Understanding and Effectively Suppressing the Noise Coupling in Mixed-Signal SoC Applications. 

By: Dr. Cosmin Iorga.

Description
 

Introduction:
 

Semiconductor industry has significantly advanced in the past decades mainly driven by computing technology, Internet, communication networks, and portable consumer electronics.  Market demands requiring increased functionality and lower costs have pushed the technology scaling down to sub-micron, deep sub-micron, and nano-scale dimensions. 
 

Technology scaling has generated the trend to integrate analog and digital blocks on the same chip, concept referred as mixed-signal integrated circuit, and is heading towards the goal of implementing a complete system in a single chip, concept referred as System-on-Chip (SoC).  SoCs eliminate the connection between multiple chips used in previous architectures, thus reducing the number of output buffers and the cost of packaging and fabrication. 
 

Besides the advantages of integrating more functionality in a single chip, mixed-signal integrated circuits and SoCs encounter tremendous challenges.  These challenges are generated primarily by the high density of circuits and include coupling interaction between blocks and interconnects, increased power supply and substrate noise, and limitations of design and verification tools. 
 

Because mixed-signal integrated circuits and SoCs implement sensitive analog circuits on the same die with high-speed digital processing circuits, the switching noise produced by the digital circuits propagates through substrate and power distribution to the analog circuits, degrading their performance.  This problem aggravates with technology scaling because larger number of transistors and more functions are implemented in the digital core resulting in more noise injected into substrate and power distribution circuits.  Device scaling increases the substrate doping concentration to reduce the transistors threshold voltage.  As a consequence, the substrate conductivity increases and provides a lower resistive path for noise coupling.  The device scaling reduces the headroom and voltage swings in the analog circuits, thus making them more sensitive to the coupled noise.  As transistor sizes are predicted to shrink to smaller and smaller dimensions, the noise coupling challenges are projected to worsen.  To overcome these challenges, the development of coupling suppression techniques and circuits less sensitive to noise is essential.  
 

This tutorial builds the skills necessary to efficiently address the noise coupling challenges in mixed-signal integrated circuits and SoCs.  During this tutorial the attendees develop a thorough understanding of the noise coupling physical mechanisms at device, chip substrate, interconnects, package, and PCB levels, and apply this knowledge to understand the applicability and limitations of various suppression techniques and simulation methodologies. 
 

After completing this tutorial, attendees are expected to be able to identify the noise coupling mechanisms in a particular design and to choose and implement the most efficient suppression techniques that match these coupling mechanisms.  Attendees are also expected to be able to select the most effective simulation tool or set of tools that cover all the noise coupling mechanisms in their specific chip/package/PCB design, and to create an efficient simulation methodology that estimates accurately the noise coupling effects after fabrication. 

Tutorial Section 1:  Overview of Noise Coupling in Integrated Circuits
 

This is an introductory section that discusses general aspects of noise coupling in mixed-signal integrated circuits and potential problems that may occur due to noise coupling.   Examples of noise coupling effects on circuits’ performance are also presented and analyzed.  Since in mixed-signal integrated circuits most of the generated noise depends on the digital activity, in large and complex chips the noise coupling issues often have an intermittent nature and are hard to duplicate.  The troubleshooting relays on identifying the sequence or combination of digital vectors that created the problem, which in many cases depend on the level of testability implemented in the digital cores.   To reduce the chances of having noise coupling issues, designers implement suppression techniques and estimate the noise coupling effects before fabrication.  This section emphasizes the importance of understanding the noise coupling mechanisms in the combined chip-package-PCB structure in order to select the right suppression techniques and simulation methodologies.

Tutorial Section 2: Noise Coupling Mechanisms
 

This section analyzes the noise generation, propagation, and reception at the physical structure levels of devices, chips, packages, and printed circuit boards. The development of equivalent circuits modeling the noise coupling mechanisms is presented.  It is also emphasized that noise couples through both the common substrate and the power distribution.  The architecture of the power distribution on chip, package, and PCB, and the choice of values and types of decoupling capacitors have a major impact on noise generation and propagation.  Power distribution resonance and noise generation dependence on the frequency characteristics of the power distribution impedance is analyzed.  Emphasis is placed on building and improving analysis skills and methodologies so that attendees can apply the knowledge and techniques learned here to existing and future devices and technologies.
  
Tutorial Section 3: Noise Coupling Suppression
 

This section presents and analyzes various noise coupling suppression techniques.   Methods of reducing the noise generation, propagation, and reception are analyzed with relation to the physical structures of devices, chip substrate, package, and PCB, discussing the advantages, disadvantages, and limitations of each method.   Traditional guard rings and shields reduce the noise coupling but do not completely eliminate it.  It is illustrated how, in some cases, guard rings and shields can actually inject additional noise into the protected regions.  To avoid this, the understanding of noise coupling and suppression mechanisms is essential.  It is also discussed the suppression by properly designing the power distribution at the system, board, package, and chip levels.  The analysis results suggest that the choice and efficiency of suppression techniques depend directly on the specific noise coupling mechanisms of each individual case.  To overcome the limitations of traditional suppression techniques, additional circuit level noise cancellation methodologies and implementation examples are presented and analyzed.

Tutorial Section 4: Noise Coupling Simulation
 

This section covers the simulation of noise coupling in integrated circuits.  The selection choice and use of conventional methods and tools are analyzed, focusing on the advantages and limitations specific to both post-layout and pre-layout stages of the design flow.  It is highlighted the importance of understanding the noise coupling mechanisms before choosing a simulation tool or set of tools.  Designers need to make sure the tools used cover all the noise coupling mechanisms in each particular chip-package-PCB structure before trusting the simulation results.  Since some decisions that affect noise coupling are typically taken in early stages of the design flow, it is desirable to be able to estimate the noise coupling during the architectural definition of the project.  Since most of the existing tools do not offer a practical approach to noise coupling simulation in early stages, this section presents a modeling methodology based only on information typically available in the architectural definition stages of projects.  The model is constructed based on the physical structure of devices, technology parameters available in the design guide, and statistical data from typical practices or previous designs.  An example showing the model construction and correlation with measurements on a test chip are also presented. 

To register for this Workshop, click here: NoiseCoupling Workshop Registration. 

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