something about guard ring, really confusing

[henrywent]

double guard ring

hi there,
which guard ring scheme is used to surround the digital part of a mixed-signal chip to prevent the switch noise from affecting the analog part, PTAP guard ring or NTAP guard ring? or which one is better?
some people told me to first put a PTAP guard ring around the digital part then a NTAP guard ring (double ring?). but for the analog part the order of the double ring is reversed, can anyone justify this please?

 

[Raviprasad_K]

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.

 

[Fady Atef]

guard ring for digital part

Hi,

There are some recommendations to use minimum width PTAP GR with very good ground connection to the ring. it is enough

Thanks,

source:
"http://books.google.com.eg/books?id=dPoJRuyiQbgC&pg=PA124&lpg=PA124&dq=optimum+guard+ring&source=bl&ots=5dBYk5C9OP&sig=1grMetZzAvGVoAQ1qnYmi38co44&hl=en&ei=sIbgSeagKYXLjAe5hJXUDQ&sa=X&oi=book_result&ct=result&resnum=2#PPA124,M1"

1st paragrah