Self-Capacitance Touch Sensor Noise Compensation Circuit
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current touch sensors face challenges in accurately detecting the presence and location of touches or proximity inputs due to interference from low-frequency noise and variations in capacitance measurements, which affect the reliability and precision of self-capacitance measurements.
Innovation Solution
The implementation of self-capacitance measurement circuits with compensation capacitors and integrator circuits that invert the polarity of integration capacitors between measurements, coupled with LF noise suppression techniques, to isolate and correct common-mode shifts and enhance signal accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If self-capacitance measurement circuits are used to detect touch inputs, then touch detection capability is enabled, but low-frequency noise and capacitance variations reduce measurement precision
Solution Approach 1:
A compensation capacitor is introduced as an intermediary element to counterbalance the harmful low-frequency noise and capacitance variations. The compensation capacitor is connected in parallel with the self-capacitance measurement circuit and adjusted to match the parasitic capacitance, thereby canceling out the noise interference and improving measurement precision.
Solution Approach 2:
The capacitance value of the compensation capacitor is dynamically adjusted to match the parasitic capacitance of the touch sensor electrode. By changing the parameter (capacitance value) of the compensation capacitor, the system adapts to varying conditions and maintains high measurement precision despite low-frequency noise and capacitance variations.
2Reliability
If compensation capacitors are added to suppress low-frequency noise, then measurement reliability improves, but device complexity increases
Solution Approach 1:
The compensation capacitor is integrated into the existing self-capacitance measurement circuit rather than being implemented as a separate system. The compensation capacitor shares the same physical substrate and control mechanisms as the touch sensor, merging multiple functions into a single unified structure that improves reliability without proportionally increasing device complexity.
Solution Approach 2:
The compensation capacitor is automatically adjusted using the touch sensor's own parasitic capacitance characteristics as the reference. The system performs self-calibration by measuring the parasitic capacitance and automatically setting the compensation capacitor value, eliminating the need for external calibration equipment or complex manual adjustment mechanisms.
3Measurement precision
If integrator circuits with polarity inversion are used to correct common-mode shifts, then signal accuracy improves, but manufacturing precision requirements increase
Solution Approach 1:
The integrator circuit inverts the polarity of the integration capacitor between measurements to counteract common-mode shifts. By reversing the capacitor connections in alternating measurement cycles, the system cancels out systematic errors and drift, improving signal accuracy. This inversion technique reduces sensitivity to manufacturing variations in capacitor values.
Solution Approach 2:
The integrator circuit operates in periodic cycles, alternating the polarity of the integration capacitor between measurement phases. This periodic inversion creates a differential measurement scheme where common-mode errors are canceled out over complete cycles, improving signal accuracy while reducing the impact of manufacturing tolerances on individual capacitor values.
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 significantly improves the reliability and precision of touch detection by effectively suppressing low-frequency noise and maintaining signal integrity, leading to more accurate determination of touch inputs and proximity measurements.
Implementation Method 1
a first amount of charge is divided between a capacitance of a touch sensor and a compensation capacitor. The division of the first amount of charge generates a first voltage at an input node that is proportional to a capacitance ratio between the compensation capacitor and the measurement capacitance
Implementation Method 2
A voltage is applied at the input node that induces a second amount of charge on an integration capacitor of an integrator circuit. The second amount of charge is proportional to a difference between the first voltage and the applied voltage
Data Source
AI summary
In one embodiment, a method includes applying a supply voltage across a compensation capacitor; dividing charge between a capacitance of a touch sensor and the compensation capacitor; and performing the application of the supply voltage and the dividing of charge a pre-determined number of times. A first amount of charge of the compensation capacitor results in a first voltage at an input node. The method also includes applying a reference voltage at the input node. The application of the reference voltage at the input node induces a second amount of charge proportional to a difference between the first voltage and the reference voltage on an integration capacitor. The method also includes determining a first difference between the first voltage and the reference voltage based on a second amount of charge on the integration capacitor; and determining whether a touch input to the touch sensor has occurred based on the difference.