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Verilog-A Introduction

Verilog-A is an analog hardware description language (AHDL) created to model, analyze, and verify analog and mixed-signal circuits. While digital design engineers rely on Verilog HDL to model and synthesize digital circuits, analog engineers use Verilog-A to model circuits. Verilog-A enables designers to quickly describe an analog behavior without having to design transistor level circuits saving significant time and effort. It helps quick validation of ideas. Verilog-A is not synthesizable.

Traditional SPICE models vs Verilog-A

Traditional SPICE simulations use C/C++ to implement device models. These models are accurate but difficult to modify. Changing or developing a new model requires extensive knowledge of C/C++ and recompiling the simulator which is very time consuming workflow for most analog designers.

Verilog-A solves following problems:

  • Transistor-level details results in slow simulation speed.
  • Creating custom models without knowing the depth of C/C++.
  • Mixed-signal Simulation.
  • Quickly validate ideas without designing transistor level circuits

Verilog-A is often used along with SPICE simulator in the same circuit which allows to retain the details of SPICE netlists and abstraction offered by Verilog-A.

Comparison against other HDLs

Verilog-A Verilog AMS Verilog HDL SystemVerilog
Purpose Describes analog circuits Combination of Verilog and Verilog-A Describe digital circuits only Superset of Verilog HDL enabling advanced verification
Synthesizable Not Synthesizable Not Synthesizable Synthesizable Synthesizable
Standard Not a IEEE standard - belongs to company Accellera Not an IEEE standard - belongs to company Accellera IEEE standard 1364 IEEE standard 1800
Simulator Spectre Spectre+Xcelium Xcelium Xcelium

Core Concepts of Verilog-A

Verilog-A models resemble the equations engineers use in circuit analysis. Understanding a few fundamental concepts is enough to start writing meaningful models.

Modules and Terminals

Every model begins with a module definition:

module resistor(p, n);
    inout p, n;
    electrical p, n;

The electrical discipline defines voltage and current attributes for each terminal.

More details about modules - Modules declaration

Analog Behavioral Block

All continuous-time equations belong inside an analog block:

analog begin
    I(p, n) <+ V(p, n) / R;
end

This statement enforces Ohm's law.

Contribution Statements

The <+ operator defines current or voltage contributions. It is central to the language and implements KCL and KVL implicitly.

  • Current contribution: I(a, b) <+ expression;
  • Voltage contribution: V(a, b) <+ expression;

Unlike SPICE subcircuits, Verilog-A models allow writing exact equations for behavior.

Built‑In Functions

Verilog-A includes tools for modeling dynamic behavior:

  • ddt(x) : time derivative
  • idt(x) : time integral
  • laplace_nd(input, num, den) : transfer functions
  • transition(x, td, tr) : smooth transitions
  • white_noise() and flicker_noise() : noise modeling

These functions enable modeling capacitors, inductors, filters, oscillators, and more.

Parameterization

Models become reusable through parameters:

parameter real gain = 10 from (0:inf);

Parameters can include bounds to help the simulator converge.

Use-Cases of Verilog-A

Verilog-A is used across semiconductor design, RF systems, power electronics, sensor design, and academic research. Here are the most impactful applications.

Behavioral Modeling of Analog Blocks

In Behavioral modeling, a circuit block is defined more by functional behavior than device-level detail. Verilog-A makes it easy to describe:

Behavioral models allow massively faster simulations compared to transistor-level implementations.

Custom Device and Sensor Modeling

SPICE does not include every possible device. Researchers rely on Verilog-A to model:

Verilog-A lets researchers implement the governing physics using their own equations.

Compact Modeling

Many industry-standard compact models have Verilog-A reference implementations:

  • BSIM-BULK, BSIM-SOI
  • BSIM-CMG (FinFET)
  • PSP
  • HICUM
  • EKV

Because Verilog-A is portable across simulators, compact models written once can run on Spectre, Ngspice, ADS, Xyce, and other tools.

Mixed-Signal Verification

Modern ICs integrate digital control logic with analog circuits. Examples include:

  • ADCs and DACs
  • PMIC controllers
  • Battery monitoring ICs
  • Sensor interfaces
  • RF transceivers

Using Verilog-A, designers replace detailed analog blocks with behavior-level models to simulate interactions with digital systems. This accelerates verification cycles and reduces simulation time by orders of magnitude.

System-Level Architectural Exploration

Before committing to a full transistor-level design, engineers explore:

  • Loop filter topologies in PLLs
  • Oversampling ratios in ΔΣ ADCs
  • Gain and bandwidth tradeoffs
  • Power supply rejection (PSR)
  • Stability and compensation strategies

Verilog-A facilitates rapid prototyping of these ideas, enabling teams to make architectural decisions early.

Advantages of Verilog-A

  • Faster simulation compared to transistor-level SPICE
  • Readable and concise equation-based modeling
  • Portable across simulators
  • Easy to modify, ideal for iterative development
  • Supports high-level system modeling

Limitations

Verilog-A is powerful but not a replacement for device-level simulation:

  • It cannot capture deep physical effects without explicit equations
  • It is not suitable for digital logic modeling
  • It requires careful writing to avoid convergence issues