Ratiometric Self-Capacitance Converter for Noise-Stable Touch Sensing
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Solution Overview
Problem
Capacitance sensing systems face challenges in accurately converting self-capacitance to digital values, particularly in detecting touch events with noise immunity and temperature sensitivity, especially in harsh conditions and low-power applications.
Innovation Solution
A ratiometric capacitance to code converter system that includes a charge transfer circuit with integration and modulation capacitors, deadband switches, and a comparator to generate a bit stream output, which is then processed to provide a digital representation of capacitance values, offering temperature insensitivity and noise immunity through a duty cycle calculation and decimation process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance sensing systems are used, then touch detection capability is provided, but temperature sensitivity and noise interference degrade measurement precision
Solution Approach 1:
The patent implements a ratiometric measurement technique that changes the measurement parameter from absolute capacitance to capacitance ratio (Csense/Cref). By measuring the ratio between sensor capacitance and reference capacitance, the system becomes insensitive to temperature variations and noise, as both capacitors experience similar environmental conditions that cancel out in the ratio calculation.
Solution Approach 2:
The patent introduces a reference capacitor (Cref) as an intermediary element that experiences the same temperature and noise conditions as the sensor capacitor. This reference capacitor serves as a mediator that allows the system to differentiate between environmental effects and actual touch events, thereby eliminating temperature sensitivity and noise interference.
2Ease of operation
If self-capacitance measurement is implemented, then single-electrode operation is enabled, but conversion to digital values with noise immunity is challenging
Solution Approach 1:
The patent replaces traditional analog-to-digital conversion mechanisms with a charge transfer and integration approach. By using capacitive charge transfer circuits and integration techniques, the system directly converts capacitance ratios to digital values through counting operations, achieving noise immunity without complex conversion circuits.
Solution Approach 2:
The patent creates a digital copy of the capacitance ratio through charge transfer operations. The analog capacitance ratio is replicated in the digital domain by transferring charges proportionally between capacitors and counting the transfer events, providing an accurate digital representation that is immune to noise and temperature variations.
3Productivity
If capacitance to digital conversion is performed, then touch detection is enabled, but temperature variations affect measurement accuracy
Solution Approach 1:
The patent changes the measurement parameter from absolute capacitance values to capacitance ratios. By measuring Csense/Cref instead of individual capacitance values, the system achieves temperature stability because both capacitors in the ratio experience identical temperature variations that cancel out, enabling reliable touch detection across varying temperature conditions.
4Measurement precision
If noise immunity is improved through filtering, then signal quality increases, but response time and processing speed decrease
Solution Approach 1:
The patent performs noise rejection in advance through ratiometric measurement before digital conversion. By establishing the capacitance ratio early in the measurement process using temperature-stable reference capacitance, the system eliminates the need for post-processing filtering, maintaining both high signal quality and fast response time.
Solution Approach 2:
The patent maintains continuous measurement capability through the ratiometric technique, allowing uninterrupted touch detection without the need for intermittent filtering operations. The continuous charge transfer and integration process provides steady signal quality while maintaining real-time response, eliminating the trade-off between filtering and speed.
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
The system effectively converts self-capacitance to digital values with improved noise immunity and temperature stability, enabling reliable touch detection in various applications, including harsh conditions and low-power scenarios.
Implementation Method 1
a charge transfer circuit with integration and modulation capacitors, deadband switches, and a comparator to generate a bit stream output
Implementation Method 2
a comparator to generate a bit stream output, which is then processed to provide a digital representation of capacitance values
Data Source
AI summary
A circuit, system, and method for converting self capacitance to a digital value may include a pair of charge transfer circuits, each including a switch network, a sensor capacitor or modulation capacitor, and an integration capacitor may be coupled to a comparator to produce a data signal representative of the capacitance of the sensor capacitor of one of the charge transfer circuits. The data signal may be used to indicate a capacitance value of the self capacitance through conversion by a circuit.