[SI-LIST] Re: Less than 100 ohms differential impedance

  • From: Joel Brown <joel.brown@xxxxxxxxxx>
  • To: billh@xxxxxxxxxx
  • Date: Tue, 25 Aug 2020 13:00:23 -0700

Best thing would be to do an end-to-end simulation.
But that is not easy and the tools are not cheap.
Depending on how critical it is for it to work reliably, you could hire a
consultant to do the simulation work.


On Tue, Aug 25, 2020 at 12:42 PM Bill Hargin <billh@xxxxxxxxxx> wrote:

And - adding to Binayak's comment - pay attention to termination values
and your connector impedances, which will be 100 ohms more often than not.
The lower speeds can typically handle a short impedance discontinuity from
the board to a connector, but you'd want to simulate the high-speed
signals, making sure to model the discontinuities and terminations.

Bill Hargin
Director of Everything
Z-zero  â–ª  Innovative PCB Stackup Design  â–ª  www.z-zero.com
billh@xxxxxxxxxx   â–ª  425-301-4425  â–ª  Skype: bill.hargin


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On
Behalf Of Binayak Shrestha
Sent: Tuesday, August 25, 2020 10:41 AM
To: jean-pierre.bouthemy@xxxxxxxxxxxxxxxxxx
Cc: Si-list <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Less than 100 ohms differential impedance

Just add to my previous comment....some high speed interface demand  a
particular impedance. Make sure you go through the spec properly on to see
if you can change them or not.
On Tue, 25 Aug, 2020, 11:08 pm Binayak Shrestha, <binayaks@xxxxxxxxx>
wrote:

Jean,

IF you are able to maintain a constant uniform impedance environment
from the driver pads to the receiver pads, it should be fine....BUT
pay attention to the "IF condition" very carefully.

- binayak

On Tue, 25 Aug, 2020, 3:33 pm BOUTHEMY Jean-Pierre, <
jean-pierre.bouthemy@xxxxxxxxxxxxxxxxxx> wrote:

Hello SI experts,
I have many differential pairs in my design that should be matched to
Zdiff = 100 ohms on the PCB.

But my stackup has very thin dielectric material and I won't be able
to reach more than 80 ohms differential impedance.
This is typical value, thus I can expect Zdiff to be as low as 70 ohms.

I would like to know the electrical and functional impact of
Zdiffp-80 ohms on following differential signals:


o   LVDS 100Mbps

o   Ethernet 100Base-TX

o   Ethernet 1000Base-BX (Xilinx Multi-Gigabit Transceiver)

o   Ethernet SGMII (Xilinx Multi-Gigabit Transceiver)

o   25MHz LVDS Clock

o   200MHz LVDS clock

o   125MHz AC-coupled LVPECL clock

Thank you for your answers.
Regards.



[cid:image019.png@01D38BAE.3AF37340]

Jean-Pierre BOUTHEMY
Ingénieur Etude et Développement en Electronique Département R&D
Centre de Compétences Groupe Microélectronique Systèmes de Mission
de Défense

Tel.: +33 (0)2 23 55 42 28
jean-pierre.bouthemy@xxxxxxxxxxxxxxxxxx<mailto:
jean-pierre.bouthemy@xxxxxxxxxxxxxxxxxx>

THALES
CAP Bretagne - Zone Artisanale  Piquet Sud
35370 ETRELLES  - France

www.thalesgroup.com/microelectronics<
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