Capacitive Sensor Sampling Circuit Eliminates Buffer Amplifier Noise

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Solution Overview

Problem

Capacitive microphone assemblies face challenges with high power consumption and noise due to the presence of a continuous time buffer-amplifier, which is a predominant source of thermal and flicker noise, and reducing the input capacitance of the buffer-amplifier increases in-band noise.

Innovation Solution

The implementation of a sampling circuit that eliminates the need for a buffer-amplifier by using a comparator, sampling capacitor, and charge and discharge circuit to produce a pulse width modulated signal, reducing power consumption and noise through a delta-sigma analog-to-digital converter.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a continuous time buffer-amplifier is used in capacitive microphone assemblies, then signal conditioning is achieved, but power consumption increases and thermal and flicker noise are generated

Engineering Contradiction:

Improvesignal qualityVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The patent removes the buffer-amplifier from the signal conditioning circuitry, extracting the noise-generating component while preserving the essential function through alternative circuit architecture using switched-capacitor techniques and correlated double sampling

Inventive Principle:

Principle #2Taking out (Extraction)

2Reliability

If the input capacitance of the buffer-amplifier is reduced, then signal attenuation is reduced, but in-band noise increases due to increased corner frequency

Engineering Contradiction:

Improvesignal attenuationVSAvoidin-band noise

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The patent replaces the continuous-time buffer-amplifier with a discrete-time switched-capacitor system that uses sampling and correlation techniques, substituting the analog continuous conditioning approach with a digital-friendly architecture that avoids the gm-related noise tradeoffs

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Data Source

PatentUS11689848B2Capacitive sensor assembly and electrical circuit therefor

Publication Date: 2023.06.27 KNOWLES ELECTRONICS LLC

AI summary

A capacitive sensor assembly includes a capacitive transduction element and an electrical circuit disposed in the housing and electrically coupled to contacts on an external-device interface of the housing. The electrical circuit includes a sampling circuit having an operational sampling phase during which a voltage produced by the capacitive sensor is sampled by a sampling capacitor coupled to a comparator and an operational charging phase during which a second capacitor is charged by a charge and discharge circuit until the output of the comparator changes state, wherein the output of the sampling circuit is a pulse width modulated signal representative of the voltage on the input of the sampling circuit during each sample period. The output of the sampling circuit can be coupled to a delta-sigma analog-to-digital (A/D) converter.