Touch Sensor Capacitance Sensing Circuit for Noise-Resistant Measurement
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Solution Overview
Problem
Existing capacitance measurement methods in touch sensors face challenges in accurately measuring capacitance variation due to external noise and parasitic currents, especially when dealing with small discharging currents, which affects touch sensitivity and noise resistance.
Innovation Solution
The solution involves a capacitive load detector that measures capacitance by sensing panel load current using a panel driver and a circuit comprising PMOS and NMOS transistors, along with a sensing capacitor, to accurately detect rising and falling current components and calculate capacitance based on load and sensing voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a small charging current is used to accurately measure capacitance, then measurement precision is improved, but the influence from external noise and parasitic current increases
Solution Approach 1:
The patent applies periodic action by using alternating charging and discharging cycles with a switching circuit. The capacitor is periodically charged through a constant current source and then discharged through a resistor, with each cycle measured by a timer. This periodic operation allows the system to use larger currents during charging phases while still achieving accurate measurements through the structured cyclic process, thereby reducing the impact of noise and parasitic currents that would be problematic with continuously small currents.
Solution Approach 2:
The patent implements continuity of useful action by maintaining a continuous measurement process through repeated charging-discharging cycles. Rather than using a single small current measurement that is vulnerable to noise, the system continuously operates through multiple cycles, accumulating measurement data and maintaining steady operation. This continuous operation ensures that the measurement process remains robust against transient noise while maintaining accuracy through the structured cyclic methodology.
2Measurement precision
If a small discharging current is used to accurately measure capacitance, then measurement precision is improved, but the discharging time increases and becomes difficult to measure due to noise
Solution Approach 1:
The patent uses periodic action by structuring the measurement as repeated charging-discharging cycles. The capacitor is charged to a reference voltage and then discharged through a resistor for a measured time period, with this process repeating continuously. This periodic structure allows the system to use larger discharging currents that produce measurable time values, avoiding the noise problems associated with very small currents while maintaining measurement accuracy through the cyclic measurement approach.
Solution Approach 2:
The patent applies parameter changes by dynamically adjusting the discharging current through the use of a switching circuit and resistor selection. Instead of using a fixed small discharging current that would be vulnerable to noise, the system can change the discharging parameters (current magnitude, time duration) based on measurement requirements. This allows optimization of the discharging process to achieve both accuracy and noise immunity by selecting appropriate current and time parameters for each measurement cycle.
3Productivity
If the capacitor is discharged in a short time, then productivity is improved, but the measurement accuracy decreases due to timer clock limitations
Solution Approach 1:
The patent introduces an intermediary element - a resistor with known resistance value - between the capacitor and ground for discharging. This resistor acts as a mediator that controls the discharging current and extends the discharging time to a measurable range. The resistor provides a stable, known resistance that allows the timer to accurately count clock cycles during the discharging process, bridging the gap between fast discharge requirements and measurement accuracy needs by creating an intermediate time scale that is both measurable and sufficiently fast for 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
This approach enables rapid and accurate measurement of capacitance variation, enhancing touch sensitivity and noise resistance while simplifying the circuit configuration and improving data processing speed.
Implementation Method 1
capacitance may be measured based on a charging time of a capacitor... the capacitor is discharged to the ground, and then charged up to a reference voltage level
Data Source
AI summary
An apparatus and method are provided to rapidly and accurately measure the variation of capacitance in a panel load of a touch sensor and thus increase touch sensitivity and noise resistance. The apparatus includes a panel driver configured to drive a panel load in the touch sensor according to a reference voltage, and a capacitive load detector configured to measure capacitance of the panel load by sensing a panel load driving current that is applied to the panel load by the panel driver.