AW: [SI-LIST] : Effect of low Zo for unterminated lines

Andreas Lenkisch (a.lenkisch@trenew.de)
Tue, 19 Oct 1999 08:57:59 +-200

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Chris,

may I ask you for the trace length and type of interconnect?
It's to know if the trace is electrically "short" or "long" and=20
if it's a point-to-point or multidrop/multipoint connection.=20
On a multipoint trace the impedance influences the propagation=20
delay at the same load.

A short trace may act as a lumped capacitance which slows the
edge rates down, a longer trace would more act as a transmission line=20
with an ohmiclike input, acting as a voltage divider with the output =
impedance.
There I would not expect a slowed down edge, but a multidrop/multipoint=20
trace acts due to the additional load capacitances as a low pass.
The lower impedance damps oscillations stronger.

regards
Andreas Lenkisch
Trenew Electronic GmbH

----------
Von: Chris Bobek[SMTP:cbobek@cadence.com]
Gesendet: Dienstag, 19. Oktober 1999 02:23
An: Si-list
Betreff: [SI-LIST] : Effect of low Zo for unterminated lines

Hi,

Because of several constraints on our stackup and line width, a 6mil
stripline trace yields a Zo =3D 29ohms. When I simulate a 29ohm,
unterminated line on our board (@ 10MHz), I see a pretty nice waveform,
albeit with slower rise/fall times (which is ok for the nets in
question). When I increase the Zo to 60 ohms, for example, I see a lot
of ringing on the same net.

>From a theoretical point of view, my guess is that this happens because
as you widen the trace, there is more capacitance, which slows down the
rise/fall times. Is this a correct assumption? Are there other factors
as well that are going on?

What I am most concerned about is the effect of a low Zo net on its
driver. For example, if I have an LVCH245 driver (3.3V) driving a 29ohm
line, will the low Zo "hurt" the driver? Or, is the only negative
effect the slower rise time? This question is a general question for
most ICs.

Thanks for your help,

Chris

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