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I need to design a system on a four layer PCB. The layer stack-up from what I know would be

  • signal / components
  • ground plane
  • power
  • signal

My problem is, I have a system with a power supply of +-5V and for one part of the system I need 3.3V analog and digital supply. What would be the best practice to realize the power distribution of this system?

My idea was to have one rail at the bottom of the power plane which is VSS and one rail at the top which is VDD and then have multiple fingers from the top to the bottom and vice versa similar to intra layer capacitors in chip design. For the 3.3V I'll just have a small area within the power plane dedicated for.

Would that make sense or is there a better solution for this kind of routing?

Edit: Here is a sketch of what I mean with multiple fingers from bottom and top of the power plane. This sketch displays my intended power plane. I hope this clarifies any misconceptions.

-------------------------
|   ______    ______      VDD = +5V
|  |  __  |  |  __  |  |
|  | |  | |  | |  | |  |
|  | |  | |  | |  | |  |
|__| |  | |__| |  | |__|
     |  |      |  |
-----´  `-----´   `-----
|_______________________  VSS = -5V 
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  • \$\begingroup\$ What does "bottom (or top) of the power plane" mean? What is VSS and VDD? "Multiple fingers from the top to the bottom" - what does this mean? \$\endgroup\$ – Andy aka Jun 23 '14 at 13:28
  • \$\begingroup\$ @Andyaka: I think top and bottom means physical "north" and "south" edges of the board. And by fingers, the author means those meanders in his ASCII art. \$\endgroup\$ – Dzarda Jun 23 '14 at 14:04
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The term that you're looking for (if you want to find more sources on this) is a split plane. It's used pretty often - you aren't doing anything out of the ordinary.

Just a few things to keep in mind:

You might be able to avoid the finger-shaped power planes if you're clever with component placement - in a pinch, you could try keeping the planes simple and using vias to connect the planes to some wide signal traces for the awkward power pins.

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