Capacitive Sensor Filtered Sampling for Op-Amp Noise Reduction

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Solution Overview

Problem

Capacitive sensors face significant noise interference from operational amplifiers and switching actions, limiting their sensitivity in detecting capacitance changes.

Innovation Solution

The implementation of passive or active filtered-sampling stages with resistive and capacitive elements, along with subtractors, to attenuate noise and isolate signal components, allowing for reduced noise output signals proportional to the capacitance difference.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If traditional capacitive sensor components (operational amplifiers, switches) are used, then the sensor can perform basic capacitance measurement, but noise from these components limits sensitivity

Engineering Contradiction:

Improvecapacitance detection sensitivityVSAvoidnoise from operational amplifier and switches

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The patent extracts and removes the noise component from the sensor output by implementing a separate noise measurement path. A second capacitive sensor measures only the noise from operational amplifiers and switches, allowing this harmful component to be isolated and subtracted from the main measurement, thereby improving capacitance detection sensitivity

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces a subtractor circuit as an intermediary that processes two separate sensor outputs. The subtractor removes the noise component measured by the second sensor from the measurement taken by the first sensor, effectively eliminating the harmful noise factor while preserving the useful capacitance signal

Inventive Principle:

Principle #24Intermediary (Mediator)

2Measurement precision

If noise reduction techniques are implemented, then sensitivity improves, but device complexity increases

Engineering Contradiction:

Improvenoise reductionVSAvoidsensor circuit complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent segments the measurement function into two independent but parallel sensor paths: one for measuring the capacitance signal and another for measuring only the noise component. This segmentation allows each sensor to be optimized for its specific function while maintaining overall system manageability despite increased complexity

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent employs identical capacitive sensor circuits for both measurement functions, making each sensor design universal. The same circuit topology serves dual purposes: primary capacitance measurement and noise characterization, reducing design complexity through reuse of proven components

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 results in a significant reduction of noise in capacitive sensor output signals, enhancing sensitivity by up to 12 dB compared to traditional capacitive sensors.

Implementation Method 1

capacitive sensors including a gain stage and a filtered-sampling stage... generating a second signal including an output component of the gain stage and the noise component... results in a significant reduction of noise in capacitive sensor output signals

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

noise from an operational amplifier... the noise component included in the output signals is large

Methodology Applied

Scientific EffectElectrical amplification:

Implementation Method 3

passive or active filtered-sampling stages with resistive and capacitive elements... to attenuate noise and isolate signal components

Methodology Applied

Scientific EffectElectrical filtering: Filter (electronic)

Implementation Method 4

filtered-sampling stages with resistive and capacitive elements... allowing for reduced noise output signals proportional to the capacitance difference

Methodology Applied

Scientific EffectRC filtering:

Data Source

PatentUS7948244B2Capacitive sensors including gain and active-filtered sampling stages

Publication Date: 2011.05.24 NXP USA INC

AI summary

Embodiments of capacitive sensors (500, 600) and methods for reducing noise in capacitive sensors are provided. Embodiments of capacitive sensors include a gain stage (510, 610), a capacitive sensor output, and an active filtered-sampling stage (550, 650). The an active filtered-sampling stage includes a first resistive element (555, 655) coupled to the gain stage output, a second resistive element (565, 670) coupled to the capacitive sensor output, a node (560, 660) between the first and second resistive elements, and a switch (575, 675) selectively coupling the first node to an integrator circuit (550, 650), where the integrator circuit is coupled to the capacitive sensor output.