From: Christian S. Rode ([email protected])
Date: Tue Nov 16 1999 - 06:06:09 PST
I think the Lucent URL is:
http://www.lucent.com/micro/hsi/docs/AP99005.pdf
They also highlight the reduced common-mode requirements of their
pseudo-ECL differential outputs compared to totem-pole devices -
this addresses an earlier question about LVDS-SSO in an earlier
thread. (Attached below -------)
sweir wrote:
>
> Chris,
>
> I reasonable tutorial is included in Lucent's quad driver / receiver
> literature. That also has the advantage of explaining some of the common
> mode range issues, as the Lucent parts are PECL.
>
> Regards,
>
> Steve.
> At 01:27 PM 11/15/99 -0800, you wrote:
> >Hi,
> >
> >I am trying to understand how differential pairs behave and how you
> >terminate them. I've read app notes and books that tell you how to
> >terminate them, but I don't know how they derived the values and
> >methodology. I would just like a general understanding of what's going
> >on. I understand how and why termination works on normal, single-ended
> >lines.
> >
> >As an example, suppose you had a 100ohm, unshielded, twisted pair cable
> >that ran for 5000 feet between two boards. Once the pair entered the
> >board, it was routed over several inches to a differential receiver.
> >The two boards do not share a ground (suppose they are battery powered).
> >
> >1) How did I know to route the traces on the board as two 50 ohm
> >traces?
> >2) How did I know to terminate the pair by placing a 100ohm resistor
> >between the two traces at the receiver?
> >3) Could I terminate each line separately as if they were two,
> >single-ended lines (say, using a 50ohm resistor to ground on each line
> >at the receiver?)
> >4) Where does the return current flow as the signal is launched down
> >the line from the driver, across the twisted pair, across the diff pair
> >on the board, and to the receiver?
> >
> >Thank you very much,
> >
> >Chris
> >
> >
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-------------------------
D. C.:
A question: Is it really practical to assume the benefits of reduced
package crosstalk with push-pull drivers? I can see that you could
tune the delays in open-collector-like technologies to maintain
a nearly continuous current flow (at least in the nominal case) but
with CMOS drivers and cross-coupled break-before-make pre-drivers
won't you always get a period where both drivers are off?
The spike in that case would be due to currents dropping, rather
than increasing and creating an initial on-chip voltage rise, rather
than a fall. David didn't characterize the nature of his spikes.
David?
Chris Rode
"D. C. Sessions" wrote:
>
> David Haedge wrote:
> >
> > Fellow SIers,
> >
> > I am working on an SSO analysis that involves large numbers of LVDS
outputs
> > switching on a die. One of the reasons to use LVDS is because the
driver is
> > basically just redirecting a 4mA current in the output loop,
hopefully
> > eliminating large di/dt's on power and ground. However, my SPICE
analysis
> > shows a 20-25mA current spike on VDD and VSS with a rise/fall time
of about
> > 230ps each time the device switches (375MHz rate). It was assumed
that we
> > could get by with a lot less VDD and VSS pads due to the expected
low di/dt's.
> > It appears now that with this large unexpected current spike, we
need to triple
> > the number of powers and grounds to achieve an acceptable voltage
drop/ground
> > bounce. Has anybody out there seen this sort of behavior in LVDS
circuitry?
> > Or is this just perhaps a quirk in SPICE (or my chosen vendor)?
>
> Assuming that you're not running the predriver on the output rails
(BAAAAAD idea)
> then it sounds like you have some rise/fall assymetry. Step one is to
fix it,
> since power noise is the least of the troubles it causes.
>
> Beyond that, you may not have a big problem Oddly enough, in a
differential
> current environment supply inductance can actually be a Good Thing
since it
> effectively forces rise/fall symmetry. Put some package inductance
into that
> simulation (with or without mutual inductance, although ignoring
mutual
> inductance is kinda silly) and see what happens. Betcha you like the
results.
>
> --
> D. C. Sessions
> [email protected]
>
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