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The usual method is to have the analog and digital circuits on different areas of the board with their own ground and power planes under them. The ground planes are connected at a single point under the part of the circuit where the analog and digital signals are interfaced. Example of interfaces are ADC or DAC.
If you need them to be electrically joined at a star point, but treated as separate nets, then what I do is to have AGND go to pin of a TestLoop component and join pin 2 of that test loop to the GND starpoint. They are then electrically different nets, but you join them with a tespoint/testloop, which is useful for testing anyway. Applies if you use planes or tracks for either or both/many grounds.
am i right to say a star topolgy is preferred? in other words, i will use different power and ground planes for analog and digital, and connect them at one single point? is using ferrite bead needed to connect between the grounds at that point? what is testloop component?
No no no no no!. A Ferrite bead will ADD inductance. For the digital side you want to keep power supply loop inductance as low as possible.
Yes, join grounds only at one place. By a TestLoop I mean a loop of tinned copper wire soldered into two pads about 150mil apart. It makes a good test point to hang a scope or meter lead on, and, as far as your PCB and Schematic program is concerned each side of that point is a different net. If one side is DGND and the other side is AGND, then that is your star point and you have fooled the program into thinking your grounds are not joined. If you join them you will get an error or worse still, they (AGND and DGND) will be routed as the same net.
Ferrite bead will add inductance. For the potential between them is there is no problem But it act as a filter in the high frequency signal.
Hence it will prevent ripple due to anlog circuit noise.
Hi all, this is good topic to disscuss. I'm also new in mixed signal design. In some mixed signal IC (ADC or Analog mux), the AGND and DGND have it's own pin but some of them only have single pin GND. So how to deal with it in term of pcb design?
Ferrite beads are also used to reduce power supply noise such as power bounce and GND bounce, ferrite beads are usually inserted between the power supply and Vcc terminal of an IC by trial and error.
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