Right; in my design the AVSS and AVDD pins are on the side of the PIC=20
oriented towards the analog area. The pins go to their own respective=20
filter caps, then from there to the power and gnd plane connections in=20
the analog area of the PCB.

I never thought of/tried  using an inductor in the power supply net;=20
that'll be something I keep in mind for the future.

I didn't use the "patch" technique for the crystal (never thought about=20
that before), but just kept the loops as short as possible, and over the=20
"digital" area of the ground plane.

How wide did you make the connection between the "patch" ground under=20
the PIC and the regular ground plane? I am guessing just not making it=20
real skinny is the main thing.....

Mark Skeels
Engineer
Competition Electronics, Inc.
TEL: 815-874-8001
FAX: 815-874-8181
www.competitionelectronics.com

On 9/2/2010 2:09 PM, Olin Lathrop wrote:
> Mark E. Skeels wrote:
>> If I read Olin's post correctly, that technique was designed to
>> minimize radiated RF, right?
> That's one part of it.  Keeping the high frequency currents off the groun=
d
> plane prevents them from generating offset voltages there, which is noise
> added to analog signals.  Remember that the ground plane impedence goes u=
p
> with frequency.  While it may be a dead short for practical purposes at D=
C,
> you can't assume that's true at 100s of MHz.
>
>> In the case of a mixed signal board I doubt this would work very well
>> w/o considering other factors; as you point out, Russel, there are
>> tradeoffs. In my case I am using the PIC's analog and PWM features at
>> the same time.
> This is a good example where you want to use the AVss and AVdd PIC pins
> properly.  If using a local ground patch under the PIC, you still connect
> the AVss pins to the main ground plane.  They don't carry much current an=
d
> therefore won't inject much noise on the main ground plane, and the A/D w=
ill
> use the true zero volts of the main ground plane as its reference.
> Similarly, AVdd is Vdd filtered by a chip inductor in series and a capaci=
tor
> to ground.
>
>
> ********************************************************************
> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
> (978) 742-9014.  Gold level PIC consultants since 2000.
--=20
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