MEMS Microphone Capacitance Linearization for HD2 Reduction
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Solution Overview
Problem
Single-ended MEMS microphones suffer from prominent second-order harmonic distortion (HD2) at certain volume levels and operating frequencies, which is related to the total equivalent parasitic capacitance at the connection node between the MEMS and its readout Application-Specific Integrated Circuit (ASIC).
Innovation Solution
An extra non-linear capacitor is added at the interface between the MEMS and ASIC to cancel out the MEMS intrinsic HD2 by maintaining a constant charge transfer, using a nonlinear capacitance component to compensate for the parasitic capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a constant-charge readout approach is used in single-ended MEMS microphones, then the circuit simplicity is maintained, but prominent second-order harmonic distortion (HD2) occurs at low-mid input sound pressure levels
Solution Approach 1:
A nonlinear capacitance component is introduced as an intermediary element between the MEMS device and the readout circuit. This component has a specific capacitance-to-voltage (CV) profile that counteracts the intrinsic nonlinearity of the MEMS device, thereby reducing HD2 distortion without requiring complex circuit reconfiguration
Solution Approach 2:
The capacitance value of the nonlinear capacitance component is specifically designed to vary with voltage according to a predetermined CV profile. This parameter change enables the component to dynamically compensate for the MEMS device's nonlinearity across different operating conditions, effectively reducing HD2 distortion
2Object-generated harmful factors
If the total equivalent parasitic capacitance at the connection node is reduced, then HD2 distortion is minimized, but additional compensation circuitry is required
Solution Approach 1:
The nonlinear capacitance component serves as a practical intermediary solution that provides HD2 compensation through its voltage-dependent capacitance characteristic, avoiding the need for complex active compensation circuits while effectively reducing distortion
Solution Approach 2:
The patent replaces complex active compensation mechanisms with a passive nonlinear capacitance component whose inherent physical characteristic (voltage-dependent capacitance) provides the desired compensation effect, simplifying the overall system
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 harmonic distortion is significantly reduced, improving system linearity by up to 20 dB, as demonstrated by system simulations.
Implementation Method 1
a first nonlinear equivalent capacitance having a first capacitance-to-voltage (CV) profile; and a nonlinear capacitance component coupled to the first node, the second node, and at least one reference voltage node, wherein the nonlinear capacitance component comprises a second nonlinear equivalent capacitance having a second CV profile
Data Source
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Figure 3A
AI summary
A microphone includes a microelectromechanical system (MEMS) device responsive to sound waves or vibrations having an output coupled to a first node; a programmable gain amplifier or source follower having an input coupled to a second node, and an output for generating an analog signal, wherein the MEMS device output and the programmable gain amplifier or source follower input comprise a first nonlinear equivalent capacitance having a first capacitance-to-voltage (CV) profile; and a nonlinear capacitance component coupled to the first node, the second node, and at least one reference voltage node, wherein the nonlinear capacitance component comprises a second nonlinear equivalent capacitance having a second CV profile.