Isolate noisy ground

[RuihW]

Hi everyone,

I'm working on a project that has a pulse generation block, a sensitive analog block and a digital block. There's also a power block that supplies power to each block using LDOs. The power block is powered by an external power supply.

In a two-layer PCB, each block's ground is isolated on the top layer and joins together on the bottom layer through vias. The entire bottom layer serves as a common ground and connects to the external power supply. The pulse generation block takes in a 50MHz sine wave from a signal generator. I'm wondering if I should place a choke between its ground and the common ground? Or is connecting the ground to the common ground with vias already enough?

Thanks!

 

[KlausST]

Hi

Show us your PCB layout.
Textual description is not useful.

Klaus

 

[RuihW]

Hi

Show us your PCB layout.
Textual description is not useful.

Klaus

The layout is still undergoing. But this one below shows the idea. All the power traces (VDDs) will be planes in the actual layout to reduce trace loss.
Each block has their own ground on the top layer, and these dedicated ground joins to the common ground on the bottom layer through vias. But the noisy signal ground (SGND) has a cutout on both layers. My question is, what's the best way to isolate the SGND while keep all the blocks on the same ground potential? I'm thinking of adding a choke between the SGND and the common ground on the bottom layer so that the ground is all connected from a DC perspective, and the noisy AC signals on the SGND will be blocked by the choke.

 

[FvM]

Most obvious deficite is lack of any bypass capacitors. Secondly, separating grounds without filtering supply lines is dysfunctional. Finally, even if you have perfectly filtered and noise-free supply voltage on each block, single ended signals crossing block borders ("digital control signals", "single pulse" above) will be affected by additional ground bounce.

I seriously doubt usefulness of the presented concept. Separated ground isles can be useful in specific cases, e.g. for switching regulators to keep commutating currents inside the block. They involve filter chokes for supply in- and output lines crossing the border.

 

[RuihW]

Most obvious deficite is lack of any bypass capacitors. Secondly, separating grounds without filtering supply lines is dysfunctional. Finally, even if you have perfectly filtered and noise-free supply voltage on each block, single ended signals crossing block borders ("digital control signals", "single pulse" above) will be affected by additional ground bounce.

I seriously doubt usefulness of the presented concept. Separated ground isles can be useful in specific cases, e.g. for switching regulators to keep commutating currents inside the block. They involve filter chokes for supply in- and output lines crossing the border.

Thanks for the feedback. There are decoupling capacitors on every VDD rails. The input from the external power supply also passes through a pi filter before going to the LDOs.
Based on your comment, is it better to connect all the ground together on the same layer and never isolate them?

 

[FvM]

Yes, I'd prefer continuous ground plane for the design.