Re: Matching
I think that in 0.18 um process and beyond, it'll always be better to use an ABAB structure than the usual ABBA structure.
Probably, this is due to the LOD effect. In processes using STI, the STI OX has a stress effect which changes the effective mobility of the transistor. The LOD effect depends on the distance between the STI isolation and the channel.
Thus , for an ABAB structure both A and B transistors will have the same average distance from STI, hence they will be better matched. The ABBA structure,however, will make each one have a different avergae distance, hence worse matching.