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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.

Tom L.
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    Why is your "bottom layer" not at the bottom? That's going to confuse a lot of people. – The Photon Sep 05 '14 at 20:32
  • Parts of digital and analog flavors on both sides or just one? – Spehro Pefhany Sep 05 '14 at 20:32
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    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 :^) – George Herold Sep 05 '14 at 20:39
  • Are components Surface Mount Technology, Thru-Hole or both? – gbulmer Sep 05 '14 at 20:50
  • Can you please reorder the list of layers in your list to match how they would actually be in the board, top to bottom? – whatsisname Sep 06 '14 at 03:02
  • @ThePhoton: Ahem, well, in fact the question is: How should I order these layers in the stack. I didn't want to direct you into any directions since I wanted unbiased opinions. But, I will put the bottom layer to the bottom. – Tom L. Sep 06 '14 at 08:09
  • @SpehroPefhany: Most of the board is analog, with only some small areas (mostly on the border) being in the digital domain. The domains are separated by ADuM(s). – Tom L. Sep 06 '14 at 08:10
  • @gbulmer: The whole board is SMT, just a single connector at the very front is a THR part. – Tom L. Sep 06 '14 at 08:13
  • @whatsisname: That is the question :-) I am looking for an optimum layout (I know it is hard with not seeing the board, it's just about getting some ideas; I would like to know your thoughts on what you would take care of) – Tom L. Sep 06 '14 at 08:14

1 Answers1

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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.

The Photon
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  • 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 .. – Tom L. Sep 06 '14 at 16:12
  • 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) – Tom L. Sep 06 '14 at 16:13
  • 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. – The Photon Sep 06 '14 at 18:05
  • 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) – Tom L. Sep 06 '14 at 22:18
  • 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. – The Photon Sep 06 '14 at 22:21
  • Putting all analog on the top side and all digital on the bottom side can be an effective way to segregate the two parts of the circuit. But until now you haven't said you're doing that. Really I don't think it's very productive to try to discuss this given the limited amount of information you're able to share about your circuit. – The Photon Sep 06 '14 at 22:23