[SI-LIST] : Is feedback really EVIL?

Charles Hill (chuckh@csn.net)
Wed, 13 May 1998 22:59:24 -0600

DC Sessions wrote:

>Eeeeyeew! Feedback! Feedback is evil. Bad driver, BAD.
>Go to bed without stability. Naughty, naughty, naughty.

>Well, OK, that's overstating it. Sometimes feedback is
>not only useful but necessary; it still makes behavioral
>modelling (a la IBIS) pretty well impossible. IMNSHO, this
>is more annoying than crippling since feedback has other
>properties that limit its usefulness in applications where
>signal integrity is a major concern, so giving up behavioral
>modelling of feedback-controlled drivers is (again, IMO) no
>great loss.

I guess one man's meat is another man's poison.

Usually I'm more interested in solving an SI problem, than ensuring that it
is easy to model. So we agree there.

So I have several questions:
1. Feedback is a very useful technique in many areas of EE. How widely is
it used in SI? What are the problems with using feedback?
2. Why is a feedback system difficult (pretty well impossible) to model
with IBIS? I know we had lots of problems achieving accurate modeling of a
feedback driver in SPICE.
3. What are the properties of feedback that limit its usefulness in
applications where signal integrity is a major concern?

DC Sessions also wrote:
>IBIS can't really deal
>with JEDEC flexible-impedance drivers, and I'm writing THOSE into
>the IEEE 1394b spec. Great fun. Still, we need to keep in mind that
>the more exotic variants that you and I deal with are of very narrow
>application.

4. What is a flexible impedance driver, and how does it work?

Charles Hill, consultant
chuckh@altaeng.com