Multi-Touch Sensor Circuit for Simultaneous Capacitance Sampling
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current multi-touch systems face challenges in simultaneous sampling of different modes such as self, mutual, and pen capacitance, leading to increased sample time and user experience issues, especially on large displays, due to high parasitic capacitances and noise interference from salt water contamination.
Innovation Solution
The implementation of a circuitry system using voltage-following sigma-delta A/D converters and sigma-delta D/A converters that allow for simultaneous transmission and reception of multiple frequencies across multiple channels, enabling simultaneous sampling of self, mutual, and pen touch signals, while also employing digital modulation and demodulation schemes to reduce noise and reject common-mode noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional sense circuits are used to measure mutual capacitance, then measurement precision can be achieved, but sample time increases significantly due to the need for multiple integration cycles to overcome parasitic capacitance effects
Solution Approach 1:
The patent uses periodic square wave drive pulses to alternately charge and discharge the sense electrode, creating periodic current flows that can be integrated. By synchronizing the integration window with the periodic drive signal, the system accumulates signal over multiple cycles while maintaining a predictable timing structure that enables faster overall sampling compared to continuous integration approaches.
Solution Approach 2:
The patent implements feedback by switching the sense electrode to the integrator input just before each falling edge of the drive pulse and uncoupling just before each rising edge. This feedback mechanism ensures that the integrator only integrates signals of one polarity, effectively rejecting common-mode noise and parasitic capacitance effects while maintaining measurement precision.
2Measurement precision
If multiple integration cycles are performed to overcome parasitic capacitance effects, then measurement accuracy improves, but the time required to determine touch location increases
Solution Approach 1:
By using periodic drive pulses at a fixed frequency (e.g., 200 kHz), the system can perform multiple integration cycles within a predictable time frame. The periodic nature allows the system to accumulate sufficient signal energy for accurate measurement while maintaining a consistent sampling rate, thus improving productivity compared to variable-length integration approaches.
Solution Approach 2:
The system performs preliminary actions by pre-configuring the integrator switch timing relative to the drive pulse edges. The switch is positioned to couple the sense electrode to the integrator just before the falling edge and uncouple just before the rising edge, preparing the integration path in advance for each cycle. This preliminary setup enables rapid sequential integration without reconfiguration delays, improving overall sampling speed.
3Device complexity
If traditional switching circuits are used for sense electrode control, then circuit complexity remains manageable, but noise interference from parasitic capacitances significantly degrades signal quality
Solution Approach 1:
The integrator circuit serves as an intermediary between the sense electrode and the digital processing stage. By placing the integrator in this intermediate position, the system converts the high-impedance, noise-sensitive capacitive signal into a voltage signal that is less susceptible to parasitic capacitance effects. The integrator's feedback capacitor acts as a mediator that accumulates charge over the integration window, effectively filtering out high-frequency noise while preserving the touch signal.
4Measurement precision
If integration time is increased to improve signal-to-noise ratio, then measurement precision improves, but user experience deteriorates due to delayed touch response
Solution Approach 1:
The system uses periodic drive pulses to rapidly accumulate signal energy over multiple cycles within a short total time. By confining the integration to specific windows synchronized with the periodic drive signal, the system achieves high signal-to-noise ratio through cumulative integration while maintaining a fast overall response time that preserves user experience.
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 enhances the speed, resolution, and sensitivity of touch sensor data acquisition, allowing for faster and more accurate multi-touch data collection across various sensor types and configurations, while reducing noise and improving user experience on large displays.
Implementation Method 1
a sigma-delta D/A converter to transform a digital signal to an analog signal to drive the row or column electrode
Implementation Method 2
The amount of current flow is directly proportional to the value of the mutual capacitance and therefore facilitates the determination of the mutual capacitance
Implementation Method 3
voltage-following sigma-delta A/D converter that detects changes in the driven signal caused by changes in the row or column electrode impedance
Implementation Method 4
employing digital modulation and demodulation schemes to reduce noise and reject common-mode noise
Data Source
AI summary
A touch sensor circuit includes a plurality of drive/receive circuits and a drive signal generation circuit. The drive/receive circuits drive and receive signals from a multi-touch sensor. A drive/receive circuit includes an analog to digital conversion (ADC) circuit and a digital to analog conversion (DAC) circuit. The ADC circuit includes a first input to receive a sensor signal on an electrode of the multi-touch sensor. The sensor signal includes a drive signal component and a receive signal component. The ADC circuit includes a second input to receive an analog reference signal from the drive signal generation circuit. The ADC circuit generates a digital signal based on the analog reference signal and the sensor signal. The DAC circuit converts the digital signal into the drive signal component and the receive signal component is representative of an impedance on the electrode detected and is indicative of a touch proximal to the electrode.