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A wireless implantable adaptive RF power converter system for monitoring real-time biological signals of an untethered small laboratory animal inside a housing cage is developed. The overall prototype sensing system exhibiting a dimension of 6 mm times 6 mm times 2 mm and a weight less than 100 mg can be implanted in the animal abdomen. The implant unit consisting of a tuned 20-turn spiral coil is inductively coupled a 4 MHz RF energy source from an external power amplifier driving a tuned 25 cm times 15 cm, 4-turn RF coil. An on-chip rectifier and linear regulator circuit convert the received AC voltage to a stable 2 V DC supply with 1 mA driving capability. Due to animalpsilas different positions and tilting angles (up to 60 degrees) inside the cage with a 1 cm nominal separation distance between internal and external coils, a large varying RF coupling strength is produced and can be detected by a power sensing circuit. The received RF power level is quantized and processed with other biological data before wireless transmission using frequency shift keying (FSK) scheme. The external power source can adaptively adjust its RF power strength based on the received one-bit power sensing data to achieve a stable and reliable voltage supply for the overall bio-implant microsystem with an optimal power coupling efficiency.