26 DESIGN007 MAGAZINE I MARCH 2021
• e edges of the board cause the greatest
amount of reflection since an edge is a
totally abrupt open circuit.
• Reflected energy is accompanied by
phase reversals in its components, and
combined reflections from the open
circuit at the edge of the board can cause
a phenomenon known as voltage
doubling creating a standing wave.
• e larger the plane area, the lower the
impedance.
• RC terminations of 3R5, in series with
10nF spaced along the board edges, are
generally sufficient to reduce the
resonance peaks.
• e addition of loss to dampen modal
resonances is more important than the
exact termination value and distribution.
• e added losses from the RC terminations
are helpful in the overall response of the
PDN as they dampen impedance peaks
arising from connecting the devices to the
board. DESIGN007
Resources
1. Beyond Design: PDN—Decoupling Capacitor
Placement, Learning the Curve, Fringing Fields, by
Barry Olney, January 2016.
2. Optimized Power Delivery Performance Using
Plane Terminations by Istvan Novak, Samtec.
3. US5708400A—AC coupled termination of a
printed circuit board power plane in its character-
istic impedance—Google Patents, Terrel Morris,
Hewlett Packard.
Barry Olney is managing
director of In-Circuit Design Pty
Ltd (iCD), Australia, a PCB design
service bureau that specializes
in board-level simulation. The
company developed the iCD
Design Integrity software
incorporating the iCD Stackup, PDN, and CPW
Planner. The software can be downloaded at
www.icd.com.au. To read past columns or
contact Olney, click here.
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