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PC Board 0V layout

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 14 August, 2009 » PC Board 0V layout « Previous Next »

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muskrat
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Username: muskrat

Post Number: 35
Registered: 06-2009

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Posted on Sunday, 09 August, 2009 - 08:41 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

When producing PC boards, which I do using Positiv 20, I have a fairly large copper area at the board edges which I connect to 0V.

Two reasons: The first is that it's easier to connect 0V tracks to the board edge rather than run tracks all over the place connecting 0V.

The other reason is cosmetic. Often, there is a build-up of photo-resist at the board edges which is too thick to react properly to the UV light, leaving "dirty" edges.

Now for the question .....

Is this acceptable practise?

I recently completed a fairly complex project which, as always, grew in its complexity as the project progressed. At the end of the job, there were several smaller boards added to the original project. One was a digital counter using CMOS logic.

I had used decoupling caps all over the place, but still got spurious counts occurring at odd times. I solved the problems by ensuring that all inputs were filtered until I finally had the thing working.

I couldn't help wondering if my 0V scheme was, in some way, a part of the problem.

The project employs analog, CMOS digital and PIC technology.

Comments would be appreciated .....
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green
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Username: green

Post Number: 87
Registered: 03-2009


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Posted on Sunday, 09 August, 2009 - 08:55 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Multiple earth paths can often cause problems on PCB's . The solution is to make sure that there is only one earth path back to the input of highest sensitivity.
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muskrat
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Username: muskrat

Post Number: 37
Registered: 06-2009

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Posted on Sunday, 09 August, 2009 - 09:02 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

So does that mean I should not use the large "earth plane" surrounding my board?

Should I make a gap in it at some point, or should I leave it and ensure that all board 0V wiring comes back to a central point?
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green
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Username: green

Post Number: 89
Registered: 03-2009


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Posted on Sunday, 09 August, 2009 - 09:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You can have as large an earth plane as you wish, providing it solves the problem. If you still have a noise problem on a high sensitivity input, then keep all connections as short as possible, screened, if possible and take them back to a common earth point close to the input. Concerning screened connections, connect the screen only at the end closest to the input, or else you introduce a multiple earth path. Use internal conductors for power supply returns. I wonder what the experts would say about this ?.

The idea of the gap is worth trying with difficult problems because it equates to the concept of a Faraday shield.

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