guard ring
Hi Henrywent,
To prevent digital noise coupling to analog section, second option is preferred.
"First put PTAP ring" - this will help in reducing the noise generation in substrate by reducing the substrate-to-ground resistance. Strong connection between PTAP ring and ground should be ensured.
"then a NTAP ring" - this will help in increasing the substrate resistance between digital and analog section. The increase in resistance is acheived by the depletion region of the diode formed between NTAP and substrate.
If PTAP ring is put outside the NTAP ring of digital, then it will result in increased resistance between substrate and ground of digital, hence producing more substrate noise.
I think even for analog section it is not good to reverse the pattern. Above pattern itself is to be used.