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Alright. I was given the task to design an analog design board which contains lots of opamps, some CMOS switches and some ADCs/DACs. It also contains 4 separate power supplies (each supplying +5V and -5V (+5VA1 .. +5VA4, -5VA1 .. -5VA4, AGND1..AGND)) plus some 3V3/ DGND logic.

Space was an issue so I was limited regarding placement and routing. I decided on 8 layers, wheras 4 of them are power planes (all of them are split planes). I designed them like this:

  • Top Layer (Signals in all directions, components)
  • MidLayer1 (Signals in horizontal direction)
  • MidLayer2 (Signals in vertical direction)
  • Power Plane 1 (AGND Plane): AGND1..AGND4
  • Power Plane 2 (POSitive Plane): +5V (digital!), +5VA1..+5VA4
  • Power Plane 3 (NEGative Plane): -5VA1..-5VA4, +3V3
  • Power Plane 4 (DGND Plane): DGND (required in large areas)
  • Bottom Layer (Signals in all directions, components)

Now I wanted to know what would be the "optimum" layer stackup. Signals are not high speed, just some rather low speed analog, relatively precise signals. Often, signals will pass from Top to Bottom (this was required because of the limited space), yet usually signals between OPamps will stay on either top or bottom layer.

Top and Bottom are given, now I was wondering if I should embed the middle layers within power pairs, or if I should put both GND planes into the middle.

Any ideas on this? There is some digital logic on the board as well, but it's mostly low speed communication.

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    \$\begingroup\$ Why is your "bottom layer" not at the bottom? That's going to confuse a lot of people. \$\endgroup\$
    – The Photon
    Commented Sep 5, 2014 at 20:32
  • \$\begingroup\$ Parts of digital and analog flavors on both sides or just one? \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Sep 5, 2014 at 20:32
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    \$\begingroup\$ I've never done more than 4, mostly 2 layer. But it's always the front end/ input that you have to think about. (follow the current.) (Well that's the part I mostly have to redo :^) \$\endgroup\$
    – George Herold
    Commented Sep 5, 2014 at 20:39
  • \$\begingroup\$ Are components Surface Mount Technology, Thru-Hole or both? \$\endgroup\$
    – gbulmer
    Commented Sep 5, 2014 at 20:50
  • \$\begingroup\$ Can you please reorder the list of layers in your list to match how they would actually be in the board, top to bottom? \$\endgroup\$
    – whatsisname
    Commented Sep 6, 2014 at 3:02

1 Answer 1

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The main consideration is that any high speed or sensitive analog tracks should run over an unbroken reference plane. This could be a power plane or a ground plane, but these sensitive tracks shouldn't run over a split in the plane.

Without knowing more about your design I can't say whether you have enough ground planes or if you need to figure out how to use fewer split planes and more ground planes or if you even need to use a 10 layer stack-up.

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  • \$\begingroup\$ Thank your for your input. I know this is pretty hard to answer but I must not post the design. Maybe it helps to give my current thoughts: I would put the AGND and DGND Layers directly next to the Top and Bottom Layer, then the Power planes and in the very middle I'd have the two signal planes. IMHO this would be a stackup which separates much of the analog design from the center. Yet, since the analog components are both on top and bottom, the return current path for the components on bottom is fairly "longer" (4 more planes to pass). The only way I could improve this is adding two more .. \$\endgroup\$
    – Tom L.
    Commented Sep 6, 2014 at 16:12
  • \$\begingroup\$ layers and duplicate the analog ground plane (AGND1..AGND4). The analog grounds are not mixed, the whole PCB is kind of split up into 4 regions from left to right, so it is easy to separate those 4 domains (also split-plane wise) \$\endgroup\$
    – Tom L.
    Commented Sep 6, 2014 at 16:13
  • \$\begingroup\$ If you use AGND as a return path for digital signals, that somewhat defeats the purpose of separating AGND and DGND. It really depends how the signals flow on your design how you should arrange AGND and DGND, and even whether you need these to be separate nets. \$\endgroup\$
    – The Photon
    Commented Sep 6, 2014 at 18:05
  • \$\begingroup\$ Maybe I didn't make that clear. I have one plane for AGND (split into 4 separate planes for AGND1..AGND4) and another plane (completely) for DGND (which is the return current for all digital parts) \$\endgroup\$
    – Tom L.
    Commented Sep 6, 2014 at 22:18
  • \$\begingroup\$ Earlier you said "I would put the AGND and DGND Layers directly next to the Top and Bottom Layer". If you route digital signals on the top layer, this makes AGND the return path (at least for high frequency components) for those signals. If all your digital tracks are on the bottom layer, then you are good. \$\endgroup\$
    – The Photon
    Commented Sep 6, 2014 at 22:21

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