Forums

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

Started by Joerg August 14, 2011
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>

-- 
Regards, Joerg

http://www.analogconsultants.com/

"Joerg" <invalid@invalid.invalid> wrote in message 
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 > 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> > > --
Convergence problems? What if you terminate it with a 1000G ohm resistor? ( or a very high resistance), instead of the diode. Cheers
On Sun, 14 Aug 2011 15:33:20 -0700, Joerg <invalid@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>
Show just the netlist of that portion. Diode? In CMOS? The "diode" has other parts ;-) ...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 08/14/2011 06:33 PM, 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> >
The convergence properties of SPICE are very delicate and hard to understand. In LTSPICE, I save many versions with different names, because the convergence and (especially) startup properties vary out of all recognition with small changes in the circuit. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
This is a multi-part message in MIME format.
--------------010203070101030705090207
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

Jim Thompson wrote:
> On Sun, 14 Aug 2011 15:33:20 -0700, Joerg <invalid@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> > > Show just the netlist of that portion. Diode? In CMOS? The "diode" has > other parts ;-) >
Ok, it's attached, I cut our the proprietary swaths. The results are very different but the basic effect is still there: Note that D2 is connected at the anode only. Run the sim, look at V(n001), three nice clean pulses. Now delete the anode connection and the diode D2 -> first pulse is hosed. -- Regards, Joerg http://www.analogconsultants.com/ --------------010203070101030705090207 Content-Type: text/plain; name="Diode_Test.asc" Content-Transfer-Encoding: 7bit Content-Disposition: inline; filename="Diode_Test.asc" Version 4 SHEET 1 2324 792 WIRE 1504 -224 1040 -224 WIRE 1904 -224 1584 -224 WIRE 1904 -192 1904 -224 WIRE 448 32 352 32 WIRE 848 32 528 32 WIRE 1040 32 1040 -224 WIRE 1040 32 944 32 WIRE 1200 32 1040 32 WIRE 1264 32 1200 32 WIRE 1376 32 1344 32 WIRE 1648 32 1456 32 WIRE 1760 32 1648 32 WIRE 1952 32 1760 32 WIRE 1648 48 1648 32 WIRE 848 64 784 64 WIRE 1040 64 944 64 WIRE 1568 64 1568 -176 WIRE 1600 64 1568 64 WIRE 352 80 352 32 WIRE 784 80 784 64 WIRE 1200 112 1200 32 WIRE 1328 112 1328 80 WIRE 1328 112 1200 112 WIRE 1520 112 1520 -176 WIRE 1520 112 1328 112 WIRE 1600 112 1520 112 WIRE 1952 128 1952 32 WIRE 1648 144 1648 128 WIRE 1760 144 1760 32 WIRE 352 176 352 160 WIRE 1040 320 1040 64 WIRE 1120 320 1120 224 WIRE 1120 320 1040 320 WIRE 1280 320 1280 80 WIRE 1280 320 1120 320 WIRE 1568 320 1568 64 WIRE 1568 320 1280 320 WIRE 1648 320 1648 224 WIRE 1648 320 1568 320 WIRE 1760 320 1760 208 WIRE 1760 320 1648 320 WIRE 1904 320 1904 -112 WIRE 1904 320 1760 320 WIRE 1952 320 1952 192 WIRE 1952 320 1904 320 WIRE 1040 336 1040 320 WIRE 1040 432 1040 416 FLAG 784 80 0 FLAG 1040 432 0 FLAG 352 176 0 SYMBOL cap 1936 128 R0 SYMATTR InstName C1 SYMATTR Value 25p SYMBOL tline 896 48 R0 WINDOW 3 -62 67 Left 0 SYMATTR Value Td=6.25n Z0=50 SYMATTR InstName T1 SYMBOL res 1024 320 R0 SYMATTR InstName R4 SYMATTR Value 1000k SYMBOL res 1632 128 R0 SYMATTR InstName R2 SYMATTR Value 