[sdiy] spice flamewars, Bob Pease vs Barrie Gilbert
Harry Bissell Jr
harrybissell at prodigy.net
Thu Dec 2 19:55:27 CET 2004
sorry this is so long... I cut out a lot of the
headers to make it more readable and to protect the
innocent from exposing their e-mail addys.
Thought a lot of folks would get a kick out of it.
I wrote to say that Spice has its place, once you KNOW
when it is or is not lying. thread follows H^) harry
From: Ward Silver
Subject: Re: ALLEGED solutions with SPICE...
The issue of SPICE lying or not lying reminds me
of Steven Hawking's quote that, "God may not play dice
with the Universe, but sometimes he throws them where
you can't see them."
Having worked with a fair number of students and
young engineers over the years, I can tell you that
any simulation at all is dangerously addictive to
them. They have absolutely no clue what the pitfalls
are, what the symptoms of breakdown are, or even how
to open the hood and really look at the data. It's
treated like a "magic box" in many cases.
Furthermore, since very few of them are experienced
builders, they will make things in ways that SPICE
assumes they won't and shouldn't. And that more
experienced engineers don't.
It's like giving a brand-new driver the keys to a
stock car. EVENTUALLY, they may figure it out, but
only after a lot of grief and after someone older and
wiser takes them in hand. It shouldn't have to be
that way at all, but...
1) SPICE (standing for simulation in general) is
introduced too early and with too few cautions in the
engineering curriculum, often in substitution for lab
work. This is BAD JUJU and leads to violations of
Will Roger's First Law - "It's not what you don't
know, it's what you know that ain't so." Simulation
should be done only in parallel with hands-on lab work
and not in substitution for it except in rare
circumstances.
2) There is effectively zero emphasis on the
simulate-build-compare cycle that shows them how the
tool's version of reality differs from what the real
electrons are doing. Without that feedback, they
don't learn when the simulation is getting into deep
water. Frankly, I think an entire quarter-long course
should be devoted solely to use and misuse of
simulation tools.
3) High-speed stuff - anything over a few MHz - is
really dangerous in the hands of inexperienced folks.
Until they get to senior-level courses, if at all, the
subjects of parasitic reactances, transmission line
effects, crosstalk, EMC, and other important are not
covered. Yet, they use models that cheerfully support
frequencies into the GHz. It's hard to explain to
them why all that stuff in the VHF spectrum is really
fiction.
It's nuts that we give this powerful, but
sensitive, tool to these young engineers with as
little guidance and caution as we do. Both industry
and academia need to work harder on dealing with this
problem.
Regards,
Ward Silver
From: Paul Rako
Subject: Re: ALLEGED solutions with SPICE...
Hmmm-- looks like we have a good fight on our
hands here. I used to try and get between combatants
and say "calm down", but today I am bored and so I say
to both these esteemed gentleman--- are you going to
take that shit from HIM???!!?!?!?!?!
Ha ha ha--
OK, on a more level keel-- I can offer a few
observations. First it is not surprising to me, an
application engineer, that Mr Gilbert thinks that
SPICE is just dandy. Most IC guys do. If you live in
the transistor design world and have 10 to 50 million
a year to feed both a Process and Modeling and CAD
(sorry for the redundant "and" Barrie) yes, SPICE can
give you a first-order indication of how an IC will
work. However, for us board-level guys-- SPICE is a
lot less useful and a lot more dangerous. At high
speeds the board is a pretty important component and
unless you drop the 30 grand for hyperlynx you can't
factor the board's effect into the design on a
computer.
I am also amazed that Analog Devices does not
have any trouble with their models-- not even on new
processes? I have seen even our huge-dollar IC spice
tell lies on a new process. Sure it gets worked out
but that can often take a couple of spins and that is
big time and big money. And the errors in SPICE are
most often in the very same corner cases where Mr.
GIlbert brags that SPICE does so well, over
temperature and process and time.
If you want to be a software engineer I think
that is a fine and honorable profession but I, like
Mr. Pease, continue to be disturbed by this modern
effete tendency by EEs to think that their job is to
sit at a workstation all day playing with a computer.
Then they toss the design over a wall to another
clueless geek that does the layout. Then everybody is
so surprised and blame the "tools" when the dang thing
doesn't work. "It can't be us-- were are so smart we
never make mistakes"-- right. It did work on the
screen after all.
