Ratiometric Capacitive Baseliner for Low-Noise Touch Sensing
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Solution Overview
Problem
Touch panels and other sensing devices face challenges in detecting small capacitance changes due to parasitic noise, especially with increasing size, which leads to saturation and reduced sensitivity in recognizing touch signals under varying environmental conditions.
Innovation Solution
A low-noise, high-resolution baseliner circuit is implemented using a two-baseliner configuration with a global and channel baseliner, employing sigma-delta modulators and ratiometric switching-capacitor technology to generate compensation currents that reduce parasitic capacitance effects, allowing for precise touch signal detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If the touch panel size is increased, then the sensing area is improved, but parasitic capacitance increases and drowns out small capacitance changes
Solution Approach 1:
The patent divides the baseliner circuit into two separate circuits: a global baseliner that compensates for common-mode parasitic capacitance affecting all channels, and individual channel baseliners that compensate for channel-specific parasitic capacitance. This segmentation allows each circuit to be optimized for its specific function, effectively reducing the impact of parasitic capacitance across the entire touch panel while maintaining high sensing area.
2Device complexity
If conventional baseliner circuits are used, then circuit simplicity is maintained, but noise increases and sensitivity is reduced
Solution Approach 1:
The patent implements feedback mechanisms where the global baseliner continuously monitors and compensates for common-mode parasitic capacitance variations, and channel baseliners provide feedback for individual channel compensation. This feedback approach enables the system to dynamically adapt to changing environmental conditions and maintain high measurement precision without requiring overly complex circuitry.
Solution Approach 2:
The patent introduces switched capacitor circuits as intermediary elements between the signal source and the measurement system. These switched capacitor baseliners act as mediators that transfer and condition the signal while reducing noise and parasitic effects, enabling high-sensitivity detection without direct complex circuitry at the sensing element.
3Measurement precision
If high-resolution capacitance measurement is pursued, then touch detection precision is improved, but computational requirements and processing complexity increase
Solution Approach 1:
The patent performs preliminary compensation actions by subtracting baseline capacitance values (measured without touch) from the raw capacitance measurements before further processing. This preliminary action removes the dominant parasitic capacitance component early in the signal chain, allowing subsequent processing to focus only on the small touch-induced capacitance changes, thereby reducing computational requirements while maintaining high precision.
Data Source
AI summary
An apparatus includes a global baseliner circuit coupled with sensing channels of a sensing device. The global baseliner circuit has a signal generator to generate a rectified sinusoidal signal and a square wave having a frequency matching that of an excitation sinusoidal signal, and is to use the square wave to modulate the excitation sinusoidal signal provided at an output of the global baseliner circuit. A channel baseliner circuit is coupled between the global baseliner circuit and a sensing channel and that includes a switched capacitor coupled between the output of the global baseliner circuit and the sensing channel; a sigma-delta modulator coupled with the signal generator and to generate, from the rectified sinusoidal signal, a density-modulated bit stream; and a pair of AND gates to use the density-modulated bit stream and non-overlapping clock signals to generate outputs including density-modulated clock signals sent to switches of the switched capacitor.