Capacitive Sensor Interface Circuit for Non-Linearity Reduction

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Solution Overview

Problem

Capacitive sensors for measuring physical parameters like acceleration and force face non-linearity issues due to stray capacitances and substrate potential, leading to reduced sensitivity and accuracy, especially in integrated semiconductor substrates.

Innovation Solution

An electronic interface circuit with a double structure featuring two integrator units and two excitation units operating alternately, which polarize capacitors symmetrically to minimize voltage offsets and substrate potential effects, allowing for improved signal-to-noise ratio and reduced non-linearity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If the electronic circuit is integrated in a semiconductor substrate, then device complexity is reduced and manufacturing is simplified, but stray capacitances are introduced that create non-linearities and reduce sensitivity

Engineering Contradiction:

Improveintegration in semiconductor substrateVSAvoidlinearity of output voltage

Core Design Contradiction:

Ease of manufactureVSMeasurement precision

Solution Approach 1:

The patent applies dynamic element matching by periodically swapping the roles of capacitors C1 and C2 through switching matrices. This converts the harmful effect of mismatched stray capacitances into a benefit by making the measurement immune to these variations. The switching operation causes the output to represent the difference between capacitors while the stray capacitances average out, eliminating their detrimental non-linear effects.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

2Ease of operation

If the substrate potential is not controlled, then the sensor operation is simplified, but non-linearity is introduced that affects the measured force

Engineering Contradiction:

Improvesubstrate potential controlVSAvoidaccuracy of measured force

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The patent eliminates the need for substrate potential control by using differential measurement and periodic capacitor swapping. The harmful substrate potential effects appear equally on both capacitors and are subtracted out in the differential operation, converting this uncontrolled parameter into a non-factor that does not affect measurement accuracy.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Device complexity

If a single integrator unit is used, then device complexity is reduced, but the signal-to-noise ratio and measurement accuracy are lowered

Engineering Contradiction:

Improvenumber of integrator unitsVSAvoidsignal-to-noise ratio

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent divides the measurement function into two separate integrator units that operate in parallel, each processing signals from different capacitor configurations. This segmentation allows independent optimization of each integrator and enables differential processing that improves signal-to-noise ratio while maintaining manageable device complexity through systematic architecture.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent combines the outputs of two integrator units through differential processing to achieve improved measurement accuracy. By merging the results from parallel integration paths and subtracting them, the system achieves better noise rejection and signal-to-noise ratio than a single integrator could provide alone.

Inventive Principle:

Principle #5Merging (Combining)

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

The symmetric operation of the electronic interface circuit enhances measurement accuracy by averaging output signals, reduces the impact of substrate potential, and provides twice the output signals compared to prior art, improving sensitivity and noise resistance.

Implementation Method 1

The capacitive sensor is composed of two capacitors mounted in differential, one common electrode of which is capable of moving between two fixed electrodes via the action for example of a force in order to alter the capacitive value of each capacitor

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

the common electrode is capable of moving a certain distance in the direction of one or the other of the fixed electrodes via the action of a force for example

Methodology Applied

Scientific EffectElectrostatic force: Electrostatics

Data Source

PatentUS7532016B2Electronic interface circuit for a capacitive sensor for measuring a physical parameter, and method for activating the electronic circuit

Publication Date: 2009.05.12 EM MICROELECTRONIC-MARIN

AI summary

An electronic interface circuit of a capacitive sensor usable for measuring a physical parameter, wherein the sensor includes two differential mounted capacitors whose common electrode moves relative to each fixed electrode in order to alter capacitive value of each capacitor. The electronic circuit includes a charge transfer amplifier unit connected to the common electrode, a first integrator unit for integrating charges supplied by the charge transfer amplifier, a first excitation unit arranged between the output of the first integrator unit and the sensor for polarizing each fixed electrode of the capacitors to a determined voltage value, a second integrator unit for integrating the charges supplied by the charge transfer amplifier, and a second excitation unit arranged between the output of the second integrator unit and the sensor for polarizing each fixed electrode of the capacitors at an opposite voltage value to the voltage value controlled by the first excitation unit.