Homebrew PCBs

Hello,

Has anyone made a circuit that has something like some digital logic 
amd a pic with some long conductor paths that will drive a fluorescent 
bulb?  Currently I am isolating the AC signal by having an analog 
ground plane that is isolated from the digital ground plane.  There is 
a common ground area in which both planes are connected by a path that 
is at least .50 inches wide.  Has anyone done something similiar and 
how much noise did the bulb create?  Any ideas on how to keep the 
noise from traveling from the AC ground plane to the Digital ground 
plane via the common ground area?  Should I keep the pathway to the 
common ground area to a certain minimum? Any help is appreciated!  
THANKS!

----- Original Message ----- 

From: "J" <wyninejr@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Tuesday, April 04, 2006 6:57 PM
Subject: [Homebrew_PCBs] fluorescent Bulbs


> Hello,
>
> Has anyone made a circuit that has something like some digital logic
> amd a pic with some long conductor paths that will drive a fluorescent
> bulb?  Currently I am isolating the AC signal by having an analog
> ground plane that is isolated from the digital ground plane.  There is
> a common ground area in which both planes are connected by a path that
> is at least .50 inches wide.  Has anyone done something similiar and
> how much noise did the bulb create?  Any ideas on how to keep the
> noise from traveling from the AC ground plane to the Digital ground
> plane via the common ground area?  Should I keep the pathway to the
> common ground area to a certain minimum? Any help is appreciated!
> THANKS!

Try connecting the digital and analogue ground planes only at the supply 
ground.

Leon 

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J wrote:
> Hello,
> 
> Has anyone made a circuit that has something like some digital logic 
> amd a pic with some long conductor paths that will drive a fluorescent 
> bulb?  Currently I am isolating the AC signal by having an analog 
> ground plane that is isolated from the digital ground plane.  There is 
> a common ground area in which both planes are connected by a path that 
> is at least .50 inches wide.  Has anyone done something similiar and 
> how much noise did the bulb create?  Any ideas on how to keep the 
> noise from traveling from the AC ground plane to the Digital ground 
> plane via the common ground area?  Should I keep the pathway to the 
> common ground area to a certain minimum? Any help is appreciated!  
> THANKS!

Your biggest challenge is predicting where the ground currents flow, and 
then try to layout the grounds to minimize voltage differences between 
the analog and digital ground pins at the uC, and simultaneously 
minimize voltage differentials across the analog grounding that lay 
between the ground of the analog signal source and uC analog ground 
pins. From memory when I build my AVR controlled 12VDC powered 18W 
fluorescent switch mode driver circuit, I managed about 5 LSB noise 
(before software filtering) out of the built in 10bit ADC. Its wasn't 
easy (at least for me), and took me three or so PCB designs.

Adam

--- In Homebrew_PCBs@yahoogroups.com, "J" <wyninejr@...> wrote:

............  >Currently I am isolating the AC signal by having an 
analog 
> ground plane that is isolated from the digital ground plane.  There 
is 
> a common ground area in which both planes are connected by a path 
that 
> is at least .50 inches wide.  .......  >Any ideas on how to keep the 
> noise from traveling from the AC ground plane to the Digital ground 
> plane via the common ground area?  ........
>


Texas Instruments have numerous documents on PCB design on their site.
The following artical addresses keeping Analogue and Digital ground 
planes seperated, amongst other points. There could be an answer here.
http://focus.ti.com/lit/an/slyt166/slyt166.pdf