Shashank,
Your description missed a few important details, so here is what I'm assuming.
The 90 Ohms is for a differential impedance.
The resulting S-parameter data will be examined for some compliance, not for a
time-domain simulation.
You are asking about the selection of normalizing impedance for the S
parameters.
The normalizing (port) impedance for S parameters doesn't change the underlying
model. If you extract the S parameters with a 50-Ohm port impedance or 40-Ohm
port impedance and then convert the 2 sets of data to Z parameters, the results
will be the same (within computational tolerance). Therefore, for a circuit
simulation, either port impedance will work fine as long as the port impedance
is within the order of magnitude of the circuit impedance. If you use the S
parameters directly for a compliance measure, such as COM, then you need to use
the 50-Ohm port impedance, because the compliance calculations assume that
value. On the other hand, these types of compliance checks also assume some
values for the component buffer impedance. If the protocol specification says
that buffers should have 100-Ohm impedance and the actual component has 90 then
the component is probably not compliant with the protocol. In that case, I
would follow the component guidelines unless your circuit goes to a connector
that is expected to meet the protocol specification on the other side.
Regards,
Weston
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Shashank D
Sent: Tuesday, May 26, 2020 9:04 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Port impedance to be considered for SI analysis
Hello All,
Hope all good!
I have a doubt in considering Port impedance for SI analysis in any tool.
I have an ASIC which specifies port impedance as 90 Ohm and Routing guideline
also specifies to route trace with 90 Ohm.
But, the IEEE spec says 100 Ohm.
Hence, can anyone please let me know what should i consider while extracting
S-Parameter from SI tool?
Thanks and Regards,
*Shashank D*
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