then the sensor structure is improved and manufacturing is easier, but the distance-to-capacitance conversion becomes approximately linear causing convex nonlinear conversion to become a disadvantage
Solution Approach 1:
The patent introduces a background capacitor compensation mechanism that dynamically adjusts the reference voltage based on the background capacitor value. This parameter change approach compensates for the linear conversion issue caused by increased dielectric thickness, restoring the desired nonlinear conversion characteristics while maintaining the easier manufacturing benefits of thicker dielectric layers.
2Manufacturing precision
If bus parasitic capacitor is calculated into the integrating capacitor, then the discreteness of the integrating capacitor is improved, but manufacturing tolerance of the background capacitor causes inconsistence among units in the fingerprint sensor array
Solution Approach 1:
The patent extracts the background capacitor influence from the measurement process by introducing a separate background capacitor measurement and compensation mechanism. This allows the integrating capacitor to maintain its discreteness benefits while the background capacitor effects are separately measured and compensated, preventing unit inconsistency in the sensor array.
Solution Approach 2:
The patent introduces a reference voltage mechanism that acts as an intermediary to compensate for background capacitor variations. This intermediary element allows each sensing unit to individually compensate for its specific background capacitor value, maintaining consistency across the entire sensor array despite manufacturing tolerances.
3Measurement precision
If Vt' causes gradual decrease of Vt, then the sensing equation is satisfied, but smaller Vt' is likely to be interfered by noise resulting in comparator flip becoming earlier or later
Solution Approach 1:
The patent implements a feedback mechanism through the integrator circuit that continuously monitors and adjusts the reference voltage based on the measured capacitance. This feedback approach maintains Vt' at optimal levels throughout the measurement process, preventing both gradual decrease and noise-induced comparator flips, thereby improving measurement precision while rejecting noise.
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
The solution achieves improved linearity and anti-noise capability, eliminating deviations caused by parasitic capacitors and ensuring consistent measurements, enhancing the sensor's sensitivity and reliability.
Implementation Method 1
a target capacitor is formed between the target electrode and the sensing electrode... and a driving capacitor is formed between the driving electrode and the sensing electrode
Implementation Method 2
a dielectric layer is formed between the target electrode and the sensing electrode... another dielectric layer is formed between the driving electrode and the sensing electrode
Data Source
AI summary
The present invention provides a “C-Q-T” type capacitive fingerprint sensor with an integrator. The integrator comprises an amplifier, an integrating capacitor, a reference voltage and a reset circuit. By applying the present invention, linearity and sensitivity of the “C-Q-T” type capacitive fingerprint sensor are improved. During a conversion process of the “C-Q-T”, through introduction of the integrator, charge transfer quantities between a target capacitor and the integrating capacitor can be consistent for each time, so that a sensing equation is optimized, and better linearity is shown in the conversion process. As influence of a background capacitor and of a bus parasitic capacitor on the sensing equation is removed, the sensitivity of the “C-Q-T” type capacitive fingerprint sensor is improved.