Separating analogue and digital ground generally has to do with return currents and ground noise. Even though we think of GROUND as having zero impendence, it is a wire like anything else. Even ground planes have some impendence, and so currents flowing through these grounds cause voltage drops in the planes.
If we have a circuit that is operating on a senative analog voltage levels, or frequencies, we might not want these unexpected voltage drops occuring in the circuit. The are often injected into the circuit by filter caps, or bypass caps that were placed into the circuit to filter a signal or chip. So, the best way to keep this noise out is to create a separate analog ground.
Separate grounds does not always mean totally separated. Most analog ground and power splits are done by either just isolating the plane and not allowing the digital signals to flow over it. There is generally a single point connetion between the DIGITAL and ANALOG grounds. More than one point opens up the possibility of ground loops which cause the ground noise form the digital side to enter to your analogue electronics ground.
You should not run digital signals over or directly next to your Analog signals or planes. This will result in capacitive/inductive coupling to the "quiet" Analog planes, and/or result in horrible signal quality on the digital signal as its return current (which normally returns on the plane directly below the signal) is forced to travel "around" the isolated analog planes.
The need to have separate grounds in a printed circuit board or in a system is usually discussed and decided upon before the design begins. Often, when noise and other problems are perceived to be a possible problem, the designer will use separate analog and digital grounds in an effort to be conservative. Sometimes he's right and sometimes the grounds are separated needlessly.
The worst possible outcome is to design and build a printed circuit board with a common ground and then find that there is a substantial amount of digital noise getting into the analog circuitry. Then the entire board design must be redone. Sometimes separate grounds are needed and sometimes they're not.
If we have a circuit that is operating on a senative analog voltage levels, or frequencies, we might not want these unexpected voltage drops occuring in the circuit. The are often injected into the circuit by filter caps, or bypass caps that were placed into the circuit to filter a signal or chip. So, the best way to keep this noise out is to create a separate analog ground.
Separate grounds does not always mean totally separated. Most analog ground and power splits are done by either just isolating the plane and not allowing the digital signals to flow over it. There is generally a single point connetion between the DIGITAL and ANALOG grounds. More than one point opens up the possibility of ground loops which cause the ground noise form the digital side to enter to your analogue electronics ground.
You should not run digital signals over or directly next to your Analog signals or planes. This will result in capacitive/inductive coupling to the "quiet" Analog planes, and/or result in horrible signal quality on the digital signal as its return current (which normally returns on the plane directly below the signal) is forced to travel "around" the isolated analog planes.
The need to have separate grounds in a printed circuit board or in a system is usually discussed and decided upon before the design begins. Often, when noise and other problems are perceived to be a possible problem, the designer will use separate analog and digital grounds in an effort to be conservative. Sometimes he's right and sometimes the grounds are separated needlessly.
The worst possible outcome is to design and build a printed circuit board with a common ground and then find that there is a substantial amount of digital noise getting into the analog circuitry. Then the entire board design must be redone. Sometimes separate grounds are needed and sometimes they're not.
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