RF DAC Shadow Capacitor Switching for Better Linearity

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Solution Overview

Problem

Code-dependent output impedance loading causes non-linearity and distortion in digital-to-analog converters (DACs), particularly at high speeds, due to fluctuating resistances and capacitances in transistor-based DAC cells.

Innovation Solution

The implementation of shadow capacitor switching in DAC current steering cells, where shadow capacitors are coupled between output nodes and ground, controlled by data inputs, to maximize switching output impedance and reduce code-dependent loading by simulating constant capacitance, thereby minimizing distortion.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If shadow capacitor switching is implemented, then linearity is improved and code-dependent output impedance loading is reduced, but device complexity increases due to additional capacitors and switches

Engineering Contradiction:

ImprovelinearityVSAvoiddevice complexity

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The DAC output impedance is segmented into multiple components by introducing shadow capacitors that can be independently switched. This segmentation allows the total output impedance to be maintained constant by compensating for variations in the main capacitive component through the shadow capacitors, thereby improving linearity while managing complexity through modular design

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention changes the parameter of output impedance by dynamically adjusting the shadow capacitor connections based on the digital input code. When the main capacitor's effective value changes with the code, the shadow capacitors are switched to compensate, maintaining a constant total output impedance and reducing code-dependent loading effects

Inventive Principle:

Principle #35Parameter changes

2Manufacturing precision

If additional bleeder currents are added to improve linearity, then distortion is reduced, but power consumption increases

Engineering Contradiction:

ImprovelinearityVSAvoidpower consumption

Core Design Contradiction:

Manufacturing precisionVSUse of energy by moving object

Solution Approach 1:

The invention replaces the electrical mechanism of bleeder currents with a capacitive switching mechanism. Instead of using additional current sources to compensate for code-dependent loading, shadow capacitors are switched to dynamically adjust the output impedance, achieving the same linearity improvement without the continuous power consumption associated with bleeder currents

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Data Source

PatentUS10944417B1Radio frequency DAC with improved linearity using shadow capacitor switching

Publication Date: 2021.03.09 XILINX INC

AI summary

A DAC current steering circuit includes a first transistor whose: drain is coupled to a first output, source is coupled to a drain of a second transistor at a first node, and gate is coupled to a data input, and a third transistor whose: drain is coupled to a second output, source is coupled to a drain of a fourth transistor at a second node, and gate is coupled to a complement of the data input. The circuit further includes first and second shadow capacitors respectively coupled, via first and second switches, between the first and second nodes and ground, the first and second switches respectively controlled by the complement of the data input, and the data input.