Forums

Unconnected part LTSpice seems to need (weird ...)

Started by Joerg August 14, 2011
Jim Thompson wrote:
> On Wed, 17 Aug 2011 20:08:11 -0400, Jim Thompson > <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: > >> On Wed, 17 Aug 2011 19:59:44 -0400, Jim Thompson >> <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >> >>> On Wed, 17 Aug 2011 16:44:36 -0700, Joerg <invalid@invalid.invalid> wrote: >>> >>>> John S wrote: >>>>> On 8/17/2011 5:40 PM, Joerg wrote: >>>>>> Fred_Bartoli wrote: >>>>>>> "Joerg"<invalid@invalid.invalid> a &#2013265929;crit dans le message de >>>>>>> news:9avccjFfkgU1@mid.individual.net... >>>>>>>> Martin Brown wrote: >>>>>>>>> On 14/08/2011 23:33, Joerg wrote: >>>>>>>>>> Folks, >>>>>>>>>> >>>>>>>>>> This is close to voodoo but repeatable. Unfortunately I can't >>>>>>>>>> disclose >>>>>>>>>> the schematic since it is for a client. Just wondering , anyone had >>>>>>>>>> this >>>>>>>>>> before? >>>>>>>>>> >>>>>>>>>> At the far end of a TX line I used to have a diode connected across >>>>>>>>>> because a previous version of a chip would have such a substrat path. >>>>>>>>>> Nice waveforms, fast sims. Everything as expected and peachy. Now the >>>>>>>>>> new iteration of the chip design won't have this diode path so I >>>>>>>>>> chopped >>>>>>>>>> off its cathode connection. Sims fine. So I deleted the diode -> >>>>>>>>>> ka-crunch ... sim slows down and the ouput is junk. >>>>>>>>>> >>>>>>>>>> If I put the diode back in and connect only its anode -> fine. If I >>>>>>>>>> leave the anode open and only connect the cathode it still sims >>>>>>>>>> but the >>>>>>>>>> results are different. >>>>>>>>>> >>>>>>>>>> <scratching head> >>>>>>>>> I am always amazed how well spice works these days. Best I can suggest >>>>>>>>> is start deleting other nodes and components to try and get a minimum >>>>>>>>> size network that shows the problem and can be disclosed publicly. >>>>>>>>> >>>>>>>> I did that a couple of days ago, it's attached again. >>>>>>>> >>>>>>>> >>>>>>>>> If you only connect the anode but to the wrong side of where the diode >>>>>>>>> goes do you get the "anode connected" behaviour or the other one? >>>>>>>>> >>>>>>>> Then the sim completely bombed on me. Like Fred said, it is possible >>>>>>>> that SPICE incudes this diode in the matrix and chokes on it. But then >>>>>>>> it's supposed to issue an error message and not just fly straight into >>>>>>>> terrain. >>>>>>>> >>>>>>> Looking a bit further (I don't use LTspice as my current production >>>>>>> tool) it happens that: >>>>>>> * a default 10^-12 gmin option is permanently set. No way to defeat it, >>>>>>> apprat from, maybe, setting it to zero. >>>>>>> * look at the log file. The unconnected node is detected. >>>>>>> >>>>>> Thanks, this would explain a lot. That's what Jim suspected, he beat you >>>>>> by 14 minutes :-) >>>>>> >>>>>> Well, if it can't be turned off I guess I'll have to learn to live with >>>>>> that stuff. No complaints because LTSpice is free and generally doesn't >>>>>> crash. Other SPICEs cost an arm amnd a leg and crash a lot. >>>>>> >>>>> In LTSpice click Tools/Control Panel. >>>>> Click the Spice tab. >>>>> Change Gmin as you wish in the upper right box. >>>>> >>>> Could be dicey. If I set it to zero other things my go kablouie because >>>> it's then universal. >>> The VALUE of gmin isn't the problem, it's that unconnected pins are being >>> connected to ground (most likely) via gmin. I basically only use LTspice as a >>> viwer and to run netlists, so I don't know the ins-and-outs of configuring >>> pins. Make sure your symbols don't default to connecting thru gmin, rather >>> than balking. >>> >>> ...Jim Thompson >>> >>> [On the Road, in New York] >> topologycheck >> Num. >> 1 >> Set to zero to skip check for floating nodes, loops of voltage sources, and >> non-physical transformer winding topology >> >> Make sure it's set to "1" >> >> ...Jim Thompson >> >> [On the Road, in New York] > > Also... > > "Mark unconnected pins: Draw a small square at each unconnected pin to flag it > as unconnected." >
