Capacitance Measurement Circuit With Switched Sensor Charging
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Solution Overview
Problem
Capacitive sensors face challenges in achieving accurate and noise-resistant capacitance measurements, particularly in determining object position and background capacitance, due to noise from ambient temperature changes and surface contaminants.
Innovation Solution
The system employs a current source to charge capacitors until a settling voltage is reached, with the sensor capacitor alternately switched between the current source and ground to reduce noise, and a counter measures the time for the voltage to reach a reference voltage, using a comparator and low-pass filter to enhance sensitivity and resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the sensor capacitor is continuously connected to the current source for charging, then the capacitance measurement can be performed, but noise from ambient temperature changes and surface contaminants degrades measurement accuracy
Solution Approach 1:
The sensor capacitor is alternately switched between the current source and ground in periodic cycles. During the charging phase, the capacitor is connected to the current source to accumulate charge. During the discharge phase, it is connected to ground to release charge. This periodic switching reduces the impact of noise from ambient temperature changes and surface contaminants by limiting the exposure time of the capacitor to noisy environments while still enabling capacitance measurement through timing the charging duration.
2Measurement precision
If the settling voltage is set close to the reference voltage to increase dynamic range, then measurement resolution is improved, but the measurement time increases
Solution Approach 1:
The system dynamically adjusts the reference voltage level based on the expected capacitance range. By calibrating the reference voltage to be just above the expected settling voltage, the system maximizes the dynamic range for capacitance measurement. This parameter optimization ensures that small capacitance changes produce measurable voltage differences, improving resolution without requiring excessively long charging times. The counter measures the time to reach this optimized reference voltage, balancing resolution and measurement speed.
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 results in high sensitivity and high resolution capacitance measurements, allowing precise detection of objects and their positions, while accounting for background capacitance and noise factors.
Implementation Method 1
a current source charges a first capacitor (e.g., a sensor capacitor) and a second capacitor (e.g., an internal capacitor) until voltages at the capacitors equilibrate at a settling voltage
Implementation Method 2
voltages at the capacitors equilibrate at a settling voltage
Implementation Method 3
Switching of the first (e.g., sensor) capacitor reduces the outside noise sources on that capacitor that could inadvertently couple into the system
Implementation Method 4
a low pass filter is coupled between the capacitor(s) and the comparator to reduce the effect of high frequency noise
Implementation Method 5
a counter counts the number of cycles generated by an oscillator as the voltage increases from the settling voltage to the reference voltage
Data Source
AI summary
A first capacitor and a second capacitor are charged until voltage at the second capacitor settles to a settling voltage. While charging, the first capacitor is alternately switched between a current source and ground. When the settling voltage is reached, charging of the first capacitor is halted. The second capacitor continues to be charged until voltage at the second capacitor reaches a reference voltage. The amount of time it takes for the settling voltage to reach the reference voltage corresponds to a measure of capacitance on the first capacitor.