Taking A Peek Under The Hood Of Your Spice Circuit-Simulation Engine

Understanding Spice numerical methods, convergence issues, and innovative simulation techniques can rev up your circuit design expertise.

Larry Meares

ED Online ID #8282

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DESIGN VIEW is the summary of the complete DESIGN SOLUTION contributed article, which begins on Page 2.

In today's quest for simplicity in "ready-to-use" EDA tools, understanding what's under the hood of a Spice circuit-simulation engine can be helpful, particularly for designs that challenge the conventional limits of Spice. Analog-centric designers will get the most out of Spice simulation by combining their knowledge and experience with an understanding of how their Spice design tool works.

Stepping back to Analog 101, Spice (Special Programs with Integrated Circuit Emphasis) is essentially an equation solver, in compliance with Kirchhoff's Current Law (KCL). The KCL matrix is modified by Spice to analyze the current through voltage sources, and with digital components (e.g., as with XSpice), it uses state variables in model extensions. For linear time-invariant circuits, the equation matrix creates a unique solution. Nonlinear circuits and operations involving integration require inserting a Norton equivalent circuit with the large-signal current summed into the matrix's right-hand side (RHS) and the small-signal conductance summed into the matrix.

That summation process is applied to the "matrix stamp" of a Spice model. Interestingly, during a Spice simulation, each model sums its contributions without knowing what the other models are doing.

The article discusses the Spice options for operating-point control to attain successful convergence and transient simulation control. Design tips are provided with various options to help make the process more efficient.

HIGHLIGHTS:
Spice Options And Design Tips For Operating-Point Control	Key Spice options and considerations are given for those trying to achieve successful dc operating-point convergence. Operating-point convergence refers to the initial stable solution for all of the dc steady-state voltages.
Spice Options And Design Tips For Transient Simulation Control	Transient simulation refers to the large-signal analysis of a design, as would be performed using instruments like an oscilloscope, a signal generator, and a dc power supply on a breadboard design.
Behavioral Hysteresis Comparator	Figure 7 shows the comparator switching with default Spice options (a). It has a rise time equal to the Spice time step when passing the switching threshold (b). Performing the same simulation with VSECTOL=50n makes the vref signal switch from 0 to 5 V in less than 10 ns (c).
Initializing Complex Circuits	The preferred method for initializing these circuits, shown in Figure 8, is to use switches that apply initial conditions at time = 0, then remove the ICs when time > 0. Here, the technique is applied to a simple R-L-C circuit.

Full article begins on Page 2