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I looked over the gerbers for Hellen154Hyundai, and as far as I can tell, all four layers have ground as the primary fill. Is there a reason that 5V or 12V isn't used one some layers to get some free capacitance? I thought having ground on neighboring planes was a bad idea as they couple together.

I saw that the middle layers for the analog input, output, mcu modules appear blank. Any reason not to use analog-ground for the analog module, or main ground for the output module?

Or maybe a better question is, what is your philosophy regarding the use of planes and isolating grounds?

For my own boards (which are fairly simple), I have moved to a 3-layer model where the top layer has components and most traces, the middle two layers are both ground, and the bottom layer is power and any extra traces I might need. Sometimes I'll use the alternate JLC stackup that moves the middle layers closer to the outer layers to get better capacitance between them. I have no data demonstrating this is better, but I read on the internet it's important and everyone knows the internet is always right.

ssmith wrote: ↑

Sun Dec 05, 2021 6:43 pm

I saw that the middle layers for the analog input, output, mcu modules appear blank. Any reason not to use analog-ground for the analog module, or main ground for the output module?

Part of the drama could be historical/technological reasons since most/many modules were originally developed for two layer only PCB.

ssmith wrote: ↑

Sun Dec 05, 2021 6:43 pm

get some free capacitance? I thought having ground on neighboring planes was a bad idea as they couple together.

do the math, the capacitance gained is hilariously small

mck1117 wrote: ↑

Sun Dec 05, 2021 7:12 pm

ssmith wrote: ↑

Sun Dec 05, 2021 6:43 pm

get some free capacitance? I thought having ground on neighboring planes was a bad idea as they couple together.

do the math, the capacitance gained is hilariously small

C=kE0A/d, E0 = 8.854e-12F/m
A = 0.0225m^2 (150mm x 150mm)

JLC7628 k=4.6 d=0.0002m capacitance=4.6nF
JLC2313 k=4.05 d=0.0001m capacitance=8nF

Sure it is dwarfed by a 100nF cap, but the plane capacitance is way more ideal than a 0603 or even 0402 bypass cap. It's the same reason you don't just drop a 10uF cap and call it good.

From what I gathered it's the difference between needing to have bypass capacitors near by vs being able to put them further away. Helps with spikes due to high clock speed MCU.