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Imad_RF , 01-18-2024, 02:06 PM

hello everyone, this is my first post in this server!

I'm designing an Audio DSP based on the ADAU1701 DSP, the board contains an XLR balanced input circuit, the DSP, Analog Filters at the output, and a MCU.

I've 3 power supply rails in this project:

+-15V to power up all the analog section, and a 3.3V to power up all the digital section that includes the DSP, a PIC18F MCU, and some low voltage op-amps for buffering only (to feed the aux ADCs of the DSP).

Now regarding the PCB, i'm planning to do the following 6-layers Stack up ( which I need your help in it )

SIG
GND
+-15V POWER PLANE (on same layer)
3.3V POWER PLANE
GND
SIG

is that ok?? is it fine to route the analog power plane over the digital plane? no crosstalk? no noise coupling etc? any other stack up ideas?

thank you very much !

Robert Feranec , 01-18-2024, 07:01 PM

I am not expert for analog, but in the cases where I had to design it (usually just some small audio circuits or simple AD inputs on big digital boards), one solid GND (not separate analog gnd) for everything was working fine (even when I had analogue at the top and digital at the bottom separated by two solid GND planes, I have not seen problems). I believe placement is important (so currents from other circuits do not mix with the currents on the analogue circuit - e.g. I usually place the analogue circuit into board corner), but this is just my experience for simple analog stuff. I hope someone else can give feedback too.

QDrives , 01-18-2024, 10:00 PM

Do you need power planes? https://www.youtube.com/watch?v=kdCJxdR7L_I
If can split the power plane, and have either L3 or L4 as Gnd too, the can use thin dielectrics between L1-L2, L3-L4 and L5-L6.
The way you have the stack now, you would need thin dielectric everywhere EXCEPT a thick one between L3 and L4.
Check out the various videos with Rick Hartley on board stackup.

Imad_RF , 01-18-2024, 10:12 PM

thank you both @Robert Feranec and @QDrives for your feedback!

I've watched almost all videos on Mr.Robert YT channel including the videos with Eric Bogatin and Rick Hartley! they were extremely helpful!.. and yes i'm not splitting the ground planes at all and sectioning my layout keeping the digital circuit far away from my analog, + I've a great power supply filtration (I referred to phil's lab mixed signal design videos).

so Mr. @QDrives ... if I use this stack-up with a thick core PCB or dielectric am I going to be fine? and if I go with your suggestion with L3 for example being ground, where am I supposed to route the 3 power rails? all on L4? this is impossible for me because of the shape of the board and the placement of the connectors as I'm restricted to a special "taplo" design. Actually i'm designing the board to be placed in a powered 2-way speaker.

QDrives , 01-19-2024, 07:28 PM

You want both the signals and power to be close to Gnd. If you keep the stack-up order the way you mention, the "core" between L2 and L3 (and L4 and L5) needs to be thin (0.1...0.2mm), the space between L3 and L4 will be big (~0.6mm).
"where am I supposed to route the 3 power rails?" -- I have never used a power plane. I always route power.

Imad_RF , 01-19-2024, 10:01 PM

thank you very much!