[SI-LIST] Re: overshoot and undershoot different swing
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 4 Apr 2007 00:50:27 -0400
> I want to know whether it is necessary that overshoot and undershoot
> values needed to be same?
I can't think of any good reason why the positive and negative overshoot
values must be the same.
On the other hand, if the driver circuit and the overshoot clamps are
supposed to be symmetrical, then equal overshoot values might be expected;
so unequal values might mean that something is wrong.
"Ideally," overshoot should be zero, meaning that the waveform swings
precisely to steady-state and stays there. That is, overshoot itself is
undesirable, or at least not desirable ... with certain exceptions. But
some overshoot is almost always tolerable; and sometimes changes to a
circuit that are otherwise beneficial, also happen to cause or increase
overshoot. So we let it happen.
I believe that negative overshoot (overshoot that dips below Vss) CAN be
more troublesome than positive overshoot, because the former can cause
effects in the substrate that can disturb logic states in other parts of the
circuit if not handled correctly. But this is a generalization and may not
be true in your case.
Jim Whitehead's discussion about overshoot vs. undershoot is correct, but
there are plenty of references (and large companies) that use the terms
incorrectly.
Regards,
Andy
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