Some beginner board layout questions

[Colin Howarth]

Hi,

I've just upgraded Eagle to the non-profit license, so I can now (in
theory do 4-layer boards.


1. If I have a ground layer and I have one part as an analogue ground
plane and another as a digital ground plane (two adjacent rectangles)
how and where should they be connected?


2. Given that I have a ground plane, should I still do copper pouring in
the signal layers (and connect them to ground)?


3. This probably isn't relevant for my first project (guitar-computer
interface: differential signal, 192 kHz, 24 bit ADC with pre-amp and
USB/FW), but which is the cleaner solution for, say, a single signal
crossing: 2 vias with a trace on the bottom signal layer, or a zero ohm
SMD with the other signal passing under it?


4. One of the circuits I am copying/modifying has a 1uF / 63V
electrolytic decoupling capacitor (directly in the signal path) before
any amplification. Wouldn't a plastic (polyethyleneterephthalate) one be
better, from a noise point of view?


5. Is it OK/possible/desirable to place a via on an SMD pad (e.g. for
resistors & capacitors to ground)?


6. I suppose it's better not to route signals underneath opamps (from a
noise point of view)? What if the path then has to become a fair bit
longer? Is there a rule of thumb here?


Thanks for any answers!

colin

 

[Colin Howarth]

Well, you paid for something that you don't own.
Cadsoft can lock you out of your work product.
http://tinyurl.com/TheEAGLE-Virus

You'd better religiously keep a backup
of your work product's **full history**.

In the old days, you could swap EAGLE library files with others.
Do that today and you could easily be screwed.

Proprietary software has gotten to a high rate of suckage.

Hmmmm. That's so odd. They don't come across as an evil company. More
the opposite in fact with their pricing structure, OSX and Linux
support, free "support" and encouragement of hobbyists.

I don't envisage much sharing of libraries myself, other than up and
downloading libs to/from their website, which I suppose should be OK.

That said, thanks for the warning. The story you linked does sound
monstrously unfair.

--colin

 

[Joerg]

Hmmmm. That's so odd. They don't come across as an evil company. ...

They aren't, it's IMHO a great company. In about 5 years I never ran
into any file corruption problems. Just make sure never to use schematic
parts and such from people that loaded a hacked Eagle onto their PCs.

... More
the opposite in fact with their pricing structure, OSX and Linux
support, free "support" and encouragement of hobbyists.

Best of all: No dongles that break off on a bumpy airplane ride and they
let you legally install a 2nd copy for the road on you laptop. Now which
other CAD company can rival that?

As for the ground split I (strongly) second John's opinion: Don't do that.

Caps in series: In the high fidelity audio world ceramics are usually
not the ticket when in the signal path, and neither are electrolytics. A
good film cap is best. At least use a smaller film cap in parallel if
you have to use an electrolytic.

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