A Wideband ADC Input Buffer Based on AC-Coupled Super Source Follower

A high-linearity input buffer is commonly used to isolate the signal source from the internal analog-to-digital converter (ADC). This work presents a high-linearity and wide-band input buffer which is based on AC-coupled super source follower (SSF) structure. Instead of the DC-coupled feedback, a coupling capacitor is adopted in the traditional SSF to interrupt the feedback loop. This structure eliminates the limitation of the fixed bias voltage via AC coupling, which enlarges the signal range. To suppress the distortion caused by the nonlinear output impedance, the auxiliary source follower is inserted. With the bootstrapped capacitor driven by the output, the drain-source voltage of the input device is forced to hold steady, which effectively expands the bandwidth and improves the linearity. The buffer is designed in 65-nm CMOS process. The post-layout simulation results with 2.5-V supply and 6 GS/s sampling rate shows that the differential input buffer achieves a Nyquist SFDR of 73.47 dB and dissipates 32.73 mW.