Sounds like inductance and ground bounce. Use small (0603 or 0402) caps close,
and try bypassing ground to VCC using a multiple via connections to the ground
plane. Ive seen Tpd measurements improve about 15% just adding 'ground bypasses'
close to the chip leg grounds.







"Brian Guralnick" <[EMAIL PROTECTED]> on 10/09/2002 12:33:14 AM

Please respond to "Protel EDA Forum" <[EMAIL PROTECTED]>

To:   "Protel EDA Forum" <[EMAIL PROTECTED]>
cc:    (bcc: Clive Broome/sdc)

Subject:  Re: [PEDA] off topic supply SOS



     I traced a nasty set of spikes on the 1.8v supply with my storage scope.
The largest is 0.11v.  This drops the 1.8v just
outside the minimum spec for the EP20K200E I'm using.

    I'm going re-hook up everything & get an FFT plot later this afternoon.

____________
Brian Guralnick
[EMAIL PROTECTED]
Voice (514) 624-4003
Fax (514) 624-3631


----- Original Message -----
From: "Terry Harris" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Sunday, September 08, 2002 11:39 PM
Subject: Re: [PEDA] off topic supply SOS


> On Sun, 08 Sep 2002 21:45:51 -0400, Brian Guralnick wrote:
>
> >Having not placed enough power supply decoupling caps on a beta PCB, I'm
looking for a temporary overpriced decoupling solution
> >until the next beta boards are printed.
>
> >I'm looking for the lowest ESR possible caps.  I found this company WIMA.
>
> >http://www.wima.de/smd2220.htm
> >1.0uf, 0.1uf, 0.01uf. -> 0.07 Ohm thru ~0.15
>
> Well who knows what your exact requirement is but putting better capacitors
> in not enough places probably isn't a fix.
>
> Also those capacitors tan delta is specified at 1, 10, 100 kHz - not
> intended for high frequency decoupling, the SMT 100n looks like it is self
> resonant around 6 or 7 MHz.
>
> Actually it looks like you read that graph to get the ESR figures you
> quoted but as I read it the line on the graph shows impedance at resonance
> not ESR.
>
> Is it high speed logic? Do you *know* you have a problem? Boards tend to be
> way over decoupled anyway especially if you have decently coupled power
> planes.
>
> I would wait till you know you have a problem and strap some extra
> capacitors across chip leads if you really need to.
>
> Cheers, Terry.
>
> ************************************************************************
> * Tracking #: 4DFFA14C1C300E4AA51F7DAF4685AD9A6EC611BC
> *
> ************************************************************************






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