But that would have to be done in the library, right? I use a ton of standard lib parts in there, which would make that cumbersome. So for now I make sure I completely delete a part for tests. A bit clumsy but man's gotta do what man's gotta do. Just got some circuit board quotes back. One of them blew me away. Companies in that field must be really hungry. -- Regards, Joerg http://www.analogconsultants.com/
On Wed, 17 Aug 2011 17:21:03 -0700, Joerg <invalid@invalid.invalid> wrote:

>Jim Thompson wrote: >> On Wed, 17 Aug 2011 20:08:11 -0400, Jim Thompson >> <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >> >>> On Wed, 17 Aug 2011 19:59:44 -0400, Jim Thompson >>> <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >>> >>>> On Wed, 17 Aug 2011 16:44:36 -0700, Joerg <invalid@invalid.invalid> wrote: >>>> >>>>> John S wrote: >>>>>> On 8/17/2011 5:40 PM, Joerg wrote: >>>>>>> Fred_Bartoli wrote: >>>>>>>> "Joerg"<invalid@invalid.invalid> a &#2013265929;crit dans le message de >>>>>>>> news:9avccjFfkgU1@mid.individual.net... >>>>>>>>> Martin Brown wrote: >>>>>>>>>> On 14/08/2011 23:33, Joerg wrote: >>>>>>>>>>> Folks, >>>>>>>>>>> >>>>>>>>>>> This is close to voodoo but repeatable. Unfortunately I can't >>>>>>>>>>> disclose >>>>>>>>>>> the schematic since it is for a client. Just wondering , anyone had >>>>>>>>>>> this >>>>>>>>>>> before? >>>>>>>>>>> >>>>>>>>>>> At the far end of a TX line I used to have a diode connected across >>>>>>>>>>> because a previous version of a chip would have such a substrat path. >>>>>>>>>>> Nice waveforms, fast sims. Everything as expected and peachy. Now the >>>>>>>>>>> new iteration of the chip design won't have this diode path so I >>>>>>>>>>> chopped >>>>>>>>>>> off its cathode connection. Sims fine. So I deleted the diode -> >>>>>>>>>>> ka-crunch ... sim slows down and the ouput is junk. >>>>>>>>>>> >>>>>>>>>>> If I put the diode back in and connect only its anode -> fine. If I >>>>>>>>>>> leave the anode open and only connect the cathode it still sims >>>>>>>>>>> but the >>>>>>>>>>> results are different. >>>>>>>>>>> >>>>>>>>>>> <scratching head> >>>>>>>>>> I am always amazed how well spice works these days. Best I can suggest >>>>>>>>>> is start deleting other nodes and components to try and get a minimum >>>>>>>>>> size network that shows the problem and can be disclosed publicly. >>>>>>>>>> >>>>>>>>> I did that a couple of days ago, it's attached again. >>>>>>>>> >>>>>>>>> >>>>>>>>>> If you only connect the anode but to the wrong side of where the diode >>>>>>>>>> goes do you get the "anode connected" behaviour or the other one? >>>>>>>>>> >>>>>>>>> Then the sim completely bombed on me. Like Fred said, it is possible >>>>>>>>> that SPICE incudes this diode in the matrix and chokes on it. But then >>>>>>>>> it's supposed to issue an error message and not just fly straight into >>>>>>>>> terrain. >>>>>>>>> >>>>>>>> Looking a bit further (I don't use LTspice as my current production >>>>>>>> tool) it happens that: >>>>>>>> * a default 10^-12 gmin option is permanently set. No way to defeat it, >>>>>>>> apprat from, maybe, setting it to zero. >>>>>>>> * look at the log file. The unconnected node is detected. >>>>>>>> >>>>>>> Thanks, this would explain a lot. That's what Jim suspected, he beat you >>>>>>> by 14 minutes :-) >>>>>>> >>>>>>> Well, if it can't be turned off I guess I'll have to learn to live with >>>>>>> that stuff. No complaints because LTSpice is free and