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I have an analog circuit with few opamps (max freq. 500Hz) and a DC-DC converter with 500 kHz frequency for power supply. It seems I need to use both sides of PCB for components and routing if I use 2 layers. Or, I can go for 4 layers with dedicated power and ground plane. I see in most cases dedicated planes are used, or at least one side is used for ground. Do I need a good ground plane in my case? Or would it be sufficient if I use thick trace for power and ground?

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    \$\begingroup\$ The devil is in the detail of the design (and you haven't shown it). \$\endgroup\$ – Andy aka May 14 at 15:13
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    \$\begingroup\$ Generally, you are after low inductance for high frequencies (not resistance which would be for DC and low frequencies). Low inductance means tight current loops and thick traces do not help with that no matter how thick they are. It's also a lot easier to route since pwr/gnd go everywhere and you don't want to be leap frogging traces everywhere. \$\endgroup\$ – DKNguyen May 14 at 15:28
  • \$\begingroup\$ If it's a simple analog circuit and you need components on the bottom, you're trying to make it too small. \$\endgroup\$ – TimWescott May 14 at 17:07
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If you really know what you're doing, and:

  1. you really need to cut down the cost (4 layer PCB is not that much more expensive)
  2. you can afford the extra work needed to route the PCB in two layers
  3. you can afford the risk of a few respins that might be needed to achieve satisfying EMC performance

the ground and power planes are not necessarily needed. I mean, it is possible to make a good design without them, but it is much much more difficult.

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The nice thing about 4 layers is you can have shielding/ground planes that create capacitance to the smaller planes. Since the smaller planes located around a DC to DC converter are usually being switched in some manner, they make excellent antennas.

Another thing that a ground plane does is act as a shield for magnetic fields and provides a path for return currents.

If the design needs to go through FCC regulatory, I would suggest a 4 layer design would be best in almost all cases.

There are some good guides on how to do routing for DC to DC converters, AN139 being one of them.

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