Capacitive Touch Controller Interference Mitigation via Spread Spectrum and Digital Filtering
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Solution Overview
Problem
Capacitive touch screen systems face interference from LCD noise and battery charger noise, leading to false touches and inadequate interference rejection due to the limited effectiveness of existing filtering methods, which are insufficient in rejecting out-of-band signals.
Innovation Solution
The implementation of spread spectrum techniques and zero-ISI digital filtering, along with the use of Hadamard matrices for panel encoding and decoding, to mitigate touch panel interference, and moving the A/D conversion to the LNA or down converter output to perform operations digitally, reducing die area and enhancing interference rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If integrate-and-dump filtering is used, then some interference mitigation is achieved, but out-of-band signals are not sufficiently rejected
Solution Approach 1:
The patent changes the filtering parameters by implementing digital filtering with adjustable cutoff frequencies and transfer functions that can be optimized for specific interference conditions. This allows sufficient rejection of out-of-band signals while preserving touch detection accuracy, resolving the contradiction between interference rejection and false touch detection.
Solution Approach 2:
The patent introduces digital signal processing as an intermediary between the analog front-end and touch detection algorithms. This digital filtering stage acts as a mediator that can selectively attenuate interference frequencies while preserving touch signal integrity, achieving both interference rejection and reliable touch detection.
2Device complexity
If A/D conversion is performed later in the signal chain, then analog circuit complexity is reduced, but interference rejection capability is limited
Solution Approach 1:
The patent performs A/D conversion early in the signal chain, before significant analog processing. This preliminary digitization allows subsequent digital filtering to effectively reject interference while keeping analog circuitry minimal. The early conversion point preserves interference rejection capability while reducing overall system complexity.
Solution Approach 2:
The patent replaces complex analog filtering circuits with digital filtering implemented in software or firmware. This substitution maintains interference rejection capability while significantly reducing analog circuit complexity and die area, as digital filters can achieve superior frequency selectivity with simpler hardware.
3Measurement precision
If excitation matrix condition number is high, then matrix inversion is possible, but signal-to-noise ratio deteriorates
Solution Approach 1:
The patent optimizes the excitation matrix parameters to achieve a low condition number while maintaining invertibility. By carefully selecting matrix elements and structure, the system achieves both accurate capacitance estimation and high signal-to-noise ratio, resolving the contradiction between measurement precision and reliability.
Solution Approach 2:
The patent implements iterative optimization of the excitation matrix based on measured signal quality and noise characteristics. The system adjusts matrix parameters to minimize condition number while maintaining measurement accuracy, using feedback from actual operating conditions to achieve optimal balance between precision and signal-to-noise ratio.
Data Source
AI summary
A touch screen controller for a capacitive touch screen panel having row conductors intersecting column conductors to form pixels, the touch screen controller comprising: a transmitting unit configured to drive the row conductors with signals formed from an excitation matrix, and a receiving unit configured to sense signals from the column conductors and to determine the capacitance of the pixels using the sensed signals. In some embodiments, the receiving unit is configured to apply zero-ISI filtering to the sensed signals prior to the capacitance of the pixels being determined. In some embodiments, the transmitting unit is configured to spread each bit of the row drive signals with a pseudo-noise code, and the receiving unit is configured to de-spread the sensed signals with the pseudo-noise code. In some embodiments, the transmitting unit is configured to pulse shape the row drive signals.