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5Spice Analysis - a Spice you can live with

Spice was written several decades ago by a diverse group at the University of California at Berkeley. It attempted to do most everything thought useful for analyzing analog behavior of circuits - to be all things to all people. Like the original C programming language, it let users try anything, then reported failures cryptically. And it assumed that circuits used the technology current at that time.

As a result, using traditional Spice requires remembering a lot of do's and don'ts to avoid frustration.

5Spice in combination with its Spice engine, WinSpice, goes a long way toward hiding Spice's quirks and provides a straight forward interface to all this power. It minimizes the number of do's and don'ts you must remember. 5Spice is focused on analysis and design at the discrete component level.

A program that supports the way engineers work

Syntax: units that follow engineering convention

Keep it simple - no user interface confusion

Keep it simple - why use a separate Spice program?

Avoiding a common Circuit Equation Matrix failure

Effectively deal with Convergence Problems

Easily use Manufacturers' models, including most containing PSpice® syntax

Run multiple Simulation types from one schematic - documenting your work

Help that is helpful

 

A program that supports the way engineers work

Engineers typically work on a circuit for a while and then put it aside for months until the next phase of the project. Then they need to pick it up promptly where they left off. 5Spice supports this through

  • an intuitive user interface that doesn't require weekly use to stay up to speed
     
  • making many Spice concepts into visible items in the schematic
     
  • providing a full size notes section that is saved with the project. each Analysis the user defines also has a small notes section.
     
  • other features discussed in this paper

To achieve this balance, 5Spice provides much of the power of Spice but not all. Features continue to be added to the program in ways that support this usage pattern.

 

Syntax:  units that follow engineering convention

Ever use M as in MHz for MegaHerz? Fine inside 5Spice but Spice sees that as milliHerz. 5Spice also accepts the Spice term Meg for the Mega  prefix. And you can have a capacitor value of 3.0F (farads) in 5Spice where Spice sees that as 3.0e-15 (femto). Note: when 5Spice reads files of subcircuits and models, it expects the standard Spice usage where M : milli, Meg : Mega, F : femto

Noise analysis values are reported in units of [volts per squareroot(Hertz)], which matches op amp and semiconductor specs, rather than Spice's traditional [volts squared per Hertz].

 

Keep it simple - no user interface confusion

Some Spice  programs have "graduated" to trying to be total schematic capture and pc board layout packages - which then dominates their user interface, adding much complexity. Or sometimes Spice comes with a "post processor" which is actually another program with a  separate user interface. 5Spice is a pure simulation product with a single, clear user interface dedicated just to analysis/simulation.

 

Keep it simple - why use a separate Spice program? (my PC board layout program includes Spice)

Experienced circuit designers usually simulate only a small portion of a large circuit. And they know that the simulation schematic is virtually never the same as the PCB schematic. You remove connectors, digital  parts, etc. you don't want to model or can't get models for. And add parts to model component imperfections plus special parts like Spice signal sources, DC supply sources and so forth. With a PC board layout package, you may also need to replace all the remaining schematic symbols with ones from a separate simulation library.

5Spice draws schematics fast. It is often quicker to draw the simulation schematic in 5Spice and start simulating. Plus it is much easier to document your simulation work in 5Spice.

 

Avoiding a common Circuit Equation Matrix failure

Certain topologies of components that seem reasonable to the user produce a singular circuit equation matrix - i.e. unsolvable. 5Spice adds a tiny resistance in series with voltage sources and a very large resistance in parallel with current sources to prevent these sorts of singular matrixes. This option may be disabled.

 

Convergence Problems

A Spice simulation may not converge to a solution - this results in an error message. 5Spice simplifies dealing with several sources of convergence problems.

  1. Spice has many default values that can be tweaked to help convergence but selecting which to use in a given analysis is anything but clear. 5Spice provides simple check-box options to enable or disable the  combinations of default values found over the years to provide the best chance of improving convergence in marginal cases. 5Spice also  provides a visual schematic component corresponding to the Spice command named NodeSet. This defines an initial voltage at a circuit node to help convergence of the circuit's DC operating point. This is usually the better solution to operating point convergence problems.
     
  2. Convergence can also fail because the voltage, current or resistance values in your circuit lie near the edge of the solution range Spice can handle. Spice has parameters that can adjust this range (remember they are set for 1970's IC technology, not a switching power supply). Books on Spice give advice for setting the relevant Spice parameters to adapt Spice to your circuit values. In 5Spice this insight is contained in a Wizard which asks you a few simple questions about your circuit’s voltage and current levels and resistance values, then sets the Spice parameters to center the solution range on your circuit nominals. And yes, you can set these values manually if you want.
     
