Analog or Digital ground?

[simce]

Hi there,
I have to design mixed signal PCB with two layers (it is not that complicated) and i don't know if it is better to put analog or digital ground on polygon on top and bottom layers?

 

[IanP]

If you are not working with very sensitive, say, 24bit AD converters or similar circuits, I would use one etire layer ac common GND for both analog and digital..

 

[jdhar]

If you can separate the Analog and digital sections, why not route on top layer, and use a ground plane on the bottomwith a split in between.

 

[simce]

To be more precise, i have to make board (B1) that is used as amplifier and filter for input analog signal. This board uses +/-12V and GND (12VGND) as supply for amplifiers, and +5V and GND (5VGND) that supplies digital part. Digital part on this card is used to drive few LEDs, and some simple logic. Board (B1) is connected to separate DSP board (B2) with inputs for analog AGND and +5V and digital DGND.
I don't know if it is better to place plane on B1 that is connected to AGND around analog part, or plane should be connected to 12VGND aroung analog part. 12VGND connects to operational amplifiers power inputs. AGND on B1 is separated and will be connected to AGND on board B2. AGND on B1 is connected to inverting inputs on OPAMPs and is used as reference for analog inputs.

 

[IanP]

You can run separate ground to the board B1; this will definitly not harm. But more important will be to use inductor-capacitor low-pass-filters on each branch of the +/-12V and 5V lines. THe values of inductor(s) can be in the range of µHs (10 -100) and capacitors should be a combination of 100nF(ceramic) and 10-47µF(tantalum)..
On the main board, however, I wouldn't bother with grounds separation, but I would still use L-C on power nodes..

 

[simce]

L-C filters are allready present on B1 with values of L=100µH and C=470nF. I know that separate grounds must be made on B1, what i don't know is: is it better to connect plane around analog part on B1 that is connected to AGND or on 12VGND?

 

[IanP]

The plane around analog circuit should be connected to 12VGND. AGND is treated as a reference voltage and therefore should not be treated as return path for any GND on board B1.

 

[newbie101]

read pdf files...

http://www.hottconsultants.com/papers.html <--- rest of papers

http://www.hottconsultants.com/tips.html <--- read the PCB stackup