A wideband fully differential source follower

Conventionally, source followers have been used at the block of track-and-hold amplifiers (THA) for high-speed 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. A small signal analysis is discussed to compare the performance. The result of Spurious Free Dynamic Range (SFDR) is -66.92 dBc for F _in= F _s/2 at F _S= 500 MHz. The SFDR of the proposed source follower is better by about 8.6 dB than the SFDR of a conventional source follower under the same experimental conditions. To verify them, an 8-b ADC which includes the proposed source follower is fabricated with a 0.13 μm CMOS technology.