Analog and digital ground saparation

[tiwari.sachin]

I have designed a PCB (4layer) as in the image.

On the bottom layer, I have the analog section and all of that is connected to analog ground.



On the top layer, I have some digital lines. These are tracks that are routed on the top side of the analog area.

Now how should I possibly connect the AGND and GND (Digital). So that there is proper return path for the digital signals.

Should I Connect AGND and GND in GND Layer as below.




Does it not completely change the concept of one point ground between AGND ad GND.


Layer Stackup

TOP
VCC
GND
BOT

I hardly is any space on the PCB to change the routing, hence I have only this option.

While for analog section, it some two thermistors that I have connected via a voltage divider.


GREEN --> TOP LAYER TRACK
PINK--> GND

BOTTOM LAYER and VCC --> NOT SHOWN in the image

 

[Kick]

Split the GND plane in to two- Digital and Analog and use via to connect with these planes from top and bottom. Also avoid the digital track above the analog section.

 

[KlausST]

Hi,

you know feeding the digital signals across the analog section is not optimal.

But it seems the signals are not extremely high frequency. So I see no big problem.

Klaus

 

[tiwari.sachin]

I dont thing any signal above that analog part go more than a few MHz.

I would have for sure considered unrouted section above analog ground but I donot have space to do so. If I chose any other method then I will have to route the digital signals on Vcc place and I dont want to to that.

KlausST.... The suggested grounding pattern is ok?? I know its not the perfect way but with a thermistor and few MHz (May be 8-10, This is at higher side) digital signal, Is that a decent option

 

[KlausST]

HI,

Either you do like kick said with the vias,

Or you use a fix connection without vias. (I usually do this).
Maybe you find a "quiet" place in the digital section. Then use this point to join both planes.
I mean: for example, if a (noisy) switching regulator is on the right side of the analog section, then connect both planes on the LEFT side of the analog section....

I think this solution(s) should be OK. But for sure in detail it depnes on how sensitive the analog part is and how noisy the digital part is.

Klaus

 

[tiwari.sachin]

I would have loved to use it that way but since its a 4 layer board and I dont intend to route on internal layer as they are dedicated Vcc and GND

Only option I am left with is top and bottom and with all the components. I could barely manage the above said routing.

The decision is to have 4 layers and 4 layers only.

- - - Updated - - -

Meanwhile the switching regulator is not close to the analog section. All the digital lines routed above analog section are either LED, switch input for few low speed signals (a couple of MHz).