This survey presents an overview of recent advances in the state of the art for computer-aided design (CAD) tools for analog and mixed-signal integrated circuits (ICs). Analog blocks typically constitute only a small fraction of the components on mixed-signal ICs and emerging systems-on-a-chip (SoC) designs. But due to the increasing levels of integration available in silicon technology and the growing requirement for digital systems to communicate with the continuous-valued external world, there is a growing need for CAD tools that increase the design productivity and improve the quality of analog integrated circuits. This paper describes the motivation and evolution of these tools and outlines progress on the various design problems involved: simulation and modeling, symbolic analysis, synthesis and optimization, layout generation, yield analysis and design centering, and test. This paper summarizes the problems for which viable solutions are emerging and those which are still unsolved.

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Computer-aided design of analog and mixed-signal integrated circuits

We describe a new method for determining component values and transistor dimensions for CMOS operational amplifiers (op-amps). We observe that a wide variety of design objectives and constraints have a special form, i.e., they are posynomial functions of the design variables. As a result, the amplifier design problem can be expressed as a special form of optimization problem called geometric programming, for which very efficient global optimization methods have been developed. As a consequence we can efficiently determine globally optimal amplifier designs or globally optimal tradeoffs among competing performance measures such as power, open-loop gain, and bandwidth. Our method, therefore, yields completely automated sizing of (globally) optimal CMOS amplifiers, directly from specifications. In this paper, we apply this method to a specific widely used operational amplifier architecture, showing in detail how to formulate the design problem as a geometric program. We compute globally optimal tradeoff curves relating performance measures such as power dissipation, unity-gain bandwidth, and open-loop gain. We show how the method can he used to size robust designs, i.e., designs guaranteed to meet the specifications for a variety of process conditions and parameters.

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Optimal design of a CMOS op-amp via geometric programming

The authors describe KOAN and ANAGRAM II, new tools for device-level analog placement and routing. Analog layout tools that merely apply known digital macrocell techniques fall short of achieving the density and performance of handcrafted analog cells. KOAN and ANAGRAM II differ from previous approaches by using general algorithmic techniques to find critical device-level layout optimizations rather than relying on a large library of fixed-topology module generators. New placement algorithms implemented in KOAN handle complex layout symmetries, dynamic merging and abutment of individual devices, and flexible generation of wells and bulk constants. New routing algorithms implemented in ANAGRAM II handle arbitrary gridless design rules in addition to over-the-device, crosstalk-avoiding, mirror-symmetric, and self-symmetric wiring. Examples of CMOS and BiCMOS analog cell layouts produced by these tools are presented. >

KOAN/ANAGRAM II: new tools for device-level analog placement and routing

The performance of high-speed electronic systems is limited by interconnect-related failure modes such as coupled noise. We propose new techniques for alleviating the problems caused by coupling between signal lines on integrated circuits. We show that models used by previous work on coupled noise-constrained layout synthesis do not allow the use of several important degrees of freedom. These degrees of freedom include the ability to utilize dynamic noise margins rather than static noise margins, the dependence of coupled noise on drive strength, and the possibility of using overlaps to reduce susceptibility to noise. We derive an expression for the coupled noise integral and a bound for the peak coupled noise voltage which shows order of magnitude improvements in both accuracy and fidelity compared to the charge sharing model used in previous work. We use the new bounds to guide a greedy channel router, which manipulates exact adjacency information at every stage, allowing it to introduce jogs or doglegs when necessary for coupled noise reduction. Experimental results indicate that our algorithm compares favorably to previous work. The coupled noise is significantly reduced on benchmark instances.

Crosstalk reduction for VLSI

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KOAN/ANAGRAM 11: New Tools for Device-Level Analog Placement and Routing

This paper describes a top-down, constraint-driven design methodology for analog integrated circuits. Some of the tools that support this methodology are described. These include behavioral simulation tools, tools for physical assembly, and module generators. Finally, examples of behavioral simulation with optimization and physical assembly are provided to better illustrate the methodology and its integration with the tool set.

A Top-Down, Constraint-Driven Design Methodology for Analog Integrated Circuits

An analytical-model generator for interconnect capacitances is presented. It obtains analytical expressions of self and coupling capacitances of interconnects for commonly encountered configurations, based on a series of numerical simulations and a partial knowledge of the flux components associated with the configurations. The configurations which are currently considered by this model generator are: (a) single line; (b) crossing lines; (c) parallel lines on the same layer; and (d) parallel lines on different layers (both overlapping and nonoverlapping). >

Automatic generation of analytical models for interconnect capacitances

An approach for generating constraints on interconnect parasitics to drive the routing of analog circuits is presented. The approach involves (a) generation of a set of bounding constraints on the critical parasitics of a circuit to provide maximum flexibility to the router while meeting the performance constraints, and (b) deriving a set of matching constraints on the parasitics from matched-node-pair and matched-branch-pair information on differential circuits. A prototype constraint generator is described. It is hoped that the constraint-based approach suggested in this paper, if applied to both placement and routing, will reduce the need of time consuming layout-extraction-simulation iterations in the physical design phase of analog circuits. >

Constraint generation for routing analog circuits

A well-defined methodology for mapping the constraints on a set of critical coupling capacitances into constraints in the vertical-constraint (VC) graph of a channel is presented. The approach involves directing undirected edges, adding directed edges, and increasing the weights of edges in the VC graph in order to meet crossover constraints between orthogonal segments and adjacency constraints between parallel segments while attempting to cause minimum increase in the channel height due to the constraints. Use is made of shield nets when necessary. A formal description of the conditions under which the crossover and the adjacency constraints are satisfied is provided and used to construct the appropriate mapping algorithms. The problem of imposing matching constraints on the routing parasitics in a channel with lateral symmetry is addressed. It is observed that perfect matching is not possible for a matched pair of nets with intersecting horizontal spans. A technique to achieve almost perfect mirror symmetry is presented for such pairs of nets. >

Constraint-based channel routing for analog and mixed analog/digital circuits

A new constraint-driven methodology for the placeinent of analog IC's is described. Electrical performance specifications are automatically translated into constraints on the layout parasitics. These constraints and the seiisiitivity iiiforinatioii of the circuit are then used to control a Simulated Annealingbased placement algorithm. At each step of the annealing a fast check on performance degradations is performed to guarantee that the tool has the necessary robur,tness.

U. Choudhury

Papers

A Top-Down, Constraint-Driven Design Methodology for Analog Integrated Circuits

Automatic generation of analytical models for interconnect capacitances

Constraint generation for routing analog circuits

Constraint-based channel routing for analog and mixed analog/digital circuits

A Constraint-driven Placement Methodology For Analog Integrated Circuits