generally doesn't >>>>>>> crash. Other SPICEs cost an arm amnd a leg and crash a lot. >>>>>>> >>>>>> In LTSpice click Tools/Control Panel. >>>>>> Click the Spice tab. >>>>>> Change Gmin as you wish in the upper right box. >>>>>> >>>>> Could be dicey. If I set it to zero other things my go kablouie because >>>>> it's then universal. >>>> The VALUE of gmin isn't the problem, it's that unconnected pins are being >>>> connected to ground (most likely) via gmin. I basically only use LTspice as a >>>> viwer and to run netlists, so I don't know the ins-and-outs of configuring >>>> pins. Make sure your symbols don't default to connecting thru gmin, rather >>>> than balking. >>>> >>>> ...Jim Thompson >>>> >>>> [On the Road, in New York] >>> topologycheck >>> Num. >>> 1 >>> Set to zero to skip check for floating nodes, loops of voltage sources, and >>> non-physical transformer winding topology >>> >>> Make sure it's set to "1" >>> >>> ...Jim Thompson >>> >>> [On the Road, in New York] >> >> Also... >> >> "Mark unconnected pins: Draw a small square at each unconnected pin to flag it >> as unconnected." >> > >But that would have to be done in the library, right? I use a ton of >standard lib parts in there, which would make that cumbersome.
That's in the setups. If you're using pre-made parts, you live by LTspice and die by LTspice ;-)
> >So for now I make sure I completely delete a part for tests. A bit >clumsy but man's gotta do what man's gotta do.
I often simply short out parts or cut them loose and tie all ports to ground.
> >Just got some circuit board quotes back. One of them blew me away. >Companies in that field must be really hungry.
It now seems suddenly hot for technologists... than God! ...Jim Thompson [On the Road, in New York] -- | 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.
Jim Thompson wrote:
> On Wed, 17 Aug 2011 17:21:03 -0700, Joerg <invalid@invalid.invalid> wrote: > >> Jim Thompson wrote:
[...]
>>> "Mark unconnected pins: Draw a small square at each unconnected pin to flag it >>> as unconnected." >>> >> But that would have to be done in the library, right? I use a ton of >> standard lib parts in there, which would make that cumbersome. > > That's in the setups. If you're using pre-made parts, you live by LTspice and > die by LTspice ;-) >
Well, yeah, but I often need to do quick sims. 1-2h, maybe 3h. So I can't spend a lot of time making library parts.
>> So for now I make sure I completely delete a part for tests. A bit >> clumsy but man's gotta do what man's gotta do. > > I often simply short out parts or cut them loose and tie all ports to ground. > >> Just got some circuit board quotes back. One of them blew me away. >> Companies in that field must be really hungry. > > It now seems suddenly hot for technologists... than God! >
It sure is. But I was surprised by the deals they cut you these days when you need boards made.
> ...Jim Thompson > > [On the Road, in New York]
Still? Whereabouts? When you reach Lake Montauk stomp on the brakes prontissimo or it'll get wet :-) -- Regards, Joerg http://www.analogconsultants.com/
On Wed, 17 Aug 2011 11:02:07 -0700, Joerg <invalid@invalid.invalid>
wrote:

>Interesting, I also grew up with good old ECA224 but that was when the >286 was the latest and greatest. It was quite a useful simulator. >Someone told me it merged into EWB but when I tried that at a client I >did not like it anymore. For my consulting office I bought MicroSim >PSpice. It came with those nice cloth covered IBM-style binders. > >I find this sim behavior odd. It seems only naturaly that, in a >"what-if" scenario, an engineer disconnects just one leg of a part. >After all, that's how we also do it in real life. SPICE is not supposed >to "partially smoosh that" and then not tell anyone.
