RE: [SI-LIST] : Decoupling:routing

Mon, 11 May 1998 10:44:18 -0600

At the risk of adding more information to this...

I have research information that shows that the addition of a ferrite
in series with the VCC pin of an IC does dramatically reduce the DM
noise produced by the chip - hence reducing the noise seen
across the planes. But the measured data showed that there was NO
reduction in the CM at all.

So, the addition of a cap using traces to the planes adds a small amount
of inductance
in series with the cap and hence a low pass filter. This may explain the
seen in the analog example. This technique, I suggest, should not be
for decoupling digital components as the signal integrity/emissions from
chip will be compromised. For digital tie the cap & the chip directly to
the planes.

As Winston Churchill once said about decouplers:

Never , in the field of engineering, has so much been written by so many
about so few.

Thank you
Charles Grasso
EMC Engineer
2270 Sth 88th Street
Louisville CO 80027 MS 4262
Symposium Website URL:

>From: Jay Diepenbrock[]
>Sent: Wednesday, May 06, 1998 7:18 AM
>To: si-list@silab.Eng.Sun.COM
>Subject: Re: [SI-LIST] : Decoupling:routing
>Regarding Tom Zimmerman's posting about the same subject, our experience is
>similar for mixed-signal designs. We have found that both noise within the
>system and EMI are improved by providing the best possible common Ground for
>all subsystems. Problems with digital noise getting into the analog
>were solved by generous use of ferrite beads and decoupling on local power
>supply connections, but we had to add local voltage regulators on the analog
>power supply lines in some cases because the noise generator (it was an
>intentional transmitter) was just so strong that filtering wasn't sufficient
>keep it out. Splitting the Ground plane with common attachment back at the
>power supply was the initial approach, but the Ground noise in the analog
>try with that arrangement was too high.
>Jay Diepenbrock
>Advisory Engineer
>Interconnect Technology & Qualification
>IBM Global Procurement, B8UA/061, RTP, NC
>Phone: 919-543-8804 Fax: 919-543-3642