Characterization of CMOS on-chip transformer for digital transmission line driver applications

This paper presents a CMOS on-chip transformer characterization for line driver applications of digital transmission using differential signals. The central-tap structure for the secondary coil is designed carefully to ensure the signal integrity after converting the signal into differential format, while the broadside-coupled configuration for the coils is utilized to enhance the transformer coupling coefficient. The final designed transformer is in 1:3 turn ratio with chip area of 265 μm × 265 μm using TSMC 0.18 μm on-poly-six-metal (1P6M) technology. The characterization is based on the differential-/common-mode thru-reflect-line calibration to acquire the actual four-port scattering parameter with extraction of the associated parameters including inductances, quality factor and coupling coefficient. The measured data are further used as a basis to simulate the high-speed digital transmission at 5 Gbps where good differential signal qualities are observed.