If sensitive analog systems are run from one supply without the sufficient bypassing to eliminate noise, undesired degradation in a system’s performance will result. This application note provides insight into suitable techniques to overcome this roadblock.
You have combined digital and analog functions together in the final stage of a system design. But then the performance of the analog function, such as an audio amplifier, deteriorates due to the noise originating from the digital circuits. This can happen even after conventional preventive measures (e.g., separating analog and digital grounds, shielding) were taken. Noise interference problems can likely be traced to the coupling of power supplies, sometimes even if separate linear regulators are used.
Eliminate Noise Through Proper Supply Bypass Filtering - [Link]