I also tend to dispute Mr. Gilbert's dismissal
of prototyping as to being a valuable contribution in
the design process. Very smart people seem to have a
hard time understanding that they are not being paid
to learn-- they are being paid to teach-- teach those
nice people in manufacturing how to make a few million
of whatever we make so any of us can get paid. The
goal is to get into a real physical piece of green
plastic or shiny silicon that can go into production.
Worshiping SPICE keeps it in that computer screen
phase for the majority of the development and then the
"genius designers" can blame layout or the tools or
something and all that happens is another 6 months
gets added to the design cycle. After working twenty
years as a consultant I can assure everyone that the
businessmen that pay the bills are lot happier to see
a air-ball prototype that works but has problems
rather then some computer print! out that shows what a
rosy world we live in. At least you can put the
air-ball into a pretty box and make the show in
Chicago. That screen is not reality-- it is a
dangerous drug and although a little can ease your
pain, a lot of SPICE and you will just be a junky with
your eyes glazed over staring into the screen and no
sense of what you are really trying to do-- like
getting real hardware going out the door.
Now Bob does like to get on his high horse
about SPICE, but you have to understand any good
preacher really can't go soft on the devil and keep
the faithful from harm. I have seen him advise young
engineers on how to mix SPICE with a few real-worlds
tests-- maybe those air-balls dead-bugged on a board,
and then from that see where SPICE lies (and it lies
by definition-- as my Professor once said--
"everything is based on linear analysis but even a
resistor goes nonlinear when you put enough juice in
it to melt it"). Then it is time for breadboards--
real green plastic boards with real black plastic
chips and you still have to count on three spins to
get everything right.
Sure SPICE is great-- when the box is small
enough. How appropriate that in college we called the
vectors and matrix classes "boxes and arrows". Yup,
that matrix is only right in a very narrow regime--
with very good models and proven processes and perfect
design rules and a manufacturing operation at 6-sigma
and on and on and on- yup-- SPICE works just fine.
But in the real analog world SPICE is doing a lot of
damage-- not only to budgets but to impressionable
young minds. And if your company is making optimum
use of it's resouces the processes are new and models
imperfect and the manufacturing is good-enough, not
perfect.
So I guess I would have to say that we
shouldn't hold a digital opinion in an analog world--
yeah, Bob does rant about SPICE but he understands in
can teach a few things, especially to idiots like me.
But the more experienced you get you may now where to
distrust SPICE, as Mr. Gilbert so well demonstrates.
I maintain that an Israeli commando with a slingshot
will be more effective thane a Boy Scout with an Uzi.
I suspect Bob sees indiscriminate SPICE use as
analogous to giving Boy Scouts Uzis.
I just did a PSPICE simulation of our very
fast LMH6624 used as a trans-impedance amp. I
integrate the output with another LMH6624 and feed
that back to the "+" pin of the main amp to servo out
the DC. It simulates just fine. But all my
experience tells me "DANGER WILL ROBINSON". I know
fast amps just don't like getting hooked tail-to-nose
like this. I will build it and report if the real
circuit works or not.
Paul
PS: Bob TRIED to toss another computer off the
third floor but security found out about the plan and
intecepted him on the way. Stay tuned.
Robert A Pease
Subject: ALLEGED problems with SPICE...
Hello to : Nishanth, Tom, Colin, Ward, Wayne, Roger,
Hal, Willy, Marty, Mark, Bob, Paul, John, Harry, Ken,
Ed, Barrie, and a couple more guys named PAUL:
I sent a copy of my comments on SPICE to
Barrie Gilbert of Analog Devices. I'll enclose his
comments and replies here. Obviously, Mr. Gilbert and
I do not agree, about SPICE and simulation.
A copy of my replies to him will follow,
shortly.
As I often say, " P.S. Strongly worded memo
to follow..."
Obviously, Mr. Gilbert likes to use SPICE,
and he has no lack of confidence in SPICE - at
least the version HE uses, that is supported by
Analog Devices - which none of the rest of us can
use.... (except maybe Paul Brokaw).
Best regards. / rap
From Barrie.Gilbert
To: "Robert A Pease"
Bob:
You write
>> *** Your comments are invited. Best
regards. / / rap
So here are some.
I believe you should give me "equal time" and
share these notes in their entirety with the folks
whom you identify only as "Tom, and Colin, and Wayne,
and Roger, and Hal, and Willy, and MARK, and Bob, and
Paul, and other FRIENDS" (eight redundant "ands").
