Sigma-Delta Sensor Feedback Circuit for Low-Noise MEMS Microphones
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Solution Overview
Problem
Conventional sensor interface topologies for MEMS microphone sensors face challenges in achieving low noise, low power consumption, and high dynamic range while processing large audio signals with minimal distortion, as they often require complex circuitry and are limited by amplifier input range and DC biasing constraints.
Innovation Solution
A sensor arrangement where a capacitive or piezo sensor is integrated within a feedback loop of a sigma-delta analog-to-digital converter, utilizing a digital-to-analog converter to generate a feedback signal that reduces the amplitude of the input signal to the amplifier, allowing for simpler amplifier design and higher acoustical overload point, with the sensor acting as a floating voltage source and eliminating the need for repeated switching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the sensor is arranged outside the feedback loop in a conventional sensor interface topology, then the amplifier can process the full sensor signal, but the amplifier input range is limited and DC biasing constraints complicate the design
Solution Approach 1:
The patent inverts the conventional approach by placing the sensor inside the feedback loop rather than outside. This inversion allows the sensor to operate with a reduced signal amplitude while the feedback mechanism restores the full dynamic range, eliminating amplifier input range limitations and simplifying DC biasing requirements.
Solution Approach 2:
The patent changes the operating parameters of the sensor by applying a reduced amplitude signal within the feedback loop. The feedback mechanism then compensates to maintain the full dynamic range, allowing the amplifier to operate in a simplified manner without complex DC biasing constraints.
2Measurement precision
If the amplifier processes the full sensor signal amplitude, then the signal dynamic range is maintained, but noise contribution from the amplifier increases
Solution Approach 1:
Instead of having the amplifier process the full sensor signal directly, the patent inverts the approach by having the sensor process a reduced amplitude signal within the feedback loop. The feedback mechanism then restores the full dynamic range, allowing the amplifier to process only quantization noise and significantly reducing its noise contribution.
Solution Approach 2:
The patent employs feedback to restore the full dynamic range after the sensor processes a reduced amplitude signal. The feedback loop compensates for the reduced signal amplitude, maintaining the full dynamic range while allowing the amplifier to operate with minimal noise contribution.
3Productivity
If repeated switching is used to charge and discharge the capacitive sensor, then the sensor signal can be digitized, but power consumption increases and noise is introduced
Solution Approach 1:
The patent applies preliminary action by pre-charging the capacitive sensor to a fixed voltage level before the measurement process. This eliminates the need for repeated switching to charge and discharge the sensor, significantly reducing power consumption and avoiding the introduction of switching noise during signal processing.
Solution Approach 2:
The patent maintains continuous useful action by keeping the capacitive sensor charged throughout the measurement process rather than repeatedly charging and discharging it. This continuous state allows for efficient signal processing without the energy losses and noise associated with repeated switching operations.
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 configuration reduces noise contribution, allows for larger sensor signal handling, and simplifies amplifier design by processing only quantization noise, while maintaining stable operation and low noise targets, even for small sensor capacitances, thus enhancing the dynamic range and reducing power consumption.
Implementation Method 1
a sensor having a first terminal and a second terminal, wherein a voltage between the terminals changes when an input quantity changes. The sensor may be a capacitive sensor or a piezo sensor. The capacitive sensor may be a two-terminal MEMS microphone sensor.
Implementation Method 2
The sensor may be a capacitive sensor or a piezo sensor. The piezo sensor may be a microphone sensor as well.
Implementation Method 3
a feedback capacitor that is coupled between the feedback circuit output and an output of the digital-to-analog converter
Data Source
AI summary
A sensor arrangement includes a sensor having a first terminal and a second terminal, and an amplifier having an amplifier input for applying an input signal and an amplifier output for providing an amplified input signal, the amplifier input being coupled to the second terminal. A quantizer having a quantizer input and a quantizer output is configured to provide a multi-level output signal on the basis of the amplified input signal and a feedback circuit having a feedback circuit input coupled to the quantizer output and a feedback circuit output coupled to the first terminal. The feedback circuit includes a digital-to-analog converter configured to generate an analog signal on the basis of the multi-level output signal, the analog signal being the basis of a feedback signal provided at the feedback circuit output, a feedback capacitor coupled between the feedback circuit output and an output of the digital-to-analog converter, and a voltage source coupled to the feedback circuit output.