Capacitive Touch Readout Circuit Signal Acquisition
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Solution Overview
Problem
Capacitive touchscreen systems face challenges with large feedback capacitors and active current division circuits, which increase area requirements and power consumption, and necessitate panel resets for each capacitance sensing cycle, complicating the handling of dynamic range signals and immunity to touch signal noise.
Innovation Solution
The capacitive touchscreen system employs a readout circuit with a gain capacitor and feedback capacitor, utilizing switch control to transfer charge representative of mutual and stray capacitances, allowing for accurate signal acquisition without large feedback capacitors or active current division circuits, and enabling simultaneous multiple drive lines and elevated drive signal voltages for improved noise immunity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional charge integrator readout circuits with increased feedback capacitor values are used to handle increased dynamic range signals, then the dynamic range handling capability is improved, but the area required in integrated circuit implementation increases
Solution Approach 1:
The patent divides the charge acquisition process into multiple phases (first charge acquisition phase and second charge acquisition phase) with different feedback capacitor configurations. During the first phase, a first feedback capacitor handles charge from drive electrodes, while during the second phase, a second feedback capacitor handles charge from sense electrodes. This segmentation allows each capacitor to be optimized for its specific phase rather than requiring one large capacitor to handle all phases simultaneously.
Solution Approach 2:
The patent dynamically switches between different feedback capacitor configurations based on the acquisition phase. The feedback capacitor value is changed from a first value during the first charge acquisition phase to a second value during the second charge acquisition phase. This dynamic adjustment allows the circuit to optimize its feedback capacitance for each specific measurement phase, improving signal handling while minimizing area requirements.
2Area of stationary object
If active current division circuits are used to reduce feedback capacitor size, then the feedback capacitor area is reduced, but additional amplifiers and resistors are required which are less operationally and temperature stable
Solution Approach 1:
The patent employs periodic action by alternating between two distinct charge acquisition phases. During the first phase, the circuit acquires charge using one feedback capacitor configuration, then during the second phase, it acquires charge using a different feedback capacitor configuration. This periodic switching allows the use of smaller capacitors in each phase while maintaining overall system stability and accuracy without requiring additional active components.
3Measurement precision
If panel reset is performed for each capacitance sensing cycle, then measurement accuracy is maintained, but the system complexity and power consumption increase
Solution Approach 1:
The patent performs preliminary charge acquisition during the first charge acquisition phase before the main measurement phase. By pre-acquiring and storing relevant charge information in the first phase, the circuit reduces the burden on subsequent phases and maintains measurement accuracy without requiring a complete panel reset between cycles. This preliminary action streamlines the overall measurement process.
Solution Approach 2:
The patent maintains continuity of useful action by seamlessly transitioning between the first and second charge acquisition phases without interrupting the overall measurement process. The circuit continuously acquires charge across both phases, eliminating the need for disruptive panel resets while maintaining measurement integrity through coordinated switching between phases.
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 solution allows for high dynamic range signal processing with reduced area and power consumption, doubling signal acquisition rates and providing better immunity to harmonic Electromagnetic Interference (EMI) without the need for panel resets, enhancing the accuracy and efficiency of capacitive touch sensing.
Implementation Method 1
each sense circuit comprising a gain capacitor having first and second terminals, a first switch, a second switch, a third switch... during a first stage the gain capacitor accumulates a charge representative of mutual capacitances and stray capacitances
Implementation Method 2
the charge accumulated in the gain capacitor during the first stage is transferred to the feedback capacitor during a second stage when the first switch is open, the second switch is closed, the third switch is open
Data Source
AI summary
Various embodiments of readout circuits are disclosed where no touchscreen or touch panel recharge is required, and the amount of time available for signal acquisition is twice that relative to prior art touchscreen or touch panel readout circuits. Voltage offsets of the integrating amplifiers may be compensated for by notch filtering signals stored in readout circuit capacitors. Some embodiments of readout circuits disclosed herein permit large dynamic range capacitive touchscreen or touch panel signals to be processed, and do not require panel reset. Readout circuits are disclosed that permit doubling of the signal acquisition rate and pre-filtering of acquired touch panel signals for improved immunity from harmonic EMI. Signal acquisition and temporary storage may be carried out using the same capacitors in such readout circuits.