Differential MEMS Sensor Amplifier Layout for Common-Mode Noise
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Solution Overview
Problem
Conventional MEMS microphone amplifier circuits face challenges in rejecting common-mode disturbances, managing power consumption, and reducing total harmonic distortion, particularly due to single-ended layouts and conflicting requirements between power consumption and distortion.
Innovation Solution
A fully differential circuit layout is implemented, featuring a capacitive microphonic sensor in a differential configuration with a feedback network of resistors and an additional amplifier stage to address common-mode noise and distortion, while maintaining low power consumption and output headroom.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-ended layout is used in the amplifier circuit, then the device complexity is reduced, but the common-mode noise rejection capability deteriorates
Solution Approach 1:
The amplifier circuit is divided into two symmetrical amplifier stages (first and second amplifier stages) that process differential signals. Each stage handles one side of the differential pair, segmenting the signal path to enable common-mode rejection while maintaining manageable complexity through modular symmetry.
Solution Approach 2:
The patent employs a fully differential configuration where the two amplifier stages are symmetrically designed but process opposite-phase signals. This controlled asymmetry in signal polarity (one in-phase, one out-of-phase) enables common-mode noise cancellation while the symmetric hardware layout maintains design simplicity.
2Use of energy by moving object
If power consumption is reduced in the amplifier circuit, then energy efficiency is improved, but total harmonic distortion increases
Solution Approach 1:
A feedback network comprising first and second resistors is implemented to provide negative feedback from the output to the input stages. This feedback mechanism linearizes the amplifier operation, reducing total harmonic distortion while the efficient differential architecture maintains low power consumption through balanced signal paths.
Solution Approach 2:
The circuit utilizes parameter optimization in the amplifier stages and feedback network to achieve low distortion at reduced power levels. By carefully selecting operating points and component values (resistor ratios, capacitor values), the circuit maintains high linearity without requiring excessive power dissipation.
3Object-affected harmful factors
If a fully differential configuration is implemented, then common-mode noise rejection is improved, but device complexity increases
Solution Approach 1:
The fully differential configuration uses symmetric hardware layout with two matched amplifier stages, but introduces controlled signal asymmetry through differential signaling. This approach achieves common-mode rejection through the signal domain rather than requiring complex asymmetric circuit topologies.
Solution Approach 2:
The differential configuration establishes equipotential virtual ground nodes at the inputs of both amplifier stages, creating balanced signal paths that naturally reject common-mode disturbances. This virtual ground technique simplifies the layout by providing stable reference points without requiring additional physical grounding complexity.
4Use of energy by moving object
If output headroom is maintained at low levels, then power consumption is reduced, but signal amplitude capability is limited
Solution Approach 1:
The feedback network parameters (resistor values and ratios) are optimized to provide sufficient gain while maintaining low output voltage levels. By adjusting the feedback factor and amplifier gain parameters, the circuit achieves adequate signal amplitude for downstream processing without requiring high power dissipation or large output headroom.
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 fully differential configuration effectively reduces common-mode noise and total harmonic distortion, enhancing the overall performance of the MEMS microphone amplifier circuit by providing improved signal rejection and distortion management.
Implementation Method 1
The microphone may include a variable capacitance with a fixed charge and a plate, called membrane, capable of bending when sound pressure is incident on the plate. In the presence of a charge, which is fixed, the variation in capacitance generates a voltage across the sensor.
Data Source
AI summary
A circuit for amplifying signals from a Micro Electro-Mechanical System (MEMS) capacitive sensor is provided. First and second input nodes receive a sensing signal applied differentially between the input nodes. A first amplifier stage and a second amplifier stage, respectively, produce a differential output signal between first and second output nodes. A common mode signal is detected at the output nodes. A voltage divider having an intermediate tap node is coupled between the first output node and the second output node. A feedback stage is coupled between the intermediate tap node of the voltage divider and the inputs of the first amplifier stage and the second amplifier stage, where the feedback line is sensitive to the common mode signal at the output nodes.