Current-Steering DAC Common-Mode Correction for Static Linearity
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Solution Overview
Problem
Existing digital-to-analog converters (DACs) face challenges in achieving accurate calibration and reducing current source matching errors, particularly in R-2R resistor ladder DACs, which affect the static linearity and accuracy of the conversion process.
Innovation Solution
The implementation of current-steering cells with bypass switches in a resistor ladder circuit allows for common-mode correction by selectively providing bypass currents to a common node, enabling improved calibration accuracy and relaxed current source matching specifications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional R-2R resistor ladder DAC is used, then the basic digital-to-analog conversion function is achieved, but current source matching errors degrade static linearity and conversion accuracy
Solution Approach 1:
The DAC is divided into multiple independent current-steering cells, each handling a specific bit weight. Each cell contains its own bypass switch and connects to specific taps on the resistor ladder, allowing independent calibration and correction of matching errors for each cell without affecting others.
Solution Approach 2:
A calibration process is performed before normal operation to pre-determine the optimal bypass current settings for each current-steering cell. This preliminary calibration establishes correction values that compensate for manufacturing variations in current sources and resistors, improving static linearity before the DAC is used for actual conversions.
2Measurement precision
If bypass switches are added to each current-steering cell for common-mode correction, then calibration accuracy and static linearity are improved, but device complexity increases
Solution Approach 1:
The bypass switches serve multiple functions: they provide common-mode correction during calibration, enable independent cell calibration, and can be used to adjust for both current source mismatches and resistor ladder errors. This multi-functionality justifies the added complexity by delivering multiple benefits from a single circuit element.
Solution Approach 2:
The bypass switches act as intermediaries between the current-steering cells and the resistor ladder circuit. They provide a controlled path for correction currents that mediate the interaction between the digital control logic and the analog output, enabling precise adjustment without directly modifying the main signal path.
3Ease of manufacture
If current source matching is relaxed to reduce manufacturing complexity, then ease of manufacture improves, but conversion accuracy deteriorates
Solution Approach 1:
The calibration process measures the actual output of each current-steering cell and uses this feedback information to determine the appropriate bypass current settings. This feedback loop compensates for manufacturing variations in current sources, allowing relaxed matching tolerances while maintaining high conversion accuracy through software-based correction.
Solution Approach 2:
The system changes the operating parameters of the current-steering cells by adjusting bypass currents to optimal values determined during calibration. This parameter adjustment compensates for manufacturing variations in current source strengths and resistor values, enabling accurate conversions even with relaxed manufacturing specifications.
Data Source
AI summary
Certain aspects of the present disclosure provide a digital-to-analog converter (DAC). The DAC generally includes a plurality of current-steering cells, each having a bypass switch, and a resistor ladder circuit having multiple segments. Each segment may include a first resistive element and a second resistive element, the bypass switch being configured to selectively provide a bypass current to a common node between the first resistive element and the second resistive element.