NI Multisim: Advanced Interactive Simulation & SPICE Analyses

Overview

Circuit simulation helps engineers uncover a variety of flaws early in the design process. Through both simple and advanced analyses, key behaviors and operations can be thoroughly tested and errors can be remedied before additional unnecessary prototypes cause costly and time-consuming rework.

National Instruments Multisim is a powerful and intuitive simulation tool that enables designers to reduce design iterations and save valuable time and money. To drive efficiency for electronics design, Multisim includes a wide variety of sophisticated simulation tools for the professional designer: a robust industry-standard SPICE engine, dozens of advanced analyses, customizable simulation options, custom component creation, and integration with industry standard test and measurement tools. In the following article, you will learn about each of these areas and how they can be applied to your design process.

 

Industry Standard SPICE Simulator

Multisim simulation is built around the mature industry standard SPICE simulation engine. SPICE simulation employs sophisticated and proven algorithms to accurately converge on a mathematical solution for a circuit's operation.

Multisim gives you the advantages of SPICE without requiring that you know low-level simulation details. You can quickly and interactively conduct SPICE simulation with the click of a mouse using Multisim virtual instruments, which are simulated bench-top instruments that will connect to your schematics and report simulation results through familiar interfaces such as an oscilloscope or spectrum analyzer. As you become more familiar with the concepts and details of simulation you can gradually ease your way into much more advanced applications.

Multisim incorporates industry-standard SPICE versions 3F5 and XSPICE at the core of its simulation engine, with customized enhancements to optimize simulation performance and improvements for convergence.

SPICE 3F5 is the most recent edition of the SPICE core designed by the University of California at Berkeley. XSPICE includes a set of powerful extensions made to SPICE under contract to the US Air Force, which includes event-driven mixed mode simulation and an end-user extensible modeling subsystem. In addition to the powerful core simulation capabilities, patented co-simulation technology enables Multisim to simulate VHDL and MCU components alongside analog and mixed-mode circuits. When simulating with Multisim you are taking advantage of over thirty years of incremental SPICE improvements, as well as a host of other benefits.

 

Advanced Analyses

A second advanced feature set of Multisim is its comprehensive suite of analyses.  As you progress beyond simple simulations, you will likely need to apply analysis techniques to get more detailed behavioral data for your circuit.  By conducting analyses in Multisim, you can uncover more subtle design errors, or refine a circuit operation. Valuable information such as the effects of component tolerances and sensitivities, the effects of variations in temperature, and a circuit's frequency response can be quickly ascertained from Multisim's 24 analyses. For a listing of Multisim's analyses, please consult Appendix A.

Multisim simplifies the procedure for an advanced analysis by providing a configuration wizard. This wizard simplifies the normally complex syntax associated with configuring an analysis such as Monte Carlo. Once the desired analysis is chosen, you need only specify the associated parameter values and output nodes - Multisim conducts the simulation and outputs results to the customizable Grapher.

 

Customizable SPICE Simulation Options: Simulation Speed versus Accuracy

Once you are comfortable performing analyses with Multisim, you may want to tweak your SPICE simulation settings to get even more out of your simulations. Although Multisim's default simulation settings will provide you with accurate results at a reasonable speed, you have the option of customizing the settings to provide better accuracy or to simulate at a faster rate. Customized simulation settings can also remedy certain simulation problems such as convergence or timestep errors.

The tradeoff between speed and accuracy is evident in many of the SPICE simulation parameters. For example, the RELTOL (relative tolerance) parameter is used when SPICE solves DC and transient analyses using its iterative algorithm. Consecutive iterations of the algorithm produce an increasingly accurate solution. When two consecutive solutions are within a value calculated from the RELTOL parameter value, then the simulation has converged and results are presented. By having a very small RELTOL, simulations may take longer but can provide more accurate results.

Multisim provides a simulation advisor to help you choose appropriate values for the many simulation parameters. The advisor also helps you to tweak parameters in order to remedy simulation problems such as those mentioned above. You can learn more about simulation options by examining the SPICE user's guide. National Instruments provides links to the SPICE user's guide as well as other fundamental SPICE topics at the SPICE Simulation Fundamentals Page.

Another invaluable resource for understanding SPICE simulation and options is The SPICE Book, by Andrei Vladimirescu (ISBN 0-471-60926-9, www.wiley.com).

