[SI-LIST] Re: Ideal driver characteristics

  • From: Scott McMorrow <scott@xxxxxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Mon, 11 Jun 2001 12:07:05 -0700

Steve and Todd,

One can, however, have a current mode driver that
is a Norton equivalent with a high shunt resistance.
In that case, if the effective shunt resistance is high enough,
then there is very little reflection coming back off the driver.
Rambus drivers, for example, are designed to have an
effective output impedance of 150 ohms.

scott


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Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
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"S. Weir" wrote:

> Todd, yes a current mode driver by itself is just as bad as a zero
> impedance voltage mode driver, neither absorbs energy and therefore require
> external terminations.  But for the impedances that we typically deal with,
> it is a lot easier to approximate an inconsequentially high Z in current
> mode, than a zero Z in voltage mode.  Then there is this little problem of
> where to package the terminations.  I agree, a driver with an approximate
> resistive curve is the most practical for point to point.
>
> Regards,
>
> Steve.
> At 12:54 PM 6/11/01 -0400, you wrote:
>
> >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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