Hi Joerg, Yes, it looks like you have hit another of those 'interesting assumptions' that always come up and bite us on the posterior. Spice has to have everything connected Somewhere, and have a path to ground from that connection. In the real world, you can have things that go nowhere, and there is no problem. Some spice progs will just give you an error message and say "Where is this supposed to connect, dummy?" while others, like you have run into, try to help out and assume you have a floating node, and it should connect to ground through a high value reisistance. I am afraid it is just a part of learning the capabilities of the tools... 8-) Charlie
On Wed, 17 Aug 2011 19:59:44 -0400, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>On Wed, 17 Aug 2011 16:44:36 -0700, Joerg <invalid@invalid.invalid> wrote: > >>John S wrote: >>> On 8/17/2011 5:40 PM, Joerg wrote: >>>> Fred_Bartoli wrote: >>>>> >>>>> "Joerg"<invalid@invalid.invalid> a &#2013265929;crit dans le message de >>>>> news:9avccjFfkgU1@mid.individual.net... >>>>>> Martin Brown wrote: >>>>>>> On 14/08/2011 23:33, Joerg wrote: >>>>>>>> Folks, >>>>>>>> >>>>>>>> This is close to voodoo but repeatable. Unfortunately I can't >>>>>>>> disclose >>>>>>>> the schematic since it is for a client. Just wondering , anyone had >>>>>>>> this >>>>>>>> before? >>>>>>>> >>>>>>>> At the far end of a TX line I used to have a diode connected across >>>>>>>> because a previous version of a chip would have such a substrat path. >>>>>>>> Nice waveforms, fast sims. Everything as expected and peachy. Now the >>>>>>>> new iteration of the chip design won't have this diode path so I >>>>>>>> chopped >>>>>>>> off its cathode connection. Sims fine. So I deleted the diode -> >>>>>>>> ka-crunch ... sim slows down and the ouput is junk. >>>>>>>> >>>>>>>> If I put the diode back in and connect only its anode -> fine. If I >>>>>>>> leave the anode open and only connect the cathode it still sims >>>>>>>> but the >>>>>>>> results are different. >>>>>>>> >>>>>>>> <scratching head> >>>>>>> >>>>>>> I am always amazed how well spice works these days. Best I can suggest >>>>>>> is start deleting other nodes and components to try and get a minimum >>>>>>> size network that shows the problem and can be disclosed publicly. >>>>>>> >>>>>> >>>>>> I did that a couple of days ago, it's attached again. >>>>>> >>>>>> >>>>>>> If you only connect the anode but to the wrong side of where the diode >>>>>>> goes do you get the "anode connected" behaviour or the other one? >>>>>>> >>>>>> >>>>>> Then the sim completely bombed on me. Like Fred said, it is possible >>>>>> that SPICE incudes this diode in the matrix and chokes on it. But then >>>>>> it's supposed to issue an error message and not just fly straight into >>>>>> terrain. >>>>>> >>>>> >>>>> Looking a bit further (I don't use LTspice as my current production >>>>> tool) it happens that: >>>>> * a default 10^-12 gmin option is permanently set. No way to defeat it, >>>>> apprat from, maybe, setting it to zero. >>>>> * look at the log file. The unconnected node is detected. >>>>> >>>> >>>> Thanks, this would explain a lot. That's what Jim suspected, he beat you >>>> by 14 minutes :-) >>>> >>>> Well, if it can't be turned off I guess I'll have to learn to live with >>>> that stuff. No complaints because LTSpice is free and generally doesn't >>>> crash. Other SPICEs cost an arm amnd a leg and crash a lot. >>>> >>> >>> In LTSpice click Tools/Control Panel. >>> Click the Spice tab. >>> Change Gmin as you wish in the upper right box. >>> >> >>Could be dicey. If I set it to zero other things my go kablouie because >>it's then universal. > >The VALUE of gmin isn't the problem, it's that unconnected pins are being >connected to ground (most likely) via gmin. I basically only use LTspice as a >viwer and to run netlists, so I don't know the ins-and-outs of configuring >pins. Make sure your symbols don't default to connecting thru gmin, rather >than balking. > > ...Jim Thompson > > [On the Road, in New York]
