Differential Touch Sensor Array for Parasitic Capacitance Cancellation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current touch sensors face limitations in resolution and size due to parasitic capacitance, which affects signal quality and noise ratio, making it difficult to achieve high-resolution touch sensing over large areas.
Innovation Solution
A sensor array with each pixel comprising a capacitive sensing electrode and a reference capacitor, along with a voltage-controlled impedance (VCI) that modulates impedance in response to a control voltage, reducing parasitic capacitance and improving signal-to-noise ratio, and incorporating a reset circuit to adjust sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the number of pixels is increased to achieve higher resolution, then measurement precision is improved, but parasitic capacitance increases additively which limits the number of pixels that can be combined
Solution Approach 1:
The patent extracts the parasitic capacitance from the signal path by using a differential sensing approach where the parasitic capacitance appears equally on both the sensing electrode and reference electrode, thereby canceling out its harmful effects and allowing more pixels to be combined without proportional increase in parasitic capacitance impact
Solution Approach 2:
The patent changes the electrical configuration from single-ended to differential mode, transforming how capacitance is measured. By measuring the difference between sensing electrode capacitance and reference electrode capacitance, the system can combine more pixels while the parasitic capacitance effects remain constant rather than adding up
2Area of stationary object
If more pixels are combined in parallel to increase sensing area, then area is improved, but parasitic capacitance combines additively which limits resolution
Solution Approach 1:
The patent segments the capacitance measurement into two separate measurements: one for the sensing electrode and one for the reference electrode. This segmentation allows the system to process each electrode's capacitance independently and then compute the difference, enabling larger sensor areas with more pixels without the parasitic capacitance limiting resolution
Solution Approach 2:
The reference electrode acts as an intermediary that provides a baseline measurement. By comparing the sensing electrode capacitance against the reference electrode capacitance, the system can eliminate common-mode parasitic capacitance effects, allowing expansion of sensor area while maintaining resolution
3Device complexity
If passive matrix capacitive touch sensing is used, then device complexity is reduced, but sensitivity to environmental noise and interference increases
Solution Approach 1:
The patent implements a feedback mechanism where the reference electrode continuously provides baseline capacitance information that is used to compensate for environmental noise and interference. This active compensation maintains high reliability while keeping the overall device complexity manageable through shared readout circuitry
4Reliability
If active matrix capacitive touch sensors are used with switching elements, then noise resistance is improved, but parasitic capacitance combines additively in each column limiting pixel count
Solution Approach 1:
The patent merges the reference electrode function into the existing pixel structure, allowing the differential measurement to be implemented without adding separate reference pixels. This combining approach maintains noise resistance while avoiding the additive parasitic capacitance problem that would result from completely separate reference 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 configuration enables high-resolution touch sensing over larger areas by minimizing parasitic capacitance and enhancing signal quality, allowing for precise detection of conductive objects, such as fingerprints, with improved noise resistance.
Implementation Method 1
Capacitance sensors such as these use capacitive effects associated with the surface contours of the fingerprint. The sensor array pixels each include an electrode which acts as one plate of a capacitor, the dermal layer (which is electrically conductive) acts as the other plate, and the non-conductive epidermal layer acts as a dielectric.
Implementation Method 2
a reference capacitor connected in series with the capacitive sensing electrode so that, in response to a control voltage, an indicator voltage is provided at the connection between the reference capacitor and the capacitive sensing electrode to indicate the proximity of the conductive object to be sensed
Data Source
AI summary
A sensor array (10) comprising a plurality of touch sensitive pixels, each pixel (12) comprising a capacitive sensing electrode (14) and a reference capacitor (16) connected in series with the capacitive sensing electrode (14) to provide an indicator voltage that is indicative of the proximity of a conductive object to be sensed.