An 8-b cascaded folding A/D converter with a new fully differential source follower

Conventionally, source followers have been used at the block of track-and-hold amplifiers (THA) for highspeed medium-bit analog-to-digital converters (ADCs). Even though the input signals of an ADC are fully differential, two single-ended source followers should be normally used. This is because differential source followers have some drawbacks. In this study, a wideband fully differential source follower is presented to obtain maximum efficiency of a fully differential THA. Because a cross coupling technique at the differential nodes is adopted, the performance of the proposed source follower is superior to the performance of conventional followers. The result of Spurious Free Dynamic Range(SFDR) is -66.92dBc for F_in=F_s/2 at F_S=500MHz. The SFDR of the proposed source follower is better by about 8.6dB than the SFDR of a conventional source follower under the same experimental conditions. To verify them, an 8-b cascaded folding ADC which includes the proposed source follower is fabricated with a 0.13um CMOS technology.