Re: gEDA-user: pcb, howto partition power planes?

[John Griessen <john@xxxxxxxxxxxxxx>]

DJ Delorie wrote:
>> In my 20+ years in engineering I have yet to see one case where
>> splitting a ground plane under high-speed ADCs has worked.
> 
> What about high precision ADCs?  I'm working on a design using ADE7753
> power monitor chips (16-bit ADCs) , and their own app note (AN564)
> shows a ferrite isolating analog ground, and a 10R resistor isolating
> AVdd.

Ask, "Which common is the one with the lowest ohms to the current source
of power?"   If it is so called "analog ground", then the ferrite will allow noise on the digital chips.
If not, the analog will be the noisy one...not as intended.

All the benefits of shields go away when you're not referring them back to one point, usually the negative terminal of a battery, 
or power supply that your other analog circuits reference.

Putting inductance on all the in or outgoing power wires except grounds, not letting ground wires go out along long wires, and 
connecting the main ground to a conductive box around your circuits will keep them quiet, and maybe even safe from RF.  And the 
star connection from one point concept like Duncan said.

I really have not had any experience where we changed a design a lot and saw the results of separate grounds planes -- I have not 
used them.

Larry Doolittle wrote:

 > I have seen exactly one case where a "split" (very carefully done)
 > on the ground plane was needed to avoid a source of ground return
 > crosstalk.

I'd like to hear more about that, if you will.

Thanks,

John Griessen


-- 
Ecosensory   Austin TX


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