Im currently building a signal generator and have a problem with placing multiple different ground planes which have different voltages.
The system is powered off of USB, so all my digital logic signals are referenced to the usb ground. I also have a virtual ground generated at half the voltage in order to power my analog circuitry. This means that all my digital signals are referenced to digital ground (0V) where as all my analog signals are referenced to virtual analog ground (2.5V). The analog signals consist of ~8MHz precision sine waves and their signal integrity takes priority.
Im developing this on a 2 layer board so I don't know if the bottom layer should be entirely digital ground, entirely analog ground, or a combination depending on the sub-system it's directly under.
Ive placed a simplified block level diagram of my solution, but let me know if you wish to know any finer details!