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Ground on top and bottom layer

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I was taking a look at a PCB and they covered their top and bottom layer with a ground plane between all the traces. It's a 4 layer board, with a PWR and GND layer (I assume). They also put vias around the perimeter of the whole PCB connecting the ground planes. Is this for electromagnetic compatibility (EMC)? Thermal? Both / neither?

Are there some good web references on the advantages / disadvantages of this approach? Thanks!!
 

ronsimpson

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(4 layer) I like to have the power and ground on the inside for the prototype. This keeps the traces on the outside so I can cut and modify traces.
To keep a board quite (EMC) I like the traces deep inside with power and ground on the outside as a shield. I have made boards where top and bottom are ground and all around the outside edge are VIAs to make a box to keep the noise inside of.

Here is a board I am working on now. I have the red layer turned off so you can't see it. Also the internal power, and ground layers are also turned off. The blue area is local ground for this area. I have some current in this layer that I don't want on the ground layer.
This IC is working at 2ghz. I have tied the local ground and the ground layer together many times impart because at 2ghz two inches of trace is an inductor/antenna. I want a teal good ground with out much inductance in it.
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So a similar local ground protection might be good for small signal analog traces - you have signals routed between the ground sandwich for this board? It makes most sense to me to route ground on top and bottom as a protective layer - if an inner layer has to get butchered for signals, I usually do it on the power layer and leave the ground layer alone.
 

ronsimpson

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In this board I have a radio receiver, and a radio transceiver. Plus a 32bit micro and PWM. + 16 bit ADCs +12bit ADCs.
To keep things from talking to each other:
Each area has a coil in the 3.3V supply. So noise on one supply will not get into another.
I also have a local ground for each area. Each ground is tied into the master ground.
If I have to make this board smaller I will give up the power layer and use it for signals and power.
 

tomizett

Active Member
Presumably a power plane is nearly as good a shield as a ground plane, provided it is well enough decoupled to ground... or is this degree of coupling just not achievable in practice (due to ESR, ESL, self-resonance etc in the capacitors)?
 

ronsimpson

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Many boards, the power and ground are connected together every inch by capacitors. This makes the two layers AC shorted together. Only at DC are they separate.
 
Do you mean because of the number of bypass capacitors in most designs, or if a board had empty space, the designer should add in additional capacitors?
 

ronsimpson

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I was thinking of the bypass capacitors on the ICs.
The two layers of copper will (also) make a capacitor.
 

spec

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Just a word of caution about ground planes (including virtual): While they have many advantages, as already outlined, and in high frequency work they are almost essential, you should always consider that a ground plane introduces a comparatively large capacitance from components to ground. So if you are dealing with high impedance circuits, say a low current operational amplifier or comparator circuit, you may need to clear the ground plane in the area of high value resistors or low value capacitors to ensure that your circuit is stable and has the frequency response you require. You may even need to clear the ground plane under the chip.

spec
 
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