Lets get all the terms correct... It is connecting to ground via a resistance of 1/gmin, i.e. the highest value resistor that it can model. Charlie
On Thu, 18 Aug 2011 14:37:45 -0700, Charlie E. <edmondson@ieee.org> wrote:

>On Wed, 17 Aug 2011 19:59:44 -0400, Jim Thompson ><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: > >>On Wed, 17 Aug 2011 16:44:36 -0700, Joerg <invalid@invalid.invalid> wrote: >> >>>John S wrote: >>>> On 8/17/2011 5:40 PM, Joerg wrote: >>>>> Fred_Bartoli wrote: >>>>>> >>>>>> "Joerg"<invalid@invalid.invalid> a &#2013265929;crit dans le message de >>>>>> news:9avccjFfkgU1@mid.individual.net... >>>>>>> Martin Brown wrote: >>>>>>>> On 14/08/2011 23:33, Joerg wrote: >>>>>>>>> Folks, >>>>>>>>> >>>>>>>>> This is close to voodoo but repeatable. Unfortunately I can't >>>>>>>>> disclose >>>>>>>>> the schematic since it is for a client. Just wondering , anyone had >>>>>>>>> this >>>>>>>>> before? >>>>>>>>> >>>>>>>>> At the far end of a TX line I used to have a diode connected across >>>>>>>>> because a previous version of a chip would have such a substrat path. >>>>>>>>> Nice waveforms, fast sims. Everything as expected and peachy. Now the >>>>>>>>> new iteration of the chip design won't have this diode path so I >>>>>>>>> chopped >>>>>>>>> off its cathode connection. Sims fine. So I deleted the diode -> >>>>>>>>> ka-crunch ... sim slows down and the ouput is junk. >>>>>>>>> >>>>>>>>> If I put the diode back in and connect only its anode -> fine. If I >>>>>>>>> leave the anode open and only connect the cathode it still sims >>>>>>>>> but the >>>>>>>>> results are different. >>>>>>>>> >>>>>>>>> <scratching head> >>>>>>>> >>>>>>>> I am always amazed how well spice works these days. Best I can suggest >>>>>>>> is start deleting other nodes and components to try and get a minimum >>>>>>>> size network that shows the problem and can be disclosed publicly. >>>>>>>> >>>>>>> >>>>>>> I did that a couple of days ago, it's attached again. >>>>>>> >>>>>>> >>>>>>>> If you only connect the anode but to the wrong side of where the diode >>>>>>>> goes do you get the "anode connected" behaviour or the other one? >>>>>>>> >>>>>>> >>>>>>> Then the sim completely bombed on me. Like Fred said, it is possible >>>>>>> that SPICE incudes this diode in the matrix and chokes on it. But then >>>>>>> it's supposed to issue an error message and not just fly straight into >>>>>>> terrain. >>>>>>> >>>>>> >>>>>> Looking a bit further (I don't use LTspice as my current production >>>>>> tool) it happens that: >>>>>> * a default 10^-12 gmin option is permanently set. No way to defeat it, >>>>>> apprat from, maybe, setting it to zero. >>>>>> * look at the log file. The unconnected node is detected. >>>>>> >>>>> >>>>> Thanks, this would explain a lot. That's what Jim suspected, he beat you >>>>> by 14 minutes :-) >>>>> >>>>> Well, if it can't be turned off I guess I'll have to learn to live with >>>>> that stuff. No complaints because LTSpice is free and generally doesn't >>>>> crash. Other SPICEs cost an arm amnd a leg and crash a lot. >>>>> >>>> >>>> In LTSpice click Tools/Control Panel. >>>> Click the Spice tab. >>>> Change Gmin as you wish in the upper right box. >>>> >>> >>>Could be dicey. If I set it to zero other things my go kablouie because >>>it's then universal. >> >>The VALUE of gmin isn't the problem, it's that unconnected pins are being >>connected to ground (most likely) via gmin. I basically only use LTspice as a >>viwer and to run netlists, so I don't know the ins-and-outs of configuring >>pins. Make sure your symbols don't default to connecting thru gmin, rather >>than balking. >> >> ...Jim Thompson >> >> [On the Road, in New York] > >Lets get all the terms correct... > >It is connecting to ground via a resistance of 1/gmin, i.e. the >highest value resistor that it can model. > >Charlie
Some of us _can_ think in terms of conductance ;-) ...Jim Thompson [On the Road, in New York] -- | 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.
