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Do U design for Analog Device Company?, I have been design for Analog Device, they like use star GND, it seem like test point, at this point they can connect AnalogGND and Digital GND each other, but i don't know why they connect it together, I will inform U when I have information
All your grounding points,. IE. transformers, smoothing, sockets, PCB boards, chassie connections all come to gether at ONE CENTRAL POINT . It takes its name from the apperance that the connections have , it looks lick a star .
This method is for noise reduction. The currents for all stages do not flow through common inductances. As the frequency becomes larger, the inductance of the ground wires to the common point makes high reacctances which affects the circuit performance. Therefore, at higher frequencies a ground plane is used for ground. As someone said above, the digital and analog ground planes are connected at a point.
It is necessary to minimise the introduction of DC offsets and noise into the signal return paths for different parts of system (analog baseband, digital baseband, audio amplifier, LCD controller, power cct, RF tranceiver - mobile phone circutry is a good example). For eg. special care must be taken to separate the signal ground from the power at a start, such that no power current will flow through any signal line ground.