Rail-to-Rail Driven Shield for Capacitance Measurement
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Solution Overview
Problem
Capacitive touch interfaces face issues with unwanted capacitive effects due to moisture on the touch-sensitive surface, which conventional driven shield techniques struggle to effectively cancel, particularly in self-capacitive sensing configurations.
Innovation Solution
Implementing a rail-to-rail driven shield circuitry that varies the shield voltage between two reference voltage rails during measurement cycles, allowing for the cancellation of mutual capacitance effects by accumulating and then subtracting the effects in subsequent measurement phases, thereby isolating self-capacitive sensing from mutual-capacitive interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional driven shield techniques are used to cancel mutual capacitive effects, then some capacitive cancellation is achieved, but moisture-related unwanted capacitive effects remain and sensing accuracy is compromised
Solution Approach 1:
The patent applies periodic action by implementing multiple measurement cycles with alternating voltage phases. The driven shield voltage is periodically switched between different levels (e.g., Vdd and GND) across successive measurement cycles, allowing the system to accumulate and then subtract mutual capacitive effects. This periodic voltage switching enables the cancellation of moisture-related capacitive interference while maintaining accurate self-capacitive sensing.
Solution Approach 2:
The patent utilizes parameter changes by dynamically adjusting the driven shield voltage parameter across different measurement phases. The shield voltage transitions between discrete voltage levels (first voltage level, second voltage level, third voltage level) depending on the measurement cycle phase. This parameter variation allows the system to differentiate between self-capacitive signals and mutual capacitive interference, effectively canceling the latter while preserving the former.
2Reliability
If analog buffers are used to track sense and drive electrode voltages for driven shield, then continuous voltage tracking is achieved, but circuit complexity and power consumption increase
Solution Approach 1:
The patent extracts the continuous voltage tracking function from the driven shield circuitry by removing the need for analog buffers. Instead of using complex buffer circuits to continuously track sense and drive electrode voltages, the invention samples voltages at specific phases and uses digital processing to generate the driven shield signal. This extraction of the tracking function reduces circuit complexity and power consumption while maintaining reliable voltage tracking through periodic sampling and calculation.
Solution Approach 2:
The patent replaces the mechanical/analog buffer-based voltage tracking system with a digital processing approach. Instead of using analog buffers that require continuous operation and consume power, the invention uses digital sampling of voltages at specific phases, followed by computational processing to determine the driven shield voltage. This substitution of analog mechanics with digital processing simplifies the circuit architecture and reduces power consumption.
3Reliability
If analog buffers are used for continuous voltage tracking, then voltage tracking reliability is improved, but power consumption increases
Solution Approach 1:
The patent applies periodic action by implementing discrete voltage sampling at specific phases rather than continuous tracking. The system samples sense and drive electrode voltages only at the phases needed for calculation, then uses these sampled values to generate the driven shield signal for subsequent measurement cycles. This periodic sampling approach maintains voltage tracking reliability while dramatically reducing power consumption compared to continuous analog buffer operation.
Solution Approach 2:
The patent replaces the power-hungry analog buffer system with a low-power digital processing approach. By using digital sampling and computational methods to determine the driven shield voltage, the system achieves reliable voltage tracking without the continuous power consumption inherent in analog buffer circuits. The digital approach processes voltage information only when needed, minimizing energy usage while maintaining tracking accuracy.
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 eliminates unwanted mutual capacitive effects, enhances sensing accuracy, and reduces the need for analog buffers, resulting in cost savings, improved power efficiency, and simplified circuitry while maintaining effective moisture rejection.
Implementation Method 1
The driven shield circuitry is configured to drive one or more inactive electrodes of the electrode circuitry between two reference voltage rails in order to induce mutual capacitive effects at an active sense electrode
Data Source
AI summary
One or more examples of the present disclosure relate generally to systems and methods for canceling mutual capacitive effects in a capacitance measurement. Some examples relate to providing a driven shield during capacitance measurement. Some examples relate to providing such a driven shield using rail-to-rail voltage.