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

Chuck Hill (chuckh@altaeng.com)
Tue, 19 Oct 1999 11:10:27 -0600

Chris,

At 05:23 PM 10/18/99 -0700, Chris Bobek wrote:
>Hi,
>
>Because of several constraints on our stackup and line width, a 6mil
>stripline trace yields a Zo = 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?

Try this way to look at it: Case 1 has low characteristic impedance, Case 2
has higher characteristic impedance. The termination impedances remain the
same. So how do the reflection coefficients change with the characteristic
impedance? If you assume the resistances are lower than the characteristic
impedance to begin with, when the characteristic impedance is lowered, the
reflections are reduced, and the ringing will also be reduced. A better
source match occurs in case 1.

I'm not sure the absolute values of 29ohms and 60ohms are correct, but I've
found the ON resistance of buffers is generally lower than the
characteristic impedance of lines (hence the often used series resistors at
the source).

Chuck

>
>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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