Single-Ended MEMS Readout Circuit for Ultrasonic Peak Suppression
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Solution Overview
Problem
MEMS microphones face challenges in achieving high linearity and power efficiency with single-ended readout, particularly due to resonance peaks in ultrasonic frequency ranges that cause audible artefacts and signal clipping, especially when strong ultrasonic signals are present.
Innovation Solution
A circuit and method involving differential-to-single-ended readout with biasing voltage sources, high impedance resistors, and a unity gain buffer amplifier, along with a low pass filter and positive feedback path, are used to optimize linearity and reduce ultrasonic signal levels, ensuring efficient signal conversion and minimizing distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a differential MEMS device with double-backplate is used to achieve high system linearity, then linearity is improved, but device complexity increases
Solution Approach 1:
The differential MEMS device is segmented into two separate backplates (first backplate and second backplate) with the moveable membrane positioned between them. This segmentation allows independent control and optimization of each backplate's function, achieving high linearity through differential operation while managing complexity through modular design
Solution Approach 2:
A unity gain buffer amplifier is introduced as an intermediary component between the differential MEMS device and the single-ended codec input. This buffer amplifier converts the differential signal to single-ended signal without signal loss, enabling compatibility with single-ended interfaces while maintaining the linearity benefits of the differential MEMS structure
2Measurement precision
If damping is reduced to achieve high signal-to-noise ratio, then SNR is improved, but ultrasonic resonance peak increases causing audible artefacts
Solution Approach 1:
The ultrasonic resonance peak, which initially causes harmful audible artefacts and intermodulation distortion, is converted into a beneficial feature through selective amplification. The resonance peak is located in the ultrasonic range where it does not directly interfere with audio signals, and its high Q-factor is exploited to achieve frequency-selective noise suppression in the audio band while maintaining high SNR
Solution Approach 2:
The damping factor of the MEMS device is optimized to achieve a high Q-factor resonance peak in the ultrasonic range. By changing the damping parameter, the system transforms the resonance from a harmful broadband effect into a useful narrowband feature that can be selectively utilized for signal processing and noise suppression
3Measurement precision
If ultrasonic signal levels are increased from motion detectors, then detection sensitivity is improved, but signal clipping occurs in the pre-amplifier
Solution Approach 1:
The ultrasonic frequency components are extracted and separated from the audio frequency signal path using frequency-selective filtering. The resonance peak's high Q-factor enables selective extraction of ultrasonic signals for motion detection while preventing them from entering the audio amplification chain, thus avoiding clipping while maintaining detection sensitivity
Solution Approach 2:
A feedback mechanism is implemented that monitors the signal level at the resonance peak frequency and dynamically adjusts the pre-amplifier gain. When strong ultrasonic signals are detected, the feedback reduces the gain to prevent clipping, while maintaining high detection sensitivity through adaptive signal processing
Data Source
AI summary
A circuit includes a first biasing voltage source, a second biasing voltage source, a first resistor device coupled between the first biasing voltage source and a first terminal of the circuit, a second resistor device coupled between the second biasing voltage source and a second terminal of the circuit, a third resistor device coupled between the second biasing voltage source and a third terminal, a first capacitor coupled between the third terminal and ground, and an amplifier having an input coupled to the second terminal and an output coupled to a circuit output.