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Old 11th May 2006, 11:28 AM   #1
Wynand is offline Wynand  South Africa
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Default Tips for routing PCBs - please add some

I thought it would be nice if some of the more experienced builders could lend some help to the newcomers in the DIY field.

Since more beginners would be starting with Chipamps than with Discrete or Class-D I thought putting this thread here would be the right place.

I've tried one or two layouts of my own. And the oscilated. So I'm a bit intimidated trying my own boards again. In the meantime I've bought some audiosector PCB and they're both running well.

Some tips would be rather nice.

For instance:

Does it make a difference when two traces cross each other (via wire link or double sided pcb) at 90 degrees as opposed to 45 degrees.

Common reasons for oscilations.

Which components too keep away from each other.

and/or component placement.

Maybe a wiki page would work for this kind of thread.
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Old 11th May 2006, 11:43 AM   #2
Wynand is offline Wynand  South Africa
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Default First one

One thing that I've learnt is about by-passing op-amps. 100nF is a commonly suggested value one between +V and GND and another between -V and GND. Placement of these caps should be as close to their respective voltage pins as possible.
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Old 11th May 2006, 12:58 PM   #3
Mad_K is offline Mad_K  Norway
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Your best tool is to understand how the circuit works. Think about where signal and power currents are flowing (and their return; the dreaded ground). The rest of the technicalities with PCB design comes with experience. Read up on how to do high speed pcb design , and if you apply this to audio circuits you will never have trouble with oscillating circuits.

Here's a nice, free book:

http://focus.ti.com/lit/an/slod006b/slod006b.pdf

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Old 12th May 2006, 03:43 PM   #4
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I agree...learning high-speed digital layout techniques will help in analog PCB design.

Good habits:

1. Only use 45 degree and 90 degree traces...they make your board look much more professional. Same goes on corners, always use 2 45 degree angle bends rather than a single right angle corner.

2. If star grounding is not practical on your board size or layout, make sure to leave a large via at any ground-reference pins of an opamp (for instance, the non-inverting input on an opamp that is tied to ground for an inverting configuration). That way, you can manually run a wire to a common star ground and avoid hiss.

3. Lay out components before routing traces and avoid autorouter and rat's nest tools. Placing the components first will allow you to visualize where all the traces are going.

4. Use all available board area! Always add in spaces for components that may be optional or that will allow you to experiment. Place random vias on certain traces that will allow you to use capacitors and resistors later on...

5. Surface-mount is your friend. It may take some practice, but 0805 and 0603 devices are easily hand-soldered. Using these sizes will make layouts CONSIDERABLY more compact.

6. Oscillations are often caused by a long feedback loop that has a lot of parastic L and C. Always try to place feedback components as close the chip's pins as possible.

7. By 1000 0603 size .1 uF capacitors. This will cost only a few dollars and they will last you forever. Then, simply stick some pads near the +/- supply pins of all chips for local bypassing.

8. CHECK AND DOUBLECHECK! You will ALWAYS have a few errors...

9. Use ground planes, they help. And keep digital and analog ground planes separate and connected at one point. Same goes for analog and digital power supply rails.

10. Place resistors, etc in rows rather than scattering them around the board. Suprisingly, your layout will be easier and will look nicer.

There are a million more, but you can only learn them by experience and experimentation!
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Old 12th May 2006, 05:49 PM   #5
VEC7OR is offline VEC7OR  Lithuania
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Try to route signal traces without jumpers and in shortest possible way, then route power then everything else.

I would say 1206 and 0805 are better for hand soldering, besides it easier to stick a trace under one of these (well its possible even with 0603, but I find it extreme).

I suggest to buy 1206/805/0603 sizes for caps, in this case you can stick those almost anywhere you want. (Personally I have bought 400 of 805 ant those really help out)

I fully agree on check and double check rule.

If you work with something that has 'digital' written on it it must be well decoupled or trouble is imminent.

Also use copper fill (connect to ground where possible), it will save etchant and reduces thermal strain on board during soldering (try tinning the whole board you'll understand that its all about, also boards dont warp too much when wave soldered)
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Old 12th May 2006, 09:48 PM   #6
ingrast is offline ingrast  Uruguay
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Agree with most posts.

I use to lay out parts with due attention to the actual schematics layout, that makes troubleshooting / developement much easier.

I rather like to run the ground and power rails first so as to keep them stright and short. Make themwider than signal traces.

Before actually routing, take your time juggling components around (to keep resistors paralell is a neat idea and works most of the time). Look for an overall pattern of short starlike traces rather than a dizzying convolute pattern of spiraling routes.

Try to use an integrated schematics capture / pcb layout package like Eagle (free edition with more than enough capabilities for most of our needs). This will take care of layout errors and the rats nest presentation helps sorting out placement as suggested in the previous paragraph.

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Old 13th May 2006, 04:27 PM   #7
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I think that Cadsoft Eagle is an awesome piece of software for creating PCBs. http://www.cadsoft.de
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Old 14th May 2006, 07:56 AM   #8
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Some very good replies so far!

Some oscillations can also be cured by limiting bandwidth, shunting HF through the feedback loop or at the input to the gain stage IC. Raising the gain can also help but isn't always reasonable.

If you wanted more specific suggestions about the circuits you had problems with, post the schematics and/or reasonably good pictures of both sides of the build.
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Old 15th May 2006, 07:42 AM   #9
Wynand is offline Wynand  South Africa
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This is marvelous responses. BTW Its not a specific problem that I've personally have at the moment. I just thought these tips will help all us newbies.

Please keep them coming, it's looking great.

Thanks
Wynand.
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Old 15th May 2006, 08:16 AM   #10
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Just to reiterate the above posts, component layout is everything. I quite often place the components, and lay tracks, then rip most of it up, move the components around to optimise the layout, route again, then repeat as required. Sometimes it takes five or six run throughs to get something decent.

Another tip. Keep heat sensitive components like input trannies and electrolytics away from heat generating components like big resistors or output devices wherever possible.

And, a minor thing, but it helps a lot in troubleshooting, place all component names next to the components, so you can read them once the PCB is populated, rather than underneath.
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