Ratiometric Self-Capacitance Converter for Noise-Stable Touch Sensing
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitance sensing systems face challenges in accurately converting self-capacitance to digital values, particularly in detecting touch events with high 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 independent of clock frequency and supply voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance-to-digital conversion is used, then the system can detect touch events, but the measurement precision deteriorates due to temperature sensitivity and noise in harsh conditions
Solution Approach 1:
The patent implements a ratiometric measurement technique that changes the measurement parameter from absolute capacitance to a ratio of capacitances (Csense/Cref). This ratio remains stable against temperature variations and supply voltage changes, as both capacitors are affected similarly by environmental factors. The conversion circuit computes this ratio to generate a temperature-insensitive digital output, directly resolving the measurement precision issue under harsh conditions.
Solution Approach 2:
The patent introduces a reference capacitor (Cref) as an intermediary element that serves as a stable benchmark for comparison. By measuring the ratio between the sense capacitor and reference capacitor rather than the absolute value, the system eliminates common-mode noise and temperature drift effects. The reference capacitor acts as a mediator that cancels out environmental variations affecting both capacitors equally.
2Measurement precision
If high-resolution capacitance conversion is implemented, then measurement precision improves, but device complexity increases due to additional circuit components
Solution Approach 1:
The patent combines multiple functions into a single integrated conversion circuit that simultaneously performs capacitance-to-voltage conversion, ratiometric calculation, and digital encoding. The circuit merges the sense capacitor, reference capacitor, integration capacitor, and digital output stage into one cohesive unit, achieving high-resolution measurement without requiring separate discrete circuits for each function.
Solution Approach 2:
The conversion circuit is designed with multi-functionality to handle both self-capacitance and mutual-capacitance measurement modes using the same hardware architecture. The circuit can operate in different measurement configurations (single-ended or differential) without requiring additional components, providing universal applicability across different touch sensor types while maintaining high precision.
3Measurement precision
If noise immunity is enhanced through filtering, then measurement precision improves, but response time increases due to processing delays
Solution Approach 1:
The patent employs periodic switching of the sense capacitor between charging and discharging phases, with the conversion circuit operating in synchronized periodic cycles. This periodic action allows the circuit to naturally filter high-frequency noise through its integration time constant while maintaining a fast response to actual touch events. The regular timing ensures that noise is averaged out over multiple cycles without significantly delaying genuine touch detection.
Solution Approach 2:
The conversion circuit incorporates feedback mechanisms where the digital output is continuously monitored and used to adjust the integration process. This feedback allows the circuit to adapt to varying noise conditions in real-time, enhancing noise immunity dynamically without requiring excessive processing time. The feedback loop ensures that once a valid touch signal is detected, the system can quickly lock onto it and provide a stable reading.
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 enhanced noise immunity and temperature stability, enabling reliable touch detection in various applications, including touch-sensing surfaces and arrays, regardless of environmental conditions.
Implementation Method 1
A ratiometric capacitance to code converter system that includes a charge transfer circuit with integration and modulation capacitors
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 deadband switch network, a sensor capacitor or modulation capacitor, and an integration capacitor may be coupled to a comparator to produce a bitstream representative of the capacitance of the sensor capacitor of one of the charge transfer circuits. The bitstream may be used to indicate a capacitance value of the self capacitance through conversion by a digitizing circuit element.