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I have been designing boards (2-6 layer). After the complete layout I usually pour a GND in the end all over the board. What are the rules governing ground pours ? In my 1st board I have 2 BLDC motors taking in about 16-20 amps. So having a gnd pour with vias stitched help in heat dissipation. But my new board consumes only about 7mApms and it has a GPS. Will a ground pour affect the performance of this GPS ? Is my understanding of ground pours clear ? Or is there any other advantage of ground pours ?

I understand that I should isolate the noisy Ground from the digital gnd. Is it ok if I route the noisy ground and digital ground seperate and then stitch them at a single point via a 0R resistor or a ferrite bead ? Is this a good design practise ?

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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.
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  1. In your case the ground plane will serve as a direct return path for the signals. This is a very good practice regarding EMC issues.
  2. Now for the custard tart about splitting ground plane: I no more doing this. Just keep analog signals far from digital ones and keep in mind that the current will ALWAYS choose the shortest path.
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  • \$\begingroup\$ "the current will ALWAYS choose the shortest path" - Really? \$\endgroup\$ – Roger Rowland Jun 1 '15 at 7:03
  • \$\begingroup\$ Current does NOT always follow the shortest path. It follows the path of least inductance. Low frequency signals spread out more, so its like as if its shortest path but high frequency signals, the return path follows the signal path. \$\endgroup\$ – efox29 Jun 1 '15 at 7:20
  • \$\begingroup\$ Ok. Let me explain some more: with the raise of frequency, the return path act more and more like a micro-strip with the ground plane. At DC, the current takes the shortest resistance of the ground plane. Then finally, that's what I say: a ground plane has a resistance and inductance density (eg ohm/m or H/m), given this, the current will go to the shortest path. But ok, at DC it will spread a bit (like in high frequency ;-) ) \$\endgroup\$ – papyDoctor Jun 1 '15 at 7:27
  • \$\begingroup\$ Thank you for all the answers. But can anyone elaborate the actual importance of ground pours ? When is it an advantage to add gnd pours and when not ? Is there a thumb rule? Another doubt. Why are split gnd planes not advised ? There are quite a number of opinions on this. What is the actual thumb rule ? \$\endgroup\$ – Board-Man Jun 1 '15 at 11:00
  • \$\begingroup\$ @efox29 ..Low frequency signals spread out more, so its like as if its shortest path but high frequency signals, the return path follows the signal path. Could you kindly elaborate this statement please. \$\endgroup\$ – Board-Man Jun 1 '15 at 14:49

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