Perhaps "Paul" is our mutual friend of
band-gap fame, ( *** Not so) so I have put him on the
CC list. I'm also sending a copy to your disciple (or,
in your words "this poor kid in India [who] is still
being
screwed up by SPICE"), and to a colleague at Analog
Devices, who has more than a passing interest in
squishing whatever bugs may still be loitering in our
own most excellent simulator, ADICE.
Herewith, a selection of your current mystical
observations:
*YOU are not being screwed up by SPICE - -
but you cannot say that SPICE is not screwing up a lot
of other people.
*Worst of all - you cannot say that engineers
in other places are not being screwed out of the art
and the pleasure of how to learn to design circuits
without SPICE.
*One of you folks, as listed as recipients,
said that "SPICE is a powerful tool". That is indeed
true. It is so powerful that it can fool a young
engineer into thinking that Black
is White, and since Black and White are the same,
then GREY is just - the natural state of Chaos.
SPICE told him that here is no way to detect the
chaos, and in fact, even an op-amp or a comparator
cannot detect the difference between Black and White.
Every op-amp is just -- a follower, EVEN WHEN you
cross-wire the inputs.
*Pissing away a $100,000 maskset because SPICE
DOESN'T CARE whether the inputs are crossed or not is
a SERIOUS BUSINESS ERROR.
*LOOK HERE, Gentlemen: I have a PC, and it is
so powerful that if there is something it does not
want me to see, it won't let me see it. It is -- yes
-- powerful -- but it is an insolent piece of SHIT.
*YOU can trust SPICE every time it helps you -
and you can DIE every time it lies to you. I never let
SPICE lie to me.
*Don't tell me that I'm incompetent at using
my computer.
>>> Bob, I don't need to.
As a child, my mother wisely taught me that
"It's a bad workman who blames his tools". And later,
as I grew up, I learned that the skilled craftsman is
he who thoroughly familiarizes himself with both the
power and the limitations of his tools, and knows
instinctively when to use each of many such to full
advantage.
I just can't understand why you continue to
rail so emotionally against the use of circuit
simulators. In all the "case studies" you've
presented, including these latest examples, the
problem lies not in the simulator - whatever version
of SPICE, or other
program - but in a profound misunderstanding of its
operation.
For some reason that escapes me, you are
determined to "advise" your congregation of faithful
followers that simulation is the Devil's most
mendacious tool, with which he lures the vulnerable
young mind to certain downfall. Instead, it appears
you'd have them all build breadboards, as the only
safe path to salvation, as we once did back in the
halcyon, simple days of the 1970's.
Never mind that the breadboard is but one
solitary instantiation of the circuit, merely a
snapshot whose behavior is as likely to be
unrepresentative as it is to be usefully informative;
Never mind that discrete components may not
match, or be almost isothermal, when it is often
essential that they should be;
Never mind that it may take many hours to find
the needed parts and build the board, plus more
frustrating delays when, due to a momentary oversight
during which the supply voltage was set a bit too
high, some (but, ah! which??) transistors were
damaged;
Never mind that numerous parasitic
capacitances and inductances of this rat's nest
introduce indeterminate spurious effects;
Never mind that examining the effects of
temperature variations may take many hours of slow
cycling, using a breadboard connected by long antennas
to a small fortune in test instrumentation;
Never mind that the breadboard is just a
dead-end, rather than a living organism, with precise
parameters that may be passed on to further steps in
the product's life cycle, such as the design of
the test vectors, the IC layout and much else ....
... all these, you'll probably say, are
merely inconveniences, nothing compared to the hugely
erroneous predictions (even death, it seems!) that can
result from the foolhardy dependence on SPICE.
