On Tue, 17 Jan 2012 22:32:53 -0500, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:> Does the wires assume real life situations to some >degree or do we need to add stray inductance, Resistors >and caps to act like a board? > > I would of thought it would assume an average lay out of one >persons worse nightmare and the actual real live circuit would >just work better, seeing that this designer would engineer proper >protocols on the board. > > It just seems that I don't see many of the small defects the >sim may show, when it is on a real board assembled using common >practices. > > Which leads me to my question, does the sim assume a worse case >scenario or maybe the sim just over exaggerates now and then? > > This would be LTspice I am referring to. I was thinking of looking at >Pspice to see how that goes. > > Jamie >Do the math... by hand... then use the sim to VERIFY! ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Still waiting for a Ltspice expert answer.
Started by ●January 17, 2012
Reply by ●January 17, 20122012-01-17
Does the wires assume real life situations to some degree or do we need to add stray inductance, Resistors and caps to act like a board? I would of thought it would assume an average lay out of one persons worse nightmare and the actual real live circuit would just work better, seeing that this designer would engineer proper protocols on the board. It just seems that I don't see many of the small defects the sim may show, when it is on a real board assembled using common practices. Which leads me to my question, does the sim assume a worse case scenario or maybe the sim just over exaggerates now and then? This would be LTspice I am referring to. I was thinking of looking at Pspice to see how that goes. Jamie
Reply by ●January 17, 20122012-01-17
I've heard of a few simulators which assumed the designer indicated wire resistance proportional to the length of the traces on the schematic drawing. Which often have no relation to actual board traces, and certainly have no relation to their widths. As far as I know, most professional environments (SPICE based) assume all nets are ideal. If the models themselves include parasitics, they will necessarily be limited to package parasitics. It's always the user's responsibility to include realistic wiring parasitics. I don't remember if LTSpice includes package parasitics. I seem to recall they mostly use MODELs rather than SUBCKTs, but they may've expanded the MODEL definition to include simple series or parallel elements. In short, no, SPICE tends to assume ideal conditions. It's a model, not a reality simulator, and it's your duty to construct a model that has any useful representation at all. If you insist on reality simulation, there are suites out there which attempt to do such things. Ansoft makes such a system, including electromagnetic (which can do static, AC and transient 3D simulations on real board layouts, and generate approximate SPICE models for the circuit), mechanical (stress/strain) and thermal (including fluid flow, expansion, tempcos to feed back into the circuit). A few others make similar suites; all are horrifically expensive, only something worthwhile if you're either making millions of top-quality, moderately complex products (cars maybe??), or a few very, VERY critical systems (avionics, aircraft, spacecraft?). Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms "Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message news:i%qRq.2757$FH7.1429@newsfe09.iad...> Does the wires assume real life situations to some > degree or do we need to add stray inductance, Resistors > and caps to act like a board? > > I would of thought it would assume an average lay out of one > persons worse nightmare and the actual real live circuit would > just work better, seeing that this designer would engineer proper > protocols on the board. > > It just seems that I don't see many of the small defects the > sim may show, when it is on a real board assembled using common > practices. > > Which leads me to my question, does the sim assume a worse case > scenario or maybe the sim just over exaggerates now and then? > > This would be LTspice I am referring to. I was thinking of looking at > Pspice to see how that goes. > > Jamie > >
Reply by ●January 18, 20122012-01-18
Jim Thompson wrote:> On Tue, 17 Jan 2012 22:32:53 -0500, Jamie > <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: > > >> Does the wires assume real life situations to some >>degree or do we need to add stray inductance, Resistors >>and caps to act like a board? >> >> I would of thought it would assume an average lay out of one >>persons worse nightmare and the actual real live circuit would >>just work better, seeing that this designer would engineer proper >>protocols on the board. >> >> It just seems that I don't see many of the small defects the >>sim may show, when it is on a real board assembled using common >>practices. >> >> Which leads me to my question, does the sim assume a worse case >>scenario or maybe the sim just over exaggerates now and then? >> >> This would be LTspice I am referring to. I was thinking of looking at >>Pspice to see how that goes. >> >> Jamie >> > > > Do the math... by hand... then use the sim to VERIFY! > > ...Jim ThompsonWell that just sounds counter productive. Why waste my time using a sim if I am going to do it by hand in the first place? You can haggle all you want Jim, It does not work on me.. I am not one of your goal post. You have JL for that :) Jamie
Reply by ●January 18, 20122012-01-18
