Projected Capacitive Touch Sensor Circuit Parasitic Capacitance
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Solution Overview
Problem
Existing projected capacitive touch sensors face challenges in accurately determining touch locations due to large parasitic capacitances, which require multiple integration cycles, increasing the time to make a determination and affecting user experience, especially in larger displays with numerous electrodes.
Innovation Solution
A circuit that continuously integrates the sense signal and samples both the maximum and minimum values of the output signal prior to the falling and rising edges of the drive pulse, respectively, allowing for the determination of mutual capacitance without analog switching, and uses computational logic to calculate the difference between these values for accurate capacitance measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple integration cycles are performed to overcome parasitic capacitance effects, then measurement precision is improved, but loss of time increases
Solution Approach 1:
The patent applies periodic action by using a drive pulse signal with periodic square wave transitions to excite the capacitive sensor. The sense circuit samples the integrator output at specific periodic intervals (at the rising and falling edges of the drive pulse), enabling accurate capacitance measurement through periodic excitation and sampling rather than requiring multiple continuous integration cycles. This reduces measurement time while maintaining precision.
Solution Approach 2:
The patent implements preliminary action by performing sampling operations at predetermined timing points (just before the rising and falling edges of the drive pulse). The integrator continuously integrates the sense signal, and the switch circuit is pre-configured to sample at these critical moments, capturing the maximum and minimum values before the drive pulse transitions. This preliminary sampling approach eliminates the need for multiple integration cycles to overcome parasitic effects.
2Device complexity
If analog switching is used to couple sense electrode to integrator, then device complexity is reduced, but measurement precision deteriorates due to parasitic capacitance
Solution Approach 1:
The patent introduces an integrator circuit as an intermediary between the sense electrode and the measurement system. The integrator continuously integrates the sense signal, and its output is then sampled by a switch circuit at specific timing points. This intermediary integration approach allows the system to tolerate the presence of parasitic capacitance in the switch and analog-to-digital converter without compromising measurement accuracy, as the integration process inherently filters out high-frequency parasitic effects.
Solution Approach 2:
The patent replaces traditional analog switching between the sense electrode and integrator with a different approach: the sense electrode is continuously coupled to the integrator, and switching occurs only at the output stage to sample the integrated signal. This substitution of the switching location and method eliminates the parasitic capacitance problem associated with input switching while maintaining circuit simplicity.
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 reduces the time required to determine touch locations by allowing for accurate mutual capacitance measurement in as little as 1µs to 10µs, minimizing the impact of parasitic capacitances and noise, and improving user experience across various display sizes.
Implementation Method 1
measures current flow between the electrodes due to mutual capacitive coupling that exists between the column electrodes and the row electrodes
Implementation Method 2
an integrator circuit with an input section that is in continuous electrical communication with the sense electrode for an amount of time required for measuring the mutual capacitance
Data Source
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AI summary
A method for measuring a mutual capacitance between electrodes of a touch sensitive device includes generating a periodic pulse, communicating the pulse to a drive electrode, and measuring a mutual capacitance between the drive electrode and a sense electrode at a point of intersection between the drive electrode and the sense electrode. Measuring further includes continuously integrating a sense signal from the sense electrode to thereby generate an integrated sense signal, periodically sampling the integrated sense signal at a first time that is prior to a falling edge of the pulse, and at a second time that is prior to a rising edge of the pulse to thereby sample a maximum value and a minimum value of the integrated sense signal, digitizing the maximum value and the minimum value, and determining a difference between the maximum value and the minimum value to thereby determine a mutual capacitance between the drive electrode and the sense electrode.