Conveniently, you ignore the facts:
Never mind that SPICE allows studies of
circuit behavior using the full range of documented
lot-to-lot process variations, and enormously
different environmental conditions, in a matter of
minutes, and far beyond what could ever be done at the
bench;
Never mind that simulation studies can provide
statistical data about performance, and thereby start
down the long and arduous road of product
characterization, and the precise determination
of test limit values to optimize yield while
maintaining a high level of performance and product
competitiveness;
Never mind that the often serious effects of
element mismatches can be studied with surgical
precision, and in depth - in cases where it is not
immediately apparent how these play out;
Never mind that one can vary an individual
device temperature, or all of them, in a matter of
minutes rather than hours in an environmental chamber;
or explore the consequences of dynamic thermal
imbalance due to transient power shifts, impossible in
any other way; or entirely remove the thermal modeling
of all or some transistors, or experimentally couple
them in various ways, with a view to minimizing
thermal errors;
Never mind that one can selectively sweep
individual parameters within a model - say, BF, to
examine (qualitatively and to some extent
quantitatively) the consequence of very low (one-third
nominal) to very high (one thousand times nominal) DC
beta and optionally, inversely couple VAF to BF; or
examine the inertial consequences of sweeping TF; or
just remove CJC when it seems that is causing the
problem - or all the junction capacitances,if you like
- and indeed, all the junction resistances, too. Is
that realistic? No. Is it rich in insights?
Unquestionably.
Never mind that one can investigate many
extreme conditions as readily as more benign ones,
asking such "What if?" questions as: "What happens at
-100C, or +200C?" (I note here that ADI's transistors
- their equations and their parameterization - and
of course the simulation engine, continue to hold up
from -270C to over +300C);
Or "What if the supply spikes to ten times its
nominal value?";
Or "What happens when an ESD event hits this
or that IC pin?"
Or ask numerous other "What if?" questions,
including the one that's surely the most powerful:
"What happens if I alter the topology - this way - or
maybe that?" - a question that is at the root of all
circuit invention, and readily performed with
the utmost precision and insight in simulation, while
at best difficult and uncontrollable when attempted on
a breadboard;
Never mind that one can include all those
immensely important but subtly-interacting parasitic
reactances of the IC package, and the detailed
parasitics back-extracted from the layout;
Never mind that the outcome of all these
simulations studies is not only a data object, which
passes effortlessly forward to all subsequent steps in
the development; but which, unlike the
dust-collecting, damage-prone, test-stressed
breadboard, can be archived in complete, pristine and
total accuracy;
And, above all,
Never mind that the student emerges from these
studies having not simply a rat's nest, to jubilantly
wave in front of one's admiring onlookers (as I've
seen you do), but RATHER, with a WHOLE NEW SET OF
INSIGHTS into clever circuit design, insights that
would almost certainly have remained hidden in a
pathetic one-off jumble of ad-hoc components...
....in spite of all these aspects of
simulation and many more, Bob, you STILL advise the
pursuit of something vaguely called "real design", and
warn that anything else will result in the designer
being (to use your charming, delicately-chosen
words)"screwed out of the art and the pleasure of how
to learn to design circuits without SPICE simulation"
(but evidently not
helped much with the construction of an English
sentence).
Frankly, the joy I extract from design is
experienced almost entirely in the hours spent
crafting the product on my screen with the assurance
that the performance of first silicon will be
excellent - and if it's not, it will because I screwed
up, not because of any fault of my simulator. During
the process, I am completely in control, no less fully
than someone poring over a stack of textbooks, lecture
notes, professional papers or whatever else supposedly
characterizes this thing called "real design". Quite
to the contrary, like any other user of a good
simulator, I have at my fingertips the knowledge of a
small army of experts in all aspects of device
modeling, and
numerous powerful mathematical algorithms and
post-processing capabilities. A seat at a simulator is
every designer's dream come true, the open sesame to
endless pleasurable invention.
I can only assume, Bob, that you've somehow
been denied this experience, but still I'm puzzled by
your extreme invective. Perhaps it is being offered
in a genuine and sincere attempt to point out the
(rare) pitfalls that may be encountered by the unwary,
inexperienced or lazy user of circuit simulators.
However, there are many dangers, much more
threatening, that might be brought to the attention of
"this poor kid in India"- or anywhere else.
I recall you telling me once, in a lunch line,
that you didn't trust calculators because "they make
mistakes", and that you preferred to do your
calculations on a slide-rule, or with a pencil and
paper. Remarkable. No: incredible! Most of us need
a bit more help in getting the job done, and I rather
suspect you do, too, but hate to admit it. At least
you have stopped throwing computers from high places
(you HAVE, haven't you?).
*******************************
I certainly agree that there is need for
considerable caution in using any tool, including, of
course, SPICE and its kin. To begin with, the user
needs to immerse himself in the learning process,
mostly by running hundreds of examples, and examining
the consequences of this or that variation, whether
these are in the circuit topology, the device types,
detailed parameter values, convergence tolerances and
much else.