  3. Convergence can fail because the ratio of largest to smallest resistance values  exceeds 1E15. The circuit equation matrix may become singular as it is being solved due to the limited resolution available with double precision numbers. The 5Spice Wizard warns you when you are using resistance values with a ratio greater than this. And the Wizard warns of the accuracy loss that occurs as ratios approach this limit.
     
  4. Convergence in Transient analysis can fail with Spice’s dreaded "time step too small" failure. 5Spice provides a "try harder" check-box which fixes the error a few circuits. 5Spice 2.0 has an improved simulation engine that corrects a conceptual error in Spice that leads to some of these failures. Using the Wizard cures some other cases. Finally there is the SMPS option for switching power supply simulation that improves speed and helps prevent this error at the cost of lower accuracy.

 

Easily use Manufacturers Models (subcircuits), including most containing PSpice® syntax

Almost every semiconductor manufacturer has Spice models for their products (models using more than one circuit element are called subcircuits in Spice). These tend to be the most accurate models since they are usually determined from actual devices, not from data sheet parameters. 5Spice makes it easy to use these models due to several features.

    1. a schematic op amp symbol and a generic subcircuit symbol are provided.

    These symbols adjust the number of their pins automatically to match the model  (subcircuit) you select. The generic symbol will connect to any subcircuit. Many Spice op amp models do not conform to the Spice standard connection order. 5Spice 2.0 can connect the op amp symbol to all these connection variations without the user modifying the subcircuit’s listing. The Professional level includes a database of connection information for ~2000 op amps and comparators.

    2. model library

    5Spice uses the manufacturer's model file as the element in its model library. You place the text file containing the model(s) under one of the existing library subdirectories on your hard drive or create a new subdirectory. Then ask the program to rebuild its library index files. The new models will show up in the library index that appears when you select a model to go with the schematic symbol.

    3. Pin names

    For any subcircuit model in the library, you can define names for the associated schematic symbol's pins and save the names to the library. They are stored in a library database so you can upgrade the model itself without losing the pin names.

    4. PSpice compatibility

    Some  manufacturer's models use elements of PSpice syntax which are not compatible with standard Berkeley Spice3 syntax. 5Spice has significant compatibility with PSpice syntax as used in Analog Devices, Linear Technology and TI subcircuit models. The few subcircuits containing unsupported syntax are identified in the library index. PSpice's digital models are not supported.

    5. how to find models

    See the Links page of this website for links to model download pages of popular manufacturers. If no luck there, do a Google search, example: ‘2N2222 model’

 

Running multiple Simulation Types from a single Schematic - documenting your work

5Spice has features which allow you to have a single schematic but run various types of simulations (AC, Transient, etc.) and graph them without having to change wiring or values in the schematic.

5Spice uses the concept of an Analysis. An Analysis defines (a) the input and output signals of your circuit, (b) the type of simulation desired and the range over which it will run (in time, frequency, or  voltage), and (c) the setup for the graph or data table it will produce. You can name and define as many different Analyses (simulations) for the schematic as you wish. For example: two different AC Analyses, three Transient Analyses and a Noise Analysis. These definitions are saved when you close the project or the program.

For input signals, there is a schematic symbol called a Signal Source which acts like a function generator in the lab. You set separate parameters for each type of analysis (AC, Transient, etc.). Further you can have more than one Signal Source - say two voltage Signal Sources in series, each set for a different waveform. Then in each Analysis, you can specify each source as enabled/disabled (a disabled voltage source acts as a short).

Output signal points are indicated by a schematic symbol called a TestPoint (like a test point in a breadboard). You may place as many TestPoints as you want in the schematic. Graph plots show the voltage or current at a particular TestPoint. In each Analysis, you can select different TestPoints and define/enable/disable associated Graph plot(s).

5Spice provides a full size Notes page for you to enter comments, notes, changes, etc. as you wish. Your notes are saved with the project. Each Analysis you define also has a small notes section that is saved with it.

Simulation data may be exported to other programs in spreadsheet compatible format. Images of graphs or the on-screen portion of the schematic may also be exported.

 

Help that is helpful

Unlike many programs, 5Spice's Help system focuses on the how and why of working with the program and Spice simulation. Topics are commonly presented in discussion blocks so you can read just the item of interest or the whole area. Help was written by an engineer who uses Spice simulation.

For complex but infrequently used parts of the program, there is often a discussion you can read as you are trying to make things happen, without going to the Help.

 

PSpice is a registered trademark of Cadence Design Systems, Inc.

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