TIA Baseline Correction for Low-Noise Capacitive Sensing

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Solution Overview

Problem

Existing global coarse baseline correction (GCBC) techniques for capacitive sensing are limited by noise susceptibility, particularly flicker noise and temperature drift, due to the use of current mirrors in current GCBC circuits.

Innovation Solution

The implementation of a transimpedance amplifier (TIA)-based GCBC system that uses signal-producing resistors with the same temperature coefficient of resistance (TCR) as the feedback resistor, eliminating noise sources by offsetting temperature drift and avoiding flicker noise through resistor-based sensing signal production.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If current mirrors are used in GCBC circuits, then baseline correction can be achieved, but noise susceptibility increases due to flicker noise and temperature drift

Engineering Contradiction:

Improvebaseline correction capabilityVSAvoidnoise susceptibility

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent extracts and removes the harmful current mirror component from the GCBC circuit, replacing it with a resistor-based sensing signal production mechanism. This eliminates the source of flicker noise and temperature drift while preserving the baseline correction function through alternative means.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent substitutes the electronic current mirror mechanism with a resistor-based voltage-to-current conversion system. The transimpedance amplifier converts voltage signals to current signals using resistors, eliminating the need for current mirrors and their associated noise issues.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Object-affected harmful factors

If transimpedance amplifier with resistors is used, then noise interference is reduced, but device complexity increases

Engineering Contradiction:

Improvenoise interferenceVSAvoidcircuit structure

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The transimpedance amplifier is designed to perform multiple functions: it converts voltage to current, provides baseline correction, and eliminates noise susceptibility. The resistor-based approach serves both as a sensing element and a conversion mechanism, reducing the need for separate components.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The patent changes the fundamental operating parameters from current-based (current mirrors) to voltage-based (resistors and transimpedance amplifier). This parameter transformation simplifies the circuit by using passive resistive elements instead of active current mirror transistors, reducing complexity while maintaining functionality.

Inventive Principle:

Principle #35Parameter changes

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 approach enhances the accuracy of capacitive sensing by reducing noise interference, allowing for more robust detection of capacitance changes in sensor electrodes, thereby improving the detection of input objects in capacitive sensing systems.

Implementation Method 1

The transimpedance amplifier is configured to receive the first current from the current source and produce a first voltage based on the first current

Methodology Applied

Scientific EffectTransimpedance conversion: Ohm's Law

Implementation Method 2

The first resistor is configured to provide a second current to the first sensor electrode based at least in part on the first voltage

Methodology Applied

Scientific EffectElectrical conduction: Conduction (electrical)

Implementation Method 3

The controller is configured to detect changes in a capacitance of the first sensor electrode based at least in part on the second current

Methodology Applied

Scientific EffectCapacitance sensing: Capacitance

Implementation Method 4

The second resistor has the same temperature coefficient of resistance (TCR) as the first resistor

Methodology Applied

Scientific EffectTemperature coefficient matching: Thermal Expansion

Data Source

PatentUS11481076B2Transimpedance amplifier (TIA)-based global coarse baseline correction (GCBC) for capacitive sensing

Publication Date: 2022.10.25 SYNAPTICS INC

AI summary

A method and apparatus of global coarse baseline correction (GCBC) for capacitive scanning. An input device may include a number (N) of sensor electrodes, a GCBC circuit, and detection circuitry. Each sensor electrode is associated with a respective channel. The GCBC circuit produces sensing signals in each of the N channels and the detection circuitry may detect changes in the capacitances of one or more sensor electrodes based on the sensing signals. In some implementations, the GCBC circuit may include a current source which outputs a first current, a transimpedance amplifier (TIA) which converts the first current to a sensing voltage, and a number (N) of resistors that can be coupled between the output of the TIA and the N sensor electrodes, respectively. The coupling of each resistor between the TIA and a respective sensor electrode produces a sensing signal in the channel associated with the sensor electrode.