A 1.1-V Analog Multiplexer With an Adaptive Digital Clamp for CMOS Video Digitizers

We present the design of an integrated multi- plexer and a dc clamp for the input analog interface of a high-speed video digitizer in the 1.1-V 65-nm complementary metal-oxide-semiconductor process. The ac-coupled video signal is dc restored using a novel all-digital current-mode charge pump. An eight-input multiplexer is realized with T-switches, each con- taining two series-connected bootstrapped switches. A T-switch's grounding branch is merged with the pull-down end of the clamp- ing charge pump. An adaptive digital feedback loop encompass- ing a video analog-to-digital converter (ADC) controls the clamp charge pump. The bootstrapped switches have been adapted to suit the video environment, allowing on-the-fly recharging. The varying ON-resistance of the conventional bootstrapped switch is utilized to linearize the multiplexer response by canceling the effect of the nonlinear load capacitance contributed by the clamp transistors. Under worst case conditions, the multiplexer main- tains a 62-85-dB spurious-free dynamic range over a range of known input video frequencies, and it reduces the second-order harmonic component upon optimization. The dc clamp provides 12-bit precision over the full range of the video ADC and can set the dc at the target level for at most 194 video lines.