It seems ADAU1701 has separate analog and digital grounds, but datasheet says they "could" be tied together. Would it make sense to separate them for better audio performance?
These grounds will be tied together internally in most cases. I would have to check if they are on this part. With more recent understanding of signal integrity issue and with mixed signal parts it is better to have one plane. It simplifies the layout issues and generally makes it easier to prevent RFI issues etc. Paying attention to good layout practices that includes being aware of where the parasitic return currents are going to flow, you end up with better signal integrity on the digital transmission lines and less noise on the power and ground planes. This in turn makes the analog portion of the part more quiet.
My having designed large analog systems, it does become difficult if the system has digital, analog and chassis grounds. You cannot combine them in a sub-PCB without causing ground loops. In that case I have simply used just one of the grounds, analog or digital for the DSP boards. Often, the DSP boards work best with only digital ground since you would have codecs and other digital parts on the PCB. The analog portion would be short and hopefully, differential. In the case of the ADAU1701, the DSP section of the design is in the part and there are no codecs on the board, then I would just use analog ground and make sure to decouple and filter the DVDD well. If you have multiple ADAU1701's that have I2S transmissions between them then I guess you would have to flip a coin! In that case I could see splitting the boards into an analog and digital portion. The analog signal would only cross the boundary close to the DSP. You have to be really careful to not cross over the planes with power planes etc. It gets really complicated having multiple planes and in consultation with customers I have seen issues with split planes.
I have seen noise issues when there is only one plane for ground and they were caused by bypass caps placed on the opposite side of the board and the vias were not placed properly where they tie to the power and ground planes. See the ADAU1452 datasheet for details. It is near the end of the datasheet in the PCB Design Considerations section. PCB layout is critical with mixed signal parts.
Thanks for listening...
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