[SI-LIST] matching differential impedance for edge-coupled lines
- From: "Perry Qu" <perry.qu@xxxxxxxxxxx>
- To: si-list <si-list@xxxxxxxxxxxxx>
- Date: Fri, 15 Jun 2001 15:05:47 -0400
Hi!
There has been a few threads about odd/even mode impedance and I would
like to throw in another one:
Basically there are two types of differential pair for edge-coupled
lines, one is loosely-coupled, with trace spacing much wider than trace
width/height above ground; I find from my 2D field solver that only in
this case you can obtain a 100 ohm differential impedance (or 50 ohm
Zodd as mentioned in TDA's paper); To match the diff pair, you can
either terminate each line with 50 ohm to ground or a 100 ohm across
plus and minus;
The second type is a tightly-coupled diff pair, with trace spacing
equivalent to trace width and trace height against ground plane. In such
case, I can't get a 100 ohm differential impedance (or 50 ohm Zodd) in
my calculation with a realistic stackup and manufacturing constaints.
Zodd is more likely in the region of 40-45 ohm range. To match the diff
pair in this case, I need to terminate each line against ground with
Zodd or 2*Zodd across plus and minus. (I did this with HSPICE on a very
simple topology: Ideal voltage source (50ps rise time step pulse) ->
transmission line -> load). This is intuitive since we can imagine that
there is a virtual ground between plus and minus.
My question:
In reality, I saw many diff pair done in second case because of the
space limit on already very dense board. In such case, I think instead
of using standard 100 ohm termination (e.g., LVDS application),
shouldn't we terminate the diff pair with smaller values to match the
differential signal ?
Thanks in advance
Perry Qu
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