[SI-LIST] Re: Combining DC and TRANS analysis
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>, "Shee Kian Wong" <SKWONG@xxxxxxxxxx>
- Date: Mon, 20 Jan 2003 22:11:23 -0500
SK,
By now you should have read the other two replies that describe how you
might save the operating point 500ns into the transient simulation.
However, my concern would be why the operating point shifts at all.
Ideally, nothing should change during transient analysis if none of the
independent sources are changing, unless the circuit is an oscillator.
But sometimes things don't work that way.
In some cases, the operating point may shift because of incorrect SPICE
control settings. If you use any .IC statements or IC= element
parameters, you may be telling SPICE to converge on the wrong operating
point, and then the transient analysis needs to spend some time to drift
to the correct operating point.
Certain .OPTIONS can cause SPICE to converge incorrectly too. I've seen
it be off by a volt or two when some of the tolerance options were much
too loose.
The other possibility, is that your circuit uses AC coupling and the
operating point shifts when the AC signal is "turned on" at t=0.
When you do the .DC analysis, do you want to sweep a voltage source, or
do you just need to know the operating point voltages and currents? You
can get the operating point information fairly easily, but making it do
a real .DC sweep may be another matter....
Regards,
Andy
> In fact, I have a circuit which needs about 500ns to settle down and
> produce
> steady output. I can only do any DC analysis on it after 500ns to get
> more
> accurate results.
>
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