Chopped MEMS Accelerometer Readout Circuit for Low-Noise Sensing
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Solution Overview
Problem
MEMS capacitive accelerometers face challenges in reducing noise and minimizing size due to the design of sensing circuits, particularly in detecting changes in capacitance caused by acceleration, which affects their performance in electronic devices.
Innovation Solution
The implementation of a capacitance-to-voltage converter circuit with a differential chopping circuit path and a differential sigma-delta analog to digital converter (ADC) circuit, which inverts the polarity of the differential chopping circuit path and samples the MEMS sensor output signal to reduce noise, specifically addressing 1/f noise and thermal noise, and includes a pseudo-random clock signal and dither noise to minimize dead-bands in the ADC output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional sensing circuit is used to detect capacitance changes in MEMS accelerometers, then the circuit can function, but noise levels increase and circuit area increases
Solution Approach 1:
The patent merges the capacitance-to-voltage conversion function and the sigma-delta ADC function into a single integrated circuit block. This consolidation eliminates the need for separate capacitor arrays for both functions, reducing the overall circuit area while maintaining low noise performance through the combined architecture.
Solution Approach 2:
The sensing circuit is designed to perform multiple functions within a single circuit structure: capacitance sensing, voltage conversion, and sigma-delta modulation. This multi-functional approach reduces the total number of components needed, thereby reducing circuit area while achieving superior noise reduction through the integrated design.
2Measurement precision
If more capacitors are used in the sensing circuit, then capacitance detection accuracy improves, but thermal noise increases and power consumption increases
Solution Approach 1:
The patent combines the capacitance sensing capacitors and the DAC capacitors into a single shared capacitor array. This merging allows the circuit to achieve high capacitance detection accuracy without requiring separate large capacitor arrays, thereby reducing thermal noise and power consumption associated with multiple capacitor banks.
Solution Approach 2:
The patent employs a sigma-delta modulation scheme that changes the effective resolution and accuracy parameters through oversampling and noise shaping. This allows high measurement precision to be achieved through signal processing rather than through increased capacitor size, thereby reducing thermal noise and power consumption.
3Measurement precision
If a high-resolution ADC is used to achieve greater than 100 decibels of dynamic range, then measurement precision improves, but the circuit requires more components increasing area and complexity
Solution Approach 1:
The patent merges the capacitance-to-voltage converter and the sigma-delta ADC into a single integrated circuit that uses a shared capacitor array. This consolidation achieves high-resolution conversion with greater than 100 decibels of dynamic range without requiring separate capacitor arrays, thereby reducing the number of capacitors and overall circuit complexity.
Solution Approach 2:
The patent uses sigma-delta modulation with periodic oversampling and noise shaping to achieve high dynamic range. This periodic action allows high measurement precision to be achieved through time-domain signal processing rather than through increased spatial resolution, reducing the number of capacitors needed.
Data Source
AI summary
An apparatus includes a capacitance-to-voltage converter circuit configured to be electrically coupled to a micro-electromechanical system (MEMS) sensor circuit. The capacitance-to-voltage converter circuit includes a differential chopping circuit path configured to receive a differential MEMS sensor output signal and invert a polarity of the differential chopping circuit path, and a differential sigma-delta analog to digital converter (ADC) circuit configured to sample the differential MEMS sensor output signal and provide a digital signal representative of a change in capacitance of the MEMS sensor.