Capacitive Sensing ADC Modulation for Low-Cost High Resolution
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Solution Overview
Problem
Existing analog-to-digital conversion methods in capacitive sensing systems face challenges in achieving high resolution at low cost, particularly in applications where high-resolution converters are economically unfeasible, and require large capacitors and resistors for generating triangular waves, which may not be available in low-cost implementations.
Innovation Solution
A method that modulates the analog signal with a triangular or saw-tooth signal, using a low-resolution ADC to produce digital samples, which are then averaged to achieve a higher resolution, with the modulating signal's peak-to-peak amplitude corresponding to an integer multiple of the quantization step size, allowing for a cost-effective increase in resolution without the need for large capacitors and resistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high-resolution analog-to-digital converter is used, then the measurement precision is improved, but the cost increases
Solution Approach 1:
The patent segments the analog signal measurement process into multiple lower-resolution measurements taken at different times, then combines these segments through digital processing to achieve high resolution. Instead of using one high-resolution ADC, the system uses multiple low-resolution ADC measurements that are computationally combined to reconstruct the high-resolution signal.
Solution Approach 2:
The patent employs periodic switching between different capacitor configurations and periodic sampling at different voltage levels. By periodically alternating the capacitor connections and taking multiple samples over time, the system accumulates information that is processed to achieve high resolution equivalent to a much more expensive high-resolution ADC.
2Ease of manufacture
If a low-resolution ADC is used, then the cost is reduced, but the measurement precision deteriorates
Solution Approach 1:
The patent introduces capacitors as intermediary elements that store and transfer charge between the sensing electrode and the low-resolution ADC. By using capacitors to hold the analog signal and repeatedly sampling the same signal under different switching conditions, the system extracts more information from the low-resolution ADC than would normally be possible.
Solution Approach 2:
The patent replaces the need for a high-resolution mechanical/electronic ADC with a combination of low-resolution ADC and digital signal processing. The physical high-resolution conversion function is substituted by a computational process that combines multiple low-resolution measurements through mathematical operations.
3Measurement precision
If triangular wave generation with large period is used to increase resolution, then the measurement precision is improved, but the device complexity increases due to large capacitors and resistors
Solution Approach 1:
The patent uses dynamic switching of capacitor connections controlled by a multiplexer to create the modulation effect, replacing the need for large-period triangular wave generation. Instead of using large capacitors and resistors to generate slow triangular waves, the system dynamically reconfigures the capacitor network at high speed to achieve the same resolution-enhancing effect.
Solution Approach 2:
The patent changes the operating parameters by using high-frequency switching instead of low-frequency triangular waves. By changing from a time-based modulation approach (large period triangular waves) to a switching-based approach (multiplexer-controlled capacitor connections), the system achieves resolution enhancement without requiring large passive components.
4Measurement precision
If additional amplifier is added to sum analog signal and triangular wave, then the measurement precision is improved, but the device complexity increases
Solution Approach 1:
The patent merges the signal summation function into the capacitor-based switching network itself. Instead of using a separate amplifier to add the triangular wave to the analog signal, the system uses the capacitors and switches to directly combine the signals in the analog domain before ADC conversion, eliminating the need for additional amplifying components.
Solution Approach 2:
The multiplexer and capacitor network serve multiple functions simultaneously: they perform signal switching, signal combining, and hold the analog value for repeated sampling. This multi-functionality eliminates the need for dedicated amplifier components that would be required in a traditional approach to signal summation.
Data Source
AI summary
A capacitive sensing system operates according to a method which uses an ADC. The analog signal to be digitized is modulated with a triangular or saw-tooth modulating signal, so that a modulated analog signal is obtained, which is sampled with the ADC. The triangular or saw-tooth signal is chosen to have a peak-to-peak amplitude corresponding at least approximately to an integer multiple L, with L≧1, of the quantization step size of the ADC. The saw-tooth or triangular signal has a number M, of periods per each sequence of N samples. M and N are chosen such that M>1 and M≠N and such that R=r*N/(k*gcd(N, M)*L), where gcd(M, N) is the greatest common divisor of N and M and where k=2 if the modulating signal is a saw-tooth signal and k=4 if the modulating signal is a triangular signal.