Capacitance Measurement Circuit With Two-Phase Sensor Charging
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Solution Overview
Problem
Existing capacitive sensor systems face challenges in accurately measuring capacitance due to interference from parasitic capacitive elements and variations in supply voltage, which affect the reliability and precision of touch or proximity sensing.
Innovation Solution
A capacitive sensor system operates in a charging mode and a transferring mode, using a switching unit to control voltage levels and a current mirror to discharge and transfer charges to a sample capacitor, with voltage references based on a bandgap voltage reference for stability, thereby reducing interference and enhancing measurement precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional capacitive sensor system charges the capacitive sensor from one end only, then the charging process is simple, but measurement precision deteriorates due to interference from parasitic capacitive elements and supply voltage variations
Solution Approach 1:
The charging process is segmented into two distinct phases: first phase charges the capacitive sensor from the first end to a first predetermined voltage, second phase raises the voltage from the first predetermined voltage to a second predetermined voltage after disconnecting the first end from the first supply voltage. This segmentation eliminates interference from parasitic capacitive elements and supply voltage variations, significantly improving capacitance measurement precision.
Solution Approach 2:
The first phase performs a preliminary charging action by charging the capacitive sensor from the first end to a first predetermined voltage before the second phase raises the voltage to a second predetermined voltage. This preliminary action prepares the capacitive sensor in a controlled state, isolating it from parasitic elements during the critical measurement phase.
2Reliability
If the system uses a single supply voltage for charging, then the system is simple to operate, but reliability deteriorates due to supply voltage variations affecting measurement accuracy
Solution Approach 1:
The system changes the voltage parameter in two distinct steps: first charging to a first predetermined voltage, then raising to a second predetermined voltage. This parameter change approach eliminates the influence of supply voltage variations on the capacitance measurement, improving measurement reliability while maintaining straightforward operational control through automated phase switching.
3Measurement precision
If the capacitive sensor is charged continuously from the first supply voltage, then the charging process is continuous and simple, but measurement precision deteriorates due to interference from parasitic capacitive elements
Solution Approach 1:
The charging process is performed periodically in two distinct phases: first phase charges the capacitive sensor from the first end, second phase raises the voltage after disconnecting the first end. This periodic action with clear separation between phases eliminates continuous interference from parasitic capacitive elements and supply voltage variations, improving measurement precision while maintaining efficient charging duration.
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
The system provides stable and precise capacitance measurements by minimizing interference from parasitic elements and supply voltage variations, ensuring accurate touch or proximity sensing across varying temperatures and processes.
Implementation Method 1
a current mirror that discharges the capacitive sensor with a discharging current until a voltage at the first end of the capacitive sensor decreases to a first predetermined voltage and transfers a number of electric charges to a sample capacitor by mirroring the discharging current
Implementation Method 2
voltage references based on a bandgap voltage reference for stability
Data Source
AI summary
A system operable between a charging mode and a transferring mode for measuring capacitance of a capacitive sensor, includes a switching unit configured to, in a first phase of the charging mode, arrange the capacitive sensor to be charged by a first supply voltage from a first end of the capacitive sensor until a voltage difference between the first end and an opposite second end of the capacitive sensor reaches a first predetermined voltage, and in a second phase of the charging mode, disconnect the first end of the capacitive sensor from the first supply voltage and couple the second end of the capacitive sensor (10) to a second supply voltage to raise a voltage at the first end of the capacitive sensor to a second predetermined voltage.