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The only difference between analog and digital grounds is what you connect to them. You connect your analog component returns to analog ground, and your digital component returns to digital ground.
The ferrite bead on the connection between the two grounds is intended to act as a high frequency filter to remove some of the noise that might be coupled from the digital side of the ground to the analog side.
The truth is that you don't need separate analog and digital reference planes if you are careful about component separation and signal routing. The critical design requirement is to prevent your analog signals from sharing a return path with the digital signals. That can be accomplished by simply avoiding routing analog signal traces over portions of the reference plane already in use by digital signals.
Digital and analog gnd separation is very important if audio signal appear on your circuit. e.g digital cordless phone
This is because most digital cordless phone operates in TDD mode ( e.g. Dect TDD switches between Tx & Rx at 10ms period ) and transmit power is relatively high. As a result, TDD switching noise leaks to audio circuit through gnd ( return path ). A strong hum of 100Hz could be heard. Therefore, carefully gnd separation in between digital and analog is necessary for system optimization.
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