High frequency currents take the least inductive path. At the presence of
decoupling capacitors across VCC/GND, there are two possible paths: through
decoupling caps or through VCC/GND planes. Caps have series inductance
(ESL), which makes them not as effective above their resonant frequencies.
Therefore, to prevent high-f current to be sourced from VCC/GND planes
causing EMI problems, one fix is to have a series inductor - typically a
ferrite bead - to increase the VCC/GND loop inductance with respect to the
decoupling cap ESL. This technique is generally applied to VCC, but could be
extended to GND as well. The app note, "CK100 Clock Buffer Preliminary EMI
Layout Guideline," available at http://developer.intel.com/ial/sdt/, gives
such an example.
-Michael T. Zhang
Platform Architecture Lab (PAL), Intel Corp
(503) 264-2301
-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Friday, March 05, 1999 8:06 AM
To: [email protected]; [email protected]
Cc: [email protected]
Subject: Re: [SI-LIST] : A Question About Power Noise.
The series addition of an inductor on the VCC line is an EMI type fix for
very
noisy IC's. This type of fix has worked for circuits for up to the 100MHz
range. Their usefuleness for higher speeds may be doubtfull. Here is the
theory.
IC's with intensive di/dt requirements (e.g. clock generator and driver) may
have the VCC fed with a series inductor to reduce Ldi/dt noise generation on
the supply plane. Although this may appear counter-intuitive, the use of
proper decoupling becomes now extremely critical for this to work. The
decoupling capacitors will now have to provide all the di/dt to the VCC pin
of
the IC that would have come from the planes. That means that the decoupler
cap
must be adjacent to both the VCC and GND pins of the IC. Reducing the round
trip inductance is a must so, for a j-leaded IC, the decoupler must be on
same
side as the IC with low inductance traces (short and wide). The vias to the
planes are placed by the j-lead but not so as to affect the L of the trace
to
the cap.
If proper decoupling had been designed in the first place, the need for an
inductor would have been eliminated.
Hans Mellberg
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