Source Follower Shield Drive for Low-Power Capacitive Sensing
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Solution Overview
Problem
Capacitive sensors in mobile devices face challenges in maintaining sensitivity while reducing power dissipation and noise, especially in detecting proximity inputs, due to the use of operational amplifiers in self-capacitance mode circuits.
Innovation Solution
A source follower circuit is introduced, replacing the operational amplifier, comprising a transistor and a switch array that operates in multiple modes to copy the sense voltage to the shield node, reducing noise and power consumption by minimizing the number of current paths and transistors involved.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an operational amplifier is used in self-capacitance mode circuit to copy voltage from sense node to shield node, then the voltage copying function is achieved, but power dissipation and noise increase
Solution Approach 1:
The patent extracts only the essential voltage copying function from the operational amplifier, implementing it through a simplified circuit consisting of a transistor and switch array. This removes unnecessary components that consume power and generate noise while preserving the core functionality of copying the sense node voltage to the shield node.
Solution Approach 2:
The patent creates a simplified copy of the operational amplifier's voltage buffering function using a transistor-based source follower circuit. This copy achieves the same voltage copying effect with significantly reduced power consumption and noise by using fewer transistors and eliminating complex operational amplifier internal structures.
2Measurement precision
If an operational amplifier is used in self-capacitance mode circuit to copy voltage from sense node to shield node, then the voltage copying function is achieved, but noise increases
Solution Approach 1:
The patent removes the noisy operational amplifier component entirely and extracts only the essential voltage copying function, implementing it through a simplified transistor and switch array circuit. This elimination of complex operational amplifier internal structures significantly reduces noise generation.
Solution Approach 2:
The patent replaces the expensive, complex operational amplifier with a simpler, less noisy transistor-based circuit. This substitution uses fewer components that can introduce noise, achieving a cleaner signal path suitable for sensitive capacitive proximity detection.
3Object-generated harmful factors
If operational amplifier with numerous transistors is used to reduce noise, then noise reduction is achieved, but power dissipation increases
Solution Approach 1:
The patent extracts only the essential voltage copying function from the operational amplifier, implementing it through a simplified circuit consisting of a transistor and switch array. This removes unnecessary components that consume power while preserving the core functionality.
Solution Approach 2:
The patent applies partial action by implementing only the necessary voltage copying function without the full operational amplifier structure. This partial implementation achieves sufficient noise reduction and voltage buffering without the excessive power consumption of a complete operational amplifier with numerous transistors.
Data Source
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AI summary
A source follower for a capacitive sensor device having a sense node and a shield node is provided. The source follower may include a transistor, and a switch array selectively coupling the transistor between the sense node and the shield node. The switch array may be configured to substantially disable current to the transistor during a first mode of operation, precharge the transistor during a second mode of operation, and enable the transistor to copy a sense node voltage to a shield node voltage during a third mode of operation.