Capacitive Touch Circuit With Integrated LPF-CDS Noise Reduction
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Solution Overview
Problem
Capacitive touch applications face challenges with high noise sensitivity, spatial non-linearity, and power consumption due to complex circuit structures and shared ADC channels, which limit conversion rate and increase surface area requirements.
Innovation Solution
A capacitive touch circuit is designed with integrated blocks, combining Low Pass Filter (LPF) and Correlated Double Sampling (CDS) functions into a single block, using a serial resistor and shared capacitors to reduce noise and power consumption, and incorporating a switched capacitor ADC with shared amplifiers to enhance conversion efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple circuit blocks (LPF, CDS, S/H, ADC) are implemented as separate stages, then each block can be optimized independently, but the total circuit area and power consumption increase significantly
Solution Approach 1:
The patent combines the Low Pass Filter (LPF) and Correlated Double Sampling (CDS) blocks into a single integrated block. The LPF transfer function H(s) = 1/(1+s/ωc) is implemented within the CDS structure, allowing simultaneous filtering and noise correlation without requiring separate physical blocks. This merging reduces the overall circuit area while maintaining signal processing quality.
Solution Approach 2:
The integrated LPF+CDS block performs multiple functions simultaneously: it acts as a low pass filter with cutoff frequency ωc, implements correlated double sampling for noise reduction, and prepares the signal for subsequent ADC conversion. This multi-functionality eliminates the need for separate dedicated blocks for each function, reducing total circuit area.
2Measurement precision
If separate circuit blocks are used for LPF, CDS, S/H, and ADC, then each block can be independently optimized, but the total power consumption increases
Solution Approach 1:
By merging LPF and CDS into one block, the patent reduces the number of independent power-consuming stages. The shared circuitry and common signal path in the integrated block eliminate redundant power consumption that would occur in separate blocks, while still providing both filtering and correlation functions.
3Device complexity
If ADC channels are shared among multiple sensing capacitors, then hardware complexity is reduced, but conversion rate decreases and spatial non-linearity increases
Solution Approach 1:
The patent segments the signal processing pipeline by implementing dedicated interface circuits for each sensing capacitor that connect to a shared ADC. Each sensing capacitor has its own charge integrator and signal conditionening circuitry, allowing parallel processing of multiple channels while sharing the final ADC resource. This segmentation maintains high conversion rates by avoiding multiplexing bottlenecks.
4Productivity
If dedicated circuit blocks are used for each sensing capacitor, then conversion rate and spatial linearity are improved, but device complexity and surface area increase
Solution Approach 1:
The patent implements local quality by providing dedicated charge integrators and signal conditioning circuits for each sensing capacitor, ensuring optimal performance at each sensing node. However, it uses a shared ADC resource with proper timing and control mechanisms, avoiding the need for fully dedicated ADCs per channel. This balanced approach maintains high conversion rates while controlling overall device 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
The integrated circuit design reduces noise levels, current consumption, and silicon area while improving conversion rate and spatial non-linearity, enabling faster and more accurate capacitive sensing with reduced hardware complexity.
Implementation Method 1
The second block 2' shown in Figure 1 is a low pass filter. Its purpose is to remove high frequency components from the Vint without significantly attenuating it.
Implementation Method 2
The third block 2" a Correlated Double Sampling (CDS) block: its function is to sample twice its input, once in a known condition and once in an unknown condition, for removing an undesired offset or noise.
Data Source
Figure 1
Figure 2
Figure 3A~3D
AI summary
A circuit for capacitive touch applications comprising: a charge integrator; a low pass-filter; a correlated double sampler comprising an input capacitor; a sampler and holder; an analog to digital converter. Said low pass-filter having a cut-off frequency lower than the Nyquist frequency of the sampler and holder. Said low pass filter comprising said input capacitor and a serial resistor.