As a matter of good practice, one's approach
to a new portfolio of simulation studies should begin
with a thorough "calibration" of the particular
situation, moving quickly through rudimentary DC
working points, to a set of transient analyses, during
which it's prudent to establish the effects of
changing the tolerance factors (such as CHGTOL,
ABSTOL, RELTOL and VNTOL), in order to determine
whether the convergence of each solution point is of
sufficient accuracy for your purposes. Thus, one would
not use large values of these factors when
investigating distortion and
intermodulation at high frequencies down to the
-120dBc level, but may elect to do so in simulating a
complex logic function, to speed the computation.
Most junior engineers have probably already
encountered your "op-amp connected backward"
situation. They quickly recognize what's up, when it
appears to behave more or less correctly as a feedback
amplifier using AC excitation. They appreciate the
limitations of restricting circuit analyses to this
particular mode of exploration. ALL
small-circuit (linearized) models and the resulting
matrices are inherently narrow in their ability to
represent actual circuit behavior. The
normally-intelligent user comes to understand this
particular peculiarity, after a short time in the
driver's seat, and learns instinctively how to avoid
falling into such rudimentary traps thereafter.
I strenuously advise my students against the
exclusive use of AC analyses, partly because in common
situations, these fail to alert the user to his OWN
silly errors in feedback polarity connections -
certainly not something to blame the simulator for! -
and partly because any signal-induced deviations from
a
small-signal operating point invariably result in
significant, frequently important, and often drastic,
changes in behavior.
Some of the "blame" for this aberrant
viewpoint can be laid at the feet of the many
textbooks which spend far too much time on the easy
notion of small-signal models, and on the predictable,
straightforward mathematics of linear networks,
to the neglect of the more "awkward" topic of
nonlinear elements and circuits, and their often
intractable equations - which of course is why we must
depend on simulators in their native role as nonlinear
equation-solvers. It is probably fair to say that no
textbook or design methodology allows one to cope with
the inherently nonlinear nature of our active elements
(and several "passive" ones, too) in an adequately
detailed and comprehensive fashion. One notably
egregious example of the importance of simulation
is that of circuit noise, invariably treated as a
small-signal phenomenon, but in fact hugely affected
by nonlinearities.
We might perhaps recommend a handful of useful
and instructive exercises for the SPICE neophyte,
suggesting, for example, to be on the lookout for
anomalies and even apparent errors in the results. It
thereby becomes a quest, an adventure of discovery.
While it's extremely unlikely any will be found, the
challenge would oblige the experimenter to take more
risks - to push the envelope, as people like to say
about doing anything original.
As one who has used simulators productively
and profitably for over thirty years (after one
memorable lunch with Don Pederson, the "father of
SPICE", at which, as a young upstart of but 27, I
opined that "SPICE is a crutch for those who can not
design
circuits"), and as a contributor to new analysis
features and operating modes, I am assured that any
apparent "errors" will invariably be entirely those of
the user: not the engine, nor the model equations, nor
their parameterization.
Once in a Blue Moon, one may find a genuine
bug. But they're very few and far between, and usually
arise when some arcane, unplanned operation is
attempted, one which the command-line parser is
unprepared for. These mishaps are mostly harmless,
causing only fleeting frustration when some needed
operation is not already built in (so one has to write
one's own macro - usually a matter of minutes).
I can't speak for run-of-the-mill circuit
simulators, such as SPICE clones, but doubt there are
genuine or serious errors in the core code for any of
them. The modeling equations could be inaccurate in
some user's environments; it is well understood,
of course, that they are incomplete, as must be any
finite set of equations and parameters describing a
physical element.
But if you can actually cite a case of a
significant error in simulator (all of your examples,
generously offered in various texts, have been only
red herrings - blatant and obvious cases of
misunderstanding, which you seem to delight in
reporting as "problems with SPICE"), I'd be extremely
interested in hearing about it. More importantly,
though, you should make a point of reporting such rare
shortcomings to those who might be able to effect a
remedy. That is admittedly less easy when one doesn't
have the source code under control. Still, unless a
defect is noted and reported, by somebody, it will
never be corrected.
I agree that there is much the experienced
analog designer can do to encourage sound design
practices. I also agree that, to a sobering extent,
these topics are rarely given the emphasis
they deserve, and that consequently new graduates know
only of the world as presented through the eye of
their computer. Many have never held a transistor in
their hand; even fewer have had the opportunity to
take it for a brisk walk on a curve-tracer, or
persuaded it to show its paces on an oscilloscope.
