MEMS Microphone Compensation Loop for Channel Mismatch 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 overall MEMS sensitivity and performance, then sensitivity and performance are improved, but sensitivity mismatch between channels increases leading to signal distortion and poor total harmonic distortion
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. 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 prevent distortion
Solution Approach 2:
The patent dynamically adjusts the bias voltage parameter based on the detected asymmetry in the differential output. By changing the bias voltage in response to measured performance degradation, the system optimizes the trade-off between sensitivity (which requires higher voltage) and linearity (which degrades at higher voltages due to mismatch)
2Stability of the object's composition
If differential MEMS device is used to achieve high system linearity, then linearity is improved, but sensitivity mismatch between channels due to capacitance, bias voltage, and membrane stiffness causes signal distortion
Solution Approach 1:
The feedback loop continuously monitors the differential output for asymmetry indicators and automatically adjusts the bias voltage to compensate for manufacturing mismatches. This allows the system to maintain high linearity while tolerating manufacturing variations in capacitance, membrane stiffness, and other parameters
Solution Approach 2:
The system performs self-compensation by using its own output signal to detect asymmetry and automatically adjusting its operating parameters. The differential MEMS device monitors itself and corrects its own performance degradation without external intervention, making the system robust against manufacturing variations
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 issues, particularly at high sound levels.
Implementation Method 1
sensing a pressure difference using a movable membrane and sensing a change in capacitance between the movable membrane an a fixed backplate
Implementation Method 2
amplifier having an input coupled to an output of the common mode coupling circuit
Implementation Method 3
feeding back the amplified common mode AC voltage to a bias input node of the differential MEMS device
Data Source
Figure 1A
Figure 1B~1C
Figure 2A
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.