Capacitive Sensor Compensation Circuit for Parasitic Capacitance
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Solution Overview
Problem
Capacitive sensor devices face challenges in resolving small changes in capacitance due to the dominance of parasitic and mutual capacitances, leading to impaired ability to detect inputs such as finger proximity or contact, as these changes are masked by larger baseline capacitance values, reducing the dynamic range of capacitance sensing systems.
Innovation Solution
A compensation circuit is implemented to minimize the effects of parasitic and mutual capacitances by supplying a calibrated compensation current that cancels out the baseline capacitance, allowing for a greater number of discrete levels to represent the dynamic range of capacitance changes, thereby improving the resolution of capacitance sensing systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance sensing is used, then the system can detect capacitance changes, but the resolution is impaired due to dominant parasitic and mutual capacitances masking small capacitance changes
Solution Approach 1:
The patent extracts and separates the parasitic capacitance component from the total measured capacitance. By measuring the parasitic capacitance independently (when no input is present) and subtracting it from the total capacitance measurement, the system isolates the small capacitance changes caused by input objects, thereby improving measurement resolution
Solution Approach 2:
The patent applies preliminary anti-action by pre-measuring and storing the parasitic capacitance value before actual sensing operations. This pre-measured parasitic capacitance is then used to compensate for its masking effect during subsequent measurements, effectively counteracting its harmful influence on detection resolution
2Loss of information
If the sensor capacitance is represented as a digital code, then the parasitic capacitances occupy a larger proportion of discrete capacitance levels, but this reduces the number of levels available to represent small capacitance changes
Solution Approach 1:
The patent extracts the parasitic capacitance contribution from the total capacitance measurement in the digital domain. By converting the measured capacitance to a digital code and subtracting the pre-stored parasitic capacitance code, the system recovers the digital representation of only the input-induced capacitance changes, increasing the effective use of discrete levels
Solution Approach 2:
The patent changes the reference parameter by using the measured parasitic capacitance value as a dynamic offset. This parameter change allows the system to adjust the baseline from which capacitance changes are measured, effectively scaling small capacitance variations to utilize the full input range of the digital code
3Measurement precision
If a compensation circuit is added to cancel baseline capacitance, then the dynamic range utilization improves, but the device complexity increases
Solution Approach 1:
The patent performs preliminary action by pre-measuring and storing the parasitic capacitance value in memory during system initialization or calibration. This pre-stored value is then used during normal operation to compensate for parasitic effects without requiring complex real-time compensation circuits, thereby improving precision while minimizing added complexity
Solution Approach 2:
The patent implements feedback by using the measured parasitic capacitance value to generate a compensation signal that is subtracted from subsequent measurements. This feedback mechanism continuously corrects for parasitic effects, improving measurement precision through a relatively simple computational approach rather than complex hardware circuits
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 enhances the dynamic range utilization of capacitance sensing systems, enabling more precise detection of input changes, such as finger proximity, by effectively scaling small capacitance variations to the full input range, thereby improving the accuracy and resolution of capacitive sensing devices.
Implementation Method 1
A compensation circuit is implemented to minimize the effects of parasitic and mutual capacitances by supplying a calibrated compensation current that cancels out the baseline capacitance
Implementation Method 2
The capacitance, detected by a capacitance sensor, changes as a function of the proximity of a conductive object to the sensor
Data Source
AI summary
A capacitive sensor may include a transmit electrode and a receive electrode capacitively coupled with the transmit electrode. A capacitance sensing circuit senses a capacitance between the transmit and receive electrodes by applying a signal to the transmit electrode and rectifying a signal induced at the receive electrode. A compensation circuit reduces the effect of a mutual and parasitic capacitances of the transmit and receive electrode pair by adding a compensation signal to the rectified signal.