This brief presents a submicrowatt, offset-free, and implantable system for a biomedical capacitive sensor. The system is powered by a 13.56-MHz radio frequency signal and performs sensor signal amplification, analog-to-digital conversion, and load-shift keying uplink data transmission within 640 mu s. An ultralow-power capacitance-to-digital converter (CDC) is designed by replacing a power-hungry operational amplifier with a subthreshold inverter in a switched-capacitor amplifier (SC-amp). A fast-response-gain compensation method is employed to reduce the gain error of the SC-amp while achieving high energy efficiency for the CDC. To eliminate the offset, a two-step autocalibration is applied. The application-specific integrated circuit is implemented in the Taiwan Semiconductor Manufacturing Company 90-nm complementary metal-oxide-semiconductor technology, and the whole system achieves an 8.02 effective number of bits with 9-bit linearity while consuming only 5.5 mu W.