Analog + Digital Ground or just only a general Ground plane in high speed designs?

[Enrique P. (flote21)]

Hello guys,

I have reading some documentation in internet and some people recommend 
to use two different ground planes (AGND + DGND) but other people 
recommend to use just one good GND plane with the digital part and the 
analog part very well placed and differenced.

In my last designs I have been using only one ground plane and I placed 
the analog part and the digital part in the right way.And I got good 
results (I follow the next TI paper: 
http://www.ti.com/lit/an/scaa082/scaa082.pdf),

However, I would like to know which are your experience?

Thanks too much.

[Peter D. (peda)]

I use always separate planes and connect them only on a single point 
(net-tie).
Since planes are also resistors and inductors, like every wire.
So digital current should not flow during the analog area.

[Alexxx (Gast)]

It depends on so much things.
Which digital frequency range you use?
How about disturbances on your ground plane because of connections with 
other devices? (Connection with PC etc.)


Best things you can employ:
- proper layout is paramount!
=> keep trace lengths as short as possible
=> keep sensible / high frequency current flows in the smallest possible 
loop area
=> use series resistance in your digital output lines to reduce slew 
rate of your signals
=> Place high speed and low level analog as far away as possible
=> shield digital signals with surrounding ground lines
=> decouple DC with ferrite beads and special low-impedance ceramic 
capacitors
For low level / high accuracy analog measurements (ADC > 14 Bits) it 
might be best practice to keep analog & digital ground (and power 
supply!)seperated and use optocouplers in between the domains, but this 
introduces time delays of 20ns to µs-seconds.

[Carsten W. (eagle38106)]

Hi!

We do use only single ground planes over a complete layer in our high 
speed designs (> 1 GHz) at work. And it works well. Having split or 
interrupted planes always gives you impedance changes at the gap and 
therefore the signal quality goes down.

And as you wrote: One single ground plane worked well in your last 
design, so why change a running system?

For ripple-pickups in audio circuits, it is a different story.

Cheers,
Carsten

[Peter D. (peda)]

Carsten Wille schrieb:
> For ripple-pickups in audio circuits, it is a different story.

I assumed, exact this was the question.
Many Notebooks, MP3-Players and so one have such problems with audible 
noise because wrong decoupling of the analog GND/VCC and digital signals 
to the analog circuits.

[Carsten W. (eagle38106)]

Peter Dannegger schrieb:
> Carsten Wille schrieb:
>> For ripple-pickups in audio circuits, it is a different story.
>
> I assumed, exact this was the question.
> Many Notebooks, MP3-Players and so one have such problems with audible
> noise because wrong decoupling of the analog GND/VCC and digital signals
> to the analog circuits.

According to the quoted datasheet, it is all about high speed design, 
not audio.

[Enrique (Gast)]

mmmm I see that there are different opnions respecting to this issue. I 
would like to know if there is some kind of gold rule regarding to the 
ground planes which is valid for all kind of designs: video, audio, etc. 
For example, in theory IPHONE(Apple) is a good product and in therory it 
is an example to follow respecting to the PCB design..., Anybody knows 
how is the ground plane here?

[Arc N. (arc)]

Enrique schrieb:
> mmmm I see that there are different opnions respecting to this issue. I
> would like to know if there is some kind of gold rule regarding to the
> ground planes which is valid for all kind of designs: video, audio, etc.
> For example, in theory IPHONE(Apple) is a good product and in therory it
> is an example to follow respecting to the PCB design..., Anybody knows
> how is the ground plane here?

Good question...
The iPhone 4 uses a 10 layer PCB
http://www.eetimes.com/document.asp?doc_id=1259324
and the tear down shows that the audio codec is more or less beneath the 
CPU
https://de.ifixit.com/Teardown/iPhone+4+Teardown/3130

For the iPhone 6
https://de.ifixit.com/Teardown/iPhone+6+Teardown/29213

[R23 (Gast)]

First look at the current flows.