Touch Panel Readout Circuit Using CDS for Charger Noise Rejection
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Solution Overview
Problem
Touch panels using self-capacitance sensing methods face challenges with narrow frequency response and interference from charger noise, making it difficult to achieve a high signal-to-noise ratio for accurate sensing.
Innovation Solution
A readout circuit incorporating a charge modulator and correlated double sampling demodulator, which converts DC sensing voltage signals to AC and filters out noise using a low-pass filter, allowing for improved signal separation and increased sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If self-capacitance sensing method is used with narrow band frequency response, then the touch panel can operate with simpler circuit design, but the driving frequency is limited and charger noise interference cannot be avoided
Solution Approach 1:
The patent applies periodic action by modulating the sensing signal with a carrier wave at a specific frequency (e.g., 1/4 of the charging frequency). This periodic modulation shifts the sensing signal to a different frequency band, allowing it to be distinguished from the charger noise through frequency separation. The modulated signal can then be filtered and demodulated to extract the touch information while rejecting the noise.
2Measurement precision
If the driving frequency is increased to improve sensing performance, then the signal-to-noise ratio may improve, but the narrow frequency response of self-capacitors and wire resistors limits the available frequency range
Solution Approach 1:
The patent changes the frequency parameter of the sensing signal by modulating it with a carrier wave. This transforms the baseband sensing signal into a higher frequency modulated signal, effectively expanding the usable frequency range beyond the natural resonance frequency of the self-capacitor. The modulation allows the system to operate at frequencies that would otherwise be unavailable due to the narrow bandwidth of the self-capacitor.
3Measurement precision
If charger noise filtering is applied to remove interference, then the signal-to-noise ratio improves, but the sensing signal and noise are mixed at the same frequency band making separation difficult
Solution Approach 1:
The patent introduces a carrier wave as an intermediary to mediate between the sensing signal and the noise filtering process. The carrier wave modulates the sensing signal, shifting it to a different frequency band where it can be easily separated from the charger noise using bandpass or low-pass filters. After filtering, the carrier is removed through demodulation, leaving only the original sensing signal with the noise eliminated.
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 enhances the signal-to-noise ratio, enabling more accurate detection of touch events by filtering out charger noise and improving the sensitivity of the touch panel's sensing capabilities.
Implementation Method 1
a driving signal generates an induced electric field on self-capacitors
Implementation Method 2
When an external conductor changes the induced electric field, it is equivalently deemed as a capacitance variation of the self-capacitor
Data Source
AI summary
A readout circuit of a touch panel is provided. The readout circuit includes a charge modulator and a correlated double sampling demodulator. The charge modulator is configured to provide a driving current to charge or discharge a sensing electrode of the touch panel and generate a sensing voltage signal. The correlated double sampling demodulator is coupled to the charge modulator. The correlated double sampling demodulator is configured to sample the sensing voltage signal to obtain a voltage variation when a touch event happens. A method for driving the touch panel is also provided.