Feedback DAC Mismatch Measurement in Delta-Sigma ADCs
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Solution Overview
Problem
Analog-to-digital converters (ADCs) using feedback digital-to-analog converters (DACs) face challenges in accurately measuring and correcting switching mismatch errors, which affect the performance of delta-sigma modulators and pipeline modulators, leading to harmonic distortions and reduced Signal-to-Noise-and-Distortion Ratio (SNDR) and spurious free dynamic range (SFDR).
Innovation Solution
A digital measurement technique is implemented to determine switching mismatch error in DAC elements by forcing them to switch a certain number of times and using the modulator to measure the errors, which are then stored in a look-up table for full correction in the digital or analog domain. This involves applying a square wave signal to each DAC element and measuring the modulator output to quantify the switching mismatch error.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If background calibration techniques are used to measure DAC switching mismatch errors, then measurement capability is provided, but the process is complex and requires modulating each DAC element sequentially with out-of-band digital sequences
Solution Approach 1:
The modulator measures its own DAC switching mismatch errors using its existing feedback loop and quantizer, without requiring external calibration equipment or complex sequential modulation of each DAC element. The system performs self-diagnosis by analyzing the relationship between DAC output codes and corresponding modulator output codes.
Solution Approach 2:
The existing modulator components (feedback loop, quantizer, output stage) are utilized for dual purposes: normal signal conversion and self-calibration of DAC switching mismatch errors. This eliminates the need for dedicated calibration circuitry and simplifies the overall system architecture.
2Measurement precision
If sequential modulation of each DAC element is performed for calibration, then measurement accuracy is achieved, but calibration time and productivity are reduced
Solution Approach 1:
The system pre-establishes the correspondence between DAC output codes and modulator output codes by analyzing normal operating data, rather than performing time-consuming sequential modulation of each DAC element. This preliminary analysis of existing operational data enables rapid error measurement.
Solution Approach 2:
The calibration process operates continuously during normal modulator operation without requiring sequential interruption to modulate each DAC element individually. The system continuously monitors and measures switching mismatch errors while maintaining normal signal conversion functionality.
3Measurement precision
If traditional calibration methods are used, then DAC errors can be measured, but harmonic distortions and reduced SNDR/SFDR performance persist due to incomplete error correction
Solution Approach 1:
The measured switching mismatch errors are fed back to correct the DAC output codes in real-time, creating a closed-loop calibration system. This continuous feedback mechanism ensures that error corrections are applied dynamically, improving SNDR and SFDR performance by eliminating the source of harmonic distortions.
Solution Approach 2:
Instead of trying to physically adjust or replace mismatched DAC elements, the system inverts the approach by measuring the mismatch errors and compensating for them through digital correction of the output codes. This software-based compensation achieves the same effect as physical adjustment but with higher precision and without hardware modifications.
Data Source
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AI summary
For analog-to-digital converters (ADCs) which utilize a feedback digital-to-analog converter (DAC) for conversion, the final analog output can be affected or distorted by errors of the feedback DAC. A digital measurement technique can be implemented to determine switching mismatch error for the feedback DAC in a continuous-time delta-sigma modulator (CTDSM) or in a continuous-time pipeline modulator. The methodology forces each DAC unit elements (UEs) to switch a certain amount times and then use the modulator itself to measure the errors caused by those switching activities respectively. The obtained errors can be stored in a look-up table and fully corrected in digital domain or analog domain.