RE: [SI-LIST] : Routing Differential Pairs as 100 differentially Vs individually 50 ohm lines

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From: Larry Miller (ldmiller@nortelnetworks.com)
Date: Mon Nov 27 2000 - 07:09:44 PST


Virtually all decent trace impedance calculators take into account the trace
thickness (i.e., it's an input parameter). You have an exaggerated situation
where your traces probably look more like square beams rather than somewhat
thin sheet-like cross-sections. Actually, this is good for controlling the
ratio between common-mode and differential impedances.

Larry Miller

> -----Original Message-----
> From: Ozgur Misman [SMTP:omism@amkor.com]
> Sent: Wednesday, November 22, 2000 3:23 PM
> To: Ingraham, Andrew
> Cc: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Routing Differential Pairs as 100
> differentially Vs individually 50 ohm lines
>
>
>
> Hi everybody,
>
> I have a circuit that I use to design differential pairs. The circuit is
> based
> on an optimization routine that minimizes the reflections ( S11) from a
> resistor
> across the differential pair. Where the relections are zero or close to
> zero
> the value of the resistor would be the differential impedance. Since we
> are in
> the packaging business our trace widths are typically between 60 to 100
> micron
> range. A higher number would lead to routing inefficiency a lower number
> would
> be very difficult to meet from manufacturing point of view in laminate
> based
> packages.
> Typically I set a target differential impedance and optimize the trace
> width and
> spacing to meet target Zdiff. The simulation tool is Microwave design
> system.
> I found out my optimum trace width and spacing greatly depends on the
> trace
> thickness as well! I have been following the discussions very closely, and
> there
> is no reference to trace thickness in the design of differential pairs!
> One possibility is that the software might be using rectangular conductor
> cross
> sections and therefore exagerates the effect of trace thickness. (We know
> a
> trapezoid would be a better aproximation)!
> Second possibilty is that since the aspect ratio is small in package
> traces as
> opposed to PCB traces, the tighter coupling between the traces results in
> strong
> dependence of Zdiff to trace height.
>
> As an example a two layer stack up would be signal-ground with 100 micron
> seperation and 75 micron trace spacing and width. ( Of course this is not
> 100
> Ohms differential impedance)
> Well, the physics supports the second option, however I am very interested
> in
> hearing any opinions or perspectives on the importance of trace thickness
> in
> Differential impedance calculations or simulations.
> Thanks
> Ozgur Misman
> Sr. Engineer
> Amkor Technology
>
>
>
>
>
>
>
>
>
> "Ingraham, Andrew" <Andrew.Ingraham@compaq.com> on 11/22/2000 02:46:37 PM
>
> Please respond to "Ingraham, Andrew" <Andrew.Ingraham@compaq.com>
>
>
>
> To: "si-list@silab.eng.sun.com"
> <si-list@silab.eng.sun.com>
>
> cc:
>
>
>
> Subject: RE: [SI-LIST] : Routing Differential Pairs as 100
> differentially Vs individually 50 ohm lines
>
>
>
>
>
>
> ...
> > Routing them close to each other produces interaction that reduces the
> > final amplitude.
> ...
>
> This "interaction" you speak of is the reduction in differential impedance
> as the two lines are brought into one another's neighborhood. Simple
> physics. You got reduced amplitude because your lines were less than 100
> ohms differential, and if you didn't take that into account, you probably
> mismatched the line's impedance.
>
> Two widely separated 50 ohm lines give 100 ohm differential impedance.
> Two less separated 60 ohm lines give 100 ohm differential impedance.
> Two even less separated 70 ohm lines give 100 ohm differential impedance.
> And so on.
>
> What's different between these, is the common-mode impedance. Matching
> the
> differential impedance is the first, and most important, step, but you
> often
> want to look at how (or whether) you matched the pair's common-mode
> impedance with the terminations at one or both ends, which affects
> common-mode noise susceptibility.
>
> There is nothing "wrong" about routing differential pairs close so they
> "interact" with one another. After all, that's what a twisted pair cable
> is. The wires are close to each other, producing interaction, and a lower
> impedance than if the two wires were far apart.
>
> Andy
>
>
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