[SI-LIST] Re: Package, pad, bond, and bond wire parasitic capacitence USB 2.0 HS

  • From: "Dima Smolyansky" <Dima.Smolyansky@xxxxxxxxxxxxxx>
  • To: <ericsilist@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 17 Feb 2004 10:03:55 -0800

Eric,

I think you have nailed it yourself - the capacitance you observe is caused
by the foot, lead frame and bond wire with combination of the choice of
pads. Overall, it is caused by your choice of the package and pinout of the
die; a simple comparison to some of your competitor's choices may make
things clearer.

It may be worth investigating what the package vendor specifies as the
values of capacitance, inductance and resistance - both for your package and
some of the alternatives. If you do not have these data, you can relatively
easily compute these values (capacitance definitely will be easy) using some
of the techniques for package modeling we discuss in our appnote on the
subject: TDR Techniques for Characterization and Modeling of Electronic
Packaging (http://www.tdasystems.com/library/appnotes/pkgm0703.pdf). It is
also possible that the way your package is mounted on the board causes the
capacitance to increase substantially.

We also have more appnotes on package-related signal integrity analysis
issues on our web site at
http://www.tdasystems.com/support/applicationnotes/byinterconnecttype.shtml#packages


Thanks,

===================
Dima Smolyansky
TDA Systems, Inc.
4000 Kruse Way Place
Bldg. 2, Suite 300
Lake Oswego, OR 97035
(503) 246-2272
(503) 246-2282 (fax)

TDA's MeasureXtractor is the EDN Product of the Year Finalist! Vote at
www.edn.com!!!


----- Original Message ----- 
From: "eric steimle" <ericsilist@xxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, February 17, 2004 7:06 AM
Subject: [SI-LIST] Package, pad, bond, and bond wire parasitic capacitence
USB 2.0 HS


> Hi,
> I'm currently working on a USB 2.0 high speed
> (480MB\s) transceiver of our own design. When I look
> at the D+ D- lines on a TDR I see a significant amount
> of capacitance which I now believe is caused by the
> foot, lead frame and bond wire in combination with our
> choice of input pads on the die. I've looked at other
> commercial usb transceivers and while you can still
> see the effect in their devices our is much more
> pronounced. I really think that this is what is
> really hurting our ability to receive (we can transmit
> but of course that's not the hard part).
>
> I work with almost all pure digital guys so anything
> like this is always a tough sell. So I was hoping
> someone could explain to me what causes the above
> mentioned capacitances. I've been reading and
> googling and so far I found a lot of information
> saying they exist but not a lot explaining why (and
> perhaps even how they change with increasing
> frequency). This may be a very basic question but
> I'm still learning so any advice or pointing in the
> right direction would be greatly appreciated.
>
> Best Regards,
> Eric
>
>
>
> __________________________________
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