nmos substrate connection

[hbk2222]

I want to ask about substrate of nmos should i connect it as a separate pin and connect it into the top level to ground or connect it directly to ground.

 

[SP24]

It depends on the block for which you are doing.
If the substrate is very noisy and you want to protect you Ground from the substrate induced noise then the best way is to have a separate pin for the subatrate at the block level and finally connect them to the gnd pin at the chip level. This type of connection is called STAR connection.