An interface circuit for a capacitive accelerometer sensor
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Solution Overview
Problem
Existing capacitive accelerometer sensor interface circuits face challenges in simultaneously achieving fine grain size programmability and range trimming for the feedback capacitor, particularly in accommodating different acceleration input ranges, due to the difficulty in managing parasitic capacitance and implementing programmability in small, floating capacitors.
Innovation Solution
The interface circuit employs three programmable capacitors, with two floating capacitors for acceleration range selection and a grounded capacitor for precise gain trimming, distributing the programmability burden and simplifying the implementation of both tasks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single feedback capacitor is used for both gain trimming and range selection, then the circuit structure is simple, but it becomes difficult to achieve both fine grain size programmability and range trimming simultaneously
Solution Approach 1:
The single feedback capacitor is segmented into two separate capacitors: a first feedback capacitor connected between the amplifier output and the first input terminal, and a second feedback capacitor connected between the amplifier output and the second input terminal. This segmentation allows each capacitor to be optimized for specific functions (one for gain trimming with fine programmability, the other for range selection), thereby resolving the contradiction between structural simplicity and functional versatility.
2Adaptability or versatility
If the feedback capacitor is made small to accommodate different acceleration ranges, then the range selection capability is improved, but the impact of parasitic capacitance increases and programmability becomes difficult
Solution Approach 1:
By segmenting the feedback path into two separate capacitors, the design can use a larger first feedback capacitor for gain trimming (reducing parasitic capacitance impact) and a smaller second feedback capacitor for range selection. This segmentation allows each capacitor to be sized appropriately for its specific function, mitigating the parasitic capacitance issue while maintaining range selection capability.
Solution Approach 2:
Different capacitors are used for different functional requirements: the first feedback capacitor is optimized for gain trimming with fine programmability and larger capacitance value to minimize parasitic effects, while the second feedback capacitor is optimized for range selection with smaller capacitance value. This local optimization of capacitor properties at different locations in the feedback path resolves the contradiction between range selection and parasitic capacitance impact.
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 allows for easier and more precise gain trimming and range selection, enabling effective accommodation of various acceleration ranges with improved programmability, reducing the complexity of implementing small capacitance values and minimizing the impact of parasitic capacitance.
Implementation Method 1
In capacitive MEMS accelerometers, a change of the MEMS capacitance in presence of the acceleration generates the electrical signal
Implementation Method 2
capacitive sensing techniques... converts this acceleration into electrical currents or voltages
Data Source
Figure 1a~1b
Figure 2~3
Figure 4~5
AI summary
The present invention relates to an interface circuit (5) for a capacitive accelerometer sensor (3) for measuring an acceleration value sensed by the sensor (3). The interface circuit (5) comprises a plurality of electrical switches (S1 - S7) and three programmable capacitors (Cf, Cref, Cp). Two of the programmable capacitors (Cf, Cref) are arranged to implement gain trimming of the interface circuit (5), while one of the programmable capacitors (Cp) is arranged to implement acceleration range selection.