On Sun, 14 Aug 2011 16:07:54 -0700, Joerg <invalid@invalid.invalid> =
wrote:

>Martin Riddle wrote: >> "Joerg" <invalid@invalid.invalid> wrote in message=20 >> news:9ar0piFma8U1@mid.individual.net... >>> Folks, >>> >>> This is close to voodoo but repeatable. Unfortunately I can't =
disclose
>>> the schematic since it is for a client. Just wondering , anyone had=20 >>> this >>> before? >>> >>> At the far end of a TX line I used to have a diode connected across >>> because a previous version of a chip would have such a substrat path. >>> Nice waveforms, fast sims. Everything as expected and peachy. Now the >>> new iteration of the chip design won't have this diode path so I=20 >>> chopped >>> off its cathode connection. Sims fine. So I deleted the diode -> >>> ka-crunch ... sim slows down and the ouput is junk. >>> >>> If I put the diode back in and connect only its anode -> fine. If I >>> leave the anode open and only connect the cathode it still sims but=20 >>> the >>> results are different. >>> >>> <scratching head> >>> >>> --=20 >>=20 >> Convergence problems? >>=20 >> What if you terminate it with a 1000G ohm resistor? ( or a very high=20 >> resistance), instead of the diode. >>=20 > >I have a 1MEG in there. It's not really a convergence issue (I think >...). Looks more like a etlist thing. On the schematic there clearly is >no connection in either case but the results are vastly different.
Just a really off the wall idea, is it possible for a no-connect to be read as a connection to ground (node 0)? ??-\
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> a &#2013265929;crit 
dans le message de news:qj1r479gronv8mp8mkeevdglo3lfdngm2j@4ax.com...
> On Thu, 18 Aug 2011 14:37:45 -0700, Charlie E. <edmondson@ieee.org> wrote: > >>On Wed, 17 Aug 2011 19:59:44 -0400, Jim Thompson >><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >> >>>On Wed, 17 Aug 2011 16:44:36 -0700, Joerg <invalid@invalid.invalid> >>>wrote: >>> >>>>John S wrote: >>>>> On 8/17/2011 5:40 PM, Joerg wrote: >>>>>> Fred_Bartoli wrote: >>>>>>> >>>>>>> "Joerg"<invalid@invalid.invalid> a &#2013265929;crit dans le message de >>>>>>> news:9avccjFfkgU1@mid.individual.net... >>>>>>>> Martin Brown wrote: >>>>>>>>> On 14/08/2011 23:33, Joerg wrote: >>>>>>>>>> Folks, >>>>>>>>>> >>>>>>>>>> This is close to voodoo but repeatable. Unfortunately I can't >>>>>>>>>> disclose >>>>>>>>>> the schematic since it is for a client. Just wondering , anyone >>>>>>>>>> had >>>>>>>>>> this >>>>>>>>>> before? >>>>>>>>>> >>>>>>>>>> At the far end of a TX line I used to have a diode connected >>>>>>>>>> across >>>>>>>>>> because a previous version of a chip would have such a substrat >>>>>>>>>> path. >>>>>>>>>> Nice waveforms, fast sims. Everything as expected and peachy. Now >>>>>>>>>> the >>>>>>>>>> new iteration of the chip design won't have this diode path so I >>>>>>>>>> chopped >>>>>>>>>> off its cathode connection. Sims fine. So I deleted