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Old 29th January 2013, 10:13 AM   #1
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Default Some PCB GND layout questions


I'm a beginner regarding PCB layout. My plan is to learn about PCB design by making a small amp with IC like LM1875 or equivalent.

What is the best way for ground layot? I made 2 examples for the powersupply part and was wondering which is best. Also where do I connect small signal ground? To one of the existing ground lines or do I have to run a seperate one to the GND connector?

Tips would be highly appreciated..
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Old 30th January 2013, 05:57 AM   #2
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I like your second example better than the first. The central vertical trace establishes a single ground for both sides of the supply. In your first example, the two sides of the supply can have different "ground" reference potentials at the IC.

In general you will separate signal ground from power ground and connect them at only one point in the circuit - sometimes called the "star ground" point. The Application Notes for your particular IC, and any threads about it on this Forum, may have suggested examples of how to do this.

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Old 30th January 2013, 07:58 AM   #3
CBS240 is offline CBS240  United States
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It is also worth having a simple CRC filter, or CRCCcc. The key is to have seperate return current paths for the first 'C' than those for the second 'C', the one that actually powers the amplifier. The GND return for the second 'C' would be where the speaker and decoupling would go. The center tap of the transformer would be at the junction of the first set of caps, and then a single seperate GND connection to the other set of caps and the main star GND for the amplifier circuit. This keeps the pulse currents from the 60Hz (120Hz) power input AC seperate from the GND return currents from the amplifier driving the speaker.
All the trouble I've ever been in started out as fun......
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Old 30th January 2013, 10:54 AM   #4
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Thanks for the replies

Very useful information. I have added some traces to the PCB, hope this is OK.
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Old 6th April 2013, 07:09 AM   #5
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I got some time to practice my PCB layout skills and came up with this design. It is a LM1875 amp on steroids, with 2 extra transistors.

Any recommendations regarding the layout would be highly appreciated.
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Old 6th April 2013, 08:14 AM   #6
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If I had to use dual layer pcb for that, I probably would consider to make every trace on one layer and reserv the other one completely as a groundplane???
Then even put a capacitor bank on the same pcb...
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Old 6th April 2013, 08:19 AM   #7
AndrewT is offline AndrewT  Scotland
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post4 is better.
Can you remove or reduce the loop areas of the +ve & return?
Same for loop of -ve and return.

Remember that in EVERY circuit there is a source voltage that drives the current out, or away, in the FLOW route and that current MUST come back in the RETURN route.
The loop formed by that flow and return MUST be kept small.
regards Andrew T.
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Old 6th April 2013, 12:51 PM   #8
alex mm is offline alex mm  Romania
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....... single side PCB without steroids and power supply , only schematic from data catalog ......
Regards Alex.
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Old 6th April 2013, 01:27 PM   #9
00940 is offline 00940  Belgium
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The big problem I see with the layout in post 5 is that the small sensitive groundplane in blue is tied to the AC ground terminal coming from the transformer. It's the worst place to connect it, as heavy charging currents run through that point. Put that groundplane back to the "green layer" and simply tie it to the main groundplane.

Even with a groundplane, it is good to think in term of a power bus when laying things down physically.

edit: CBS240 already gave a detailed explanation on the reasons why.

Last edited by 00940; 6th April 2013 at 01:30 PM.
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Old 6th April 2013, 02:01 PM   #10
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Thanks for the tips.

So now I changed the ground and power layout as a starting point.

And tie the sensitive signal ground plane to the top part of the ground plane near the small 100n capacitors, right?
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