 

Customizing Components

As shown above, analyses and customized simulation options can give you a better understanding of your circuit's operation and increase the accuracy or speed of that simulation. In addition, customizing components affords you an entirely new vector to improve simulations. By either existing parts or creating new parts, you will be able to incorporate components that do not exist in the Multisim database, thereby extending the functionality your simulation tool. You can implement your SPICE knowledge to create your own models and account for more sophisticated or real-world effects that would otherwise go unnoticed until a prototype was made.

You can easily edit the properties of components by double-clicking on them. When editing a part, you can customize all properties, such as the symbol, electronic parameters, user fields, simulation model, and layout footprint. Should you desire to modify the SPICE model for a component, you can consult the SPICE user's guide or look in the Multisim help file for a detailed explanation of the meanings of the various SPICE model parameters. After your modifications are made, you can save the customized part to a personalized user database and share it with others in a corporate database stored on a network disk.

You can also create custom components to model specific devices, effects, or to account for real-world non-idealities that occur under specific circumstances. The easy-to-use component wizard guides you through the process of defining parts for placement, simulation, and PCB layout. The wizard guides you through the creation process in which you can specify as much or as little information as needed for your application and the part's intended use. You can download SPICE models from a variety of online resources, including vendors, and assign them to your own components in this wizard. You can view a tutorial on this topic: Creating a Custom Component in NI Multisim

Part manufacturers and chip vendors are an excellent resource for SPICE or other simulation models for components. However, there are often cases where no model is available. In order to streamline the process of component creation Multisim includes 24 model makers that will automatically generate a SPICE model for you based on the datasheet parameters for that component. Simply launch the model maker and specify parameters values and a SPICE model will be generated. For a list of model makers, consult Appendix B.

 

Integration with Industry Standard Test & Measurement Tools: LabVIEW and Signal Express

In addition to useful and versatile simulation-specific features, Multisim is tightly integrated into the complete design chain, enabling you to achieve even greater efficiency in your design cycle. For example, there are times when you may find it difficult or impossible to model real-world inputs to a circuit---Multisim delivers the ability to examine a circuit's response to such inputs through powerful integration with the test and measurement standard graphical programming environment - LabVIEW.

You can place LabVIEW-based custom virtual instruments on your schematic diagrams in Multisim that interactively measure real-world signals and input them to simulations. This means that Multisim can simulate the circuit's response to real data to produce results that will more closely match a prototype's actual behavior. You even have the ability, with native file sharing and custom virtual instruments, to send the outputs of your simulation to the real world. With this methodology you can essentially create a virtual prototype to see how your circuit will interact with other system-components in the real world long before prototypes are manufactured. By delaying costly prototypes until later in the design cycle, you can uncover and remedy design errors early-on in the process, minimizing the amount of costly and time-consuming rework done to prototypes.

Once a prototype is created, you typically verify and validate its operation through physical measurements. To ensure correct operation the measurement results are compared to simulations. Multisim can output simulation data to a standard LabVIEW Measurement File which is read easily by both LabVIEW and SignalExpress.

 

Conclusion

Multisim is a circuit simulator that drastically increases in versatility as you become more familiar with its many advanced features. At any level, from basic interactive simulation to sophisticated system-level simulation, Multisim is a powerful tool for the electronics designer.

 

Appendix A - List of Multisim Analyses

DC operating point	Noise	Batched
AC Sweep	Distortion	User Defined
Transient	DC Sensitivity	Noise Figure
DC Sweep	AC Sensitivity	Nested Sweep
Fourier	Param Sweep	IV Analysis
Worst Case	Temp Sweep	3 dB Point
Monte Carlo	Pole Zero	Stabilty (circles/Smith charts/matching networks)
Trace Width	Transfer Function	Gains

 

Appendix B - List of Multisim Model Makers

AC Motor	Interdigital Capacitor	Operational Amplifier
BJT	Linear Transformer with Neutral Terminal	RF Spiral Capacitor
Boost Converter	Lossy Line	SCR
Buck Converter	Microstrip	Strip Line
Buck-Boost Converter	MOSFET	Stripline Bend
Cuk Converter	Multiple Winding Transformer	Two Winding Linear Transformer
Diode	Non-linear Transformer	Waveguide
Ideal Transformer	Open End Microstrip Line	Zener

 

 

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