I'm combining 2 ground planes. Analog and Digital. What potential problems can h

[tdengineer]

AGND AND DGND

BOTH ARE FULL PLANES TAKING UP THE ENTIRE SPACE OF THE BOARD

COLORED OUTLINES ARE JUST WHERE COMPONENTS WILL CONNECT TO THEIR RESPECTIVE GND'S

TOP LEFT WHITE ARROW IS POINTING TO NET TIE'S CONNECTING ANALOG AND DIGITAL GROUND ONLY IN THAT AREA OF THE BOARD



The ic on the top left is my analog power supply stuff, the one on the top right is the digital supply stuff



electrode inputs and array on the left side of board GREEN

MCU connections , spi, digital stuff on right side of the board RED



LAYER 1 = SIGNALS

LAYER 2 = DGND

LAYER 3 = AGND

LAYER 4 = SIGNALS

[uer166]

You'll have no problems combining those planes and stitching them together. You will have lots of problems if you don't do that though, this AGND and DGND rubbish is out-dated advice that had dubious hand-wavey justifications.

Just keep analog bits away from digital on the same GND reference plane for lowest cross-coupling and best return path impedance. The current from DGND won't magically jump into your analog circuitry through the same reference plane.

[thm_w]

https://www.eevblog.com/forum/eda/please-review-my-first-pcb-eeg-analogdigital/
https://www.eevblog.com/forum/projects/how-do-i-make-a-via-connect-ground-1-and-ground-2-planes-without-making-them-the/
https://www.eevblog.com/forum/projects/do-intersecting-spi-signal-return-paths-cause-major-issues-or-do-they-kind-of-ju/

All of these threads are about the same project. No need for new threads.

For return paths you can watch feranecs video, same with ground planes

[jonpaul]

star gnd, single junction of AN, DIG, PWR.

j

[radiolistener]

If AGND and DGND are not routed properly, you can end up with significant noise on the ADC and PLL jitter issues.
I once bought a cheap 14-bit high speed DAC board from AliExpress where the ground layout was done very poorly — clearly by someone with no RF or high-speed layout experience. As a result, the board would ring like a resonant circuit during fast signal transitions, purely due to the ground geometry.