DAC Delta-Sigma Calibration for Bit Weight and Timing Errors
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Solution Overview
Problem
Data converters, particularly digital-to-analog converters (DACs), suffer from imperfections such as amplitude and timing errors, leading to noise and spurs in the output, which degrade the converter's performance and can affect other parts of the signal chain.
Innovation Solution
The use of delta-sigma patterns to drive reference and test DAC cells, allowing for the calibration of amplitude and timing errors by generating test signals that cancel out at the analog output, leaving a residual signal representative of the errors, which can then be sensed and corrected.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If traditional calibration methods using square wave signals are used, then the calibration process is simple to implement, but the method cannot be scaled to perform amplitude calibration between bit cells having different bit weights and is limited in frequency to integer fractions of the sampling clock
Solution Approach 1:
The patent changes the calibration signal from traditional square waves to delta-sigma modulated sequences, transforming the signal characteristics to enable scalable amplitude calibration across different bit weights while avoiding frequency limitations. This parameter change allows the same calibration approach to work for MSB, ISB, and LSB segments with different weights.
Solution Approach 2:
The patent applies segmentation by treating different bit weight groups (MSB, ISB, LSB) as separate segments that can be calibrated independently using the same delta-sigma pattern methodology. Each segment receives appropriately scaled test signals that account for its specific bit weight, enabling versatile calibration across the entire DAC range.
2Measurement precision
If additional DAC cells are added to perform calibration, then measurement precision improves, but device complexity and area increase
Solution Approach 1:
The patent implements self-service calibration where the existing DAC cells serve dual purposes: normal conversion operation and calibration measurement. By injecting delta-sigma test patterns into the existing DAC structure and measuring the analog output, the system calibrates itself without requiring external test equipment or additional dedicated calibration cells.
Solution Approach 2:
The patent makes the existing DAC cells universal by enabling them to perform both their primary conversion function and calibration measurement function. The same DAC cells that convert digital audio signals also process delta-sigma calibration patterns, eliminating the need for separate calibration hardware while maintaining measurement precision.
3Measurement precision
If complex calibration hardware is used, then measurement precision improves, but ease of manufacture and device complexity worsen
Solution Approach 1:
The patent merges the calibration function with the existing DAC structure and control logic. The delta-sigma test pattern generation is integrated into the existing digital signal processing path, and the calibration measurements are performed using the same analog output infrastructure already present in the DAC, simplifying manufacturing while maintaining precision.
Solution Approach 2:
The patent uses the delta-sigma modulated signal as an intermediary that bridges the digital control domain and analog measurement domain. This intermediary signal enables precise error measurement through standard existing infrastructure without requiring complex dedicated calibration hardware, facilitating easier manufacturing integration.
Data Source
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AI summary
A digital to analog converter (DAC) maps a digital word to an analog output. The DAC bits may have amplitude and timing errors. These errors (or sometimes referred herein as "non-idealities") result in distortion and degradation of the dynamic range in DACs. To reduce these negative effects, delta-sigma patterns can be provided to two bit cells, a reference bit cell and a bit cell under calibration, to perform, e.g., amplitude calibration and timing skew calibration. Delta-sigma patterns are particularly advantageous over square wave signals, which cannot be scaled to perform amplitude calibration between bit cells having different bit weights and are limited in frequency to integer fractions of the sampling clock.