Electronic – Help with Metal-Core PCB

in my experience having truly separate AGND and DGND nets almost never works out well in practice. 90% of the designs i see that try to do this end up with current loops that introduce EMI issues and can generate more noise in the analog portions of the circuit than using a single ground with careful part placement would.

Having two GND planes also creates a problem for routing in that signals referenced to a particular ground should only ever be run on layers that are adjacent to this plane or its associate power plane. This can result is a pretty funky stack up that can limit where you can run traces. Your best answer would be AGND,signal,?GND,POWER,signal,DGND but thats funky to layout, uses lots of vias, only gives 2 signal layers to route on.

What i would recommend is a single solid ground plane and careful part placement. High speed digital signals and noise will follow the path of least inductance to ground not the path of least resistance. The path of least inductance is the smallest loop area, for signals this is directly under the trace on the adjacent ground plane. In some cases a ground pour on top, bottom, or both can be helpful in reducing noise pick up as well. This is dependent on the components and the design layout.

Create virtual partitions, keep out areas, where you only run either analog or digital signals, keeping in mind that the return current path for the low frequency analog signals is the path of least resistance, while the return path for the high speed digital signals is the path of least inductance. As long as your careful to ensure that the return current paths don't cross, especially a digital return running under your analog sections. You shouldn't get much noise pick up at all.

If your have a particular device that is very sensitive to noise, such as a high resolution ADC, you can use a ground island to increase noise immunity, like this: alt text http://www.hottconsultants.com/techtips/a-d%20gnd%20plane.gif

In cases where i have some sensitive analog circuitry i will usually also use a power island that is separated from the digital power supply by an LC filter of some sort, depending on the digital frequencies i'm wishing to block.

I'll just comment on the design:

1. Replace C5 with a 100nF ceramic capacitor and place it close to the power supply pin of the MCP6022.
2. Put the designators on the PCB-Design, not values. Make it far easier to understand the layout.
3. Avoid 90° trace bends, they can cause problems when etching the board. They're also bad for high-speed stuff (at least that's the common opinion on the matter). Use two 45° bends instead.
4. Consider flooding one side of the board with a GND-Plane.
5. Use wide short traces for power supply connections.
6. Use one side of the board for mostly vertical traces and the other side for horizontal traces.
7. Take more care of component placement. Place them in a way where they are easier to route. Component placement is 70% of the job. Place them BEFORE starting to route a single trace (Won't always work out). Just use the ratsnest (the lines which indicate connections which are not routed yet) as a rough guideline.
8. Do not see a trace which is already routed as something which is fixed. If its in the way or you don't like they way it looks, rip it up and try again.
9. When in doubt, start from scratch, try not to rescue something which can't be rescued anymore.
10. Rule of thumb: Create something which pleases the eye. Others will have an easier time to understand it and sometimes it will even work better.

There are two Books i can highly recommend for learning Electronics/PCB Design: The Circuit Designer's Companion and EMC for Product Designers. While the second one more about EMC compliance it helps to understand WHY these things should be done in a certain way.