PCB ground layer design

[alireza7]

hi
consider a pcb with fpga microcontroller  adc and dac and other parts 

how to design ground  layer ?
just a complete plane in ground layer ?
or it's better to route ground trace of each component separately from the whole pcb input ground to the corresponding component so the ground traces are joint in just one initial point?

[dmills]

Solid plane in the first instance, it is far more likely to be right then anything else you might try if you are asking this question.

I **Might** neck down the plane between the analog and digital sections, but if you go here it is vital that NO signal traces pass over the slots in the plane, in the first instance just do a solid plane, 99% of the time it will be fine.

Now optimal part placement, that is a whole other conversation....

Regards, Dan.

[Nitrousoxide]

I'd personally have two separate ground domains, one for the analog and one for the digital. You must make sure the planes are both connected, but you may route out a division between them. And as already mentioned, you should pass all the signal traces through that gap for optimum return impedance results.

Star grounding (as you mentioned last) can be appropriate for several instances, say when board complexity is low or a well defined current return path is required (say a high power motor current return).

Regardless, make sure you properly via stitch the bottom and top layers if they are both grounds.

I think part and trace placement, minimising dI/dt and dV/dt and decoupling would play an equivalently significant factor.

[b_force]

Actually, some manufactures datasheets are a bit odd about grounding.
It all depends what kind of analog (and digital) signals you have.

In general, I like to use with separate grounds and stich them together at proper points (mostly close to the ADC/DAC with proper capacitor buffering).
If that's not really possible, use one big ground plane.

[Rerouter]

Everyone likes throwing rules around, without explaining the why

Ok so basics, ground planes are used to work with the path of least inductance, as the edge rate of the signals on your boards get faster, less of the ground current follows the path of least resistance, and instead follows the path of least inductance.

In general this path will be directly underneath your signal trace,

Ok so the ground current wants to be directly underneath the signal, however you generally end up routing yourself in to a corner and needing via's to hop out, meaning you generally need to leave a small sliver out of the ground plane under your signal to allow another signal to hop under,

So how can you keep the ground return path happy in this instance?, hop the ground to be adjacent to your signal on the top layer where the signal passes under it, following in the trace beside it will still be lower inductance than hopping around your gap, so it minimises loop area and a few other pains that cutting up ground planes normally cause.