what is the difference between anolog ground and digital ground
: can any one tell me the difference between analog ground and digital ground,why it should be seperate line and finally it has shorted at one point???
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
anything else. Even ground planes have some impendence, and so currents flowing through these grounds cause voltage drops in the planes.
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.
This single point keeps the ground potentials of those different ground planes same but keeps the transfer of digital noise to analogue side minimum.
Your digital circuit noise can get to your analogue signal path if you don't use separate grounding systems for digital and alalogue parts (those are interconnected only in one place). Digital grounds are invariably noisier than analog grounds because of the switching noise generated in digital chips when they change state. For large current transients, PCB trace inductances causes voltage drops between various ground points on the board (aka "ground bounce"). Ground bounce translates into varying voltage level bounce on signal lines. For digital lines this isn't a problem unless you cross a logic threshold. For analog it's just plain noise to be added to your signals.
How to eliminate this problem ? why the suggestion of keeping analog and digital grounds separate is made by having them separate on the IC, so use separate grounds. Two completely separate supplies allows switching currents to be speced and optimized for at the regulator, and to have analog supply optimized to be nice and quiet. Filtering and bypassing can help to short out some ground bounce currents. And if you connect the analogue and digital grounds together do it only in one single well-designed place.
If you are building an analogue to digital converter circuit, then you have to be very careful in keeping the analogue and digital grounds separare or otherwise you will not get the resolution you expect to get because of the noise. The AGND and DGND pins should connect to the analogue ground, preferably with it flooded under the whole IC package. If you don't do this, noise on the DGND pin fom the digital supply will get injected into the analogue section inside the IC.
The digital input lines should be close to each other and connect to the nearby digital IC. Underneath these lines, the analogue and digital ground planes should CONNECT. The reason for this is to keep the radiating loop area of the digital current path to a minimum. If you don't do this, every time a digital edge occurs, the pulse will either flow around rather a large path or get injected into the IC's analogue section, depending on your implementation.
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http://www.epanorama.net/documents/groundloop/in_circuit.html 