[SI-LIST] Re: 50 ohm and 130 ohm on the same board

  • From: Chris Cheng <Chris.Cheng@xxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Wed, 2 Jun 2004 18:31:29 -0700

I also think it is 130ohm differential.
But on the subject of making >100 ohm trace and 50 ohm traces on the same
layer, I've known people who will cut the reference plane underneath the
>100ohm microstrip to force it to reference the next power/gnd plane beneath
and thereby creating a super high impedance. I wouldn't do it personally but
people have shipped product like that.

-----Original Message-----
From: steve weir [mailto:weirsp@xxxxxxxxxx]
Sent: Wednesday, June 02, 2004 4:14 PM
To: msharpes@xxxxxxxxxx; prasadsa@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: 50 ohm and 130 ohm on the same board

M, that is an interesting application model.  I think the prevailing wisdom 
would have been to use a lower impedance driver and transmission line to 
minimize the discontinuity caused by load capacitances.  What you did plays 
on paper.  I would be wary of how accurately the real loads match the line, 
especially with tolerancing, but if it worked, that is what counts.

Still 130 ohms seems like a value derived from a pair and not a single 
ended.  I think Prasad is in India, and we won't see a response until


At 05:00 PM 6/2/2004 -0600, msharpes@xxxxxxxxxx wrote:
>Although 130 Ohms seems on the high side, through some experience I can
>talk of designing higher impedance transmission lines (in the 100 Ohm
>neighborhood) for a fairly large distributed load.  So, the concept was
>that the capacitive nature of each of the distributed loads served to
>lower the characteristic along the line so much so in some cases that
>impedances of 80-100 Ohms were desired for a 50 Ohm 'effective'
>impedance.  The trick to doing this is to space the loads in an equal
>pitch with the pitch being something less than an inch for the
>transmission line to 'look' like something around 50 Ohms.  Now, this
>isn't portable to extremely fast designs, but we have had success with
>terminated transmission lines of this type with the right drive strength
>and source impedance to move 2V/ns at each load in a 3.3V system.
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