Capacitive Transducer Circuit with Switched Capacitor Biasing

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Solution Overview

Problem

Capacitive transducer circuits, such as those in MEMS microphones, face challenges in providing a predictable DC output voltage with low added noise due to high impedance and leakage currents, which can lead to signal distortion and reduced dynamic range.

Innovation Solution

A capacitive transducer circuit incorporating a switched capacitor circuit between the voltage source and the transducer, along with a feedback loop using a digital filter and current-output DAC, to manage bias voltage and minimize noise and leakage effects.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a high resistance value (e.g., 8 GΩ or higher) is used to set the corner frequency at 20 Hz, then the low-frequency response is improved, but the DC output voltage becomes unpredictable and noise increases due to leakage currents

Engineering Contradiction:

Improvecorner frequency accuracyVSAvoidDC output voltage stability

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent implements a feedback mechanism where the DC output voltage is monitored and compared against a target value. A feedback current is generated based on the voltage error and injected into the high-impedance node to correct deviations. This closed-loop control stabilizes the DC output voltage despite leakage currents, while maintaining the high resistance value needed for accurate corner frequency response.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent introduces an intermediate feedback circuit that acts as a mediator between the high-impedance transducer node and the output. This intermediate circuit generates compensating currents without directly loading the high-impedance node, thereby maintaining the corner frequency characteristics while providing DC voltage stabilization through the feedback mechanism.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Measurement precision

If the bias resistance is increased to guarantee minimum corner frequency performance, then the low-frequency response is improved, but leakage currents cause larger DC offsets and reduced dynamic range

Engineering Contradiction:

Improvecorner frequency responseVSAvoidDC offset from leakage current

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The feedback mechanism detects the DC offset caused by leakage currents and generates a compensating feedback current. This feedback current is injected into the high-impedance node to counteract the leakage effect, thereby eliminating the DC offset while preserving the high bias resistance value needed for accurate corner frequency response.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent converts the harmful effect of leakage currents into a useful signal by using the feedback mechanism to detect the voltage deviation caused by leakage and generate a compensating current. The leakage current, which would normally cause DC offset, becomes the basis for the feedback correction that eliminates the offset.

Inventive Principle:

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

3Power

If a voltage source with high output impedance is used to maximize voltage across the transducer, then the signal amplitude is improved, but noise from the voltage source becomes indistinguishable from the transducer signal

Engineering Contradiction:

Improvevoltage across transducerVSAvoidnoise from voltage source

Core Design Contradiction:

PowerVSObject-affected harmful factors

Solution Approach 1:

The patent introduces a buffered intermediate stage between the voltage source and the transducer. This buffer provides low output impedance to the voltage source, isolating the transducer from voltage source noise while maintaining the high voltage across the transducer. The buffer acts as an intermediary that decouples the noise characteristics from the signal path.

Inventive Principle:

Principle #24Intermediary (Mediator)

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 solution provides a stable DC output voltage with reduced noise, improved linearity, and increased dynamic range by effectively managing bias voltage and leakage currents, enhancing the performance of capacitive transducer circuits.

Implementation Method 1

a switched capacitor circuit provided between the voltage source and the capacitive transducer

Methodology Applied

Scientific EffectSwitched capacitor circuit:

Implementation Method 2

a feedback loop using a digital filter and current-output DAC

Methodology Applied

Scientific EffectDigital filtering: Filter (electronic)

Implementation Method 3

current-output DAC

Methodology Applied

Scientific EffectDAC conversion:

Data Source

PatentUS8068623B2Capacitive transducer circuit and method

Publication Date: 2011.11.29 CIRRUS LOGIC INC

AI summary

A capacitive transducer circuit includes a capacitive transducer having first and second electrodes. The first and second electrodes are biased by respective first and second bias voltages. An amplifier is connected to receive a first analog signal on an input terminal, the first analog signal being generated by the capacitive transducer, and to generate a second analog signal on an output terminal. A digital feedback circuit is connected between the output terminal of the amplifier and the input terminal of the amplifier. The digital feedback circuit is configured to provide one of said first or second bias voltages. A switched capacitor filter circuit may be arranged between the voltage source and the transducer and may be arranged to filter the output of the voltage source.