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

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.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg
Loading thread data ...

Convergence problems?

What if you terminate it with a 1000G ohm resistor? ( or a very high resistance), instead of the diode.

Cheers

Reply to
Martin Riddle

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.
Reply to
Jim Thompson

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
Reply to
Phil Hobbs

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.

Reply to
Joerg

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/
Reply to
Joerg

Yes, but it sure converges better than PSpice. Plus there is a major price difference :-)

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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

Reply to
petrus bitbyter

"Joerg" wrote in message news: snipped-for-privacy@mid.individual.net...

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

Reply to
Martin Riddle

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.
Reply to
Jim Thompson

I really hate to ask this, but have you "simulated" your LTSpice results in hardware yet?

Also, what's the RF voltage level on the line?

Mark L. Fergerson

Reply to
alien8752

The Spice transmission line is weird in that it has, essentially, an ideal 1:1 transformer built in. There's no continuity in the "shield" from end to end. That can have weird effects, including "singular matrix" types of errors that you wouldn't expect from a real hunk of coax.

John

Reply to
John Larkin

I have no access to abse, but that kind behavior is exactly what I meant. Different sim results while the netlist should not have caused a difference. Maybe I stumbled upon a bug here and if this is corroborated by others I should send it in for Mike's crew.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Thanks ... but ... if you add a diode in your version and disconnect the cathode you get a change again: A Mohawk hair-style on the third pulse. That's not supposed to be there.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

You do need an R to GND on TX lines, else they cause SPICE to abort.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

No, can't do that yet. It's an IC and that is not taped out yet.

About 100V, and then from microvolts up to a volt during receive depending on signal strength coming in.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Well, you normally will not have the diode in the circuit like that anyway. Same goes for the resistor. This is just a SPICE thing. I would not worry too much about it.

Cheers

Reply to
Martin Riddle

SPICE uses iterative techniques to solve large, sparse, nonlinear systems. That's a hard problem in general, and disconnected nodes make even the linearized matrices singular. LTSPice is generally very good at finding those sorts of things and ignoring them, but like other solver packages, it isn't perfect.

A nearly-singular system of equations is a very powerful noise amplifier, so having a bit of fur show up now and again may simply be amplified roundoff, or it may be actual numerical instability. What integrator are you using?

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
Reply to
Phil Hobbs

I got the same results as you. I didn't try different diode types.

Put a .001 R in series in the cathode path of that "floating diode" (for lack of a better term) and you get a ~ 3.6 KV spike a bit after .3 uS. I tried a number of different values for that added R and the only value that affected the thing was .001 ohms. .000999 did not, nor did .0011. Weird.

Ed

Reply to
ehsjr

Dang. Do you have an example of the old setup handy on which you can try your diode changes as above?

I thought it might be high. Think; what happens with a real diode connected as described above in such a field?

Anyway, if it were me I'd stop fiddling with the diode and just use a terminating resistor.

Of course, once the chip was ready I'd try it with the diode, just to see. ;>)

Mark L. Fergerson

Reply to
alien8752

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.