SAR ADC Correction DAC for DNL and Code Transition Linearity
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Solution Overview
Problem
Digitally corrected successive approximation analog to digital converters (ADCs) face challenges in eliminating differential non-linearity (DNL) errors due to errors in bit weights, which require redundancy and post-processing correction, affecting the accuracy and linearity of the conversion process.
Innovation Solution
Incorporating a correction digital to analog converter (DAC) that calculates and applies a residue correction value based on the difference between actual and ideal bit weights, effectively reducing DNL errors by adjusting the output to ensure equally spaced code transitions and improving integral non-linearity (INL).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If digital correction is applied to remove bit weight errors, then integral non-linearity (INL) is improved, but differential non-linearity (DNL) errors remain due to unchanged code transition points
Solution Approach 1:
The patent segments the correction process into two distinct parts: (1) digital correction of bit weight errors through post-processing to improve INL, and (2) a separate correction DAC to adjust code transition points for DNL improvement. This segmentation allows each correction mechanism to address its specific error type without interfering with the other, resolving the contradiction between INL and DNL performance.
Solution Approach 2:
The patent introduces a correction DAC as an intermediary component between the main ADC and the output. This correction DAC receives digital correction values and converts them to analog signals that are added to the main ADC output, thereby adjusting code transition points to improve DNL while preserving the digital correction benefits for INL.
2Reliability
If redundancy bits are added to avoid missing codes, then reliability is improved, but device complexity increases
Solution Approach 1:
The patent applies partial redundancy by adding only enough correction bits to prevent missing codes without fully oversampling. Specifically, for an N-bit ADC, the patent uses an (N+1)-bit correction DAC, providing just sufficient redundancy to eliminate missing codes while minimizing the increase in device complexity. This partial action approach balances reliability improvement with acceptable complexity increase.
3Measurement precision
If correction DAC output spans full range, then correction capability is maximized, but device complexity and power consumption increase
Solution Approach 1:
The patent applies local quality by making the correction DAC's output range adaptive to the specific error characteristics of the main ADC. Instead of designing the correction DAC to cover the full analog range, the patent sizes the correction DAC to cover only the residual error range after digital correction, which is typically a small fraction of the full scale. This reduces the correction DAC's complexity and power consumption while maintaining sufficient correction capability for the remaining errors.
Solution Approach 2:
The patent changes the parameter of correction DAC output range from full-scale to residual-error-scale. By calculating the maximum expected error after digital correction and sizing the correction DAC accordingly, the patent optimizes the correction DAC's resolution and bit-width to match the actual correction needs, thereby reducing device complexity and power consumption while maintaining adequate correction capability.
Data Source
AI summary
An analog to digital converter having improved differential non-linearity is provided. The converter has a memory which is used to look up the actual weight or a weight error corresponding to the bits that have been kept as part of the SAR process to form an output correction value A part of this, for example a residue (the part following the decimal point in a decimal representation) is used to drive a correction DAC which causes a correction to be applied to the trial value presented to a comparator used by the ADC.