On Tue, 17 Jan 2012 22:32:53 -0500, Jamie wrote:> Does the wires assume real life situations to some degree or do we need > to add stray inductance, Resistors and caps to act like a board? > > I would of thought it would assume an average lay out of one > persons worse nightmare and the actual real live circuit would just work > better, seeing that this designer would engineer proper protocols on the > board. > > It just seems that I don't see many of the small defects the > sim may show, when it is on a real board assembled using common > practices. > > Which leads me to my question, does the sim assume a worse case > scenario or maybe the sim just over exaggerates now and then? > > This would be LTspice I am referring to. I was thinking of looking at > Pspice to see how that goes.Wires in LTSpice (and PSpice, as far as I know), are made of Magic Stuff. A node is a node is a node (in Spice-land) and has no voltage variations across it. Jim's right, although I would state it differently: the simulation is help for those who already know what's up; try something that the simulation isn't prepared to give a correct answer for, and you'll get a wrong answer. It _is_ handy for complex, nonlinear circuits (like switchers), but you still have to pose questions that it can answer sensibly. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●January 18, 20122012-01-18
On Jan 17, 8:32=A0pm, Jamie <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:> =A0 Does the wires assume real life situations to some > degree or do we need to add =A0stray inductance, Resistors > and caps to act like a board? > > =A0 =A0I would of thought it would assume an average lay out of one > persons worse nightmare and the actual real live circuit would > just work better, seeing that this designer would engineer proper > protocols on the board. > > =A0 =A0It just seems that I don't see many of the small defects the > sim may show, when it is on a real board assembled using common > practices. > > =A0 =A0Which leads me to my question, does the sim assume a worse case > scenario or maybe the sim just over exaggerates now and then? > > =A0 This would be LTspice I am referring to. I was thinking of looking at > Pspice to see how that goes. > > =A0 JamieDon't forget to add, "delay" All these simulators keep forgetting about that pesky quirk of reality, "It takes time for a signal to go from here to there." And, oh, yes, don't forget to add that even peskier term, "radiation loss" That way when you simulate charging a cap from another cap, you can reconcile the apparent loss of energy.
Reply by ●January 18, 20122012-01-18
On Tue, 17 Jan 2012 23:43:42 -0500, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:>Jim Thompson wrote: > >> On Tue, 17 Jan 2012 22:32:53 -0500, Jamie >> <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: >> >> >>> Does the wires assume real life situations to some >>>degree or do we need to add stray inductance, Resistors >>>and caps to act like a board? >>> >>> I would of thought it would assume an average lay out of one >>>persons worse nightmare and the actual real live circuit would >>>just work better, seeing that this designer would engineer proper >>>protocols on the board. >>> >>> It just seems that I don't see many of the small defects the >>>sim may show, when it is on a real board assembled using common >>>practices. >>> >>> Which leads me to my question, does the sim assume a worse case >>>scenario or maybe the sim just over exaggerates now and then? >>> >>> This would be LTspice I am referring to. I was thinking of looking at >>>Pspice to see how that goes. >>> >>> Jamie >>> >> >> >> Do the math... by hand... then use the sim to VERIFY! >> >> ...Jim Thompson >Well that just sounds counter productive. > > Why waste my time using a sim if I am going to do it by hand in the >first place? > > You can haggle all you want Jim, It does not work on me.. > > I am not one of your goal post. You have JL for that :) > > Jamie >"goal post"? Bwahahahahahaha! So show us your by-hand math. You're showing your inexperience by continuing to argue. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●January 18, 20122012-01-18
On Tue, 17 Jan 2012 23:43:42 -0500, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:>Jim Thompson wrote: > >> On Tue, 17 Jan 2012 22:32:53 -0500, Jamie >> <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: >> >> >>> Does the wires assume real life situations to some >>>degree or do we need to add stray inductance, Resistors >>>and caps to act like a board? >>> >>> I would of thought it would assume an average lay out of one >>>persons worse nightmare and the actual real live circuit would >>>just work better, seeing that this designer would engineer proper >>>protocols on the board. >>> >>> It just seems that I don't see many of the small defects the >>>sim may show, when it is on a real board assembled using common >>>practices. >>> >>> Which leads me to my question, does the sim assume a worse case >>>scenario or maybe the sim just over exaggerates now and then? >>> >>> This would be LTspice I am referring to. I was thinking of looking at >>>Pspice to see how that goes. >>> >>> Jamie >>> >> >> >> Do the math... by hand... then use the sim to VERIFY! >> >> ...Jim Thompson >Well that just sounds counter productive. > > Why waste my time using a sim if I am going to do it by hand in the >first place? > > You can haggle all you want Jim, It does not work on me.. > > I am not one of your goal post. You have JL for that :) > > Jamie >If you don't understand your circuit maths, how do you know if the simulation is close to reality???? Many times, you can do this in your head, but sometimes you need to write down a bunch of maths to understand issues you may run in to. As an excercise, try to temperature stabilize a simple one transistor amplifier without understanding the basic transistor equations. Do you blindly trust your models??? If you do, beware, they are often wrong. With experience, you will learn a healthy distrust in your simulator and test equipment.
