Ratio-Metric Capacitance Converter for Low-Noise Sensing
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Solution Overview
Problem
Capacitance sensing systems face challenges in achieving high sensitivity and low noise levels due to variations in power supply voltages, clock frequencies, and reference voltages, which affect the signal-to-noise ratio, especially in high-sensitivity and fast-scanning applications.
Innovation Solution
The implementation of a ratio-metric capacitance-to-digital converter (CDC) that uses a sensor cell and a reference cell with modulation capacitors to generate a bitstream representing the ratio of sensor capacitance to reference capacitance, independent of direct current voltage sources, clock frequencies, and supply voltage variations, allowing for common mode noise cancellation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance sensing systems use direct current voltage sources and clock frequencies for conversion, then the system can achieve basic capacitance measurement functionality, but the signal-to-noise ratio deteriorates due to variations in power supply voltages, clock frequencies, and reference voltages
Solution Approach 1:
The patent transforms the conversion mechanism from using absolute voltage and frequency parameters to using ratio-based parameters. The capacitance-to-digital converter measures the ratio of sensor capacitance to reference capacitance, making the measurement independent of power supply voltage variations and clock frequency drift. This parameter transformation eliminates the sensitivity to environmental variations while maintaining measurement functionality.
Solution Approach 2:
The patent introduces a reference cell as an intermediary element that mediates the measurement process. By comparing the sensor cell capacitance against a stable reference cell capacitance through ratio-metric conversion, the system achieves immunity to common-mode noise from power supply and clock variations. The reference cell acts as a mediator that cancels out environmental variations affecting both measurement paths equally.
2Measurement precision
If the system uses ratio-metric conversion independent of voltage sources and clock frequencies, then noise immunity improves, but the device complexity increases due to additional reference cells and modulation capacitors
Solution Approach 1:
The patent segments the conversion process into distinct functional blocks: sensor cell, reference cell, modulation capacitors, and ratio-metric converter. This segmentation allows each component to perform a specific function independently, making the overall complex system manageable and enabling modular design. The segmentation also facilitates independent optimization of each block for noise immunity.
Solution Approach 2:
The reference cell serves multiple functions: it provides a stable reference for ratio-metric conversion, cancels common-mode noise, and compensates for environmental variations. The modulation capacitors also serve dual purposes in charge transfer and signal conditioning. This multi-functionality reduces the need for separate compensation circuits and simplifies the overall design despite the ratio-metric approach.
Data Source
AI summary
Apparatuses and methods of capacitance-to-digital code conversion are described. One apparatus includes a bridge circuit and a modulator front-end circuit. The bridge circuit includes a first terminal to couple to a reference cell and a second terminal to couple to a sensor cell. The modulator front-end circuit includes a comparator coupled to the bridge circuit, a first modulation capacitor coupled to a first input of the comparator, and a second modulation capacitor coupled to a second input of the comparator. The modulator front-end circuit provides a digital bitstream. A duty cycle of the digital bitstream is representative of a ratio between a capacitance of the sensor cell and a reference capacitance of the reference cell.