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

[Joerg <joergsch@xxxxxxxxxxxxxxxxxxxxx>]

Bob Paddock wrote:
> 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.
> 

There is a lot of dissent in the analog world about this. Almost as much 
as there is in political discussions right now :-)

I found that a good 0603 cap can carry its weight well into the GHz 
range and at some point the remaining stub plus bond wire wins anyhow.


> 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.
> 

But only on a vintage machine somewhere in a shed that's heated by a 
rickety coal stove ...

Seriously, I've run lots of rather mixed boards and the assemblers never 
had any beef with that.


> 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.
> 

0402 is fairly standard these days, even 0201 is. The next smaller size, 
no that's a different matter where you can go to just any placement shop 
anymore.

And DJ's pick and place machine is a pair of tweezers ;-)

At the end of the day the only thing that counts is whether it's good 
enough and it looks like DJ's board should perform pretty well now. 
Well, at least as long as none of his kids decide to fire up a 500W CB 
amplifier.

-- 
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.



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