Capacitance Measurement System Using SAR Converter for Touch Sensing
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Solution Overview
Problem
Existing capacitance measurement systems for touch screens and touch buttons are susceptible to noise and have varying measurement times based on capacitance values, lacking sensitivity and accuracy across a broader range of capacitances.
Innovation Solution
A capacitance measurement system utilizing a successive approximation register (SAR) converter with a capacitor digital-to-analog converter (CDAC) and auto-zeroing circuitry, which precharges capacitors to redistribute charges and generate a digital representation of capacitance, providing consistent measurement times and improved sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If prior art capacitance measurement systems are used, then measurement can be performed, but measurement precision is degraded due to noise susceptibility and varying measurement times
Solution Approach 1:
The system performs preliminary actions by precharging capacitors to a known voltage state before the actual measurement. The SAR converter initializes its internal capacitors and establishes a reference state, ensuring that measurements start from a consistent, noise-free baseline condition, thereby improving measurement precision and reducing noise susceptibility.
Solution Approach 2:
The SAR converter employs feedback mechanisms where the digital output from the comparator is fed back to control the switching of capacitors in the next iteration. This iterative feedback process allows the system to converge on the precise capacitance value while filtering out noise through multiple measurement cycles and digital processing.
2Adaptability or versatility
If prior art measurement systems are used, then a wide range of capacitances can be measured, but measurement time varies significantly based on capacitance value
Solution Approach 1:
The SAR converter operates through periodic cycles of capacitor switching and comparison. Each cycle tests one bit of the digital output, and the process repeats for all bits in a fixed sequence. This periodic action ensures that measurement time is determined by the number of bits (fixed) rather than the capacitance value, providing consistent timing across the full measurement range.
Solution Approach 2:
The system replaces analog timing mechanisms with digital counting and binary search algorithms. Instead of using variable time constants or mechanical counters that depend on capacitance value, the SAR converter uses digital logic to perform a binary search through the capacitance range, substituting mechanical time-based measurement with a fixed-duration digital process.
3Measurement precision
If prior art systems are used, then basic capacitance detection is achieved, but sensitivity is insufficient for precise touch location detection
Solution Approach 1:
The SAR converter dynamically adjusts the capacitor configuration during measurement, switching capacitors in and out based on the binary search algorithm. This dynamic reconfiguration allows the system to probe different capacitance ranges with optimal sensitivity, adapting the measurement process to detect subtle changes in capacitance that correspond to precise touch locations.
Solution Approach 2:
The system transitions from direct analog voltage measurement to digital domain processing. By converting the capacitance measurement problem into a digital binary search problem, the system gains an additional dimension of precision through digital resolution, allowing for much finer discrimination of capacitance changes that correspond to subtle touch location variations.
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 achieves consistent and accurate capacitance measurements across a broader range, with enhanced sensitivity and reduced noise immunity, allowing for precise touch location detection in touch screens and touch buttons.
Implementation Method 1
precharges first terminals (21-0 . . . 21-k . . . 21-n) of a plurality of capacitors (25-0 . . . 25-k . . . 25) to a first voltage (VDD) and also precharges a first terminal (3-j) of a capacitor (CSENj) to a second voltage (GND). The first terminals of the CDAC capacitors are coupled to the first terminal of the capacitor to redistribute charges therebetween so as to generate a first voltage on the first terminals
Data Source
AI summary
A capacitance measurement system precharges first terminals (21-0 . . . 21-k . . . 21-n) of a plurality of capacitors (25-0 . . . 25-k . . . 25), respectively, of a CDAC (capacitor digital-to-analog converter) (23) included in a SAR (successive approximation register) converter (17) to a first voltage (VDD) and pre-charges a first terminal (3-j) of a capacitor (CSENj) to a second voltage (GND). The first terminals are coupled to the first terminal of the capacitor to redistribute charges therebetween so as to generate a first voltage on the first terminals and the first terminal of the capacitor, the first voltage being representative of a capacitance of the first capacitor (CSENj). A SAR converter converts the first voltage to a digital representation (DATA) of the capacitor. The capacitance can be a touch screen capacitance.