simpson@sunee.waterloo.edu (KFS Lam) (11/10/90)
Hi, I am currently doing a passive filter design with Pspice on a PC. Unfortunately, Pspice performs DC analysis automatically at the beginning. Since my circuit is passive, there are no DC sources and Pspice starts to compliant about floating nodes. (Some of my nodes have capicitor connected to other nodes, so no DC can get through. Therefore, Pspice thinks they are floating.) I get around it by putting a HUGE resistor from that node to ground. Then Pspice starts to complaint about convergence problem in DC analysis. Is there any way you can shut off the DC analysis after checking the circuit? Or there are ways to get around that? simpson@sunee.waterloo.edu
jws@mintaka.mlb.semi.harris.com (James W. Swonger) (11/10/90)
The Large Resistor To Ground trick is one of the most straightforward ways of improving convergence. You may be able to explicitly set initial conditions on your PSpice element cards (current for inductors, voltage for caps). If you know the initial conditions this should help quite a bit. Also see whether PSpice has a store/restore capability with which you can save the DC solution and use it to initialize the circuit state for the next. Finally, see what you have available to you as far as setting the number of DC iterations, required tolerances for "convergence", etc. You may just need more iterations. Note also that a filter with only AC components is going to be hard to solve for a computer - any error is going to make everything "sing". Try putting large resistors in parallel with the caps and very small resistors in series with the inductors (small being a judgment call based on frequency of interest). This may provide a means for the errors to damp out.
mark@mips.COM (Mark G. Johnson) (11/11/90)
In article <1990Nov9.160343.3301@sunee.waterloo.edu> simpson@sunee.waterloo.edu (KFS Lam) writes: >I get around it by putting a HUGE resistor >from that node to ground. Then Pspice starts to complaint about >convergence problem in DC analysis. Is there any way you can Another standard stunt is to use large (10 Henry) inductors from "floating" nodes to voltage sources. This sets the voltage at time zero and then keeps out of the way during other analyses. -- -- Mark Johnson MIPS Computer Systems, 930 E. Arques M/S 2-02, Sunnyvale, CA 94086 (408) 524-8308 mark@mips.com {or ...!decwrl!mips!mark}
FC138001@ysub.ysu.edu (Phil Munro) (11/13/90)
> Hi, I am currently doing a passive filter design with Pspice > on a PC. Unfortunately, Pspice performs DC analysis automatically > at the beginning. Since my circuit is passive, there are no > DC sources and Pspice starts to compliant about floating > nodes. ... Is there any way you can > shut off the DC analysis after checking the circuit? ... > > simpson@sunee.waterloo.edu This is an interesting problem with Spice. I look forward to solutions to this *other* than introducing large resistors or inductors into the circuit. Perhaps you should consisder using another good simulator. I like WATAND which runs under CMS on our mainframe and is an *interactive* simulator. With WATAND, frequency analysis (FR) can be run by specifying the Q-point, and for a linear circuit it defaults to ZERO. It is not necessary to run DC analysis, unless one wants to use the DC solution as a Q-point. Therefore, your problem doesn't occur with WATAND because the operating point can be taken as ZERO. WATAND is used at, and available (I think) from the Univ of Waterloo. It should be pointed out, however, that having a node which floats with respect to dc is not really a normal design. So it is not unreasonable to add the large resistor in parallel with the capacitor, and to add the small resistor in series with the inductor. This just makes the models for those elements more accurate. Of course the other side of this is that many capacitor types have so little dissipation that the added resistor is not really necessary to get good simulation.
raoul@eplunix.UUCP (Nico Garcia) (11/14/90)
In article <1990Nov9.160343.3301@sunee.waterloo.edu>, simpson@sunee.waterloo.edu (KFS Lam) writes: > Hi, I am currently doing a passive filter design with Pspice > on a PC. Unfortunately, Pspice performs DC analysis automatically > at the beginning. Since my circuit is passive, there are no > DC sources and Pspice starts to compliant about floating > nodes. (Some of my nodes have capicitor connected to other Hmmm. The big resistor to ground trick works well, and is actually closer to physical situations (there is always *some* radiation of AC, and leakage of DC). You might also incorporate leakage/series resistances for your caps, depending on what kind of caps you use, and series resistances for your inductances. Also, it makes sense to put *some* kind of connection to ground on floating circuits. Who knows what sort of static charges could build up and fry your caps while you're not looking? -- Nico Garcia Designs by Geniuses for use by Idiots eplunix!cirl!raoul@eddie.mit.edu