Capacitive Sensor Sampling Across Frequencies for EMC Robustness
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Solution Overview
Problem
Capacitive sensors face challenges in electromagnetic compatibility (EMC) susceptibility, particularly due to aliasing effects caused by interference frequencies equal to or multiples of the sampling frequency, which affect spectral noise sensitivity.
Innovation Solution
Implementing a capacitive integrating converter with multiple sampling frequencies and a controller to calculate a digital representative of the input signal as a reverse weighted average of these frequencies, reducing spectral noise sensitivity by calculating differences between samplings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a low-pass anti-aliasing filter is used before digitalization, then aliasing effects are reduced, but the device complexity increases and high-frequency signal loss occurs
Solution Approach 1:
The patent extracts and eliminates the need for external anti-aliasing filters by implementing the aliasing reduction functionality directly within the capacitive integrating converter through multiple sampling frequencies. This removes the separate filter component while achieving the same EMC robustness goal.
Solution Approach 2:
The patent replaces the physical analog filter structure with a digital signal processing approach using multiple sampling frequencies and spectral averaging. This substitutes mechanical/analog filtering with an electronic/digital method that achieves aliasing reduction without the associated complexity and signal loss.
2Reliability
If RC filter with spread spectrum technology is used, then aliasing is reduced for higher multiples of sampling frequency, but the measurement precision deteriorates and the method is only partially effective
Solution Approach 1:
The patent implements a universal solution that handles all aliasing cases (not just higher multiples) by using multiple sampling frequencies in combination with spectral averaging. This multi-functional approach simultaneously reduces aliasing across the entire frequency spectrum while maintaining measurement precision, unlike the limited RC filter approach.
3Device complexity
If single frequency sampling is used, then the device complexity is low, but spectral noise sensitivity increases and EMC robustness deteriorates
Solution Approach 1:
The patent segments the single sampling operation into multiple sampling operations at different frequencies. By dividing the measurement process into multiple frequency segments and combining their results through spectral averaging, the system achieves enhanced EMC robustness while keeping each individual sampling operation relatively simple.
Solution Approach 2:
The patent employs periodic sampling at multiple different frequencies in a cyclic manner. The capacitive integrating converter alternates between different sampling frequencies periodically, and the spectral averaging process combines these periodic measurements to reduce noise and improve reliability.
Data Source
AI summary
A method is provided for improving the EMC robustness of Integrated Capacitive Sensor systems with a sensor Signal-Conditioner (SSC). The SSC is connected with a capacitive integrating converter to convert a received signal into a bit stream. An oscillator provides a plurality of sampling frequencies. A counter connected with the capacitive integrating converter collects the bit stream and calculates the digital representative of the physical input which is than stored in an output register. The method includes performing some conversions with different sampling frequencies from the oscillator or a frequency divider by the capacitive integrating Signal-Converter; storing the results of the samplings and using the results in the following cycle to calculate for each sampling frequency a difference to the prior sampling of the same frequency; and calculating the digital representative of the input signal from the external sensing capacitor as the reverse weighted average of the samplings of the different frequencies.