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

Larry Smith ([email protected])
Tue, 19 Oct 1999 09:55:14 -0700 (PDT)

Chris - The driver has some output impedance. It has an IV curve that
is well represented by an ibis model, but can be approximated with a
resistance, probably around 10-20 Ohms, depending on the strength of
the driver. For the inital rise or fall time, we have a voltage
divider between the output impedance of the driver and the Z0 of the
transmission line. The driver will drive a 60 ohm line a lot harder
than a 29 Ohm line. The initial voltage transient for the 60 Ohm line
is a lot greater.

You mention that the line is unterminated. In that case, the voltage
doubles and reflects back to the near end. The driver does a fairly
good job of "source" terminating the 29 Ohm line, but a much poorer job
of terminating the 60 Ohm line. Termination occurs when energy from
the transmission line is obsorbed by a resistor and turned into heat.

So, not only do you have a stronger signal driven into the 60 Ohm line,
but you have less efficient termination when the reflected signal
returns. That is why the 60 Ohm line rings more than the 29 Ohm line
with the same driver. The low impedance net will probably not hurt the
driver, but it will consume more power each time the driver switches.
If you have many drivers in parallel, SSN can become an issue becuase
all of the drivers are contributing more di/dt.

Larry Smith
Sun Microsystems

> Date: Mon, 18 Oct 1999 17:23:51 -0700
> From: Chris Bobek <[email protected]>

> 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?
> 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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