Homebrew_PCBs

I often see ground traces expanding out to cover large areas of
PCBs. I have a half-idea that this is to clean up the ground when
using analog parts, is this correct? If not, whats the actual
reason? How much does it really help?

I can't offer all the reasons, but high frequency stuff will effect
the low frequency stuff. you have 2 choices, physical seperation or
an antenna that will adsorb that HF stuff. Often, you'll see the
circuit board is cut with a physical seperation.

AC on the board can and will effect low voltage ADC circuits and low
voltage logic. putting a ground plane around the lines reducies
noise on those lines.

long traces runnning next to HF stuff will be effected so keeping the
traces sperated 1/4 wavelenght is a minimum, more is better, a ground
inbetween even better.

I put moats around my ADC stuff. the terminals, filters, ADC are all
surrounded by a ground line. My outputs by another, and my power
supply by another and if needed, the micro gets one too.

There are impedance reasons for the ground plane and RF and emf
reasons for a ground ring. Also a ground matrix is sometiems used.
this is a cross hatched area, not just solid copper.

Hope this helps.
and I hope others fill in the blanks.

Dave



--- In Homebrew_PCBs@yahoogroups.com, "ghidera2000
<ghidera2000@y...>" <ghidera2000@y...> wrote:

> I often see ground traces expanding out to cover large areas of
> PCBs. I have a half-idea that this is to clean up the ground when
> using analog parts, is this correct? If not, whats the actual
> reason? How much does it really help?

Thanks Dave, good explanation. I was looking up HF topics and saw how
high frequency tends to make the electrons move only at the of a
conductor. The wider the conductor the more edge it has so the less
impedance it presents to the current. Pretty cool concept really!

I also hadn't thought about using the grounds as shielding against RF
from other traces. Makes sense once pointed out though I guess. I'll
have to try that on a noisy board and see what kind of difference it
makes. It'll make routing an even bigger hell than usual though :X

--- In Homebrew_PCBs@yahoogroups.com, "Dave Mucha <dave_mucha@y...>"
<dave_mucha@y...> wrote:

> I can't offer all the reasons, but high frequency stuff will effect
> the low frequency stuff. you have 2 choices, physical seperation

It makes a world of difference in critical analog circuitry and in
digital circuit to reduce EMC issues. With the ground plane, you
reduce the inductance of the traces, and if you run the numbers, you
and create a controlled impedance trace which is critical for high
speed or RF circuits.

Another reason is to reduce the amount of copper in the waste stream.
I found lots of dead copper in consumer gear years ago in addition to
ground plance, and found out it was a way of reducing cleanup costs.
Important for the homebrewer who treats the waste himself, as well as
the high volume consumer places trying to save microcents.
Ron