The present invention relates to an optical sensor that may be implanted within a living animal (e.g., a human) and may be used to measure the concentration of an analyte in a medium within the animal. The optical sensor may wirelessly receive and may be capable of bi-directional data communication. The optical sensor may include a semiconductor substrate in which various circuit components, one or more photodectors and/or a light source may be fabricated. The circuit components fabricated in the semiconductor substrate may include a comparator, an analog to digital converter, a temperature transducer, a measurement controller, a rectifier and/or a nonvolatile storage medium. The comparator may output a signal indicative of the difference between the outputs of first and second photodetectors. The measurement controller may receive digitized temperature, photodetector and/or comparator measurements and generate measurement information, which may be wirelessly transmitted from the optical sensor.