RE: [SI-LIST] : Decoupling:routing

Grasso, Charles ([email protected])
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
[email protected]
Symposium Website URL:

>From: Jay Diepenbrock[SMTP:[email protected]]
>Sent: Wednesday, May 06, 1998 7:18 AM
>To: [email protected]
>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
>Email: [email protected]