I am using a microcontroller with an integrated DAC. On my 4 layer PCB, I have connected the digital and the analog ground planes below the microcontroller. The microcontroller and the DAC are supplied by a 3.3V regulator. The DAC amplifier is supplied by a 5V regulator.
See the following picture?
- Which ground plane (GND or AGND) should the 5V regulator be referenced to?
- Which ground plane should the DAC amplifier supply (and its decoupling capacitor) be referenced to?
Answer
In general, the point of separating ground planes is to reduce the influence of any noisy components on the noise-sensitive ones. Thus, the voltage regulators and primary micro circuitry should use the normal ground, and the micro analog, amplifier, amplifier decoupling capacitor and voltage divider resistors should use the analog ground. This is what you have already illustrated. (Note that the passive components in the analog portion can introduce noise as well, but there is little you can do to avoid that, since grounding them to the general plane will just couple in even more noise.)
Another strategy that is often used for noise reduction is to group and separate the noisy components from the sensitive ones. This could be as simple as placing the power components on one side of the micro, and the amplifier and friends on the other side. Make sure the routing is direct and as short as possible, and decrease the impedance by increasing trace widths or copper weight wherever possible. Note that this can be used independently of, or along with, the separate ground planes.
This e2v application note is a very good place to start when laying out out a mixed signal board.
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