RE: [SI-LIST] : Timing on a large board

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From: Tom Dagostino ([email protected])
Date: Mon Oct 09 2000 - 08:58:10 PDT


oops, sorry I saw an ECL part number and made an assumption.

AC coupled means the same thing a AC terminated in this context. AC
coupling comes from the linear amplifier world where signals are either DC
coupled, where there is a DC current path between the driver and the
receiver or AC coupled where there is a DC blocking capacitor between the
driver and receiver. In this case the termination is AC coupled to the
driver/transmission line.

Tom Dagostino
IBIS and Tau Modeling Manager
SDD
Mentor Graphics Corp.
503-685-1613
[email protected]

-----Original Message-----
From: Michael Nudelman [mailto:[email protected]]
Sent: Friday, October 06, 2000 4:49 PM
To: Tom Dagostino
Cc: 'Juergen Hannappel'; [email protected]
Subject: Re: [SI-LIST] : Timing on a large board

Tom,
we are talking of terminating TTL output of ECL (or LVDS) to TTL converter.
Which is a totem pole driver. Which CAN be AC terminated (nobody said -
"coupled"). AC termination means - series resistor and capacitor link in
parallel to the transmission line.

Mike.

Tom Dagostino wrote:

> ECL outputs cannot be AC coupled to the termination. The output is an
open
> emitter that needs the DC current path to Vcc - 2 Volts.
>
> Tom Dagostino
> IBIS and Tau Modeling Manager
> SDD
> Mentor Graphics Corp.
> 503-685-1613
> [email protected]
>
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Michael Nudelman
> Sent: Friday, October 06, 2000 2:21 PM
> To: Juergen Hannappel
> Cc: [email protected]
> Subject: Re: [SI-LIST] : Timing on a large board
>
> The 50-Ohm info:
>
> Pages 3-90, 3-91, 3-92 of MC10125 datasheet, lool at the bottom of the
table
> row
> "switching times". The measurement condition is with 50 Ohm load.
>
> If it is not point-to-point, use Thevenin split - it will save you drive
> current (cuts
> it in half). Though with 100 Ohm and this driver you may be able to use AC
> termination. I
> like 220 - 300pF cap value; seems to be optimal. With lower values the
> overshoot is too
> high; with much higher - the front gets a "step".
> The driver also seems to be nice - the rise/fall time is not too fast -
> 2.5ns typical,
> and can drive up to 10 TTL loads.
>
> Mike.
>
> Juergen Hannappel wrote:
>
> > Michael Nudelman <[email protected]> writes:
> >
> > > Juergen,
> > >
> > > How many stubs do you have on any given LVDS line on your backplane?
If
> it is not
> > > point-to-point, in the first place equalizing LVDS diff pairs on the
> board would be
> > > a bad idea, since you want to keep stubs on your boards 1" or shorter.
> Again, only
> > > if it is not point-to-point. Otherwise it's fine.
> >
> > The LVDS outputs are point-to-point. On the edge of the board i have a
> > connector, and from there the signals go via twisted pair flat cables
> > to other boards wich each have input termination.
> >
> > >
> > > The 7ns difference would account for 42 (3.5 feet) inches, and you
said
> your board
> > > is roughly foot and a half. If the traces do not wiggle, and it is not
> some kind of
> > > reflection you see - where the 7ns skew comes from? YOu should see no
> more than 3ns
> > > skew worst case..
> >
> > I am also astonished by this, but the pattern of delays is the same
> > for a number of boards (4), so it is not an erratic effect and also
> > cannot be caused by the discriminator modules.
> >
> > >
> > > The driver of 10125 can drive 50 Ohm line; if you have space problem -
> use 75/100
> >
> > Thanks for that info, i was unable to extract it from the data sheet...
> >
> > > Ohm lines (will make traces thinner and you could pack more of them),
> and since you
> > > are point-to-point (TTL-to-LVDS input), use series termination; it
will
> save you
> > > instant drive current versus end-AC termination and power consumption
> versus
> > > Thevenin split.
> >
> > It would not be point-to-point, but rather a 3 to 4 drop
> > configuration, because i need 3 or 4 LVDS versions of the same TTL
signal.
> >
> > >
> > > Mike.
> >
> > --
> > Dr. Juergen Hannappel Office: W148 Phone: +49 228 73 2447 FAX +49 228
73
> 7869
> > mailto:[email protected] Physikalisches Institut der
Uni
> Bonn
> > http://lisa2.physik.uni-bonn.de/~hannappe Nussallee 12, D-53115 Bonn,
> Germany
> > CERN: Phone: +412276 76461 Fax: 77930 Bat. 892-R-A13 F-01631 CERN CEDEX,
> France
>
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