Segmented Current-Steering DAC With Auxiliary Switching for SFDR
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Solution Overview
Problem
High-speed current-steering digital-to-analog converters (DACs) face limitations in dynamic performance due to intrinsic effects that generate third-order harmonic distortion, particularly from variations in summing node potential and output impedance, leading to signal-dependent switching activity and distortion.
Innovation Solution
The introduction of a segment switch architecture with a main switch and an auxiliary switch, where either the main or auxiliary switch is toggled in each clock cycle, ensuring constant switching activity independent of the output signal, effectively canceling harmonic distortion by creating the same transients as the main switch without altering the output current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional current-steering DAC uses a single main switch per segment, then the device complexity is low, but third-order harmonic distortion increases due to signal-dependent switching activity
Solution Approach 1:
The DAC is divided into multiple segments, each with its own main and auxiliary switches. This segmentation allows independent control of switching activity in each segment, enabling distortion cancellation while maintaining overall system manageability. The segmentation principle is applied by dividing the current-steering DAC into N segments with 2N switches total.
Solution Approach 2:
An auxiliary switch is introduced as an intermediary element that generates transients identical to the main switch but with opposite polarity. This intermediary switch cancels the harmful third-order distortion components without affecting the fundamental output signal, effectively mediating between the main switch and the output node.
2Productivity
If the DAC operates at high conversion rates, then the productivity increases, but dynamic performance deteriorates due to intrinsic effects generating distortion
Solution Approach 1:
The auxiliary switches are designed to operate continuously at the same switching rate as the main switches, ensuring that distortion cancellation is active during every transition. This continuous operation maintains constant switching activity regardless of the input signal pattern, eliminating signal-dependent distortion even at high conversion rates.
Solution Approach 2:
The switching activity parameter is changed from signal-dependent to constant by introducing auxiliary switches that toggle independently of the input data. This parameter change ensures that the switching frequency and pattern remain uniform, canceling third-order distortion components while maintaining high conversion rates.
3Object-generated harmful factors
If auxiliary switches are added to each segment for distortion cancellation, then the third-order distortion is reduced, but the device complexity increases
Solution Approach 1:
The main and auxiliary switches in each segment are merged into a unified control structure where both switches share the same control logic and timing. This merging reduces the overall control complexity despite doubling the switch count, as the paired switches operate in a coordinated manner to cancel distortion.
Solution Approach 2:
The auxiliary switch is essentially a copy of the main switch with identical switching characteristics but opposite polarity. This copying approach simplifies the design by using replicated circuit blocks rather than requiring complex custom circuits for distortion cancellation, making the increased complexity more manageable.
Data Source
AI summary
A digital-to-analog converter (DAC) includes, in a segment of the DAC, a first switch and a second switch. The first switch includes a first pair of transistors having a first set of inputs and has a first output connected to an output of the DAC. The second switch includes second and third pairs of transistors having second and third sets of inputs, respectively, and has a second output that is connected to the output of the DAC. A driver module generates control signals to drive the first, second, and third sets of inputs based on data received by the DAC for conversion from digital to analog format at a conversion rate determined by a clock. The control signals toggle one of the first and second switches during each cycle of the clock.