RE: [SI-LIST] : Preferred PWB impedances

Mellitz, Richard (
Thu, 20 Nov 1997 10:24:10 -0500

What I expect to end up with on a gamut of boards is impedance between
50-75 ohms. The actual choice is driven more from connector and stackup
mechanicals than electrical desires. A typical target impedance will be
between 55 and 65 ohms. Usually we can we get +-/ 10% from there. The
issue that most often brings closure to driver selection is the delta
between best and worst (whatever that may specifically mean.)

The following are the classes of lines I often deal with (excluding
spec'd buses):
1) Short point to point, one 3-10 pf load, 1-3" unterminated and
reflected wave.
I can use weak drivers for this say around 4 ma.
2) Long point to point, one 3-10 pf load, 3-25", usually tune for
speed. Ergo, I perform trade off analysis as to what termination scheme
is required. Usually series termination or no termination wins.
3) Short point to point, 3 to 16 3-10 pf loads(i.e. rebuffering), 1-3"
mostly unterminated.
4) Long point to point, 3 to 16 3-10 pf loads(i.e. rebuffering),
3-25", usually tune for speed. Ergo, trade off analysis for termination
schemes is required.
5) Driving memories like devices. Up to 36 loads, 5-14" long. Usually
get I get stuck with unterminated lines but if I have a choice, I go for
a terminated one.
6) Spaghetti runs, If I don't care about speed, I go for as weak as I
can get.
7) Spaghetti runs. If I do care about speed I've got my work cut out. I
guess its called engineering :-)
8) Lines that need to be wired or'ed. As strong a drive as I can muster
if I'm looking for < 10 ns release times.
Usually we don't have many of these.

So what do I want in driver?

For source terminated transmission, ( I guess you can call it reflected
wave) I want to match the transmission line. It doesn't have to be an
exact match as long as I have margin from reflections. The gotcha is
usually the weak silicon case. I'll usually lower the driver impedance
to where I have a little margin but and still maximize speed. So TI
LVT162244 type of drivers do well for #2, #3, #4. i.e. some where around
40-65 ohms seen by the reflected wave. Symmetric drive is more desirable

Terminated drivers are another story. They offer the most in speed for
loaded lines. So for these I look for as strong and fast of a driver I
can have with out utterly being killed by SSO and power dissipation.

Oh, also we can't forget a fast buffer delay and low pin to pin skews.

... Rich Mellitz


>From: D. C. Sessions[]
>Sent: Tuesday, November 18, 1997 11:04 AM
>To: SI-List
>Subject: [SI-LIST] : Preferred PWB impedances
>I had an interesting meeting late last week. VLSI is
>drafting the specs for our newest I/O library and rather
>than wet-finger it, we're asking for a little help in
>making them meet real-world needs.
>Here's the question for the floor:
>If you could have only four 3.3v CMOS driver types, what
>would you have. (Aside from HSTL, SSTL, etc.) One obvious
>candidate is a reflected-wave driver for 50-ohm nominal
>lines; that comes to about 10mA at 350mV from the rails
>under worst case conditions. A 65-ohm reflected-wave
>driver would run about 25% lighter, or about 8mA.
>Incident-wave is another matter entirely.
>SO! the floor is open for nominations. Keep in mind that
>excessively strong drivers are both inherently slower and
>more vulnerable to SSO degradation.
>D. C. Sessions