DEM Encoder Topology for Quasi-Constant DAC Switching Transitions
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Solution Overview
Problem
Existing dynamic element matching (DEM) techniques in digital-to-analog converters (DACs) face limitations in achieving high-linearity due to mismatches and couplings, particularly in digital-controlled oscillators (DCOs) used in digital phase locked loops (DPLLs, which degrade phase noise and affect radar functionality.
Innovation Solution
A DEM encoder system with bypassable switching blocks and DEM encoders that scramble the input control-word to randomize capacitor usage, minimizing capacitive mismatches and maintaining a quasi-constant number of transitions, thereby improving linearity and reducing phase noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If existing DEM techniques are used to scramble capacitor usage patterns, then non-linear distortion is converted to high-frequency noise, but linearity degradation occurs due to mismatches and couplings between individual capacitive cells
Solution Approach 1:
The capacitor bank is divided into multiple groups, with each group controlled by a separate DEM encoder. This segmentation allows independent optimization of each group's capacitor selection, reducing the impact of mismatches within individual groups and improving overall linearity while maintaining effective noise shaping.
Solution Approach 2:
The patent implements a dynamic capacitor selection mechanism where the DEM encoders continuously adjust which capacitors are activated based on the input control word and previous state. This dynamic switching pattern randomizes the usage of individual capacitive cells, converting systematic mismatch errors into pseudorandom noise that appears as high-frequency phase noise rather than low-frequency distortion.
2Manufacturing precision
If DEM encoders scramble capacitor usage to reduce mismatch effects, then non-linear distortion is reduced, but the number of transitions varies which can cause spectral spreading
Solution Approach 1:
The patent modifies the transition behavior by changing the parameter of transition counting and control. The DEM encoders are designed to monitor and control the number of transitions, adjusting the capacitor switching pattern to maintain a quasi-constant transition count. This parameter control prevents excessive spectral spreading while preserving the mismatch randomization benefits.
3Ease of operation
If individual capacitive cells are selectively enabled or disabled to regulate total capacitance, then frequency control is achieved, but capacitive mismatches among cells result in non-linear distortion
Solution Approach 1:
The DEM encoders implement a feedback mechanism where the current state and input control word are used to determine the next capacitor configuration. This feedback approach ensures that transitions are minimized and controlled, reducing the impact of capacitive mismatches on spectral purity while maintaining accurate frequency control through the capacitor bank.
Data Source
AI summary
A dynamic element matching (DEM) encoder system configured to convert an N-bit control word into a pattern of 1-bit values. The DEM encoder system includes a plurality of bypassable switching blocks comprising an encoder input configured to receive the N-bit control word and a plurality of control outputs configured to provide a plurality of intermediate control values based on the N-bit control word, wherein the plurality of bypassable switching blocks are connected in a series; and a plurality of DEM encoders configured to receive the plurality of intermediate control values and generate a plurality of encoder output values based on the plurality of intermediate control values, wherein each encoder output value is a respective 1-bit value of the pattern of 1-bit values.