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 Forum index » DIY Hardware and Software » Developers' Corner
PCB GND planes layout advices needed...
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jpdesroc



Joined: Dec 05, 2007
Posts: 53
Location: Canada

PostPosted: Mon Dec 07, 2009 12:48 pm    Post subject: PCB GND planes layout advices needed... Reply with quote  Mark this post and the followings unread

I need your advice on the following:
I just completed the PCB layout of my new polyphonic (6 voices) MIDI2CV dotcom type module.
It's a 7" X 8" 2 layers PCB.
I'm using a power connector with the following specs:

Molex 0.156" spacing
Pin# name
1 -15VDC
2 ANALOG GND
3 ANALOG GND
4 +15VDC
5 DIGITAL GND
6 +5VDC

Both ANALOG & DIGITAL GND's are wired separately until they reach
the remote power supply using different wire paths.
There (and only there), they are wired together.

The board includes analog IC's (opamps, DAC's, transistors, etc..)
and digital IC's (PIC microcontroller, gates, opto, etc..)
Now that my OrCAD soft has helped me place
both layer traces and since the ANALOG & DIGITAL grounds are 2 separate nets,
I'm doing a 'copper pour' on the TOP side with
the ANALOG GND net and a 'copper pour' on the BOTTOM side using the DIGITAL GND net.
That's what are normally called GND planes..
I know the actual TOP layer ANALOG GND generated plane will
help a lot getting rid of noise all around the board, my question is:

Is it a good idea to generate a DIGITAL GND plane on the BOTTOM side too
or leave the DIGITAL GND paths not covering most of the BOT surface???
(Remember ANALOG & DIGITAL GND'S are NOT wired together on the board)

Thanks
JP
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DrJustice



Joined: Sep 13, 2004
Posts: 2114
Location: Morokulien
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PostPosted: Mon Dec 07, 2009 7:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

The method I'm most familiar with is using a "split ground plane", which means having all the digital circuitry in one area with a ground plane of its own, and the analogue circuitry in another area with its own ground plane. The adjacent ground planes are then connected at only one point. Some mixed signal chips may (require to) be mounted across the two planes; the chip manufacturer may have a recommended layout in these cases. In complex constructions there could be more than two areas. You can find many articles about split ground planes on the net. You can also have the analog an digital circuitry in different areas without a split in the ground plane. Both methods have their use. I'm not an expert on any of this, so consult literature or others for more detail.

DJ
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fretless



Joined: Nov 29, 2009
Posts: 19
Location: Boston

PostPosted: Thu Dec 10, 2009 5:38 pm    Post subject: Re: PCB GND planes layout advices needed... Reply with quote  Mark this post and the followings unread

-----------
Is it a good idea to generate a DIGITAL GND plane on the BOTTOM side too
or leave the DIGITAL GND paths not covering most of the BOT surface???
(Remember ANALOG & DIGITAL GND'S are NOT wired together on the board)
------------

The two ground planes will act like a capacitor. Spike noise on the digital has lots of frequency so something will couple across. Better to keep D and A ground planes on the same side of the board. That way they will not share the same area, current loops are in their own places.
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jpdesroc



Joined: Dec 05, 2007
Posts: 53
Location: Canada

PostPosted: Fri Dec 11, 2009 7:07 am    Post subject: PCB GND planes layout advices needed... followup Reply with quote  Mark this post and the followings unread

Thanks all for your good advices!

I see that effectively stray capacitive effect
on stacked PCB layers is a serious issue
between analog & digital circuitry..

One thing I forgot to mention is
my PCB's DIGITAL area occupies around 20%
of total area and is wired all in the same space.
The remaining area is ANALOG stuff.

According to what I've read so far I'm thinking of
doing copper pour on both sides of DIGITAL area
and only TOP copper pour on ANALOG areas.

What do you think ?

Remember, both DIGITAL & ANALOG gnd's
are separated and wired together ONLY at power supply.
JP
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fretless



Joined: Nov 29, 2009
Posts: 19
Location: Boston

PostPosted: Fri Dec 11, 2009 8:07 am    Post subject: Re: PCB GND planes layout advices needed... followup Reply with quote  Mark this post and the followings unread

jpdesroc wrote:


According to what I've read so far I'm thinking of
doing copper pour on both sides of DIGITAL area
and only TOP copper pour on ANALOG areas.

JP


One ground plane does a lot of work. Two is just incremental. Remember, the two ground planes are only connected together at a few points, so it still acts like a cap. The return currents follow the shortest path, so if it is really a plane then the path is a straight line. If it so happens that power is a straight line too then the magnetic loop is just two straight lines and the thickness of the PCB. For digital, why not try a ground plane and a power plane? Anyway, what clock rate are we talking about here?
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jpdesroc



Joined: Dec 05, 2007
Posts: 53
Location: Canada

PostPosted: Fri Dec 11, 2009 9:14 am    Post subject: PCB GND planes layout advices needed... Reply with quote  Mark this post and the followings unread

The clock of the PIC microcontroler is 20mhz.

So maybe 2 separate TOP ground planes
for each ANALOG & DIGITAL areaas would do. ?
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fretless



Joined: Nov 29, 2009
Posts: 19
Location: Boston

PostPosted: Fri Dec 11, 2009 9:32 am    Post subject: Re: PCB GND planes layout advices needed... Reply with quote  Mark this post and the followings unread

jpdesroc wrote:
The clock of the PIC microcontroler is 20mhz.

So maybe 2 separate TOP ground planes
for each ANALOG & DIGITAL areaas would do. ?


Yes. The analog supply can also be filtered locally with a series inductor, ferrite bead or RC filter. These types of power supply filters compensate for the slope of PSRR of op amps.
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