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Grounds Loops and stack up

batistalex , 07-05-2019, 02:43 PM
Hello guys.
I am working on a project where I have a commonly used stackup for 4-layers PCB(1.6mm thickness):

L1 - signal
----- PREPREG ----
L2 - Power
----- CORE -----
----- PREPREG ----
L4 - signal

Just to know, I have an analog and digital grounds separated and the analog and digital circuits are connected through an optocoupler (I know this is a whole new topic, let's talk about this later)

I see some problems in this stackup that will for sure cause EMI problems. I'm willing to solve one in special: For example, the signals in L1 doesn't have a proper return path because GND is way too far from the signals on L1 and since the return path will run through the least impedance path, in this case, the Power Plane will be the chosen one.

To solve this without changing this stackup too much I did the following:

L1 - signal/GND/GNDA
----- PREPREG ----
L2 - Power
----- CORE -----
----- PREPREG ----
L4 - signal/GND/GNDA

By doing this I have the electromagnetic fields contained because now I have less inductance in the PCB due power and Gnd on layers L1 and L2. But now 2 questions come up:

1. The return path for the signals on L1 will be the pour GND on L1 or still will be on L3?
2. Because I have GND/GNDA on layer L3 and L4 am I creating some kind of ground loops?

In this video: https://www.youtube.com/watch?v=5jlvRhXLVls @robertferanec talked about ground loops and I thought I understood well but turns out I didn't. I understand the concept but when applied on PCB I have a lot of doubts.

Can anyone help me with that?

Ps: The images show the layers of my PCB.
batistalex , 07-05-2019, 02:52 PM
[UPDATE] Just to complement. In the image attached on L1 the tracks are surrounded by GNDA , on L2 there is power pour, on L3 GNDA plane. Some GNDA vias connect L1 and L3. This kind of configuration creates ground loops?
robertferanec , 07-08-2019, 01:34 AM
I am not sure about your project, however when I have a board which is optoisolated, then my layout are usually like two completely separate PCBs placed on one PCB (separated for example by 1mm gap or more if required or possible) and these two circuits are only connected by the optocouplers. When you place this PCB in front of light, you will see that the two circuits are completely separated.

But I am not analogue expert.
batistalex , 07-08-2019, 07:15 AM
Thanks for the tip @robertferanec I really appreciate that. However, my main question is about the ground loops. The two previous questions are still open. I don't know if my questions are very clear but my concern here is about if I'm creating ground loops due to the chosen stackup and pouring GND around tracks. I made two images trying to explain what I'm talking about. I hope this clarifies.
Paul van Avesaath, 07-10-2019, 12:29 AM
GND return path will try and run beneath the trace you are using. ONLY if you force it to the way in you drawings it will go that way.. but normally it would go the way beneath it
robertferanec , 07-08-2019, 07:52 AM
I am not sure what is on your board and how complicated it is, but for most digital designs one GND solid plane should be fine. I would not overthink the design unless you have there controlled impedance, high speed interfaces or it has signals over 3 - 5 GHz. For analogue, I would try to do most on one side of the board and had AGND on the layer below.
Paul van Avesaath , 07-10-2019, 12:31 AM
did I just see an AC-Neutral plane on your boards, as in 220V ? then you need way more clearances in you plane... (just making sure here.. )
batistalex , 07-10-2019, 07:59 AM
Thanks @robertferanec.

Yes, it is @Paul van Avesaath. I put more clearance already. Thanks for the tip.
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