Chained Wheatstone Bridge Pressure Sensor
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Solution Overview
Problem
Conventional MEMS pressure sensors with Wheatstone bridge configurations face challenges in achieving high sensitivity while maintaining cost-effectiveness and linearity, as increasing diaphragm size enhances sensitivity but leads to fragility, larger size, and degraded performance.
Innovation Solution
A multiple Wheatstone bridge configuration where the outputs are directly chained, allowing the Wheatstone bridges to float with current sources, resulting in a composite output with enhanced sensitivity and improved signal-to-noise ratio, achieved by interconnecting the negative output node of one bridge with the positive output node of the next, effectively combining differential voltage outputs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the lateral dimensions of the diaphragm are increased to enhance sensitivity, then the sensitivity and resolution are improved, but the die becomes more fragile, the die size increases leading to higher cost, and the linearity performance degrades
Solution Approach 1:
The patent divides the sensing element into multiple Wheatstone bridge circuits (e.g., four bridges) instead of using a single large diaphragm. Each bridge contains piezoresistors positioned at specific locations on the diaphragm. This segmentation allows the system to achieve high sensitivity through multiple measurement points while maintaining a smaller, more robust diaphragm structure, thus resolving the contradiction between sensitivity and die fragility.
Solution Approach 2:
The patent combines the outputs of multiple Wheatstone bridges by chaining them together (connecting the negative output of one bridge to the positive output of the next). This merging of multiple bridge outputs effectively multiplies the voltage output and enhances sensitivity without requiring a larger diaphragm, thereby improving sensitivity while maintaining die strength and reducing cost.
2Measurement precision
If the lateral dimensions of the diaphragm are increased to enhance sensitivity, then the sensitivity and resolution are improved, but the die size increases leading to higher cost
Solution Approach 1:
The patent uses multiple compact Wheatstone bridge circuits distributed across the diaphragm surface, each with its own set of piezoresistors. This segmentation enables high sensitivity measurements without requiring a large continuous sensing area, thus achieving enhanced sensitivity with a smaller overall die size and lower manufacturing cost.
Solution Approach 2:
Instead of increasing the lateral dimensions of a single diaphragm area, the patent distributes multiple sensing elements (Wheatstone bridges) across the diaphragm surface. This spatial distribution in multiple dimensions allows the system to achieve high sensitivity equivalent to a much larger single bridge while maintaining a compact die footprint.
3Measurement precision
If the lateral dimensions of the diaphragm are increased to enhance sensitivity, then the sensitivity and resolution are improved, but the linearity performance degrades
Solution Approach 1:
The patent divides the sensing function into multiple independent Wheatstone bridge circuits, each measuring pressure at its specific location. By distributing piezoresistors at strategically positioned high-stress regions across the diaphragm and combining their outputs, the system achieves high sensitivity while each individual bridge maintains good linearity, thus resolving the contradiction between sensitivity enhancement and linearity preservation.
4Measurement precision
If multiple Wheatstone bridges are used with chained outputs to enhance sensitivity, then the voltage output is multiplied and signal-to-noise ratio is improved, but the device complexity increases
Solution Approach 1:
The patent combines multiple Wheatstone bridge outputs by directly chaining them together, where the negative output of one bridge is connected to the positive output of the next. This merging approach multiplies the voltage output and improves signal-to-noise ratio while using a relatively simple interconnection scheme, thus achieving enhanced measurement precision with moderate increase in device complexity.
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 configuration enhances sensitivity by multiplying the voltage output and improves signal-to-noise ratio, while reducing noise and offset errors, thereby achieving better performance with potential cost savings.
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.
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
A pressure sensor includes a diaphragm suspended across a cavity in a substrate. A first group of piezoresistors is provided in the diaphragm, the piezoresistors of the first group being coupled to one another to form a first Wheatstone bridge having first positive and negative output nodes. A second group of piezoresistors is provided in the diaphragm, the piezoresistors of the second group being coupled to one another to form a second Wheatstone bridge having second positive and negative output nodes. The first negative output node of the first Wheatstone bridge is electrically connected to the second positive output node of the second Wheatstone bridge to directly chain the outputs of the Wheatstone bridges. The first and second Wheatstone bridges each produce an output signal as a function of an external pressure stimulus that is combined via the chained arrangement of the Wheatstone bridges to produce a composite output signal.