Re: [SI-LIST] : A Question About Power Noise.

HMellberg@aol.com
Fri, 5 Mar 1999 11:05:50 EST

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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