Advice on PCB layout

Community PCB Design
1

Hi all,

I am relatively new to electronics and I think I have figured out the “making a functional PCB proto” part. Now, what I want to learn is “how to make the best possible PCB from a particular schematics”. It’s hard to find information about that - what are the best practices to minimize noise for example. The project I am working on is a monophonic MIDI synth. I have tried to divide clearly my PCB into 4 sections: power supply, MIDI in/out, digital processing, and analog audio processing. The current PCB is here:

http://dl.dropbox.com/u/612135/PCBs/main_board.png

My questions are:

  • I used longer traces (16 mil) in the power supply up to the most power-hungry component (the molex connector on which I connect a back-lit LCD), then 8 mil since the rest of this path doesn’t take more than 100mA. Does it make sense?

  • I have distinct ground planes for each section. What I want to avoid is getting digital noise in the audio part for example. Will it really help? Which other things should I take care of to avoid getting digital artifacts (like the “buzzing” of a clock signal toggled at a frequency in the audio range…) in the audio path? Should there be only one small trace between the ground planes or is it OK to have many as in the current design?

  • I have tried to keep the ground-planes as “connected” as possible, even if sometimes the traces on the top layer are more convoluted. What are the benefits of having a “well-connected” ground plane?

Thanks in advance for your help!

2

8 mil for standard trace width is on the small size, especially for an uncrowded and large board such as yours. I’d go no smaller than 10 mils.

Split ground traces can be problematic. This is a good design manual for higher frequency digital combined with analog:

http://www.sparkfun.com/datasheets/Brea … design.pdf

3

theatrus:
8 mil for standard trace width is on the small size, especially for an uncrowded and large board such as yours. I’d go no smaller than 10 mils.

Split ground traces can be problematic. This is a good design manual for higher frequency digital combined with analog:

http://www.sparkfun.com/datasheets/Brea … design.pdf

Thanks! I have switched to 10mils. What do you mean “split ground traces”? Having two ground planes? Or having two ground planes with multiple “bridges” between them?

4

Unless you are working in a fully isolated setup, your ground planes better have connections, or things are not going to work correctly :shock:

5

theatrus:
Unless you are working in a fully isolated setup, your ground planes better have connections, or things are not going to work correctly :shock:

Sorry for not being clear… When I meant “two ground planes” I meant “two large ground planes connected by a thin trace”.

6

I often find it to be useful to place a component, such as a SMT or a through-hole inductor or low value resistor to connect the split grounds. This way, one can use something as basic as a small inductor/chip bead or low ohmic resistor to tie the two grounds together for a simplistic, but adjustable filtering scheme. Having the pads there and available for multipurpose useage is handy. If you don’t end up needing any filtering - which could be the case- you can simply put in a short piece of jumper wire (which will still have a tiny bit of inductance at very high frequencies).

7

ftsolutions:
I often find it to be useful to place a component, such as a SMT or a through-hole inductor or low value resistor to connect the split grounds. This way, one can use something as basic as a small inductor/chip bead or low ohmic resistor to tie the two grounds together for a simplistic, but adjustable filtering scheme. Having the pads there and available for multipurpose useage is handy. If you don’t end up needing any filtering - which could be the case- you can simply put in a short piece of jumper wire (which will still have a tiny bit of inductance at very high frequencies).

Thanks! Sounds like a good idea!