Analog Ground & Digital Ground....

[tariq7868]

Hello, I have sim 300c gsm module ... it has 2 pins (51 & 52 ) along with some connections in refrence headset interfacing circuit are connected to "Agnd" in its refrences manual.....can i connect it to ground of supply?

 

[dknguyen]

When in doubt, connect both digital and analog grounds together consistently and evenly. No split planes or anything. Using those improperly will result in more problems.

They are still connected in normal use, but having them separate gives you the option of connecting them only at a single point to keep noise currents flowing through the ground of the digital system from flowing through the ground of the analog system. This can be done with split planes connected only at a single point, but if you don't know what you're doing (because this split plane places limitations on how you can route the signals of these two systems relative to the split plane), then it makes things worse.

http://www.hottconsultants.com/techtips/split-gnd-plane.html

 

[Ubergeek63]

generally you want to split the plane but the analog stuff should be compressed and the analog plane should only be under the analog. Modern ADCs run have 40uV resolution and need to be treated as RF devices.

EVERY copper trace is an antenna and needs to be treated as such since it does not take much to generate 40uV regardless of if you are looking at nois on the ground, reference, or signal.

Dan