Electronic engineers use advanced simulation tools to create virtual representations of complex electronic circuits. These “digital twins” streamline the circuit design process, accurately modeling silicon behavior and performance before physical fabrication. This article reviews the three primary circuit simulation categories: analog, digital, and mixed. It also discusses different simulation levels, such as block and chip, and explores how artificial intelligence (AI) and cloud computing are further accelerating electronic design automation (EDA).

Analog circuit simulation: methods and tools
Analog circuit simulation is typically based on linear, non-linear, and table-based models. Tools such as SPICE and FastSPICE analyze circuit behavior with continuously varying signal values in AC, DC, and transient modes. Known for its precision and accuracy, SPICE uses complex Forward Euler, Backward Euler, and Newton-Raphson methods, along with sophisticated matrix decomposition techniques, to compute the response of an entire circuit at each simulation point.

Conversely, FastSPICE employs advanced algorithms and simpler models to partition circuits and selectively evaluate them at specific time points. Both SPICE and FastSPICE are crucial for designing key electronic components, from operational amplifiers and filters to oscillators and voltage regulators. Notably, popular SPICE variants — such as LTspice, HSPICE, and SmartSpice — offer additional features and specific optimizations to meet the diverse requirements of analog circuit simulation.

The necessities of analog simulation
Analog simulation tools typically operate at the transistor level, treating circuits as matrices of differential equations and solving these equations in response to given stimuli. Although this method provides high accuracy and verification fidelity, it requires significant expertise in analog design for effective modeling, simulation execution, and debugging.

Moreover, overall performance is limited by computational complexity, while many analog simulators lack automated test pattern generation and code coverage analysis capabilities. Nevertheless, simulators are indispensable for analog circuit design. They ensure reliability, efficiency, and accuracy, with detailed modeling providing verification, signal integrity assessment, and thermal behavior analysis.

Digital circuit simulation: Techniques and applications
Most digital simulation tools use hardware description languages such as VHDL and Verilog to test circuits with discrete signal levels — ensuring timing precision, logical correctness, and signal integrity. Unlike analog simulation’s continuous signal propagation, digital simulation employs simpler models with minimal discrete voltage levels (primarily logic 0 and logic 1). This method enables the rapid and efficient simulation of larger circuits with fewer resources.

There are two primary types of digital logic simulation: event-driven and cycle-based. Event-driven simulation views the world as a series of discrete events, reacting to changes in input values by evaluating digital functions and scheduling future events. Cycle-based simulation computes the circuit’s steady-state value for each clock cycle.

Notably, digital simulators typically work with four logic values: 0, 1, X (unknown), and Z (high impedance), focusing on the discrete nature of digital logic. These simulators excel in functional verification for digital circuit design, rapidly delivering accurate results for automated stimulus generation, coverage result assessment, and regression testing execution.

Mixed-mode simulation: Bridging analog and digital
Mixed-mode simulations intelligently partition a circuit’s digital twin into analog and digital components, enabling faster, more efficient simulation of larger circuits. By combining analog and digital techniques, mixed-mode circuit simulation streamlines the design of analog-to-digital converters (ADCs), digital-to-analog converters (DACs), and mixed-signal integrated circuits (ICs).

These simulators play an essential role in verifying the interaction between analog and digital components, confirming their seamless integration and functionality in complex devices such as smartphones, automotive systems, and medical equipment. Moreover, mixed-mode simulations facilitate the precise modeling of signal transitions and timing analysis, critical for optimizing the performance and reliability of systems that integrate both analog and digital functionalities.

The many levels of circuit simulation
Whether analog, digital, or mixed, each circuit design stage requires a different level or type of simulation, including:

• Block-level: Focuses on individual circuit blocks, verifying functionality before integration into larger systems.
• Chip-level: Analyzes the performance of entire chips, including interactions between different blocks.
• Memory: Assesses key parameters such as access time, data integrity, power consumption, and read/write cycle endurance.
• Mixed signal: Targets circuits with both analog and digital elements, validating coexistence and interplay.
• RF: Tests radio frequency components crucial for wireless communication systems.
• Power management: Measures power consumption and distribution within the circuit, particularly essential for battery-operated devices and energy-efficient systems.
• Sensor integration: Simulates sensor behavior and interactions within a circuit for IoT devices, environmental monitoring systems, and automated controls.

The evolution of circuit simulation
Ever-faster CPUs, GPUs, and purpose-built accelerators are significantly boosting simulation speeds. Concurrently, advanced AI EDA tools are facilitating smarter, more efficient design processes, with sophisticated machine learning (ML) algorithms automating routine tasks and optimizing simulation tools. Moreover, cloud-based EDA now offers electronic engineers a collaborative environment with easy access to flexible, unlimited hardware resources for compute-intensive circuit simulations.

Summary
Electronic engineers rely on simulation tools to create virtual representations of complex electronic circuits. These ‘digital twins’ streamline the design process, accurately modeling silicon behavior and performance before circuits are physically fabricated. Whether analog, digital, or mixed, each stage of circuit design typically requires different levels or types of simulation.

References
What is Circuit Simulation?, Synopsys
Demystifying Mixed-Signal Simulation for Digital Verification Engineers, Cadence
How Does Circuit Simulation Work?, Sierra Circuits
What is Circuit Simulation? Its Advantages and Types, Electronics Hub