Re: analog power ring
Hi,
It is common for analog IP to have guard rings inside it. A guard ring protect the analog IP from digital nosie. It is also important if the analog IP is operating at different voltage. The gaurd ring consists of diffusion, well and metal (usually metal 1).
For mix signal layout, it is not necessary to run a power (metal) ring around the analog IP. You do this if you need to supply power to the IP, and not for isolation. However, it is good to keep a gap between between the analog IP and the digital layout. Note that this is not always necessary as the design of the IP might have a wide guard ring around it, and thus the digtal layout cannot get close to the analog circuit.
There is also concern of the pad power ring (this is the pad power that drive the digital output, and also for ESD protection). It is a common practise to break the pad power ring between the digital and analog domain. Break it physically or with a diode-breaker cell.
Obviously, the power has to be connected eventually. This is usually done on the PCB. The futher away this connection , the better the power isolation.
So, there are quite a few type "power ring" we are talking about. They are different and should not be confused in a discussion.
Regards,
Eng Han
www.eda-utilities.com