Calibrated Capacitance-to-Digital Converter for High-PSRR Linearity
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Solution Overview
Problem
Conventional capacitance-to-digital converters (CDCs) face challenges in achieving high resolution and low noise, particularly in maintaining high power supply rejection ratio (PSRR) and linearity, especially with single-ended capacitive sensors.
Innovation Solution
The proposed CDC incorporates a calibration unit that reduces the influence of input capacitor variations and non-linearity, utilizing a sigma-delta modulator with an input-common-mode feedback circuit and digital calibration, and employs a fully-differential architecture to improve PSRR and linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional CDCs generate an intermediate voltage for digitization, then the conversion process is simplified, but the power supply rejection ratio (PSRR) and linearity deteriorate
Solution Approach 1:
The patent replaces the conventional voltage-based intermediate conversion with a charge-based direct digitization approach. The sigma-delta modulator directly processes charge from the capacitive sensor without generating intermediate voltage signals, thereby eliminating PSRR issues associated with voltage generation while maintaining conversion simplicity through direct charge-to-digital conversion.
Solution Approach 2:
The patent introduces a charge redistribution mechanism as an intermediary between the capacitive sensor and the digital converter. This charge-based intermediary allows direct coupling to the sigma-delta modulator input, avoiding the need for voltage generation stages and their associated PSRR limitations while preserving measurement accuracy.
2Device complexity
If single-ended capacitive sensors are used, then the sensor structure is simplified, but achieving high PSRR and linearity becomes difficult
Solution Approach 1:
The patent transforms the single-ended sensor output into a differential signal domain through the charge redistribution mechanism. By mapping the single-ended capacitance variation onto differential charge components that feed the sigma-delta modulator, the system achieves high linearity and PSRR performance typically associated with differential sensors while maintaining the structural simplicity of single-ended sensors.
Solution Approach 2:
The patent changes the operating parameter domain from voltage (conventional approach) to charge (innovative approach). This parameter transformation allows single-ended sensors to achieve differential-like performance characteristics in terms of linearity and PSRR, as the charge-based processing inherently rejects common-mode disturbances and linearizes the sensor response.
3Measurement precision
If high resolution is achieved through conventional methods, then measurement accuracy improves, but noise performance deteriorates
Solution Approach 1:
The patent substitutes voltage-based signal processing with charge-based processing throughout the conversion chain. This substitution eliminates noise sources associated with voltage amplification and buffering stages, as charge signals can be directly processed by the sigma-delta modulator with high resolution while maintaining low noise performance through direct coupling and minimal signal conditioning.
Data Source
Figure 1A~1B
Figure 1C~1D
Figure 2A~2B
AI summary
A capacitance-to-digital converter (10) comprises a capacitor arrangement (30), a converter (1) that is coupled on its input side to the capacitor arrangement (30) and a calibration unit (13) that is coupled on its input side to the converter (1). The capacitor arrangement (30) comprises an input capacitor (16).