Current-Steering DAC Level Shifting for Higher Swing Linearity
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Solution Overview
Problem
Current steering Digital-to-Analog Converters (DACs) face limitations in maximum output amplitude, which restricts their linearity performance, particularly when the voltage swing exceeds 1.6V, causing degradation in Spurious-Free Dynamic Range (SFDR) and Third-Order Intermodulation (IM3) metrics.
Innovation Solution
The integration of level shifters with regulated supply and ground, along with additional devices and small currents between switches and resistor loads, allows for programmable expansion of output amplitude and enhancement of linearity, achieved through the use of regulators and amplitude sampling circuits to maintain switches in the saturation region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the output voltage swing is increased beyond 1.6V, then the output amplitude is expanded, but the switch moves into the linear region causing degraded linearity performance
Solution Approach 1:
An intermediate voltage adjustment stage is introduced between the digital input and the current steering switches. This stage uses regulated supply voltages (AVDD1, AVDD2) and ground references to shift the voltage levels, allowing the switches to maintain saturation region operation even when the output voltage swing exceeds 1.6V. The intermediate stage acts as a mediator that decouples the output amplitude from the switch operating region.
Solution Approach 2:
The supply voltages and ground references for the voltage adjustment stage are made programmable and adjustable. By changing these parameters (AVDD1, AVDD2, and ground levels), the maximum output amplitude can be programmed to different values while maintaining switch saturation. This parameter adjustment capability allows the system to adapt to different output amplitude requirements without sacrificing linearity.
2Device complexity
If conventional current steering DAC configuration is used, then the circuit structure is simple, but the maximum output amplitude is limited to around 1.6V
Solution Approach 1:
The DAC circuit is divided into distinct functional stages: a digital input stage, a voltage adjustment stage with level shifters, and a current steering stage. This segmentation allows each stage to be optimized independently - the voltage adjustment stage handles amplitude expansion while the current steering stage maintains simple switch-based operation. The segmentation enables output amplitude exceeding 1.6V without complicating the overall circuit structure.
Solution Approach 2:
The voltage adjustment stage with regulated supplies serves multiple functions simultaneously: it provides level shifting for the switches, sets the maximum output amplitude through programmable supply voltages, and maintains switch biasing in the saturation region. This multi-functionality achieves amplitude expansion without proportionally increasing circuit complexity.
Data Source
AI summary
A method of expanding current steering Digital-to-Analog Converter (DAC) output amplitude and enhancing linearity performance. Level shifters with regulated supply and ground voltage are inserted before current source latches. Extra devices and small current are placed between switches and resistor load to enhance the linearity of current steering DAC.