Single-Node Channel Driver Circuit for Fast Capacitive Touch Sensing
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Solution Overview
Problem
Projected capacitive touch sensors face challenges in accurately determining touch locations due to large parasitic capacitances, which require multiple integration cycles, increasing the time to make a determination and affecting user experience, especially in larger displays with numerous electrodes.
Innovation Solution
A channel driver circuit is implemented that employs a differential module, sigma-delta module, and driver module to generate a low impedance virtual signal on a load while receiving and outputting load-modified signals, effectively subtracting the reference signal to output only load-affected information, and incorporates noise shaping to reduce noise interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple integration cycles are used to determine touch locations, then measurement precision is improved, but loss of time increases
Solution Approach 1:
The patent changes the electrical parameters of the sensor circuit by introducing variable gain amplifiers and adjustable integration cycles. The system dynamically adjusts the gain and integration duration based on signal conditions, allowing faster determination when possible while maintaining precision when needed. This resolves the contradiction by making the integration process adaptive rather than fixed.
Solution Approach 2:
The patent replaces the traditional mechanical integration process with an electrical signal processing approach using operational amplifiers and feedback circuits. Instead of relying solely on multiple slow integration cycles, the system uses electrical amplification and filtering to achieve precise measurements more quickly, substituting electrical mechanisms for temporal integration.
2Device complexity
If large parasitic capacitances are present in the sensor circuit, then device complexity is reduced, but measurement precision deteriorates
Solution Approach 1:
The patent introduces feedback circuits that continuously monitor and compensate for parasitic capacitance effects. The feedback mechanism detects errors caused by parasitic capacitance and applies corrective signals, maintaining measurement precision without requiring complex shielding or isolation structures. This resolves the contradiction by using active compensation rather than passive prevention.
Solution Approach 2:
The patent extracts and separates the parasitic capacitance effect from the measurement signal by using differential measurement techniques. The system measures both the signal and parasitic components separately, then subtracts the parasitic portion mathematically or electrically, leaving only the true touch signal. This allows simple circuit structures to achieve high precision by removing the harmful effect rather than avoiding it.
3Measurement precision
If high gain is used to amplify weak sensor signals, then measurement precision is improved, but noise interference increases
Solution Approach 1:
The patent employs periodic modulation and demodulation techniques where the sensor signal is modulated at a specific frequency, amplified, then demodulated back to baseband. This periodic action allows the system to achieve high effective gain only at the modulation frequency, rejecting noise at other frequencies. The noise is averaged out over multiple cycles while the signal is coherently amplified.
Solution Approach 2:
The patent introduces an intermediary carrier signal that mediates between the weak sensor signal and the amplification process. The sensor signal modulates this carrier, which then undergoes amplification. After amplification, the original signal is extracted through demodulation. This intermediary approach allows high gain amplification while the carrier filtering properties reject noise, solving the contradiction between gain and noise.
Data Source
AI summary
A channel driver circuit includes a differential module and a driver module. In some examples, the channel driver circuit also includes a sigma-delta module. The differential module receives, via a single node of a load, a channel driving signal that is provided to the load at the single node (e.g., that is based on an electrical characteristic of the load) and generates an analog error signal that is based on the channel driving signal and a reference signal. The driver module is coupled to the differential module and generates the channel driving signal based on the analog error signal or a digital error signal corresponding to the analog error signal and transmits the channel driving signal via the single node to the load. The channel driver circuit simultaneously transmits the channel driving signal to the load at the single node and senses the channel driving signal at the single node.