Re: PCB Stack

jai.dhar@xxxxxxxxx schrieb:
1 GND plane in an 8-layer stack? I was under the belief that yes, a
power plane can serve as a return path for a signal, but it's not
preferred or equal over a GND plane. I would think partitioning the
power planes is a safer bet than cutting another GND layer.
Nope.

As a power supply for a high speed circuit you need a low inductance
power supply. A CMOS circuit is completly symmetric between VCC and GND.
The magnitude of electrical effects will depend on the maximum of the
VCC and GND inductance.

Driving a falling output for example will make the GND plane bounce up,
driving a rising output will make the VCC blane bounce down. Both
changes the input threshold voltage by the same magnitude introducing
jitter and reducing the noise margin.

Therefore design all supplies for the same inductance. The only
difference is that GND usually is common for the whole board, whereas
sometimes certain power supply voltages are only need in some areas of
the board so that you can have one plane in one half of the board and
another in the other half.

Islands are possible but very dangerous. Remember that you can not have
a high speed signal cross the island boundary on the adjacent routing
layer. Haveing a seprate layer for each supply therefore simplifies
routing a lot.

Also consider microvias. They save a lot of area, are great for signal
integrity and do not cost much extra.

Kolja Sulimma

.

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