Charge Removal Circuit Parasitic Capacitance Cancellation
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Solution Overview
Problem
Touch-sensitive capacitive matrices face noise issues due to parasitic capacitance, which affects signal accuracy and dynamic range in detecting touch events.
Innovation Solution
Incorporating current sources and charge removal circuits to cancel parasitic capacitance by applying a current for a predetermined time, and using switched-capacitive supply circuitry to mitigate clock jitter noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If parasitic capacitance is present in the capacitive matrix, then the device complexity is reduced, but the measurement precision deteriorates due to noise in the signals
Solution Approach 1:
The patent applies preliminary action by performing charge removal before the actual capacitive measurement. A current source is activated to remove parasitic charge from the sense line capacitance prior to measuring the touch-induced capacitance change. This ensures that the measurement reflects only the touch signal rather than being contaminated by residual parasitic charge, thereby improving signal accuracy without requiring complex shielding or isolation structures.
Solution Approach 2:
The patent extracts the parasitic capacitance effect from the measurement system by introducing a dedicated charge removal mechanism. Instead of trying to eliminate parasitic capacitance physically, the invention actively removes its accumulated charge through a controlled current source that drains the parasitic capacitance of its stored charge before measurement begins. This separates the parasitic effect from the useful signal in a controlled manner.
2Measurement precision
If current sources are used to remove parasitic charge, then the measurement precision improves, but the device complexity increases due to additional circuit components
Solution Approach 1:
The patent applies universality by designing the charge removal circuit to serve multiple functions. The same current source circuit that removes parasitic charge also helps establish a known initial state for the capacitance measurement and can be integrated with the existing drive and sense circuitry of the capacitive matrix. This multi-functional approach reduces the net increase in device complexity while maintaining improved measurement precision.
Solution Approach 2:
The patent implements feedback by using the output amplifier to monitor the sense line voltage and control the current source accordingly. The feedback mechanism ensures that the charge removal process is precisely controlled and that the sense line is properly initialized before each measurement cycle. This feedback control optimizes the signal-to-noise ratio while keeping the circuit implementation efficient.
3Measurement precision
If charge removal is performed for a predetermined time, then the measurement precision improves, but the duration of action increases, reducing productivity
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the charge removal time based on the operational requirements. The predetermined time parameter is optimized to be sufficient to remove parasitic charge but not excessively long to significantly delay measurements. The system can adjust this timing parameter based on factors such as the magnitude of parasitic capacitance, the required measurement precision, and the overall measurement cycle speed, thereby balancing precision and productivity.
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
Improves signal-to-noise ratio and dynamic range of capacitive measurements, enhancing the accuracy of touch detection by effectively removing unwanted parasitic charge and reducing noise from clock jitter.
Implementation Method 1
Parasitic capacitances may exist for each pixel element and/or sense line in a touch-sensitive capacitive matrix
Implementation Method 2
applying a current from the current source to the sense channel for a predetermined amount of time so as to cancel a parasitic charge associated with the parasitic capacitance
Data Source
AI summary
Apparatus and methods to measure capacitance changes for a touch-sensitive capacitive matrix are described. Charge-removal circuits and measurement techniques may be employed to cancel deleterious effects of parasitic capacitances in the touch-sensitive capacitive matrix. Capacitively switching a supply during timed charge removal may be used to cancel unwanted effects due to clock jitter. The apparatus and methods can improve signal-to-noise characteristics, sensitivity, and/or dynamic range for capacitive measurements relating to touch-sensitive capacitive devices.