Self-Capacitor Sensing Noise Suppression via Passive Mixing
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Solution Overview
Problem
Existing capacitive touch panels face increasing challenges in providing low-noise read-out circuits due to rising display noise coupling, particularly in self-capacitor sensing, which involves smaller signal levels and requires effective noise cancellation to detect small changes in capacitance.
Innovation Solution
The implementation of a discrete-time self-capacitor sensing system with a noise-suppressed discharge current and a passive mixer that up-converts signals, allowing for differential voltage sampling and amplification to indicate touch events, thereby reducing noise interference and enhancing signal detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If self-capacitor sensing is used to detect touch events, then alternative touch-sensing information is provided (especially when mutual-capacitor sensing is inaccurate), but the signal levels are smaller and noise cancellation becomes more challenging
Solution Approach 1:
The patent converts the harmful display noise into a beneficial reference signal by capacitively coupling it to a reference node. This reference noise signal is then subtracted from the sensor channel signals, transforming the previously harmful noise into a useful component for noise cancellation, thereby improving the detectability of small touch-induced capacitance changes in self-capacitor sensing
Solution Approach 2:
The patent changes the timing parameters of signal sampling and processing to optimize noise rejection. By specific timing of sampling the reference noise signal and the sensor channel signals, and by processing them through differential amplification, the system enhances the signal-to-noise ratio for small capacitance changes, making touch detection more precise
2Productivity
If conventional read-out circuits are used for self-capacitor sensing, then basic touch detection is possible, but display noise coupling increasingly degrades the noise floor and limits detection accuracy
Solution Approach 1:
The patent directly addresses the worsening display noise coupling by capturing this harmful noise through capacitive coupling to a reference node. The captured noise signal is then subtracted from the sensor readings, converting the previously detrimental noise into a useful reference for cancellation, thereby maintaining touch detection capability despite increasing noise floors
Solution Approach 2:
The patent introduces an intermediary reference noise signal that mediates between the display noise source and the sensor channels. This reference signal acts as a mediator that carries the noise characteristics to the subtraction stage, enabling the system to compensate for display noise coupling without directly modifying the sensor channels themselves
3Measurement precision
If noise cancellation techniques are applied to self-capacitor sensing, then small capacitance changes become detectable, but the circuit complexity increases
Solution Approach 1:
The patent applies preliminary action by pre-capturing the display noise signal through capacitive coupling to a reference node before the noise degrades the sensor signals. This advance capture of the noise reference allows for subsequent subtraction from the sensor channels, enabling detection of small capacitance changes without requiring complex real-time noise filtering circuits
Solution Approach 2:
The patent uses an intermediary reference noise signal as a mediator between the display noise source and the sensor channels. This intermediary carries the noise characteristics to the differential amplification stage, enabling noise cancellation through simple subtraction rather than complex adaptive filtering, thus balancing measurement precision with circuit complexity
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 effectively reduces noise interference, improving the signal-to-noise ratio and enabling reliable detection of touch events even with small changes in capacitance, thus enhancing the accuracy of capacitive touch sensing.
Implementation Method 1
display noise capacitively coupled onto the channel from the display panel
Implementation Method 2
passively mixing at least the discharge voltage level to produce a pair of up-converted channel signals
Data Source
AI summary
Techniques are described for low-noise self-capacitor sensing in a capacitive touch panel array integrated with a display panel. Each channel of the array has a self-capacitance (Ci) that changes responsive to presence or absence of a local touch event local. Each channel is read by an analog front-end (AFE) by using a locally noise-suppressed discharge current for a discrete discharge time to discharge Ci to obtain a discharge voltage level that differs with presence or absence of the local touch event, and outputting a voltage output for the channel based on the discharge voltage level by passively mixing at least the discharge voltage level to produce a pair of up-converted channel signals, sampling the pair of up-converted channel signals to obtain a differential voltage sample, and amplifying the differential voltage sample to generate the Vout as indicating absence or presence of the touch event local to the channel.