Hi all,
I've come across this simple circuit on the 'net that illustrates that Spice can't perform this kind of simulation without a DC path to ground. The narrative says that in the real world, the two 'bogus' resistors shown on this diagram wouldn't be necessary. I've not encountered this before. Has this defect in Spice been corrected since the article was written??
Here's the circuit:
-- "What is now proved was once only imagin\'d." - William Blake, 1793.
It only needs one, not two.
In general it is not a defect. Its due to basic physics principles. For AC analysis one can technically avoid this issue, but for DC or transient, its inherent. What voltage w.r.t ground do you actually propose *should* exist at the junction of two capacitors? Once you understand this, you will understand why Spice can't tell you either.
Kevin Aylward snipped-for-privacy@anasoft.co.uk
Paul,
If you're doing a .ac analysis, the linearized ac potentials are defined at all frequencies (except 0Hz). LTspice runs the circuit just fine. Most other SPICE programs aren't smart enough to see it as a linear circuit and that therefor it doesn't need to know the DC potentials.
--Mike
Thanks, Mike. I suspected as much!
-- "What is now proved was once only imagin\'d." - William Blake, 1793.
[...]
The current in inductor loops are undefined at DC, too ;-)
So LTspice takes a shortcut in the case of purely linear circuits? Perfectly reasonable, of course.
What would LTspice do if the capacitors were voltage dependent? (Say, CMOS transistor gates...)
Best Regards
Jens
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Jens,
It doesn't take the short cut. That would be an error. If you have non-linear reactances, LTspice needs the DC solution. When LTspice is compiling the circuit for execution, it notices whether or not the circuit is linear and behaves accordingly.
--Mike
So if LTS *needs* a DC path to ground for any reason in any circumstance to give a valid result; will it generate an error message flagging the problem or will we just get erroneous output values which might go unnoticed?
Thanks,
p.
-- "What is now proved was once only imagin\'d." - William Blake, 1793.
Paul,
If LTspice *needs* the DC solution, then it *will* do a DC solution. But it still won't give you give you an error message for your circuit. LTspice can usually find the voltage of "floating" nodes by assuming the circuit was build without charge on the capacitors and then turned on. Try it.
--Mike
Jens,
Yes, but to get the current LTspice release to run this, you have to run off topology checking. The topology of the circuit is checked in LTspice after it is compiled. One check is for a loop of voltage sources and inductors with no series resistance.
For example, this deck will run in LTspice:
LTspice skips the topology check, so it doesn't quit when it sees L1 and V1 in parallel, and then LTspice sees this is a .ac analysis of a linear circuit, so the DC .op point isn't required. Then it plots the correct .ac data for a circuit without a defined DC solution.
If the ".options topologycheck=0" isn't there, then you will get an error message that says, "Voltage source V1 and inductor L1 are paralleled making an over-defined circuit matrix. You will need to correct the circuit or add some series resistance."
The next LTspice release will (i) document the topologycheck option in the help and (ii) no longer require you to turn it off for linear circuits that don't require a DC solution.
--Mike
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