MEMS Compensation Loop for Differential Microphone Distortion
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Solution Overview
Problem
Differential MEMS microphones experience sensitivity mismatch between channels due to capacitance, bias voltage, and membrane stiffness mismatches, leading to signal distortion and poor total harmonic distortion (THD) at high sound levels.
Innovation Solution
A circuit and method that includes a differential amplifier, a common mode coupling circuit, and an amplifier to sense and amplify the common mode AC voltage, feeding it back to the bias input node of the MEMS device, forming a dynamic compensation loop to correct asymmetries and reduce distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If higher bias voltages are used to increase MEMS sensitivity and performance, then sensitivity is improved, but sensitivity mismatch between channels increases leading to signal distortion and poor THD
Solution Approach 1:
The patent implements a feedback mechanism where the differential output signal is fed back through a common mode coupling circuit and amplifier to adjust the bias voltage applied to the MEMS device. This closed-loop feedback system dynamically compensates for sensitivity mismatches between channels, allowing the system to maintain high bias voltages for sensitivity while automatically correcting the resulting asymmetry to preserve signal quality and low distortion
Solution Approach 2:
The patent dynamically adjusts the bias voltage parameter applied to the MEMS device based on the detected asymmetry in the differential output. By monitoring the output signal characteristics and modifying the bias voltage in response, the system optimizes the operating point to maintain both high sensitivity and low distortion, effectively changing the operating parameters to resolve the contradiction
2Stability of the object's composition
If a differential MEMS device is used to achieve high system linearity, then linearity is improved, but sensitivity mismatch between channels occurs due to capacitance, bias voltage, and membrane stiffness variations
Solution Approach 1:
The feedback circuit continuously monitors the differential output signal for asymmetry caused by manufacturing variations in capacitance, membrane stiffness, or bias voltage. When mismatch is detected, the feedback mechanism adjusts the bias voltage to compensate, thereby maintaining system linearity despite manufacturing tolerances. This dynamic compensation allows the system to achieve high linearity without requiring extremely tight manufacturing precision
Solution Approach 2:
The system performs self-diagnosis and self-correction by monitoring its own output asymmetry and automatically adjusting the bias voltage through the feedback loop. This self-service capability enables the differential MEMS device to compensate for its own manufacturing variations, maintaining linearity and sensitivity balance without external intervention or complex calibration procedures
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 effectively reduces signal asymmetry and distortion, improving the linearity and performance of MEMS microphones by compensating for amplitude, phase, and DC offset, thereby enhancing the overall signal quality.
Implementation Method 1
a common mode coupling circuit coupled to an output of the differential amplifier
Implementation Method 2
amplifier having an input coupled to an output of the common mode coupling circuit and an output configured to be AC coupled to a bias input node
Implementation Method 3
feedback the amplified common mode AC voltage to a bias input node of the differential MEMS device
Data Source
AI summary
In accordance with an embodiment, a circuit includes a differential amplifier having inputs configured to be coupled to an output of a differential microelectromechanical systems (MEMS) device; a common mode coupling circuit coupled to an output of the differential amplifier; and an amplifier having an input coupled to an output of the common mode coupling circuit and an output configured to be AC coupled to a bias input node of the differential MEMS device.