Capacitance Sensing Ratio Measurement for Oscillator Mismatch Compensation

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Solution Overview

Problem

Capacitance sensing systems face limitations in accuracy due to component mismatch and variations such as PVT variations, which affect the ability to measure small capacitances or changes in capacitance, especially in capacitive interfaces like touch pads and accelerometers.

Innovation Solution

A capacitance sensing system that includes a mismatch compensation arrangement, using a pair of oscillators and switching arrangements to measure the number of oscillations from both a reference capacitance and an unknown capacitance, allowing for the determination of the unknown capacitance through their ratio, thereby compensating for oscillator mismatch and relaxing matching requirements.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a pair of relaxation oscillators is used to sense capacitance, then the system can measure capacitance values, but component mismatch and PVT variations cause measurement errors that limit accuracy

Engineering Contradiction:

Improvecapacitance measurement accuracyVSAvoidmeasurement reliability under PVT variations

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The system uses feedback by measuring the same capacitance with both oscillators and using the ratio of their oscillation counts to compensate for mismatch. The measurement process continuously compares the two oscillator outputs and adjusts for deviations, ensuring accurate capacitance sensing despite component variations.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The invention changes the measurement parameter from absolute oscillation frequency to the ratio of oscillation counts between two oscillators. By measuring the ratio rather than individual frequencies, the system eliminates the effect of PVT variations and component mismatch, achieving high measurement precision.

Inventive Principle:

Principle #35Parameter changes

2Measurement precision

If high-precision matched oscillators are used to reduce mismatch error, then measurement accuracy improves, but device complexity and manufacturing cost increase

Engineering Contradiction:

Improvecapacitance measurement accuracyVSAvoidoscillator circuitry complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The invention converts the harmful effect of oscillator mismatch into a beneficial measurement technique. By deliberately using two potentially mismatched oscillators and measuring their ratio, the system achieves high precision without requiring expensive matched components. The mismatch becomes irrelevant through the ratio measurement approach.

Inventive Principle:

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

Solution Approach 2:

The system replaces expensive, precisely matched oscillators with cheaper, standard oscillators that have relaxed matching requirements. The invention shows that high measurement accuracy can be achieved using inexpensive components, reducing manufacturing cost and device complexity.

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

3Ease of manufacture

If standard oscillators with relaxed matching requirements are used, then device complexity and cost decrease, but mismatch errors increase measurement uncertainty

Engineering Contradiction:

Improveoscillator circuitry manufacturing easeVSAvoidcapacitance measurement accuracy

Core Design Contradiction:

Ease of manufactureVSMeasurement precision

Solution Approach 1:

The ratio measurement technique acts as a feedback mechanism that automatically compensates for oscillator mismatch. By continuously comparing the oscillation counts of both oscillators over the same time interval, the system eliminates the impact of component variations, achieving high precision with standard oscillators.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The invention changes from measuring absolute frequency to measuring the ratio of oscillation counts. This parameter transformation eliminates the influence of oscillator mismatch and PVT variations, allowing standard oscillators with relaxed matching requirements to achieve high measurement precision.

Inventive Principle:

Principle #35Parameter changes

4Measurement precision

If the oscillation counting method is used to measure capacitance, then the system can detect small capacitance changes, but oscillator mismatch introduces errors that limit detection capability

Engineering Contradiction:

Improvesmall capacitance change detectionVSAvoidmismatch error impact on measurement

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The invention converts the harmful mismatch error into an irrelevant factor by using ratio measurement. The oscillation count ratio automatically compensates for mismatch, allowing the system to detect small capacitance changes with high precision despite using standard oscillators with inherent variations.

Inventive Principle:

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

Data Source

PatentUS8040143B2Capacitance sensing with mismatch compensation

Publication Date: 2011.10.18 NXP USA INC

AI summary

Systems and methods are provided for determining the value of a capacitance. A system for sensing capacitance comprises an oscillator arrangement comprising a plurality of oscillators and a mismatch compensation arrangement coupled between the oscillator arrangement and a first capacitive element having a first capacitance. The mismatch compensation arrangement is configured to selectively couple the first capacitive element to a respective oscillator of the plurality of oscillators, wherein an oscillation frequency of the respective oscillator is influenced by the first capacitance.