RE: [SI-LIST] : Differential pair - How to compensate for differe nt length?

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From: Aubrey_Sparkman@Dell.com
Date: Mon Jan 29 2001 - 09:59:03 PST


Vinu,
What you have said:
"The trace that is shorter on the PCB is connected to a longer
trace on the BGA."
is probably, but not always true. Not all BGA connections can be made with
shortest path.

But it is appropriate to consider the total length difference (silicon pad
to pad) and how that compares to the rise time of the signals.

Aubrey Sparkman
Signal Integrity
Aubrey_Sparkman@Dell.com
(512) 723-3592

> -----Original Message-----
> From: Vinu Arumugham [mailto:vinu@cisco.com]
> Sent: Monday, January 29, 2001 11:39 AM
> To: Shahar Eytan
> Cc: 'si-list@silab.eng.sun.com'
> Subject: Re: [SI-LIST] : Differential pair - How to compensate for
> different length?
>
>
> You may not need any compensation. The trace that is shorter
> on the PCB is
> connected to a longer trace on the BGA.
>
> Thanks,
> Vinu
>
> Shahar Eytan wrote:
>
> > Hi All,
> >
> > I need to connect high speed lines from a Fine BGA
> to a connector
> > using differential lines. The positive and negative outputs
> from the FBGA
> > resides in row 1 and 2 of the FBGA, that means ~35 mils of
> difference
> > between the lines.
> > What is the right way to compensate for this difference?
> > I was thinking that the best way is to go strait
> out with from the
> > internal ball, while the second one (from the external row)
> will "pay" for
> > his shorter way by few turns as close as possible to the
> source. This way
> > the differential pair will be coupled together all the way
> to the connector.
> > Same for the receiver.
> > What do you think?
> >
> > Shahar Eytan
> > Mellanox Technologies Ltd.
> > <mailto:shahar@mellanox.co.il>
> > Tel: 972-4-9593244 ext 264
> > Fax: 972-4-9593245
> >
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