Re: gEDA-user: powermeter board, with less ground planes :-)

[Bob Paddock <bob.paddock@xxxxxxxxx>]

On Friday 31 October 2008 03:43:20 pm Joerg wrote:

> Much better. If you want to be extra good provide another 0.1uF parallel
> to the AVDD caps, close to pin 3. C13 in your channel.sch file. However,
> 10uF cermamics in SMT are already quite good these days, and cheap. If
> you can get C13 closer to pin 3 that would also help.

"A bypass-capacitor dialogue peels back the layers, Part 1"
http://www.planetanalog.com/article/printableArticle.jhtml?articleID=207602816&printable=true

Interesting article that says it is better to use bypass caps in different 
size packages due to ESL effects.

Part 2:
http://www.planetanalog.com/article/printableArticle.jhtml?articleID=208402807&printable=true

...
"David: Follow the currents. There is always a loop for current, there are 
always fringe currents inside the ground plane, and voltage drops associated 
with those currents. The cuts prevent the currents from mingling so they 
could no longer couple directly from Input A to Input B (or the reverse).

Tamara: So this was an example of a ground plane gone wrong. The ground plane 
with cuts in it actually performed better.

David: Yeah, most people believe that providing a low-impedance connection to 
ground (like a plane) is enough. Sometimes that is true, but it's too one 
sided. If you need really high isolation, you need to follow and control the 
currents around the entire loop. "
...

Part 3:
http://www.planetanalog.com/article/printableArticle.jhtml?articleID=209600453&printable=true

Do a search for "Bypass" on Planet Analog for related articles.

Doesn't look like they used PCB in the screen shots. ;-)

> Might want to rotate C11 and squeeze it in between R18/C12 so it's
> closer to pin 4.

When "Squeezing" parts between things, consider what happens at the assembly 
stage.  For example you put a tiny 0402 0.1 cap between to comparatively 
large parts like a 1206 resistor and a large tantalum capacitor.

This could create a problem with the Pick&Place machine where your board
might need two passes on the machine, increasing your assembly charges.

The 0402 cap will need a tiny nozzle to place it and fit between the larger 
parts, while the larger parts need larger nozzle due to the part mass.
If the 0402 part is on the outside of the larger parts then it is possible
that the larger nozzle could be used for all operations in that area,
this saves time hence cost.


-- 
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 http://www.softwaresafety.net/ http://www.designer-iii.com/
                 http://www.unusualresearch.com/


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