[SI-LIST] Re: Ideal driver characteristics
- From: Bill.Cohen@xxxxxxxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 11 Jun 2001 13:49:24 -0400
Todd,
When we design curent source IO cells (e.g. LVDS & PECL) the termination
is usually part of the IO cell. For example LVDS has a NE and a FE
terminator. This will limit the mismatch and absorb reflections. If the
terminator is not on chip, then it usually is placed as close as possible
to the package.
The output current is usually set to the termination resistance. Thus we
can
control the voltage swing by matching the current to the termination. Of
course
the optimum situation is to control both the termination impedance and
output
swing. This can be quite difficult due to technology reasons. For example,
LVDS
requires differential termination. To design a contollable impedance
not connected to the power supply can become quite complicated. Thus, we
usually
use a resistor with a very low temperature coefficient. For impedances
connected
to the power supplies, the design reduces in complexity. It is still quite
difficult
to match impedances over the full frequency range (>500Mhz) due to
parasitics such as package and ESD.
Best Regards,
Bil Cohen
Toshiba America
Circuit Design Group
Marlborough, MA
"Todd Westerhoff" <twester@xxxxxxxxxxx>@freelists.org on 06/11/2001
12:54:35 PM
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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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