DAC Thermometer Coding Segmentation for Precision and Area
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Solution Overview
Problem
Existing digital-to-analog converters, such as R-2R DACs, face limitations in achieving high precision and monotonicity due to output impedance matching issues and increased noise, while thermometer coders require large integrated circuit areas and complex layout schemes.
Innovation Solution
A digital-to-analog converter system combining fractional-bit and multi-bit DAC cells controlled by thermometer coders, which reduces the number of semiconductor devices needed and optimizes current distribution for precise conversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a thermometer coder is used to ensure monotonicity and reduce noise, then precision and signal quality are improved, but the integrated circuit area and device complexity increase significantly
Solution Approach 1:
The digital input is divided into two portions: a first portion (MSBs) converted by a thermometer coder to generate first control signals, and a second portion (LSBs) converted by binary-coded DAC cells. This segmentation allows the system to achieve high precision through the thermometer coder for the significant bits while using fewer resources for the less significant bits, thereby reducing overall integrated circuit area while maintaining conversion precision.
Solution Approach 2:
Different conversion approaches are applied to different portions of the digital input signal. The most significant bits, which have greater impact on precision, are processed by the thermometer coder for high accuracy, while the less significant bits are processed by simpler binary-coded DAC cells. This local differentiation optimizes the balance between precision and area usage.
2Measurement precision
If a conventional segmented DAC is used to achieve higher resolution, then conversion precision is improved, but the layout complexity and integrated circuit area increase
Solution Approach 1:
The DAC is segmented into fractional-bit DAC cells controlled by thermometer-coded MSBs and multi-bit DAC cells controlled by binary-coded LSBs. This segmentation achieves high resolution through the fractional-bit cells while the multi-bit cells handle the remaining precision requirements with simpler architecture, thereby reducing overall layout complexity compared to a fully segmented approach.
3Ease of manufacture
If an R-2R DAC is used for binary-to-analog conversion, then implementation simplicity is improved, but output impedance matching and noise performance deteriorate
Solution Approach 1:
The R-2R DAC structure is segmented such that only the multi-bit DAC cells use the R-2R resistive network, while the fractional-bit DAC cells use a different architecture controlled by thermometer coding. This segmentation reduces the overall noise generated by the R-2R network while maintaining implementation simplicity for the majority of the conversion process.
Data Source
AI summary
A digital-to-analog converter comprising a digital input; a first thermometer coder for generating a set of first control signals based on a first portion of the digital input; at least one fractional-bit DAC cell controlled by one or more of the first control signals for providing a fractional-bit current based on the first portion of the digital input; at least one second thermometer coder for generating a set of second control signals based on a second portion of the digital input; and at least one multi-bit DAC cell controlled by one or more of the second control signals for providing a multi-bit current based on the second portion of the digital input, wherein the fractional-bit current and the multi-bit current are combined to form an output of the DAC corresponding to the digital input.