the diode -> >>>>>>>>>> ka-crunch ... sim slows down and the ouput is junk. >>>>>>>>>> >>>>>>>>>> If I put the diode back in and connect only its anode -> fine. >>>>>>>>>> If I >>>>>>>>>> leave the anode open and only connect the cathode it still sims >>>>>>>>>> but the >>>>>>>>>> results are different. >>>>>>>>>> >>>>>>>>>> <scratching head> >>>>>>>>> >>>>>>>>> I am always amazed how well spice works these days. Best I can >>>>>>>>> suggest >>>>>>>>> is start deleting other nodes and components to try and get a >>>>>>>>> minimum >>>>>>>>> size network that shows the problem and can be disclosed publicly. >>>>>>>>> >>>>>>>> >>>>>>>> I did that a couple of days ago, it's attached again. >>>>>>>> >>>>>>>> >>>>>>>>> If you only connect the anode but to the wrong side of where the >>>>>>>>> diode >>>>>>>>> goes do you get the "anode connected" behaviour or the other one? >>>>>>>>> >>>>>>>> >>>>>>>> Then the sim completely bombed on me. Like Fred said, it is >>>>>>>> possible >>>>>>>> that SPICE incudes this diode in the matrix and chokes on it. But >>>>>>>> then >>>>>>>> it's supposed to issue an error message and not just fly straight >>>>>>>> into >>>>>>>> terrain. >>>>>>>> >>>>>>> >>>>>>> Looking a bit further (I don't use LTspice as my current production >>>>>>> tool) it happens that: >>>>>>> * a default 10^-12 gmin option is permanently set. No way to defeat >>>>>>> it, >>>>>>> apprat from, maybe, setting it to zero. >>>>>>> * look at the log file. The unconnected node is detected. >>>>>>> >>>>>> >>>>>> Thanks, this would explain a lot. That's what Jim suspected, he beat >>>>>> you >>>>>> by 14 minutes :-) >>>>>> >>>>>> Well, if it can't be turned off I guess I'll have to learn to live >>>>>> with >>>>>> that stuff. No complaints because LTSpice is free and generally >>>>>> doesn't >>>>>> crash. Other SPICEs cost an arm amnd a leg and crash a lot. >>>>>> >>>>> >>>>> In LTSpice click Tools/Control Panel. >>>>> Click the Spice tab. >>>>> Change Gmin as you wish in the upper right box. >>>>> >>>> >>>>Could be dicey. If I set it to zero other things my go kablouie because >>>>it's then universal. >>> >>>The VALUE of gmin isn't the problem, it's that unconnected pins are being >>>connected to ground (most likely) via gmin. I basically only use LTspice >>>as a >>>viwer and to run netlists, so I don't know the ins-and-outs of >>>configuring >>>pins. Make sure your symbols don't default to connecting thru gmin, >>>rather >>>than balking. >>> >>> ...Jim Thompson >>> >>> [On the Road, in New York] >> >>Lets get all the terms correct... >> >>It is connecting to ground via a resistance of 1/gmin, i.e. the >>highest value resistor that it can model. >> >>Charlie > > Some of us _can_ think in terms of conductance ;-) >
Yup. But to be more precise, gmin is an additional conductance set *across* every junction, while the additional path to ground if set by rshunt (or gshunt in LTspice). -- Thanks, Fred.
