FW: [sdiy] PCB GND planes layout advices needed...

[Jerry Gray-Eskue]

-----Original Message-----
From: Jerry Gray-Eskue [mailto:jerryge at cableone.net]
Sent: Monday, December 07, 2009 9:33 AM
To: jpdesroc at oricom.ca
Subject: RE: [sdiy] PCB GND planes layout advices needed...


Jean-Pierre,

I see that you have two pins for ANALOG GND, what I would do in this case is dedicate one of the pins to "Quite Ground" and use that for the copper pour on both top and bottom. The quite ground should carry no current analog or digital and have a very low impedance to your ground "Star Point Connection". This will act as a classic ground shield design. 

I would hesitate to use Digital and Analog ground planes on opposite sides of the board, this forms a low value capacitor and spikes may transfer to a degree between the ground planes. 

Another common practice is to place all the digital and analog in separate sections of the board and use the corresponding ground for ground planes in each area. 

Some of the most important design considerations for quite boards are using significantly larger tracks for power and ground traces, and spacing sensitive analog paths further from other tracks. The greater the gap between traces, the less capacitive and emf coupling of signal transitions. You can also use Guard Ring/Traces for sensitive analog traces and pin connections. In cases where I have a major noise source such as the system clock I may use a Digital ground guard trace to limit its radiated energy.

- Jerry

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Jean-Pierre
Desrochers
Sent: Monday, December 07, 2009 8:07 AM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] PCB GND planes layout advices needed...



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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