Touch Sensing Guard-Domain Circuitry for Stray Capacitance Control
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Solution Overview
Problem
Capacitive touch sensor panels face errors and offsets due to parasitic or stray capacitances between sensing electrodes and other components, which can affect the accuracy and dynamic range of touch detection.
Innovation Solution
Incorporating guard circuitry referenced to earth or chassis ground and touch sensing circuitry referenced to guard ground, with shielding electrodes to mitigate stray capacitance, and using a voltage driver, differential amplifiers, and ADCs to process touch signals effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If touch sensing circuitry is integrated into the display pixel stackup, then the transparency and integration of the touch screen is improved, but parasitic capacitances between sensing electrodes and other components increase
Solution Approach 1:
The patent divides the touch sensing system into multiple power domains: a first power domain for the display and a second power domain for the touch sensing circuitry. This segmentation isolates the sensing circuitry from parasitic capacitances in the display stackup while maintaining integration benefits.
Solution Approach 2:
The patent introduces a level shifter as an intermediary component between the first power domain (display) and the second power domain (touch sensing). This level shifter converts signals between the different power domains, enabling communication while preventing parasitic capacitance interference.
2Measurement precision
If guard circuitry referenced to earth or chassis ground is used, then parasitic capacitances are reduced, but the device complexity increases
Solution Approach 1:
The patent creates a guard ring circuit that maintains equipotential conditions around the touch sensing electrodes. By keeping the guard ring at the same potential as the sensing electrode, electric field distortion is minimized and parasitic capacitance effects are reduced.
Solution Approach 2:
The guard ring circuit is configured to preemptively counteract parasitic capacitance effects by establishing a controlled electric field environment before touch detection occurs. This preliminary configuration reduces measurement errors.
3Measurement precision
If multiple power domains are used for touch sensing, then signal-to-noise ratio is improved, but the device complexity increases
Solution Approach 1:
The patent segments the device into multiple power domains: a first power domain for the display and a second power domain for the touch sensing circuitry. This segmentation isolates noise sources while maintaining functional integration.
Solution Approach 2:
The level shifter acts as an intermediary between the first and second power domains, enabling signal transmission while preventing noise and parasitic capacitance from the display power domain from affecting the touch sensing power domain.
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 reduces parasitic capacitances, improves signal-to-noise ratio, and enhances the dynamic range of touch detection, allowing for more accurate and reliable proximity and touch sensing.
Implementation Method 1
parasitic or stray capacitances can exist between the sensing electrodes used for sensing touch on the touch sensor panels, and other components of the devices in which the touch sensor panels are included
Implementation Method 2
one or more shielding electrodes coupled to the guard signal and configured to mitigate stray capacitance at the touch node electrodes and/or at one or more routing traces coupled to the touch node electrodes
Data Source
AI summary
The disclosure relates to a touch and/or proximity detection system having some components operating in the guard domain and other components operating in the earth or chassis ground domain. A guard chip in the earth or chassis ground domain can include a voltage driver configured to produce a guard signal, for example. In some examples, the guard signal can be coupled to one or more shielding electrodes of a touch screen and to the ground pin of one or more touch sensing chips of the touch and/or proximity detection system. In this way, for example, the touch sensing chips, which can include sense amplifiers coupled to one or more sensing electrodes of the touch screen, can operate in the guard domain. In some examples, the guard chip can further include differential amplifiers and/or ADCs, allowing these components to operate in the earth or chassis ground domain.