That is not the fault of the curve-tracer, nor the
fault of the scope;
equally, "bad design" cannot be blamed on one's
simulator. It is just as easy to do silly things with
paper and pencil (and, yes, a slide-rule if you
insist) as it is using SPICE.
I believe the way to address these issues is
by taking positive remedial action, pin-pointing each
of the specific concerns as it comes along. It is
certainly never a reason for blaming our tools, nor an
excuse for empty belly-aching or pointing fingers
at imaginary faults. While undeniably colorful, Don
Quixote was in the final analysis a pitiable
character. History celebrates those who build, and
quickly forgets those who would tear down.
Barrie
-----Original Message-----
From: Robert A Pease
Subject: SPICE??? ???
Subject: SPICEY thoughts...
*** Hello, Barrie,
Yeah, I thought I really ought to send a
copy of this to YOU.
As well as to a dozen people who thought
I was being "too negative" about SPICE....
*** YES, you may say that YOUR colleagues in
the Simulation or Design Automation Groups at ADI
have
protected and defended YOU from such troubles. But
this poor kid in India is still being screwed up
by SPICE. Lots of the people in the world are. YOU
can say that YOU are not being screwed up by SPICE
- - but you cannot say that SPICE is not screwing
up a lot of other people.
AND:
*** Worst of all - you cannot say that
engineers in other places are not being screwed out
of the art and the pleasure of how to learn to
design circuits without SPICE. Which we both agree
is of great value.
*** Your comments are invited. Best
regards. / /rap
From: Robert A Pease
Subject: BAD SPICE...
&&& Hello to Tom, and Colin, and Wayne,
and Roger, and Hal, and Willy, and MARK, and Bob, and
Paul, and other FRIENDS:
&&& Okay, Gentlemen:
Some of you guys think I am "much too
negative" about SPICE.
Some of you have been saying that there is
"nothing wrong with SPICE". If I have been having
little problems, maybe I am just using "LOUSY
models...."
&&& Here is a poor lad in a far-off place,
and SPICE had just about convinced him that the
inputs of an op-amp don't care if you connect them
in the right sense, or the wrong way.... Do you
think he was using the "wrong models" - ??
&&& Let me assure you, that if SPICE starts
lying this way, it doesn't make any difference if
you got a macromodel from NSC, or from any other
company, or just "Vout = (gain) x (V1 - V2) x (lag)",
or a real schematic of an op-amp made with
transistors.
SPICE is capable of screwing up --
and is capable of screwing YOU up.
It's not just the ORCAD PSPICE VER9.1. I've been told
that other kinds of SPICE are still misfiring like
this.
If you doubt it, go ahead and put your
favorite op-amp model into SPICE, and make sure it's
right by putting a ramp-step into the + input.
(With the output tied to the - input.) OK, it's a
follower. (But don't put in a tiny Time-Step - just
let it run in Default.)
&&& NOW swap the inputs. Does it "follow", or
does it PEG? Some people still have it "following",
if they are using ORCAD PSPICE VER9.1. Or similar
flawed versions. Hell, we don't even know which
version of BERKELEY SPICE has fixed this... (if
any....)
------------------------------------------------------------
&&& One of you folks, as listed as
recipients, said that "SPICE is a powerful tool".
That is indeed true. It is so powerful that it can
fool a young engineer into thinking that Black is
White, and since Black and White are the same , then
GREY is just - the natural state of Chaos. SPICE
told him that here is no way to detect the chaos, and
in fact, even an op-amp or a comparator cannot detect
the difference between Black and White. Every op-amp
is just -- a follower, EVEN WHEN you cross-wire the
inputs.
&&& - - Isn't that what you wanted it to do?
- - - - -
YES: SPICE is a very powerful tool. It
can screw you up, in strong, powerful ways...
---------------------------------------------------------------
LOOK HERE, Gentlemen: I have a PC, and it is
so powerful that if there is something it does not
want me to see, it won't let me see it. It is -- yes
-- powerful -- but it is an insolent piece of SHIT.
Don't tell me that I'm incompetent at using my
computer. Sometimes I tell the computer EXACTLY what
to do, and it EXACTLY refuses to do it. With friends
like that, who needs
enemies? Anemones? Enemas?
*** Best regards. / rap / Robert A. Pease,
Staff Scientist, NSC.