Reply by ●January 18, 20122012-01-18
On Wed, 18 Jan 2012 10:57:36 -0800, qrk <SpamTrap@spam.net> wrote:>On Tue, 17 Jan 2012 23:43:42 -0500, Jamie ><jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: > >>Jim Thompson wrote: >> >>> On Tue, 17 Jan 2012 22:32:53 -0500, Jamie >>> <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: >>> >>> >>>> Does the wires assume real life situations to some >>>>degree or do we need to add stray inductance, Resistors >>>>and caps to act like a board? >>>> >>>> I would of thought it would assume an average lay out of one >>>>persons worse nightmare and the actual real live circuit would >>>>just work better, seeing that this designer would engineer proper >>>>protocols on the board. >>>> >>>> It just seems that I don't see many of the small defects the >>>>sim may show, when it is on a real board assembled using common >>>>practices. >>>> >>>> Which leads me to my question, does the sim assume a worse case >>>>scenario or maybe the sim just over exaggerates now and then? >>>> >>>> This would be LTspice I am referring to. I was thinking of looking at >>>>Pspice to see how that goes. >>>> >>>> Jamie >>>> >>> >>> >>> Do the math... by hand... then use the sim to VERIFY! >>> >>> ...Jim Thompson >>Well that just sounds counter productive. >> >> Why waste my time using a sim if I am going to do it by hand in the >>first place? >> >> You can haggle all you want Jim, It does not work on me.. >> >> I am not one of your goal post. You have JL for that :) >> >> Jamie >> >If you don't understand your circuit maths, how do you know if the >simulation is close to reality???? Many times, you can do this in your >head, but sometimes you need to write down a bunch of maths to >understand issues you may run in to. As an excercise, try to >temperature stabilize a simple one transistor amplifier without >understanding the basic transistor equations. > >Do you blindly trust your models??? If you do, beware, they are often >wrong. With experience, you will learn a healthy distrust in your >simulator and test equipment.I don't think Jamie is even a technician, but he _is_ successfully becoming a reductive emulation of John Larkin ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●January 18, 20122012-01-18
On Jan 17, 7:32=A0pm, Jamie <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:> =A0 Does the wires assume real life situations to some > degree or do we need to add =A0stray inductance, Resistors > and caps to act like a board?No.> =A0 =A0I would of thought it would assume an average lay out of one > persons worse nightmare...One persons' nightmare is not "average." WTH? No, what you suggest -- "average" is whackadoodle. There is no "average" layout.> =A0 =A0It just seems that I don't see many of the small defects the > sim may show, when it is on a real board assembled using common > practices.There may be standards and usual practice for a variety of tasks -- the more simple the more likely it can be handled by standards. "Simple" is limited. The harder the job, the more diverse opinion you'll get regarding what is a proper approach.> =A0 =A0Which leads me to my question, does the sim assume a worse case > scenario or maybe the sim just over exaggerates now and then?"Worst case" and "one persons' nightmare" doesn't mean "average" or "common." If your wrong views were more coherent, then they would be easier to pick apart! You have 1 pF of pad capacitance, and you think it may matter? then put it in You care about 0.7 nH of via inductance? then put it in You care about the inductance of a long trace? then put it in The models are just *models* -- not the real thing. How good are they? After all, that is all spice has to work with. Models are never perfect -- what should spice assume as "average" about them? Caveat emptor: you need to have some idea of what you're doing in the first place. I think your question is really: Can I get SPICE to think for me? No, you can't.
