Remote powering and power management of a passive sensor system employing antenna-rectifier co-design is presented. The operating frequency of the system is chosen at 2.45 GHz in order to reduce the antenna size. A co-design approach enables the coupling of the antenna and the rectifier for maximum power efficiency. The presented tag consists of an antenna, RF rectifier, power management circuit and a ring oscillator. Temperature sensing with the passive sensor system is demonstrated by the on-chip ring oscillator. The RF sensitivity of the integrated sensor system is -11.4 dBm while the average DC power consumption is measured as 25 mu W. The measurements prove the effectiveness of the method for maximum transfer efficiency and the use of ring-oscillator based temperature sensor for in-vivo temperature monitoring.