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  On 9/2/2010 10:00 AM, RussellMc wrote:
> [SNIP]

> As Olin notes, you can "island" the processor and feed power current to a=
nd
> from it via a single "highway".

If I read Olin's post correctly, that technique was designed to minimize=20
radiated RF, right?

In the case of a mixed signal board I doubt this would work very well=20
w/o considering other factors; as you point out, Russel, there are=20
tradeoffs. In my case I am using the PIC's analog and PWM features at=20
the same time. Previous to my current design I have never needed a 4=20
layer pcb, but with the first 2 layer layout attempt, the noise coming=20
from the power PWM sections was corrupting the ADC inputs with noise and=20
I couldn't get them to settle down.

I wrestled with this and eventually realized I would need a 4 layer pcb=20
and a very careful layout to achieve the lower noise I needed for the=20
analog measurements I needed to make.

I ran across this:

http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf

....which turned out to be the key to my noise issues.

I made a layout with the two PWM output channels on one edge of the=20
board, analog processing  for each channel  to either side, the PIC in=20
the center of the board, and a line forming a physical barrier between=20
digital and analog circuitry which formed a u-shaped area around the=20
bottom of the pcb for the analog. I placed mixed signal parts, like the=20
pic and adcs, dacs, etc, on the line and routed analog through the=20
U-shaped area and around to the the isolated analog sections to either=20
side of the PWM channels.

(See attachment for a sketch of this...)

I created essentially 1 ground plane (well, it's a bit more complex than=20
that....) and controlled where the return currents were flowing in it by=20
this locating method.

To my delight, after creating this pcb layout, the analog noise problems=20
I was having disappeared.

I found this article extremely useful and I bet there are some others on=20
the PICList who might also be able to profit from it......

Mark






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