Adaptive Digital Pre-Distortion for Delta-Sigma DAC Linearity
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Solution Overview
Problem
Digital audio systems face linearity issues due to nonlinearities in digital to analog converters (DACs), particularly in n-well resistors, which vary with voltage, temperature, and manufacturing process corners, affecting the system's performance over time.
Innovation Solution
An adaptive digital pre-distortion block implemented in digital components with programmable registers that dynamically or statically adjusts the pre-distortion transfer function to correct nonlinearities, ensuring a linear response by modeling and compensating for second-order or higher-order polynomial nonlinearities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If n-well resistors are used in the current-to-voltage converter, then ease of manufacture is improved, but linearity performance deteriorates due to voltage-dependent resistance variations
Solution Approach 1:
The patent applies preliminary action by pre-characterizing the nonlinear behavior of n-well resistors during manufacturing and storing correction coefficients in lookup tables. The DAC system then uses these pre-computed correction values to compensate for nonlinearities in real-time operation, eliminating the need to switch to more complex resistor types while maintaining linearity performance.
2Manufacturing precision
If silicide block poly resistors are used to improve linearity, then linearity performance is improved, but device complexity and manufacturing cost increase due to additional processing steps
Solution Approach 1:
The patent substitutes the physical/structural solution (complex silicide block poly resistors) with a digital/software-based solution. Instead of using physically complex resistors to achieve linearity, the system uses digital lookup tables and correction algorithms to compensate for nonlinearities, thereby maintaining linearity performance while reducing device complexity and manufacturing cost.
3Device complexity
If standard DAC design is used, then device complexity is minimized, but linearity performance deteriorates due to inherent nonlinearities in the current-to-voltage conversion stage
Solution Approach 1:
The patent introduces an intermediary correction mechanism between the digital input and analog output stages. Lookup tables containing pre-computed correction coefficients act as an intermediary that modifies the digital signal to compensate for the nonlinear behavior of the n-well resistors, thereby maintaining linearity without adding physical complexity to the DAC structure.
4Volume of moving object
If nonlinear resistors are used to minimize device size, then device size is reduced, but linearity performance deteriorates due to voltage-dependent resistance variations
Solution Approach 1:
The patent converts the harmful nonlinear characteristic of n-well resistors into a beneficial feature by characterizing their specific nonlinear behavior and using it to drive correction algorithms. The voltage-dependent resistance variation, which is normally harmful, becomes predictable and compensatable through lookup tables, allowing the system to maintain linearity while benefiting from the compact size of n-well resistors.
Data Source
AI summary
An adaptive digital pre-distortion block is used to cancel device nonlinearities to improve the overall linearity of a Delta-Sigma DAC system. In particular, the pre-distortion block may be implemented all in digital components and utilize programmable registers that change the pre-distortion transfer function either statically or dynamically, or both. Static changes can be for variation in process corners during production, whereas, dynamic changes can be used to correct nonlinear changes that can occur from environmental reasons such as voltage, temperature, aging and device stress.