Best Practices in Simulation

Best Practices in Simulation

Upon capturing a schematic, circuit designers are able to utilize simulation in the form of SPICEand XSPICE simulation to predict the behavior of a circuit and analyze the effects of various components and signals upon the design. This is an important step in the modern design process because it allows one to emulate the performance of a device before it is even physically built. A design topology can be tested immediately to see if it needs to be modified. Simulation can therefore save both time and money. Simulation can help uncover the most uncommon flaws quicker, and prior to costly prototyping.

However, what exactly is simulation? Simulation is a mathematical model, for example of a transistor, that approximates its real-world counterpart at specific conditions and is accurate for certain temperatures or operating conditions. 

1. Learn more at the SPICE Simulation Fundamentals page
2. Understand more about Cosimulation in NI Multisim

The accuracy of the model defines how closely a component will resemble the real-world performance in the simulation environment, and thereby will affect an engineer’s analysis of a design. Certain models for BJTs, FETs and operational amplifiers are defined to accurately predict the behavior of the device with a great deal of precision such that it effectively mirrors the physical real world. These models tend to encompass various parasitic effects and complex behavior.

Traditionally EDA tools require engineers to have an intermediate, to intimate, knowledge of SPICE (or one of its proprietary variants) in order to effectively simulate circuit behavior. SPICE is a text-based language, and therefore an engineer may have to write multiple lines of code to simulate a device. This can be time consuming and difficult. The graphical nature of NI Multisim removes the difficulty of SPICE and provides engineers from novice to expert levels an easy interface to simulation.

In NI Multisim during the schematic capture process industry standard symbols (ASCII or DIN) are connected together with nets. In the background this symbolic drawing of a design immediately builds the SPICE code in the background. This is because each symbol in Multisim is associated with a SPICE model (effectively simulation code) which defines a netlist to be simulated. This netlist is what is passed to the simulation engine to emulate circuit behavior.

The most effective simulation method is iterative. Combining interactive SPICE simulation and analyses (as seen in figure 6) an engineer can systematically study a design and explore the changes and improvements that will improve the circuit.

Figure 6 - The Iterative Nature of Simulation

The best practice with a traditional iterative simulation approach is to:

1. Build the circuit schematic
2. Utilize Interactive simulation and SPICE based instruments to drive, measure, and investigate circuit behavior. SPICE simulation instruments such as an AC voltage source, Function Generator, Oscilloscope etc… drive and visualize the transient circuit characteristics.
3. Feedback changes and improvements to the circuit schematic in the design flow
4. Employing SPICE Analyses such as AC, Monte Carlo, and Worst Case provide advanced understanding of circuit behavior.
5. Feedback changes and improvements to the circuit schematic in the design flow
6. Build and validate the prototype

This iterative approach during the simulation stage of the design process ensures that one is systematically and methodically uncovering flaws, and validating design behavior. As mentioned previously, simulation is considered a best practice as it can allow an engineer to uncover flaws earlier in the design flaw – prior to physically prototyping. The physical prototype is an important and necessary step in the design flow, which simulation alone cannot replace. However simulation does allow one to uncover common flaws which may normally only be found at the physical prototype stage, thereby reducing iterations of PCBs.

Therefore by using the simulation instruments and analyses one can reduce errors.