Double guard ring should be used to sorround 5V N/MOS connected to I/O pad double guard ring defintion
for NMOS ,double guard ring includes P+ pickup (tied to vss ) and N+ in NW (tied to Vdd)
for NMOS ,double guard ring includes N+ pickup (tied to vss ) and P+ in PW (tied to Vdd)
smic180
原版IO ESD MOS 也没有double guard ring