On Mon, 15 Aug 2011 07:29:01 -0700, Joerg <invalid@invalid.invalid> =
wrote:

>nuny@bid.nes wrote: >> On Aug 14, 3:33 pm, Joerg <inva...@invalid.invalid> wrote: >>> Folks, >>> >>> This is close to voodoo but repeatable. Unfortunately I can't =
disclose
>>> the schematic since it is for a client. Just wondering , anyone had =
this
>>> before? >>> >>> At the far end of a TX line I used to have a diode connected across >>> because a previous version of a chip would have such a substrat path. >>> Nice waveforms, fast sims. Everything as expected and peachy. Now the >>> new iteration of the chip design won't have this diode path so I =
chopped
>>> off its cathode connection. Sims fine. So I deleted the diode -> >>> ka-crunch ... sim slows down and the ouput is junk. >>> >>> If I put the diode back in and connect only its anode -> fine. If I >>> leave the anode open and only connect the cathode it still sims but =
the
>>> results are different. >>> >>> <scratching head> >>=20 >> I really hate to ask this, but have you "simulated" your LTSpice >> results in hardware yet? >>=20 > >No, can't do that yet. It's an IC and that is not taped out yet. >
Is someone of crazy here (very possibly me, i have not done an actual IC design)? How can you dare try tapeout before having a believable simulation? Can't you get adequate device models without tapeout? Do they even know which fab and process they are going to use?
> >> Also, what's the RF voltage level on the line? >>=20 > >About 100V, and then from microvolts up to a volt during receive >depending on signal strength coming in.
On Mon, 15 Aug 2011 09:44:56 -0700, Joerg <invalid@invalid.invalid> =
wrote:

>nuny@bid.nes wrote: >> On 8/15/2011 7:29 AM, Joerg wrote: >>> nuny@bid.nes wrote: >>>> On Aug 14, 3:33 pm, Joerg<inva...@invalid.invalid> wrote: >>>>> Folks, >>>>> >>>>> This is close to voodoo but repeatable. Unfortunately I can't =
disclose
>>>>> the schematic since it is for a client. Just wondering , anyone had >>>>> this >>>>> before? >>>>> >>>>> At the far end of a TX line I used to have a diode connected across >>>>> because a previous version of a chip would have such a substrat =
path.
>>>>> Nice waveforms, fast sims. Everything as expected and peachy. Now =
the
>>>>> new iteration of the chip design won't have this diode path so I >>>>> chopped >>>>> off its cathode connection. Sims fine. So I deleted the diode -> >>>>> ka-crunch ... sim slows down and the ouput is junk. >>>>> >>>>> If I put the diode back in and connect only its anode -> fine. If =
I
>>>>> leave the anode open and only connect the cathode it still sims but=
the
>>>>> results are different. >>>>> >>>>> <scratching head> >>>> >>>> I really hate to ask this, but have you "simulated" your LTSpice >>>> results in hardware yet? >>> >>> No, can't do that yet. It's an IC and that is not taped out yet. >>=20 >> Dang. Do you have an example of the old setup handy on which you can >> try your diode changes as above? >>=20 > >No, this IC will be totally new turf. Reason for my sims is the we now >have to take care of the design of the connecting electronics while >it'll be in production. That way it'll all come together at roughly the >same time. > > >>>> Also, what's the RF voltage level on the line? >>> >>> About 100V, and then from microvolts up to a volt during receive >>> depending on signal strength coming in. >>=20 >> I thought it might be high. Think; what happens with a real diode >> connected as described above in such a field? >>=20 > >In the end we'll have to live with whatever the IC has, there is >practically no space for any other parts to the right of the TX line. > > >> Anyway, if it were me I'd stop fiddling with the diode and just use =
a
>> terminating resistor. >>=20 >> Of course, once the chip was ready I'd try it with the diode, just =
to
>> see. ;>) >>=20 > >I might. But we can't place diodes because it's multi-channel and that >would be lots of parts in a space that isn't there :-) > >It's no problem because the signals going up can be shaped accordingly. >I was just wondering why LTSpice is producing inconsistent results here. >It's ok if it runs into a dead end with some calcs but I'd have thought >that would caused the usual error messages. Yet I get none of those.
I can totally understand being uneasy committing to an IC design with a sim that i could not trust. ?-/