Capacitive Touch Circuit With Integrated LPF and CDS Sampling
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Solution Overview
Problem
Capacitive touch applications face challenges with noise immunity, spatial non-linearity, and high power consumption due to complex circuit structures and shared ADC channels, which limit conversion rate and increase surface area requirements.
Innovation Solution
Integration of Low Pass Filter (LPF) and Correlated Double Sampling (CDS) functions into a single block, combined with a switched capacitor ADC, sharing capacitors and amplifiers to reduce noise and power consumption, and optimizing ADC resolution through error signal generation and residue evaluation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple circuit blocks (LPF, CDS, S/H) 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) functions into a single integrated block. The CDS circuit inherently provides low-pass filtering characteristics through its transfer function, eliminating the need for a separate LPF stage. This merging reduces the total circuit area while maintaining signal processing quality.
Solution Approach 2:
The integrated CDS block performs multiple functions simultaneously: it acts as both a Correlated Double Sampler and a Low Pass Filter. The same circuit components (switches, capacitors, amplifiers) used for CDS operation also provide the filtering function, achieving multi-functionality with a single block.
2Measurement precision
If multiple circuit blocks are implemented as separate stages, then each block can be optimized independently, but the total power consumption increases
Solution Approach 1:
By merging LPF and CDS into one block, the patent eliminates redundant circuitry and reduces the total number of active components. The integrated design shares amplifiers and capacitors between functions, reducing overall power consumption while maintaining signal processing quality.
Solution Approach 2:
The multi-functional CDS block uses the same hardware resources for both CDS and LPF operations, eliminating the need for separate power supplies for each block. This universal approach reduces total power consumption while achieving both signal sampling and filtering objectives.
3Device complexity
If ADC channels are shared among multiple sensing capacitors, then the number of ADCs is reduced, but the conversion rate decreases
Solution Approach 1:
The patent segments the signal processing pipeline by placing an integrated LPF+CDS block at the input of each ADC channel. This segmentation allows each ADC to process signals independently at full speed while the integrated blocks handle preprocessing, enabling parallel operation and maintaining high conversion rates.
Solution Approach 2:
The integrated LPF+CDS blocks perform preliminary signal conditioning and noise filtering before the ADC conversion. By preprocessing the signals in advance, each ADC can operate at its maximum conversion rate without being bottlenecked by shared resources, as the preprocessing is already completed.
4Device complexity
If ADC channels are shared among multiple sensing capacitors, then the number of ADCs is reduced, but spatial non-linearity increases
Solution Approach 1:
By assigning dedicated integrated LPF+CDS blocks to each ADC channel, the patent ensures that each sensing capacitor has its own preprocessing path. This segmentation isolates the signal paths, preventing cross-channel interference and reducing spatial non-linearity errors.
Solution Approach 2:
The integrated CDS blocks perform preliminary offset cancellation and noise filtering for each channel independently before ADC conversion. This preliminary action removes channel-specific errors early in the signal path, reducing spatial non-linearity before the signals are combined or multiplexed.
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 simplifies the circuit structure, reduces noise, power consumption, and spatial non-linearity, enabling faster digital processing and higher conversion rates with improved resolution and reduced surface area.
Implementation Method 1
The second block 2' shown in FIG. 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'' is 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
AI summary
A circuit for capacitive touch applications has a charge integrator, a low pass-filter, and a correlated double sampler having an input capacitor. The circuit also includes a sampler and holder, and an analog to digital converter. The low pass-filter has a cut-off frequency lower than the Nyquist frequency of the sampler and holder, and the low pass filter includes input capacitor and a serial resistor.