Multiple energy-efficient CDCs are proposed for millimeter sensor nodes. Compared to the state-of-the-art, these CDCs achieves excellent energy efficiency, high SNR, and wide input range with a variety of techniques. These include correlated double sampling in front of a SAR ADC, incremental delta-sigma conversion with a zoom-in SAR converter, energy-efficient dual-slope conversion, and fully-digital iterative delay-chain discharge conversion.