Sigma-Delta Modulator Excitation for Linear Capacitance Sensing
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitance-to-digital converters using sigma delta modulators in capacitance sensors face issues with non-linear digitized signals, low sensitivity, and limited excitation signal levels, which hinder high-resolution pressure measurements, especially in applications requiring dual absolute differential pressure sensing.
Innovation Solution
The proposed solution involves alternative excitation schemes and circuit architectures for sigma delta modulators, including first and second-order two-phase modulators with programmable ratio-metric excitation voltage generators and adaptive excitation voltage control, which improve transfer function linearity, dynamic range, and sensitivity by increasing the excitation signal magnitude.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional sigma delta modulators are used for capacitance-to-digital conversion, then the circuit can operate with simple excitation schemes, but the digitized signal exhibits non-linearity and low sensitivity
Solution Approach 1:
The patent implements adaptive excitation voltage control that dynamically adjusts the excitation signal magnitude based on the measured capacitance value. The excitation voltage is increased for smaller capacitance measurements and decreased for larger capacitance measurements, optimizing the signal-to-noise ratio across the full measurement range and improving both linearity and sensitivity without requiring a completely complex circuit architecture.
Solution Approach 2:
The patent changes the excitation signal parameters (magnitude and phase) to improve measurement performance. By using two-phase excitation signals with programmable ratio-metric voltage generators, the system achieves better transfer function linearity and sensitivity. The excitation voltage magnitude is specifically adjusted to optimize the dynamic range for different capacitance measurement ranges.
2Measurement precision
If the excitation signal magnitude is increased to improve sensitivity, then measurement sensitivity improves, but the dynamic range becomes limited
Solution Approach 1:
The adaptive excitation control dynamically adjusts the excitation signal magnitude based on the measured capacitance value. For small capacitance measurements, a larger excitation magnitude is applied to improve sensitivity. For large capacitance measurements, a smaller excitation magnitude is used to maintain adequate dynamic range. This dynamic adaptation resolves the contradiction between sensitivity and dynamic range.
Solution Approach 2:
The patent uses two-phase excitation signals that periodically switch between different voltage levels and phases. This periodic action allows the system to extract more information from each measurement cycle and optimize the transfer function across different operating points, effectively expanding the usable dynamic range while maintaining high sensitivity through programmable ratio-metric voltage generation.
3Device complexity
If simple excitation schemes are used, then the circuit design is simpler, but the transfer function linearity and sensitivity are degraded
Solution Approach 1:
The patent employs two-phase excitation signals that periodically switch between different voltage levels. This periodic excitation scheme improves transfer function linearity by ensuring that the capacitance measurement is performed under optimized conditions for each phase, allowing the system to achieve better linearity without requiring excessively complex circuitry.
Solution Approach 2:
The patent uses programmable ratio-metric voltage generators to precisely control the excitation signal parameters. By programmatically adjusting the voltage ratios and magnitudes, the system achieves improved transfer function linearity and sensitivity while keeping the overall circuit architecture relatively simple and based on conventional sigma delta modulator building blocks.
4Measurement precision
If conventional excitation levels are used, then the circuit operates within standard voltage ranges, but the resolution for pressure changes is insufficient
Solution Approach 1:
The adaptive excitation control system dynamically adjusts the excitation signal magnitude based on the measured capacitance value, which correlates with pressure. For measurements requiring higher resolution (smaller capacitance changes), the system increases the excitation magnitude to improve the signal-to-noise ratio. This dynamic adjustment achieves high pressure measurement resolution while managing energy consumption by only using higher excitation levels when necessary.
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
These improvements result in higher resolution and sensitivity for capacitance measurements, enabling better detection of pressure changes and addressing the limitations of existing sensor circuits.
Implementation Method 1
The variable capacitance produced by a capacitance sensor must be converted to an electrical signal
Implementation Method 2
Capacitance-to-digital (C/D) converters using sigma delta modulators have been used in conjunction with capacitance sensors
Data Source
Figure 1A~1B
Figure 2
Figure 3A~3B
AI summary
A single plate capacitance sensor includes a sensor capacitor and a reference capacitor that share common plate. A capacitance-to-digital sigma delta modulator provides separate sensor excitation and reference excitation signals to the sensor capacitor and the reference capacitor to provide high resolution detection. Programmable ratio-metric excitation voltages and adaptive excitation voltage sources can be used to enhance modulator performance.