Capacitance Measurement Circuit with Differential Amplifier Feedback
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Solution Overview
Problem
Conventional capacitance sensing methods, such as transcapacitive and absolute capacitance sensing, face inefficiencies in measurement cycles and interference susceptibility, particularly in touch-based input devices, leading to reduced accuracy and increased sensing time.
Innovation Solution
A hybrid capacitive sensing method utilizing a differential amplifier with a feedback capacitance and modulated reference voltage, allowing for a reduced sensing cycle and increased transmitter signal frequency, which decouples the sensor electrode during the reset phase and balances voltages to measure absolute capacitance efficiently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional transcapacitive or absolute capacitance sensing methods are used, then capacitance measurement can be performed, but the measurement cycle is inefficient and sensing time is increased
Solution Approach 1:
The patent implements periodic switching between two sensing modes (transcapacitive and absolute capacitance sensing) in alternating measurement cycles. The system switches modes based on the presence or absence of a reference electrode, enabling efficient measurement by selecting the appropriate mode for each phase of the measurement cycle, thereby reducing overall sensing time while maintaining measurement accuracy.
2Measurement precision
If conventional capacitance sensing methods are used, then capacitance can be measured, but the system is susceptible to interference
Solution Approach 1:
The patent employs feedback mechanisms where the system continuously monitors the measurement environment and adjusts the sensing mode accordingly. When interference is detected or the reference electrode is absent, the system switches between transcapacitive and absolute capacitance sensing modes to maintain measurement accuracy, effectively compensating for harmful interference factors.
3Ease of operation
If the sensor electrode is continuously coupled to the differential amplifier, then measurement can be performed, but the reset phase cannot be initiated
Solution Approach 1:
The patent segments the measurement process into distinct phases (reset phase and measurement phase) using a switch that couples or decouples the sensor electrode from the differential amplifier. This segmentation allows the system to perform initialization and calibration during the reset phase when the electrode is decoupled, and then perform accurate measurement during the measurement phase when coupled, effectively managing the trade-off between operational ease and 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 approach enhances the signal-to-noise ratio, increases proximity sensing distance and accuracy, and coordinates interference avoidance for both transcapacitive and absolute capacitance sensing modes, enabling faster and more accurate capacitance measurements.
Implementation Method 1
A feedback capacitance disposed between the second input and the output is reset to a first level of charge
Implementation Method 2
the differential amplifier is utilized to integrate charge on the sensor electrode, such that an absolute capacitance is measured
Data Source
AI summary
A capacitance measurement circuit comprises a differential amplifier with first and second inputs and an output, first and second feedback capacitances, and a reset mechanism. The first input is coupled to a modulated reference voltage and the second input is coupled with a sensor electrode. A first feedback capacitance is coupled between the output and the second input. A second feedback capacitance is coupled between the output and the second input. The reset mechanism resets the first feedback capacitance to a first level of charge and the second feedback capacitance to a second level of charge. During an absolute capacitance measurement phase, the differential amplifier charges the sensor electrode while balancing voltages on the first and second inputs to a voltage level associated with the modulated reference voltage and integrates charge on the sensor electrode to measure capacitance corresponding to a coupling between the sensor electrode and an input object.