Microcontroller Guard Ring Circuit for Parasitic Capacitance Reduction
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Solution Overview
Problem
Conventional capacitive sensors face challenges in high noise environments due to parasitic capacitance, which reduces the resolution and sensitivity of capacitive measurement systems, particularly in microcontrollers with capacitive touch detection capabilities.
Innovation Solution
A microcontroller system with a digital processor, sample and hold capacitors, and an analog-to-digital converter (ADC) is designed to include a guard ring and resistor network, along with programmable switches, to minimize parasitic capacitance by maintaining the same voltage potential on the guard ring as the capacitive sensor, thereby reducing noise interference and enhancing detection resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitive sensors are used in high noise environments, then basic touch detection is achieved, but parasitic capacitance reduces measurement resolution and sensitivity
Solution Approach 1:
The guard ring is driven to the same voltage potential as the capacitive sensor by connecting it through a resistor network to the sensor's voltage node. This equipotential connection eliminates voltage differences between the sensor and adjacent conductors, thereby minimizing parasitic capacitance effects and improving measurement resolution in noisy environments.
Solution Approach 2:
A resistor network is introduced as an intermediary element between the digital output drivers and the guard ring. This resistor network allows digital outputs to control the guard ring voltage while minimizing direct capacitive coupling, thus reducing parasitic capacitance interference while maintaining the ability to drive the guard ring to the appropriate voltage potential.
2Measurement precision
If a guard ring is added to reduce parasitic capacitance, then measurement sensitivity improves, but device complexity increases
Solution Approach 1:
Existing digital output drivers within the microcontroller are utilized to drive the guard ring, making these drivers serve dual purposes: controlling external outputs and managing the guard ring voltage. This multi-functionality approach reduces the need for additional dedicated hardware components, thereby limiting the increase in device complexity while still achieving improved measurement sensitivity.
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 solution effectively reduces parasitic capacitance, improving the resolution and sensitivity of capacitive measurements, and provides enhanced noise shielding without compromising detection accuracy in high noise environments.
Implementation Method 1
A first charge is settled between the sample and hold capacitor and the capacitive sensor
Implementation Method 2
One technique utilizes capacitive voltage division (CVD) to evaluate whether a capacitive touch element has been touched or not
Implementation Method 3
parasitic capacitance may pose a problem in many capacitive sensor applications. Parasitic capacitance is generated whenever a conductor adjacent to the sensor (or its connection to the microcontroller) is at a different voltage potential then the sensor
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
A guard ring (320) is provided around each capacitive sensor plate and charged to substantially the same voltage as a voltage on the capacitive sensor plate. The guard ring reduces parasitic capacitances of the capacitive sensor plate caused by differences in voltage potentials between the capacitive sensor plate, and adjacent circuit conductors, ground planes and power planes. Two digital outputs (724, 728) and associated voltage divider resistors (708, 710) are used to drive the guard ring voltage to substantially the same voltage as the voltage on the capacitive sensor plate.