List,

I am about to attempt my first 4 layer pcb.

The design is a dual channel PWM power apparatus.

Switching frequency is ~60 KHz.

I'm using a PIC24H at 40 MHz internal, 8 MHz external clock oscillator.

After that, the worst digital stuff on the PCB is the LCD communication, 
or the I2C.

Can't say too much more about that. :-)

Noise and grounding issues associated with a mixed signal design like 
this, coupled with a denser-than-ever-before layout requirement have 
driven me to consider the advantages of a 4 layer pcb; particularly the 
better grounding capabilities achieved by isolating return current flows 
through analog/digital circuitry placement isolation and use of a ground 
plane, as described by Henry Ott and others.

http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf

I plan to use a single ground plane and isolate power, digital and 
analog portions of the circuit to their own areas, being careful to 
route digital and analog traces over isolated, separate areas of the 
ground plane.

The main objectives are to quiet ADC circuitry and to a lesser degree, 
to enhance noise immunity. On the existing 2 layer prototype, the PIC 
can be reset at times when higher output currents are generated.

The power plane will be partly VCC and other power supply distribution 
and partly high current switching signals, isolated to their own end of 
the PCB.

Ok, so my question is about the stack up order.

I would have thought this was the best for my needs:

1. Signal #1
2. Ground
3. Signal #2
4. Power/VCC.

In my mind, it places the two signal layers in the closest proximity to 
the ground plane, and since I have reasonably low (relatively speaking) 
intentional RF type activity on the PCB, I am most concerned with 
isolating the digital and analog return currents and separating digital 
and analog areas of the PCB, and not too concerned about what I 
understand as the advantageous capacitive or shielding actions of having 
the ground plane/power plane interaction.

However, I see nobody doing this.

Instead, I see the most frequently recommended stack up order is

1. Signal #1
2. Ground
3  Power/VCC
4. Signal #2

Even my PCB program (Altium Designer) defaults to this configuration.

I have also seen a few other stack-ups but by placing the planes on the 
outside, (for example) they force the breakup of the plane for component 
connections, which I see as not very useful for me, all things considered.

Does anybody have any input or advice on this? Thanks?

-- 
Mark Skeels

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