Afshin Salehi wrote:

> Just out of curiosity, what types of things do you guys run simulations on?
> What drives you to run a simulation on that specific device?  How accurate
> is the simulation to a real world bread boarded device?
>
> Jon Elson said it takes a day at first then maybe an hour or so each time to
> remember things, how is that justified to your boss?  I am really just
> curious as to what things people run sims on, how complex those circuits
> that are simulated are, and if the tests are worth while?

If you've used SPICE before, then things are a lot easier.  Most of the
basic (passive) components are installed correctly, IF you remember to
pull all the resistors, caps, etc. from the right library.  There are several
libraries that have SPICE in the name.  One of them will generate a
good SPICE file for export, but doesn't have the file names built into
the components to like up with the internal model files that are part of
Protel.  The other one does have this extra information, but it does not
always have the precise path name to the required file.

These problems are not too tough, once you've solved it the first
time.  Getting vendor's macromodels loaded into Protel so they can
be fetched automatically by a library component is not much harder,
once you've examined how the file structures are built, and how they are
called up from the library component.

As for the simulation accuracy, it can range from a precise transistor-level
simulation of reality, with all parasitic capacitances, inductances and
resistances
included, or a gross oversimplification, with million amp power supplies
delivering terawatts to improperly connected transistors.  SPICE can
definitely produce meaningless garbage from a poorly constructed
schematic.  It can also produce meaningless garbage from poorly
built or badly documented macromodels.  For instance, dependant
current sources are often used in macromodels to produce offset
and bias currents in the simulated IC.  When the device is operated
in an unusual operating mode, such as wrong power supply connections,
a chip can become a DC power source, a physical impossibility.  For instance,
you can have a chip connected to ground and +5 V, and it produces a
-12 V output!  Clearly impossible!  But, this isn't a SPICE defect, but a
poorly built macromodel.  Generally, if you use the macromodeled
chip in the "normal" manner, these defects in the macromodel
won't show up.

But, all simulation results require a sanity check, and when things are
not looking sane, see if you have some wiring errors in the schematic.

Jon


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