Question
Please can you clarify the layout requirements for one of your parts? I cannot
resolve the various requirements.
The part in question is the AD73360 Analog Front End, and the question concerns
grounds and power. Page 26 of the data sheet gives quite a lot of information
that, in most circumstances, I think I would be sufficient. The probelm occurs
when a single supply is used for AVDD and DVDD.
To quote the data sheet, page 26, "In systems where a common supply voltage is
used to drive both the AVDD and DVDD of the AD73360, it is recommended that the
systemâ€
s AVDD supply be used." Then decoupling is discussed. In this case,
do the discussions about separate ground in the previous paragraphs still
apply? If the grounds are still separate, then AVDD will overlap both planes -
not good practice. Or should AGND be used for both supplies as suggested in
your tutorial MT-031?
To add to my confusion, the data sheet also states "The analog ground plane
should be allowed to run under the AD73360 to avoid noise coupling" even though
figure 29 above this shows the split ground under the device.
Answer
In the case where a single supply (the system analog supply) is used for both
AVDD and DVDD, a single ground (the system analog ground plane) should be used
and the AVDD and DVDD supplies should be decoupled to it.
In the special case of evaluation boards, there is a slight performance
advantage obtained by using separate supplies for AVDD and DVDD, since both of
these supplies will be relatively quiet. However in the case of a real system,
where the Digital system supply and ground plane are shared with processors,
logic, digital IO etc, the optimum strategy is to power the AD73360 from a
single quiet supply and to us a single quiet ground plane.
Our best reference regarding grounding mixed-signal components is the Mixed
signal seminar, which can be found on the following web page.
http://www.analog.com/media/en/training-seminars/design-handbooks/MixedSignal_Se
ct10.pdf
Pages 23 - 31 explain the apparent contradiction when the eval board shows one
grounding strategy wheras the datasheet recommends another for the customers
system.