Shielded Capacitor Array Layout for Monotonic Capacitance Steps
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Solution Overview
Problem
Digital capacitor arrays face limitations in precision and monotonicity due to manufacturing process variations and lack of effective electrical shielding, leading to non-monotonic transitions and increased stray capacitances.
Innovation Solution
A combination binary-thermometer coding scheme and a common-centroid capacitor array layout with individual shielding of unit capacitors are employed to minimize process variations and ensure monotonic capacitance steps, using a combination of binary and thermometer coded bits to balance signal routing complexity and precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If unit capacitors are made with smaller capacitance to increase resolution, then the minimum capacitance step decreases, but stray capacitances become significant and manufacturing variations increase
Solution Approach 1:
The capacitor array is divided into multiple groups with different weighting schemes (binary-weighted and thermometer-coded segments). Each group handles specific bit ranges, allowing optimization for different precision requirements while managing stray capacitance effects independently in each segment.
Solution Approach 2:
Different coding schemes are applied to different portions of the capacitor array. Binary-weighted coding is used for less significant bits where smaller capacitance values are needed, while thermometer coding is used for more significant bits where larger capacitance values require better matching. This local optimization resolves the contradiction between resolution and manufacturing precision.
2Device complexity
If digital capacitor array uses simple parallel connection of unit capacitors, then circuit complexity is reduced, but monotonicity and precision are compromised due to process variations
Solution Approach 1:
The capacitor array is segmented into multiple groups with different coding schemes. Binary-weighted segments use fewer capacitors with larger values, while thermometer-coded segments use more capacitors with smaller values. This segmentation maintains monotonicity by ensuring that transitions between capacitance values are controlled and predictable, even with process variations.
Solution Approach 2:
The patent employs asymmetric coding schemes where different portions of the capacitor array use different weighting patterns. The binary-weighted portion uses exponential scaling while the thermometer-coded portion uses linear scaling. This asymmetric approach optimizes for both monotonicity and precision without requiring uniform treatment of all capacitors.
3Area of stationary object
If capacitor array elements are closely spaced to reduce area, then chip area is reduced, but electrical shielding becomes insufficient and stray capacitances increase
Solution Approach 1:
Grounded shielding structures are introduced as intermediary elements between adjacent capacitor groups. These shielding structures act as electrical barriers that prevent capacitive coupling between neighboring elements. By placing these intermediaries strategically, the patent reduces stray capacitance effects while maintaining compact spacing between functional capacitor elements.
4Device complexity
If binary-weighted coding is used for all bits, then routing complexity is minimized, but precision and monotonicity deteriorate due to accumulated manufacturing variations
Solution Approach 1:
The binary-weighted capacitor array is segmented and combined with thermometer-coded segments. The binary-weighted portion handles less significant bits where routing complexity is a primary concern, while the thermometer-coded portion handles more significant bits where precision and monotonicity are critical. This segmentation allows each segment to be optimized for its specific requirements.
Solution Approach 2:
The patent changes the coding parameter (weighting scheme) used for different bit positions. Instead of uniformly applying binary-weighted coding, it transitions to thermometer coding for significant bits where precision matters most. This parameter change optimizes the trade-off between routing complexity and measurement precision by adapting the coding scheme to the significance of each bit position.
Data Source
AI summary
A digital capacitor array with individually shielded unit capacitors and combination binary—thermometer coded addressing is disclosed. Such a capacitor array may be part of a digitally controlled oscillator in a MEMS-based frequency reference.