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There are three basic types of circuit simulation: analog, digital, and mixed-mode.

Analog circuit simulation involves the use of highly accurate models (i.e., representations) of the electronic circuit to achieve high accuracy. The models include non-linear, linear, and simpler table-based representations of the various electronic devices in the circuit. Analog simulation can run in different modes. These include AC (frequency domain), DC (non-linear quiescent), and transient (time-domain). All analog simulators employ algorithms to mathematically analyze the behavior of the electronic circuit in these different modes. They all share the quality of solving matrices to predict the performance of the electronic circuit. Signals are propagated as continuously varying values.

There are two primary types of analog circuit simulators: SPICE and FastSPICE. SPICE simulators use highly accurate non-linear and linear models of electronic devices to analyze the behavior of the circuit. SPICE simulators employ many different integration methods, such as Forward Euler, Backward Euler, and Newton-Raphson as well as matrix decomposition techniques to compute the response of the entire circuit (i.e., mathematical representation) at every single time point in the simulation period of interest.

By contrast, FastSPICE simulators use simpler table-model representations of electronic devices to analyze circuit behavior. They employ sophisticated algorithms to reduce the complexity of the circuit and partition the circuit based on various criteria, essentially creating a simpler and more modular circuit representation. This representation is then selectively evaluated at a given time point in the simulation period of interest, a process that greatly improves the performance and capacity of the simulation. FastSPICE simulators offer various simulation knobs to help balance the tradeoffs between simulation accuracy and performance.

Digital circuit simulation involves the use of simpler models of the electronic circuit. These models are typically created using HDL. In digital simulation, rather than propagating continuously varying signals, a few discrete voltage levels (primarily logic 0 and logic 1) are propagated. The methods to propagate these signals have varying degrees of accuracy regarding the propagation delay of the logic levels through the circuit.  This method of simulation allows much larger circuits to be simulated in less time with fewer compute resources when compared to analog simulation.

Mixed-mode circuit simulation combines the analog and digital simulation approaches. The circuit is partitioned between the two regimes to support the correct level of analysis detail for each part of the circuit. Analog simulators (SPICE and FastSPICE) are used for the analog analyses and a digital simulator is used for the digital analyses. This method of simulation allows for much larger circuits to be simulated in less time with fewer compute resources when compared to analog simulation. 

Circuit simulation provides a critical view into the behavior of electronic circuits. Given the expense and time involved in fabricating electronic circuits, especially ICs, it’s much more practical to validate circuit behavior and performance via circuit simulation prior to manufacturing.

Some of the specific areas of validation include:

• Memory Performance. The read and write access times and latency of memory devices are built from analog circuit simulation of the bit cells and read/write paths inside these memories.
• Overall Digital Simulation Accuracy. Digital circuit simulators model the propagation of voltage for logic level 1 and logic level 0. Analog circuit simulation is used to determine the time it takes for a circuit to transition between these voltage levels. This forms the basis for the overall accuracy of the digital circuit simulator.
• Noise and Crosstalk. Higher level models for noise and crosstalk are developed based on the detailed circuit level analysis of these parasitic effects from analog circuit simulations.
• Optimization of High-Frequency and High-Power Circuits. These types of circuits must undergo detailed continuous time analysis to determine their behavior and performance criteria. Analog circuit simulation delivers these important analyses.
• The overall performance and behavior of complex digital circuits (core processors and AI accelerators are examples) are verified with digital circuit simulation.