-----------------------------------------------
P.S. - You guys might think I am very
PICKY. Nobody really cares if the op-amp's gain is
depicted properly, + or -.
WRONG.
I was once working on a project with only
5,000 transistors. Continuity and integrity was
assured because SPICE said it was working. SURE.
Then somebody spotted that one of the
op-amp's inputs had been crossed. Yet SPICE said it
was OK.
Who spotted the flaw? The senior engineer?
SPICE? ME? No. It was Carlos, the senior
technician. Thank God...
Pissing away a $100,000 maskset because
SPICE DOESN'T CARE whether the inputs are crossed or
not - is a SERIOUS BUSINESS ERROR.
YOU trust SPICE?
YOU can trust SPICE every time it helps
you - and you can DIE every time it lies to you. I
never let SPICE lie to me. / rap
-----------------------------------------------
>From nishanth
Subject: Re: opamp doubt
Dear Sir,
I am happy that i didnt get
fooled.hahahahaha......hmmm....
*** Well, you were sure fooled for a WHILE!!
*** What do you say about the way that your
MATH agreed with that LYING SPICE? You shouldn't
have
let the computer play tricks on your pencil-and-paper
math.
I think, as u said,I had been trying out the
circuit with wrong time step size value.I tried with
run time = 10ms, start saving data after 0.1ns,
maximum
step size = 0.1ms.It seems to work as a voltage
follower.
The input signal is sine wave with 0.5v
amplitude,1v offset and 100hz frequency.I tried with a
model of LM324 with single supply(5v) operation.
Then, i tried with a maximum step size of 0.01
ms.Now it shows an output saturated in the negative
side(0v). WOW...it is working!!!hmmmm....
I wonder if ur NSC spice also gives the same
results with the same settings?I am using ORCAD PSPICE
VER9.1.
*** NSC's versions of SPICE do not now permit
it to run like a "follower".
I still feel like a fool....,who doesnt know
how to use his tool.hmmm...any way.
*** No, your tool was LYING to you. Never
trust SPICE.
thanx buddy,
luv,
nishanth salahudeen
*** Be sure to warn all your buddies that
SPICE can LIE LIKE A RUG!
*** And, let me know what "ORCAD" says when
you complain about
this.../rap
----- Original Message -----
From: "Robert A Pease"
Subject: Re: opamp doubt
Dear sir,
I am a 'HARD'ware design engineer in a
company in INDIA. I was always interested in analog
design and have read ur book on trouble shooting.
Now i have a design doubt and would like to
ask you before puting it physically on board.The basic
question is given below.I am talking about
voltage feedback opamps here.
1)if i connect output of an opamp to it's
inverting input and apply a signal to its
non-inverting input, it works as a voltage follower.I
understand this as,"the output rises until the
difference signal seen by the opamp input
stage tends to zero and thus input and output are
same, ignoring the offset
".Fine every body knows this!!!. But what
happens if i short the output to the non inverting
input and apply signal to the inverting input?Math
equations and spice simulation seems to show that it
still works as a voltage follower.But I cant visualize
it as before.Is this the TRUTH? Or is spice fooling
me? As i understand it, "the output was zero at the
beginning of time.The output goes down to the negative
side as i apply a positive signal to the inverting
input.Now the difference signal seen by the
input stage is increasing which further accelerates
the output more to the negative side until it
saturates". Is this explanation wrong?!! If so, Why?
If it actually works as a voltage follower, is'nt
there any practical
advantage/disadvantage for this particular method?
How to avoid being HOAXED - by SPICE.
*** Hello, Nishanth,
I have heard that some kinds of SPICE will
servo "backwards" in a case like this, if the
circuit and conditions are entered into SPICE in the
simplest way. This may be related to the DEFAULT
value of time-step. That may cause "false gain" as
you describe.
Here at NSC, our SPICE will not give the false
answer you saw.
Please tell me EXACTLY what kind of SPICE you
are using...
Now, here is the TRICK: SET the time-step to a
smaller value. If the rise-time of the input step is 1
usecond, set the "time-step" to a much smaller value,
such as 0.1 or 0.01 Usecond, or even smaller. The
op-amp may start (properly) going to limit,rather than
acting like a follower, with + gain.
As I always say - SPICE can lie - and lie
BADLY.
YOU should never trust SPICE. Or, in
general, never trust any computer.
Best regards. / rap
*** Strongly worded memo, as a reply, SOON. /
rap
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