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Q. What about the analog and digital power-supply requirements of these devices? Should I buy separate analog and digital power supplies or use the same supply?
A. This really depends on how much noise is on your digital supply. The ADSP-21msp50, for example, has separate pins for the +5- V analog supply and the +5-V digital supply. If you have a relatively quiet digital supply, you can probably get away with using it for the analog supply too. Be sure to properly decouple each supply pin at the device with a 0.1-µF ceramic capacitor. Remember to decouple to the analog ground plane, not the digital ground plane! You may also want to use ferrite beads for further isolation. The diagram below shows the proper arrangement. A much safer solution is to use a separate +5-V analog supply. You can generate the +5 V from a quiet +15-V or +12-V supply using a three-terminal regulator, if you can tolerate the extra power dissipation.
If you are using mlti-layer boardwith ground plane, your ground plane can be both analog and digital ground. However, it would be useful, if you group the analog circuits together.
If your are using a 2-layer board, you should make a star-ground. Star ground means that there is only one physical connection between the AGND and DGND. Typically, this connection is made at the power supply (or battery).
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