Fully Differential MEMS Accelerometer Common Mode Rejection
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Solution Overview
Problem
Existing MEMS accelerometers face challenges in accurately sensing multi-axis accelerations due to common mode signal interference and increased complexity in sensing circuitry, particularly when using single proof masses and constant voltage shielding, which fails to mitigate parasitic coupling and crosstalk.
Innovation Solution
A multiple-mass, multi-axis MEMS accelerometer with a fully differential sensing design that employs two proof masses arranged side-by-side, applying differential drive signals and combining capacitance signals from sense fingers to cancel out common mode signals and reduce noise, while using a split shield for separate shielding of proof masses and sense fingers to minimize electrostatic interactions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single proof mass with constant voltage shielding is used, then the device structure is simpler, but common mode signal interference and parasitic coupling increase
Solution Approach 1:
The single proof mass is divided into two separate proof masses (first and second proof masses) that are differentially driven. This segmentation allows independent control of each mass, enabling common mode rejection while reducing parasitic coupling effects on the substrate.
Solution Approach 2:
The two proof masses are configured as counterweights that move in opposite directions when differential acceleration is applied. This anti-weight configuration naturally cancels common mode signals and reduces net parasitic coupling to the substrate, improving measurement precision.
2Measurement precision
If separate shielding for proof masses and sense fingers is implemented, then electrostatic interactions are minimized, but device complexity increases
Solution Approach 1:
The shielding structure is segmented into separate shields for the first proof mass, second proof mass, and sense fingers. Each shield is independently controllable, allowing optimization of electrostatic shielding without requiring a single complex unified shield structure.
Solution Approach 2:
Shielding structures are introduced as intermediary elements between the proof masses and the substrate, and between sense fingers and the substrate. These intermediaries reduce direct electrostatic interactions and parasitic coupling, improving signal-to-noise ratio.
3Measurement precision
If differential drive signals are applied to multiple proof masses, then common mode signals are attenuated, but sensing circuitry complexity increases
Solution Approach 1:
The sensing circuitry for multiple proof masses is merged into a unified capacitive sensing system. Capacitance signals from sense fingers associated with different proof masses are combined and processed together, simplifying the overall sensing circuitry while maintaining common mode rejection through differential operation.
Solution Approach 2:
The sense fingers are designed with multi-functionality, serving both to sense motion of proof masses and to provide capacitive coupling for differential signal detection. This universal design reduces the need for separate sensing circuits for each proof mass.
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
This design effectively attenuates common mode signals, simplifies sensing circuitry, reduces noise, and enhances accuracy in multi-axis acceleration measurements by eliminating the need for compensation circuits and improving system noise performance.
Implementation Method 1
a first plurality of sense fingers coupled to the substrate, capacitively coupled with the first and second proof masses, and configured to sense X-direction movement of the first and second proof masses
Implementation Method 2
a split shield may be provided, with a first shield underneath a proof mass coupled to the same drive signal applied to the proof mass and a second shield electrically isolated from the first shield provided underneath the sense fingers and biased with a constant voltage to provide shielding for the sense fingers
Data Source
AI summary
Disclosed herein are aspects of a multiple-mass, multi-axis microelectromechanical systems (MEMS) accelerometer sensor device with a fully differential sensing design that applies differential drive signals to movable proof masses and senses differential motion signals at sense fingers coupled to a substrate. In some embodiments, capacitance signals from different sense fingers are combined together at a sensing signal node disposed on the substrate supporting the proof masses. In some embodiments, a split shield may be provided, with a first shield underneath a proof mass coupled to the same drive signal applied to the proof mass and a second shield electrically isolated from the first shield provided underneath the sense fingers and biased with a constant voltage to provide shielding for the sense fingers.