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AD7663: PCB layout

Q 

We are now trying to design a PCB for the three AD7633s and associated
circuitry. Our customer has asked us to look at your reference design as
detailed in document, EVAL-AD76XXCBZ. However, I would like to ask you (or your
colleagues) about the peculiarness with the ground planes. From what I can see,
there are four layers, and layer 2 is ground. The only issue I have regarding
your board are the connections to ground from layers 1 and 3, as the multiple
connections from these layers will cause ground looping problems within the
PCB. I'm therefore reluctant to start using this grounding scheme due the
grounding issues. 
Finally, if we were to go forward with this type of grounding scheme, our
customer actually wants to use all 4 layers as ground - I wonder whether you
could comment on this please.

 

A 

The EVAL-AD76XXCBZ has been designed, layout and have been tested for optimum
performance by our product line and layout group. I have a good confidence that
there is no foreseeable issues. And if there is any issues we would have found
it by now (fingers cross).

The EVAL-AD76XXCBZ doc and layout gives a good explanation as to this. See page
3, "power supply and grounding" page 4. Fig 2 to fig 10 give all info wrt to
the board. If you are referring to layer 1 and 3 to top_side_layer and
shield_layer respectively.
As far as I know it, the shielded layer need not be connected to ground, (hence
as the name implies). But you need to double check this, as I do not have the
Gerber files and appropriate cad viewer.

As for your customers request in using all 4 layers of ground will have routing
issues (if it were based from our eval board design). From design and layout
experience, pending on one's design & complexity, i.e. components, speeds etc.
do and will dictate the layers and grounding requirements. I believe for this
EVAL-AD76XXCBZ is sufficient. But for your design which may have more
additional components/complexity and extra parts, could well be differ.
Therefore (maybe) you may in fact need to increase your layers and also
increase your ground layers.

As your client may feel there is an issue as your design may differ from our
ref design as suggested above. Therefore as an independent reviewer. I would
consider the above first before deciding and maybe looking at the option in
increase to 6-8 layers.
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