100 ohms natters if you are using 100 ohm media feeding 100 ohm
terminations. But inside your board you can use 80 ohm lines and 80 ohm
terminations all you want and nobody cares as long as the drivers can take
the extra load (which they almost always can). In the instance that you must
drive 100 ohm external media despite 80 ohm traces, place the driver right
at the connector to keep the 80 ohm trace lengths less than a prop delay
(not hard at 100 Mb/s).
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On Behalf
Of BOUTHEMY Jean-Pierre
Sent: Tuesday, August 25, 2020 3:02 AM
To: 'si-list@xxxxxxxxxxxxx' <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Less than 100 ohms differential impedance
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@fr.thale
sgroup.com>
THALES
CAP Bretagne - Zone Artisanale Piquet Sud
35370 ETRELLES - France
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