Re: Wanted: LM-709 (Spice model) National Op-Amp

Robert <Robert@xxxxxxxxx> wrote:
: What he said finally drove him up a wall was he was trying to get a circuit
: to converge with great frustration and IIRC he came in one morning and the
: previous night's run had converged.

: When he examined the netlist he found that he had (during the
: troubleshooting effort) left in a couple of components (a resistor and
: capacitor?) connected to ground with the other ends disconnected. They
: should have had no effect on a real circuit.

: That made it converge. Taking the components completely out of the circuit
: caused the original non-convergence.

: That kind of non-real World physical behavior, he calls it "lying", drives
: him crazy.

I'll add my $0.02 here; perhaps it is useful.

On the SPICE vs. no SPICE debate: Designing modern analog ICs [1]
would be well-neigh impossible without SPICE due to modern circuit
size and complexity. Other posters have already pointed this out.
Also, when designing an IC, you control nearly
all parameters of the components you use, and you have highly accurate
models of your transistors available. Therefore, SPICE can do a good
job predicting circuit behavior.

Designing analog boards, on the other hand, is different. Most of the
time, the models you have at your disposal are vendor macromodels,
which are not device-level models of the actual components you use.
Rather, they are idealizations which attempt to model the important
features of the device's performance in its operating region.
Vendors won't give you real device-level models of their components
because then you could reverse-engineer their circuits. Therefore,
the SPICE models you use in board design are generally useful, but are
not totally accurate.

Also, when designing boards, stray capacitances are not as well
understood or controlled as they are when designing ICs. (Perhaps if
you purchase a $100K tool from one of the big EDA vendors you can
extract the strays from a PCB layout, but I have never seen that done
in real life.) Therefore, the fabbed board will always act
differently from any SPICE simulation, particularly if your circuit is
sensitive to strays.

Therefore, for IC design, SPICE is indespensible. For board
design SPICE provides good guidance, but isn't the last word in
predicting circuit performance.

As for the issue of convergence mentioned above: My experience is
that if your SPICE simulation behaves strangely or doesn't converge,
it is likely that you have a fundamental problem with your circuit.
When a circuit doesn't converge, besides looking for floating nodes, I
always examine my circuit thoroughly looking for subtle mess-ups such
as two different current sources in series, or two different voltage
sources in parallel. More often than not, I find that I have
committed some kind of error.

Stuart

[1] Note bene: I am not an IC designer, so others can speak with
more authority about this. Nonetheless, my point is general enough to
not require detailed, expierential knowledge of IC design.
.