Digital and analog Grounds

[redhat]

i have read that analog and digital grounds should be separated to decrease noise,but how to do that if the analog and digital circuits use the same supply?
thanks in advance

 

[sigint]

I have done designs where there were separate Analog and Digital planes - these were connected together only at the power supply connector, you either have to trick your layout software or connect them in gerber. It's supposed to reduce noise, quite frankly I don't believe it does, but it's a bit of blackmagic the old time engineers like to use.

SiGiNT

 

[redhat]

hi sigint
i want to know how to trick the layout software for example orcad layout or will it be easier to connect them in gerber because they are the same net so, orcad will link them together if i want to make a copper pour for ground net

 

[trigger74]

Routing these kind of lines.... you need to route by your own, not by the auto-routing functions and I will have separate copper pour.

I don't believe this much also, but as we put the digital and analog parts in different locations on the PCB, making such routing won't make any difficulties....

Just think if there is trouble of noise, we can immediately cut the track (just for testing) and to clarify which part is the main source of trouble.....

 

[vdaniel]

You can make an artificial part, named, for example "bridge" to connect two nets: GND and AGND. For schematics it can be just a line segment, for PCB router something like 0805 footprint, with overlapping pads. Then you have only to ignore verification message about clearance violation.

Varuzhan

 

[IanP]

Analog-Digital IC manufacturers recommend thet AGD should be connected with DGND only at one point using thin truck that should act as a small inductor, so for DC these grounds will be at the same level and for HF they will be sort of separated.
You will find several interesting points of view on this subjec by searching this forum. Use: "analog AND digital AND GND"

 

[vdaniel]

Though it is easy (as I wrote above) to make separate ground planes even automatically, I use another approach. Even with two-sides board, if good traced, it is possible to make a most part of the bottom layer a large ground plane. Especially, if the most passive components are SMD. But such a design requires
to work very accurate and redesign the ground planes many time to get as much
connections of the ground between other wires, as possible.
You can see such a plane in attachment.
With such technique I have nice working analog/digital boards with more than 12 bit resolution.
Sure, such work cannot be done blindly. Better if the same person designs both schematics and PCB.
Varuzhan

 

[Moss]

It is not necesery in all casses to separate AGND and DGND. Basic and most important rule is to prevent digital return current to "flow" trouhg anolog return paths. read some book like "High-Speed Digital Design: A Handbook of Black Magic"

 

[Sergiu_Q]

It is not necesery in all casses to separate AGND and DGND. Basic and most important rule is to prevent digital return current to "flow" trouhg anolog return paths. read some book like "High-Speed Digital Design: A Handbook of Black Magic"

Moss have right...I found some documents where explain all this stuff and many time this method to do separate ground can cause a more problem than we will use analog and digital ground together