Guarding Signal Mitigates Parasitic Capacitance in Touch Displays
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Solution Overview
Problem
Capacitive sensing in input devices, such as touchpads and touch screens, is hindered by parasitic capacitance from display electrodes, leading to increased dynamic range requirements and settling times, and erroneous processing due to environmental and temperature-related changes.
Innovation Solution
Driving a guarding signal with similar characteristics to the capacitive sensing signal on display electrodes helps mitigate parasitic capacitance effects by maintaining a consistent voltage difference, thereby isolating the capacitance measurement from coupling capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If parasitic capacitance from display electrodes is present during capacitive sensing, then the sensing region can be integrated with display electrodes, but measurement precision deteriorates due to coupling capacitance and environmental changes
Solution Approach 1:
A guard electrode is introduced as an intermediary element between the sensor electrode and the display electrode. This guard electrode is driven with a guard signal that compensates for the parasitic capacitance coupling between the sensor electrode and display electrode, thereby isolating the capacitance measurement from the harmful coupling effects while allowing the sensing region to remain integrated with the display electrode structure
Solution Approach 2:
The voltage parameter of the guard electrode is dynamically changed by driving it with a guard signal that has specific voltage characteristics. This parameter change in the guard electrode's voltage allows the system to compensate for environmental and temperature-related changes in parasitic capacitance, maintaining measurement precision despite the integrated configuration
2Productivity
If display electrodes are driven with normal display signals during sensing, then display function is maintained, but settling time increases due to parasitic capacitance effects
Solution Approach 1:
The system employs periodic action by alternating between display update mode and capacitive sensing mode. During sensing periods, the guard signal is applied to compensate for parasitic capacitance, allowing rapid settling. The display updates are performed during other periods, maintaining high productivity while minimizing settling time requirements through time-division multiplexing of the electrode driving signals
3Device complexity
If no guard signal is applied to display electrodes, then device complexity is reduced, but dynamic range requirements increase due to parasitic capacitance variations
Solution Approach 1:
The guard electrode serves as a mediator that handles the compensation function, allowing the sensor electrode to use a smaller dynamic range for the sensing signal. The guard signal absorbs the variations in parasitic capacitance, thereby reducing the dynamic range requirements of the sensing signal while adding only minimal complexity through an additional electrode and control circuit
4Device complexity
If parasitic capacitance is present, then display and sensing functions share electrodes, but sensitivity of capacitive sensing decreases due to coupling capacitance
Solution Approach 1:
The guard electrode is positioned between the sensor electrode and display electrode to act as an intermediary that blocks parasitic capacitance coupling. By driving the guard electrode with an appropriate guard signal, the system maintains the sensitivity of capacitive sensing while allowing display and sensing functions to share the same physical electrode structure, thus preserving device complexity benefits without sacrificing sensing performance
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 improves the accuracy of capacitance measurements, reduces power consumption, and enhances the sensitivity and speed of capacitive sensing by eliminating the impact of parasitic capacitance.
Implementation Method 1
the processing system includes sensor circuitry configured to be coupled to a plurality of sensor electrodes and configured to drive a first sensor electrode of the plurality of sensor electrodes and a second sensor electrode of the plurality of sensor electrodes for absolute capacitive sensing
Implementation Method 2
Capacitive sensing in input devices, such as touchpads and touch screens, is hindered by parasitic capacitance from display electrodes, leading to increased dynamic range requirements and settling times
Data Source
AI summary
Embodiments described herein mitigate the effect of a coupling capacitance between a sensor electrode in a touch sensor and a display electrode in a display screen. An input device, which includes the touch sensor and display screen, may transmit a guarding signal on the display electrodes when performing capacitive sensing. In one embodiment, the guarding signal may have similar characteristics as a modulated signal (e.g., similar amplitude and/or phase) driven on the sensor electrode to detect interaction between the input device and an input object. By driving a guarding signal that is similar to the modulated signal onto the display electrodes, the voltage difference between the sensor electrode and display electrode remains the same. Accordingly, the coupling capacitance between the sensor electrode and the display electrode does not affect a capacitance measurement used to detect the user interaction.