FreeLists / si-list / [SI-LIST] Re: Ideal driver characteristics

[Larry Miller <ldmiller@xxxxxxxxxxxxxxxxxxxx>]

The SSTL "current source" drivers I have been looking at are far from
infinite-impedance. Indeed, per their IBIS V-I data they seem to be in the
neighborhood of 45 to 55 ohms near the middle of the switching range. The
main difference from CMOS LVTTL "voltage" drivers is that their output
impedance does not drop down to 10 or 20 ohms near ground or 20 to 30 ohms
near Vdd. They seem to hold a more constant driving impedance that is closer
to our favorite (50 ohm) PCB trace surge impedance over the range of signal
levels used. In the case of SSTL this is a rather narrow range of voltages,
and the circuits appear to be operating non-saturated.

Some of the really small-geometry ASIC companies actually do get a pretty
high output impedance, and they recommend a parallel terminating resistor.
Still others allow you to set the apparent impedance (by programming current
mirrors that are in series with the switching transistors, I am guessing).

I don't think that you are missing anything, but that the devices are not as
"pure" as you might think, especially at high speeds. Your thought of
avoiding trace lengths that have a round-trip propagation delay equal to the
rise time of the signal might indeed be a good idea. It would certainly be a
worse problem if the drivers were more ideal current sources.

One comparison is the current drivers used for 100BASE-TX. Now these are
true analog current generators, at least to the extent that you have at
least 10 dB of return loss on an S11 measurement at 80 MHz (more at lower
frequencies). In reality, these generators have an output impedance that is
3 to 10 times the 50-ohm common-mode impedance (100 ohms line to line). And
--guess what? You have to terminate them.

But most logic circuits do not spend nearly the effort and money to make
such idealized drivers.

Larry Miller

-----Original Message-----
From: Todd Westerhoff [mailto:twester@xxxxxxxxxxx]
Sent: Monday, June 11, 2001 9:55 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Ideal driver characteristics



Hi all,

We were talking about driver characteristics a few weeks back, and Arpad
questioned why someone would consider a current source to be an ideal
characteristic driver.

I'm wondering the same thing.  Since a current source's output is
independent of the voltage at its output, it seems to me that such a driver
would do a pretty poor job of absorbing reflections.  In fact, it seems to
me that such a driver would't absorb them at all, it would reflect them the
same as an open-ended line.  Am I correct in this assumption?

Looking at the IBIS curves for any number of models, it seems to me that the
transistors are usually saturated, and therefore acting as current sources,
when they first turn on.  Thus, it follows (I think) that any reflections
that hit the driver when it is first transitioning are likely to be
reflected instead of absorbed - providing one very good reason to avoid line
lengths that are 1/2 of the cycle time.

It seems to me if that I'm looking for an "ideal" driver, I'm looking for
one whose V/I characteristic is a staight line from the zero-current point
out to the maximum current (driving + absorbing reflections) at which it
will ever be operated.  I'm not likely to find such a device, but that seems
to me to be the perfect "linear" driver we'd like to have.

What am I missing?

Todd.

Todd Westerhoff
SI Engineer
Hammerhead Networks
5 Federal Street
Billerica, MA  01821
twester@xxxxxxxxxxx
ph: 978-671-5084


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