Opto-Isolator – Using Multiple Ground Planes with Optocoupler

As a rule, the ground of the circuit serve as a reference point for all signals. That is why it must have very low voltage difference across the circuit. It is generally not true for the Vcc - as long as no signals are measured relative to the Vcc, some bigger voltage differences are acceptable.

The total current in the Vcc line is equal to the current of the ground line. But because of the above paragraph reasons, the ground line must have much lower resistance (actually impedance) than the Vcc line, where high inductance together with decoupling capacitors is even good.

That is why, the ground is often routed as an area, but Vcc simply as thick enough tracks.

This all was in theory. If your PCB is not dealing with high currents and/or very high frequencies, you probably can route it in whatever manner you want and everything will be OK. So, don't be paranoid.

Footnote: If you have so big empty areas, isn't it better to change the design in order to make the PCB smaller?

Why do you pour all the PCB in the ground plane at the end? I see a lot of people doing that but without any particular reason, the reason is usually "We have better and bigger ground".

I agree with you, you should place as many via as you can when driving 20 Amps because of thermal relief.

Supposing you have GPS antenna on your PCB, keep the trace between the chip and antenna as small as possible, impedance controlled 50 Ohms(you need to have full ground plane under the trace in order to achieve that), do not pour ground around it before you are 100% sure what are you actually doing and put stich vias around the trace as desribed here Via fences for noise reduction of a chip antenna?

Now regarding EMI, keep HF currents off the reference plane. You want reference plane not antenna. Connect all bypass capacitors of a chip together and connect them with reference plane in just one point. Notice I use the term reference plane, not the ground plane! Do not use separate analog and digital ground if you don't need them. You don't need them if your analog circuitry is measured in 12bit resolution or less. If you have more, you don't what to separate them either but take a much more care about current return paths around the amplifier, A/D etc. It is a good practice to draw return current path on the PCB when designing and pour the ground afterwards. For HF, the current will flow on least impedance path, not resistance!

Now:

• Connect the drivers with thermal vias
• Use full ground plane
• Connect bypass capacitors VDD and VSS together and place a just one via connecting them to reference planes(VDD and GND). Additionally, pour the local ground if you have enough space. If you don't, leave them connected with traces.