100 SYMBOL sw 1648 32 R0 SYMATTR InstName S1 SYMATTR Value SW1 SYMBOL res 1472 48 M270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R5 SYMATTR Value 25 SYMBOL sw 1248 32 R270 SYMATTR InstName S2 SYMATTR Value SW2 SYMBOL diode 1744 208 M180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D1 SYMATTR Value MMSD4148 SYMBOL sw 1600 -224 M270 SYMATTR InstName S3 SYMATTR Value SW1 SYMBOL voltage 1904 -208 R0 WINDOW 123 24 132 Left 0 WINDOW 39 24 44 Left 0 WINDOW 3 25 102 Left 0 SYMATTR SpiceLine Rser=20 SYMATTR Value SINE(0 .5 20MEG 0 0 0 500) SYMATTR InstName V4 SYMBOL voltage 352 64 R0 WINDOW 39 24 44 Left 0 WINDOW 3 25 102 Left 0 SYMATTR SpiceLine Rser=1 SYMATTR Value PULSE(0 100 200n 10n 10n 35n 100n 3) SYMATTR Value2 AC 1 SYMATTR InstName V1 SYMBOL res 544 48 M270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R1 SYMATTR Value 50 SYMBOL diode 1104 224 M180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D2 SYMATTR Value MMSD4148 TEXT 520 -240 Left 0 !.tran 1u TEXT 1144 360 Left 0 ;R4 only for SPICE purposes TEXT 1384 -432 Left 0 !.model SW1 SW(Ron=1Meg Roff=1 Vt=15 Vh=-.4) TEXT 1384 -400 Left 0 !.model SW2 SW(Ron=1 Roff=1Meg Vt=15 Vh=-.4) TEXT 1288 144 Left 0 ;Pass-FET TEXT 1456 200 Left 0 ;Shunt-FET TEXT 1144 392 Left 0 ;Diodes representing substrate paths --------------010203070101030705090207--
Martin Riddle wrote:
> "Joerg" <invalid@invalid.invalid> wrote in message > 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 >> 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> >> >> -- > > Convergence problems? > > What if you terminate it with a 1000G ohm resistor? ( or a very high > resistance), instead of the diode. >
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. -- Regards, Joerg http://www.analogconsultants.com/
Phil Hobbs wrote:
> On 08/14/2011 06:33 PM, 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> >> > > The convergence properties of SPICE are very delicate and hard to > understand. In LTSPICE, I save many versions with different names, > because the convergence and (especially) startup properties vary out of > all recognition with small changes in the circuit. >
Yes, but it sure converges better than PSpice. Plus there is a major price difference :-) -- Regards, Joerg http://www.analogconsultants.com/
"Joerg" <invalid@invalid.invalid> schreef in bericht 
news:9ar2nqF3ocU1@mid.individual.net...
> Jim Thompson wrote: >> On Sun, 14 Aug 2011 15:33:20 -0700, Joerg <invalid@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> >> >> Show just the netlist of that portion. Diode? In CMOS? The "diode" has >> other parts ;-) >> > > Ok, it's attached, I cut our the proprietary swaths. The results are > very different but the basic effect is still there: > > Note that D2 is connected at the anode only. Run the sim, look at > V(n001), three nice clean pulses. Now delete the anode connection and > the diode D2 -> first pulse is hosed. > > -- > Regards, Joerg > > http://www.analogconsultants.com/
<snip> I got an extra 120V peak on the second pulse and a sharp dip in the third though not after removing the diode but before removing it. (See abse) Removing the diode makes the extra's disappear. Connecting the diode gives the weird pulses back. It make's a difference what type diode is chosen. A 1N914 and a 1N4148 give the same results. Several Schottkys and other silicons however seem to do no harm. petrus bitbyter

