Capacitance-to-Code Converter for High Resolution Touch Sensing
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Solution Overview
Problem
Capacitance sensing systems face challenges in accurately determining the location and type of touch events on capacitive touch-sensing surfaces, particularly in distinguishing between conductive objects and styluses, and in efficiently processing touch data to provide precise coordinates and gestures.
Innovation Solution
The system employs a processing device with a capacitance-sensing circuit that measures both mutual and self-capacitance, using a charge to code converter to convert touch data into digital values, allowing for the detection and tracking of conductive objects and styluses, and generating 2D capacitive images to determine precise coordinates and gestures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If capacitance sensing systems use traditional conversion methods, then the system structure is simpler, but the measurement precision of touch location and object differentiation is insufficient
Solution Approach 1:
The capacitance-to-code conversion process is divided into multiple sequential stages: charge accumulation phase, hold phase, and decode phase. Each stage is handled by dedicated circuit blocks (charge pump, hold capacitor, decode logic) that process specific portions of the conversion task, thereby improving measurement precision through staged processing while managing overall system complexity through functional decomposition
Solution Approach 2:
A hold capacitor is introduced as an intermediary element between the charge pump and decode logic. This hold capacitor stores the accumulated charge during the measurement period and provides a stable input to the decode logic, enabling high-resolution capacitance measurements without requiring complex real-time conversion circuits
2Measurement precision
If the system processes all touch data with high resolution, then the measurement precision improves, but the loss of time in processing increases
Solution Approach 1:
The capacitance conversion operates in periodic cycles consisting of charge accumulation, holding, and decoding phases. During each cycle, charge is accumulated over multiple periods to build up measurement resolution, then quickly held and decoded. This periodic operation allows high-resolution measurements to be achieved without requiring continuous high-speed processing, thereby reducing overall processing time while maintaining precision
Solution Approach 2:
Charge accumulation is performed in advance during dedicated charge pump cycles before the actual decoding occurs. By pre-accumulating charge proportional to the capacitance value over multiple periods, the system prepares the measurement data beforehand, allowing the subsequent decode phase to quickly convert the accumulated charge to a digital value without time-consuming real-time calculations
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 solution enables accurate and precise detection of touch events, including the differentiation between fingers and styluses, and provides reliable operation under harsh conditions, improving user interface capabilities in various devices.
Implementation Method 1
Capacitance sensing systems can sense electrical signals generated on electrodes that reflect changes in capacitance. Such changes in capacitance can indicate a touch event (i.e., the proximity of an object to particular electrodes).
Data Source
AI summary
An integration circuit including a first capacitor is operatively coupled to a comparator. The comparator is configured to compare a first capacitor voltage of the first capacitor to a reference voltage and produce a first comparator output based on the comparison. A current generator is operatively coupled with the integration circuit and configured to balance charge on the first capacitor. A control unit is operatively coupled to the comparator and the current generator and configured to balance charge on the first capacitor by sensing the first comparator output and controlling the current generator based on the first comparator output.