Twisted transformers for low coupling RF and mixed signal applications

As CMOS technologies and their respective supply voltages continue to scale downwards, new methods for designing RF analog and mixed-signal circuits must be found. A promising technique that has emerged is the use of monolithic transformers. Unfortunately, due to parasitic magnetic coupling, there can be a substantial amount of cross-talk between transformers and other, nonrelated, devices resulting in the requirement for large separation distances and hence, wasted silicon. This paper presents a new winding scheme that reduces the parasitic magnetic coupling by approximately 1000X (32 dB) and allows the separation distance between transformers to be reduced by 10X.