"Joerg" <invalid@invalid.invalid> wrote in message 
news:9ar2qaF3a2U1@mid.individual.net...
> Martin Riddle wrote: >> "Joerg" <invalid@invalid.invalid> wrote in message >> 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 >>> 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> >>> >>> -- >> >> Convergence problems? >> >> What if you terminate it with a 1000G ohm resistor? ( or a very high >> resistance), instead of the diode. >> > > 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. > > -- > Regards, Joerg > > http://www.analogconsultants.com/
Its an unterminated tx line. You need some R for spice to behave. See this..... Cheers Version 4 SHEET 1 2324 792 WIRE 1504 -224 1040 -224 WIRE 1904 -224 1584 -224 WIRE 1904 -192 1904 -224 WIRE 448 32 352 32 WIRE 848 32 528 32 WIRE 1040 32 1040 -224 WIRE 1040 32 944 32 WIRE 1072 32 1040 32 WIRE 1200 32 1072 32 WIRE 1264 32 1200 32 WIRE 1376 32 1344 32 WIRE 1648 32 1456 32 WIRE 1760 32 1648 32 WIRE 1952 32 1760 32 WIRE 1648 48 1648 32 WIRE 848 64 784 64 WIRE 1040 64 944 64 WIRE 1568 64 1568 -176 WIRE 1600 64 1568 64 WIRE 352 80 352 32 WIRE 784 80 784 64 WIRE 1200 112 1200 32 WIRE 1328 112 1328 80 WIRE 1328 112 1200 112 WIRE 1520 112 1520 -176 WIRE 1520 112 1328 112 WIRE 1600 112 1520 112 WIRE 1952 128 1952 32 WIRE 1072 144 1072 32 WIRE 1648 144 1648 128 WIRE 1760 144 1760 32 WIRE 352 176 352 160 WIRE 1040 320 1040 64 WIRE 1072 320 1072 224 WIRE 1072 320 1040 320 WIRE 1280 320 1280 80 WIRE 1280 320 1072 320 WIRE 1568 320 1568 64 WIRE 1568 320 1280 320 WIRE 1648 320 1648 224 WIRE 1648 320 1568 320 WIRE 1760 320 1760 208 WIRE 1760 320 1648 320 WIRE 1904 320 1904 -112 WIRE 1904 320 1760 320 WIRE 1952 320 1952 192 WIRE 1952 320 1904 320 WIRE 1040 336 1040 320 WIRE 1040 432 1040 416 FLAG 784 80 0 FLAG 1040 432 0 FLAG 352 176 0 SYMBOL cap 1936 128 R0 SYMATTR InstName C1 SYMATTR Value 25p SYMBOL tline 896 48 R0 WINDOW 3 -62 67 Left 0 SYMATTR Value Td=6.25n Z0=50 SYMATTR InstName T1 SYMBOL res 1024 320 R0 SYMATTR InstName R4 SYMATTR Value 1000k SYMBOL res 1632 128 R0 SYMATTR InstName R2 SYMATTR Value 100 SYMBOL sw 1648 32 R0 SYMATTR InstName S1 SYMATTR Value SW1 SYMBOL res 1472 48 M270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R5 SYMATTR Value 25 SYMBOL sw 1248 32 R270 SYMATTR InstName S2 SYMATTR Value SW2 SYMBOL diode 1744 208 M180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D1 SYMATTR Value MMSD4148 SYMBOL sw 1600 -224 M270 SYMATTR InstName S3 SYMATTR Value SW1 SYMBOL voltage 1904 -208 R0 WINDOW 123 24 132 Left 0 WINDOW 39 24 44 Left 0 WINDOW 3 25 102 Left 0 SYMATTR SpiceLine Rser=20 SYMATTR Value SINE(0 .5 20MEG 0 0 0 500) SYMATTR InstName V4 SYMBOL voltage 352 64 R0 WINDOW 39 24 44 Left 0 WINDOW 3 25 102 Left 0 SYMATTR SpiceLine Rser=1 SYMATTR Value PULSE(0 100 200n 10n 10n 35n 100n 3) SYMATTR Value2 AC 1 SYMATTR InstName V1 SYMBOL res 544 48 M270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R1 SYMATTR Value 50 SYMBOL res 1056 128 R0 SYMATTR InstName R3 SYMATTR Value 1000G TEXT 520 -240 Left 0 !.tran 1u TEXT 1144 360 Left 0 ;R4 only for SPICE purposes TEXT 1384 -432 Left 0 !.model SW1 SW(Ron=1Meg Roff=1 Vt=15 Vh=-.4) TEXT 1384 -400 Left 0 !.model SW2 SW(Ron=1 Roff=1Meg Vt=15 Vh=-.4) TEXT 1288 144 Left 0 ;Pass-FET TEXT 1456 200 Left 0 ;Shunt-FET TEXT 1144 392 Left 0 ;Diodes representing substrate paths
On Sun, 14 Aug 2011 16:06:33 -0700, Joerg <invalid@invalid.invalid> wrote:

>Jim Thompson wrote: >> On Sun, 14 Aug 2011 15:33:20 -0700, Joerg <invalid@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> >> >> Show just the netlist of that portion. Diode? In CMOS? The "diode" has >> other parts ;-) >> > >Ok, it's attached, I cut our the proprietary swaths. The results are >very different but the basic effect is still there: > >Note that D2 is connected at the anode only. Run the sim, look at >V(n001), three nice clean pulses. Now delete the anode connection and >the diode D2 -> first pulse is hosed.
I'm seeing no difference. I have v4.12 Maybe it's your wondeful system ;-) ...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.