Capacitive Touch Controller Offset Cancellation Circuit
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Solution Overview
Problem
Capacitive touch screens face sensitivity issues due to significant offset capacitance, which overwhelms signal capacitance, preventing accurate sensing of touch events.
Innovation Solution
A touch screen controller is designed to cancel offset capacitance using a code generator, switched-capacitor arrays, and an integrator, converting digital codes into electric charges and voltages to subtract offset capacitance from the touch sensor capacitance, allowing for precise signal detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If offset capacitance is not cancelled, then the circuit is simple, but the signal capacitance cannot be sensed and sensitivity is poor
Solution Approach 1:
The patent applies preliminary action by performing offset capacitance cancellation before signal detection. The controller measures and stores the offset capacitance value during an initialization phase, then subtracts this stored value from subsequent touch sensor readings to isolate the signal capacitance component.
Solution Approach 2:
The patent extracts the offset capacitance component from the total sensor capacitance by measuring it separately during a calibration phase and storing it in memory. This extracted offset value is then subtracted from operational readings to isolate the signal capacitance.
2Loss of time
If conventional offset cancellation is used, then offset capacitance is reduced, but calibration time is long
Solution Approach 1:
The patent performs offset measurement and storage in advance during an initialization routine, allowing rapid offset compensation during actual touch operations without repeated calibration sequences.
Solution Approach 2:
The patent creates a digital copy of the offset capacitance value and stores it in memory for reuse. This copied offset value eliminates the need to repeatedly measure and process the same offset data, significantly reducing calibration time.
3Manufacturing precision
If small unit capacitors are used for precise offset cancellation, then cancellation precision is high, but manufacturing is complex and control signals are numerous
Solution Approach 1:
The patent changes the parameter of unit capacitor size to a practical range (10-100 fF) that balances manufacturing ease with sufficient precision. The system achieves accurate offset cancellation through digital signal processing and multiple capacitor combinations rather than relying on extremely small individual capacitor values.
Solution Approach 2:
The patent segments the offset cancellation function across multiple switched capacitors that can be independently controlled and combined. This allows the system to achieve fine resolution through digital control of multiple larger capacitors rather than using a single small capacitor.
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 effectively cancels offset capacitance, enhancing the sensitivity of capacitive touch screens by allowing for accurate detection of touch events and reducing the calibration time, while also allowing for the use of larger unit capacitors, which simplifies manufacturing and reduces the number of control signals needed.
Implementation Method 1
A switched-capacitor array is configured to convert the time-invariant digital code and the time-variant digital code into an amount of electric charges corresponding to the offset capacitance
Implementation Method 2
The touch screen controller may further include a capacitance-to-voltage converter configured to convert the amount of the electric charges into a voltage
Data Source
AI summary
A touch screen controller for cancelling an offset capacitance of a capacitive touch screen panel includes a code generator configured to generate a time-invariant digital code and a time-variant digital code during an offset cancellation time. A switched-capacitor array converts the time-invariant digital code and the time-variant digital code into an amount of electric charges corresponding to the offset capacitance.