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Q:limits of spice simulation

Discussion in 'CAD' started by Michael Schuster, Aug 18, 2004.

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  1. Hello,
    where can I find some infos about some limits of spice simulation? I
    need it for a comparision of different simulators.

    Any link or comment (especially if you've faced a certain problem) is

    Thanks in advance

  2. What do you mean by limits?

    For example, the main error sources for a simulation are the models.
    Models are pretty much independent of the simulator, although some
    simulators have some minor variations on how the handle such models.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  3. Michael,
    Maybe you're trying to get at the trade between
    accurate transistor-level simulation with a global
    Jacobian as used in SPICE and partitioning solvers
    and/or gate-level solvers which don't have SPICE
    accuracy but the capability to simulate orders of
    magnitude larger designs. You might look at the
    sales collateral from vendors selling these
    solvers. I remember seeing one in the Anagram
    brochure. It was a 2D plot of accuracy vs.
    circuit size with a regions marked that were
    viable for various solver methodologies.

  4. Ken Smith

    Ken Smith Guest

    Here's a few:

    In general, simulators never disprove something the models assume.

    Spice models usually do not sumulate behavour outside the normal operating
    conditions of the part. Things like the output side characteristics of an
    op-amp when Vcc is much below operating would be an example.

    Thermal issues are generally not included. This can lead to missing real
    life problems like thermally caused low frequency distortion.

    Op-amp models often do not include things like power supply rejection.

    Some models, of things like swither chips, assume that the chips ground is
    connected to the global ground.

    Models of noise are usually very simplified views of what really happens.

    Simulator software can't really handle much of a mixed digital and analog
    system. Take a system with a few op-amps, an ADC, a 512 macro cell CPLD,
    a DC-DC converter, and an 8051 as an example. It is fairly hopeless
    trying to model the whole thing without making so many simplifying
    assumptions that your model is mostly a fairy tail.
  5. I mean e.g. the following:

    I want to simulate behaviour of a resistor which characteristic (U:
    Voltage, I current) is given by
    I = f(U)
    U = g(I)

    Can I feed these characteristics directly into spice or would I have
    to convert e.g. g(I) to its inverse function in order to model this
    resistor with spice?

    Or a component is given by:
    I = d/dt(I) * d/dI(f(I))
    U = d/dt(U) * d/dI(g(U))

    Possible or not?
    Thanks for your answers!
  6. Jim Thompson

    Jim Thompson Guest

    Read up on "Behavioral Modeling" in the PSpice manual. Powerful, yet

    ...Jim Thompson
  7. Yes. You can make a model using the B source, or a gm source.
    Yes. Any standard spice will allow you to model this simply by using
    capacitors/inductors in a little ".subckt". XSpice based ones (with is
    just about all of them) will also allow a second method, that is by
    using laplace S transforms. Look up the help in what ever spice you use

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  8. Engr4fun

    Engr4fun Guest

    This would be simulator dependent. I know with Micro-Cap, you can put
    in an equation directly into a resistor to define the resistance, and
    I believe PSpice does the same although I'm not 100% sure on that.

  9. Jim Thompson

    Jim Thompson Guest

    Yes, PSpice can do equations directly.

    ...Jim Thompson
  10. Thanks for your answers

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