Stylus Differential Amplifier for Capacitive Noise Rejection

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Solution Overview

Problem

When using a stylus with a capacitive touch sensor, user interactions such as touching the device edges or placing a palm on the sensor can cause interfering signals, leading to noise in the received signals.

Innovation Solution

The stylus employs two receive electrodes with different degrees of capacitive coupling to the touch sensor and a differential amplifier to process the signals, thereby reducing noise and interference from user-induced signals.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If a single receive electrode is used in the stylus, then the device complexity is reduced, but user interactions cause interfering signals and noise in the received signals

Engineering Contradiction:

Improvestylus structureVSAvoidnoise in received signals

Core Design Contradiction:

Device complexityVSObject-affected harmful factors

Solution Approach 1:

The single receive electrode is segmented into multiple receive electrodes (first receive electrode and second receive electrode) with different capacitive coupling degrees to the touch sensor. This segmentation allows the system to differentiate between user-induced signals and actual touch sensor signals, reducing noise while maintaining manageable device complexity through systematic electrode arrangement.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The differential amplifier acts as an intermediary that processes signals from multiple receive electrodes. It calculates the difference between signals received by electrodes with different capacitive coupling degrees, effectively filtering out user-induced interference while preserving authentic touch sensor communications.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Object-affected harmful factors

If multiple receive electrodes with different capacitive coupling are used, then noise and interference are reduced, but the device complexity increases

Engineering Contradiction:

Improveuser-induced noiseVSAvoidsignal processing system

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The system changes the capacitive coupling parameter by positioning receive electrodes at different locations within the stylus. The first receive electrode is positioned to have a first degree of capacitive coupling while the second receive electrode is positioned to have a second degree of capacitive coupling. This parameter variation enables noise differentiation without requiring complex processing electronics.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent replaces complex electronic filtering mechanisms with a simpler differential measurement approach. By using the natural capacitive coupling differences of strategically positioned electrodes and processing signals through a differential amplifier, the system achieves noise rejection without requiring sophisticated electronic noise cancellation circuits.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Measurement precision

If the stylus is isolated from user-induced noise, then signal reception accuracy is improved, but the ease of operation may be reduced

Engineering Contradiction:

Improvesignal reception accuracyVSAvoiduser interaction with device

Core Design Contradiction:

Measurement precisionVSEase of operation

Solution Approach 1:

The stylus system performs self-diagnosis and self-filtering by using its own receive electrodes to identify and eliminate user-induced interference. The differential amplifier automatically distinguishes between genuine touch sensor signals and noise generated by user handling, maintaining high signal reception accuracy without requiring the user to adjust any settings or perform additional actions.

Inventive Principle:

Principle #25Self-service

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 configuration effectively isolates the stylus from user-induced noise, ensuring accurate signal reception and processing even when the user is holding the stylus and interacting with the device.

Implementation Method 1

The first sensor is adapted to receive a first receive signal via a first capacitive coupling with a touch sensor of a device. The second sensor is adapted to receive a second receive signal via a second capacitive coupling with the touch sensor of the device.

Methodology Applied

Scientific EffectCapacitive coupling: Capacitance

Implementation Method 2

The stylus also includes a differential amplifier which takes the received signal from both receive electrodes as an input and produces an output signal that amplifies the difference between the two received signals.

Methodology Applied

Scientific EffectDifferential amplification:

Data Source

PatentUS20250147607A1Touch sensor and method for deriving a stylus position

Publication Date: 2025.05.08 WACOM CO LTD

AI summary

A stylus includes a first sensor configured to receive a first receive signal from a touch sensor of a device, and a second sensor configured to receive a second receive signal from the touch sensor of the device. The stylus includes an amplifier coupled to the first and second sensors and configured to produce a third signal by amplifying the difference between the first receive signal and the second receive signal. The stylus includes a controller configured to decode information encoded in the first receive signal and the second receive signal by processing the third signal.