We present a 67-pJ/bit 435-MHz modulator for 16 quadrature amplitude modulation with a fast and low-energy start-up for ultra-low-power wireless medical capsule endoscopy application. For a fast start-up, an auto-calibrated transient DC error correction is proposed to minimize the envelope error at the modulator output node occurring right after the initiation of the start-up process. Once the DC error correction is optimized through the calibration procedure, start-up energy of 50 pJ and start-up time of 18 ns are achieved, which enable symbol-level duty cycling. Implemented in a 180-nm CMOS process, the modulator performs 40-Mbps data transmission with–5-dBm output power while consuming 2.66 mW from a 1-V supply.