Differential Single-Ended Charge Converter for Touch Screen Noise Rejection
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Solution Overview
Problem
As touch sensitive displays become thinner, they experience increased parasitic capacitances that couple noise from the display layer to the sensing layer, degrading the accuracy of touch sensing due to existing methods' inability to effectively reject this noise.
Innovation Solution
A differential single-ended charge converter is introduced, comprising an input stage with NMOS and PMOS transistors and an output stage with current mirrors, along with a method of using switches to decouple the reference generation circuit during scanning periods to reject common mode noise, thereby isolating the sensing layer from display noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If touch sensitive displays are made thinner to meet consumer demands, then device thickness is reduced, but parasitic capacitances increase causing display noise to couple through to the sensing layer, degrading touch sensing accuracy
Solution Approach 1:
The patent applies common mode noise rejection techniques to convert the harmful display noise that couples through parasitic capacitances into a rejectable common mode signal. By sensing both the sense line and reference line and rejecting their common mode components, the system transforms the noise coupling problem into a solvable common mode rejection problem, thereby maintaining touch sensing accuracy despite the increased parasitic capacitances in thinner displays
Solution Approach 2:
The patent changes the electrical parameters of the sensing system by introducing common mode rejection circuitry that measures and subtracts the common mode voltage component from the sense line signal. This parameter transformation allows the system to eliminate the noise component while preserving the differential touch signal, thus maintaining measurement precision in thinner display configurations
2Measurement precision
If fully differential sensing techniques are used to reject display noise, then noise rejection improves, but power consumption increases and acquisition time increases
Solution Approach 1:
The patent implements partial differential sensing by applying common mode rejection only to the reference line and selectively to specific sense lines, rather than implementing fully differential sensing across all lines. This partial application of the technique achieves sufficient noise rejection for the application while consuming less power and requiring shorter acquisition times compared to complete differential sensing of all signal paths
3Measurement precision
If fully differential sensing techniques are used to reject display noise, then noise rejection improves, but acquisition time increases
Solution Approach 1:
The patent implements partial differential sensing by applying common mode rejection only to the reference line and selectively to specific sense lines, rather than implementing fully differential sensing across all lines. This partial application of the technique achieves sufficient noise rejection for the application while consuming less power and requiring shorter acquisition times compared to complete differential sensing of all signal paths
Solution Approach 2:
The patent employs periodic sampling and updating of the common mode reference voltage at optimized intervals, rather than continuously updating it. This periodic action maintains effective noise rejection while reducing the acquisition time penalty associated with continuous differential sensing, allowing the system to balance noise rejection performance with faster touch response times
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 solution effectively attenuates display noise, improving the accuracy of touch sensing without the power consumption and acquisition time drawbacks of fully differential sensing techniques, allowing for robust noise rejection in thinner touch screen designs.
Implementation Method 1
a first capacitive coupling between the gate of the first NMOS transistor and the source of the first NMOS transistor such that a transient component of noise received from the sense line is applied to both the gate and the source of the first NMOS transistor
Implementation Method 2
a first current mirror PMOS transistor having a source coupled to a supply voltage, a drain coupled to a drain of the first NMOS transistor, and a gate coupled to the drain of the first current mirror PMOS transistor
Data Source
AI summary
An electronic device includes a plurality of charge-to-current converters each including a first NMOS transistor having a source coupled to a sense line, a first capacitor between a gate and source of the first NMOS transistor so that a transient component of noise from the sense line is applied to both, a first PMOS transistor having a source coupled to the sense line, a second capacitor between a gate and source of the first PMOS transistor so the transient component of the noise is applied to both, a first current mirror having an input coupled to a drain of the first NMOS transistor and an output coupled to an output for that charge to current converter, and a second current mirror having an input coupled to a drain of the first PMOS transistor and an output coupled to the output for that charge to current converter.