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Old 26th November 2010, 09:52 PM   #1
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Question From schematic to layout - Am I doing it right?

So far I only have copied layouts from present PCBs, but this is one of my first attempts to create a layout from a schematic. Here I just followed my intuition and tried to create a sort of star ground and get short traces. Differently from the schematic, I included some additional resistor pads for possible paralleling to obtain non-standard values. I also included the possibilty to use a trimmer at the adjust pins of the voltage regulators.

Please have a look and rate it. I'm not really sure how to translate the "parts sequence" of a schematic to a layout. Any hint is highly appreciated. Thanks in advance!

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Last edited by Groundloops; 26th November 2010 at 10:04 PM. Reason: typo and confusing wording
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Old 27th November 2010, 06:57 PM   #2
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Maybe this picture has a better visibility:
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Old 27th November 2010, 07:37 PM   #3
godfrey is offline godfrey  South Africa
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Hi

Tracks shown in red below should be replaced by tracks shown in green.

With the existing layout: Voltage ripple/spikes at the inputs cause current ripple/spikes through C4/5, which causes voltage ripple/spikes on C11/12 due to the shared copper tracks. This is bad as it injects garbage into the "adj" pins of the regulators.

Regards - Godfrey
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Old 27th November 2010, 08:53 PM   #4
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Aye, fully understood! Thank you very much. The rest of the layout is alright?
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Old 27th November 2010, 09:43 PM   #5
godfrey is offline godfrey  South Africa
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Hmm.... You've got a handful of little decoupling caps at the output, but nothing at the input. Maybe shunting C4 and C5 with 0.1uF would be smart? Those regs aren't very good at HF rejection.
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Old 28th November 2010, 02:22 AM   #6
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Not layout related, but you should add diodes to protect the regulator against discharge from the adjust and output caps - see data-sheet.

E.g.

http://tangentsoft.net/elec/tread/misc/sch-v1.1.pdf

(Tangent Regulator, Adjustable, plus Diode bridge)
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Old 28th November 2010, 07:17 AM   #7
marce is offline marce  United Kingdom
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If you are going to star point the best place is the power supply caps, not the middle of the board. Follow some of the current loops and you will see that a lot of paths cross.
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Old 28th November 2010, 03:36 PM   #8
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The star ground should start at the PS input connections. I prefer supplie lines to be wider traces ond common to be twice that width (where possible). I also prefer the most direct route of a connection, meaning abandoning "45s", especially in analog circuitry and most definetly in RF.
Onto the schematic drawing: Do not use X-type connections, only Ts. This is detailed in the ANSI and DIN standards. I know that even the most seasond designers do it, but after a few photo copies you may not see the connection or see one that is not there.
Enjoy pcb design and develop your creative side! E
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Old 28th November 2010, 04:05 PM   #9
AndrewT is offline AndrewT  Scotland
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I disagree with posts 7 & 8 re placing the main ground at the input smoothing caps.

As God said earlier, this is where the worst charging spikes are and you must not contaminate the clean side with this dirty signal.
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regards Andrew T.
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Old 28th November 2010, 07:20 PM   #10
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by mickeymoose View Post
... meaning abandoning "45s", ...
What's a "45"?
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