Signal Interface Circuit for MEMS Pressure Sensor
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Solution Overview
Problem
Conventional MEMS pressure sensors with Wheatstone bridge configurations face challenges in increasing sensitivity while maintaining robustness and linearity, as larger diaphragms enhance sensitivity but result in fragile devices with higher costs and degraded performance.
Innovation Solution
A pressure sensor system incorporating a multiple Wheatstone bridge configuration with an array of switched capacitors and a two-stage readout mechanism, where individual bridge output voltages are stored and then summed in a series chain to generate a readout voltage, enhancing sensitivity and robustness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the lateral dimensions of the diaphragm are increased to increase sensitivity, then device sensitivity is improved, but the device becomes more fragile and the die size increases leading to higher cost
Solution Approach 1:
The patent divides the sensing function into multiple independent Wheatstone bridge circuits (first bridge, second bridge, third bridge, fourth bridge) with separate piezoresistor sets. Each bridge operates independently and contributes to the overall sensitivity through signal combination, allowing the use of smaller, more robust diaphragm sections rather than requiring one large fragile diaphragm.
Solution Approach 2:
The patent combines the output signals from multiple independent Wheatstone bridge circuits through a signal combination circuit. By merging the differential voltage outputs from all four bridges, the system achieves enhanced overall sensitivity without requiring each individual bridge to be based on a large fragile diaphragm, thus resolving the contradiction between sensitivity and fragility.
2Measurement precision
If the lateral dimensions of the diaphragm are increased to increase sensitivity, then device sensitivity is improved, but the die size increases leading to higher cost
Solution Approach 1:
The patent segments the sensing area into multiple discrete piezoresistor locations distributed across the diaphragm, with each location forming an independent Wheatstone bridge. This segmentation allows the sensing function to be distributed over a larger area without requiring a single large diaphragm, thereby reducing the die size while maintaining sensitivity through multiple contribution points.
Solution Approach 2:
Instead of increasing sensitivity by expanding the diaphragm area in two dimensions, the patent adds a fourth dimension by creating multiple independent sensing channels (four separate Wheatstone bridges). This dimensional transition from single-area to multi-channel sensing achieves enhanced sensitivity without proportionally increasing the die footprint.
3Measurement precision
If the lateral dimensions of the diaphragm are increased to increase sensitivity, then device sensitivity is improved, but linearity performance is degraded
Solution Approach 1:
The patent segments the stress sensing function into multiple distributed piezoresistor sets, each experiencing localized stress changes. This segmentation allows each bridge to operate within a more linear stress range, and the combined output from multiple bridges maintains overall linearity while achieving high sensitivity through the cumulative effect of multiple independent sensing points.
Solution Approach 2:
The patent applies local quality by positioning piezoresistors at specific locations on the diaphragm where stress distribution is optimized. Each Wheatstone bridge is configured with piezoresistors at locations experiencing controlled stress changes, ensuring that each local sensing point operates within linear limits while the aggregate system achieves enhanced sensitivity.
4Measurement precision
If multiple Wheatstone bridges are used to increase sensitivity, then signal-to-noise performance is improved, but device complexity increases
Solution Approach 1:
The patent merges the output signals from multiple Wheatstone bridges through a signal combination circuit that integrates all bridge outputs into a single differential voltage signal. This merging approach enhances signal-to-noise performance by combining multiple sensing contributions while managing complexity through a unified readout mechanism rather than requiring separate processing paths for each bridge.
Solution Approach 2:
The patent implements a universal signal combination circuit that handles all four Wheatstone bridge outputs through a common readout mechanism. This multi-functional approach allows the system to process multiple sensing channels through a single integrated path, improving signal-to-noise performance while avoiding the complexity of separate dedicated circuits for each bridge.
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 approach improves sensor sensitivity and signal-to-noise performance while maintaining robustness to process variations, offering a cost-effective solution by integrating the sensor and ASIC portion directly.
Implementation Method 1
Conventional piezoresistive pressure sensors are formed by a Wheatstone bridge that includes four piezoresistors. These four piezoresistors are placed near the edge of a deformable membrane, i.e., a diaphragm, where the stress change is high under external pressure.
Implementation Method 2
Of the four piezoresistors, two are oriented to provide an increase in resistance when external pressure is applied to the diaphragm and two are oriented to provide a decrease in resistance under the same applied external pressure. Accordingly, the output of the Wheatstone bridge is a differential voltage that changes with external applied pressure.
Implementation Method 3
a deformable membrane, i.e., a diaphragm, where the stress change is high under external pressure
Data Source
Figure 1~2
Figure 3
Figure 4
AI summary
A sensor includes groups of sense elements coupled to one another to form multiple Wheatstone bridges, each being configured to produce an output voltage across first and second output nodes. A signal interface circuit for the sensor includes switched capacitor structures, one each of the switched capacitor structures being associated with one each of the Wheatstone bridges. Each switched capacitor structure includes a capacitor having first and second terminals, a first switch for selectively interconnecting the first node of an associated Wheatstone bridge with the first terminal of the capacitor, and a second switch for selectively interconnecting the second node of the associated Wheatstone bridge with the second terminal of the capacitor. A switch state element toggles the first and second switches between a charge state and a readout state to provide a readout voltage that is equivalent to a summation of the voltage outputs of the Wheatstone bridges.