Nowadays, the efficiency of Radio Frequency (RF) energy harvesting circuits is continuously increasing and, at the same time, the energy consumption of connected devices is drastically decreasing. Despite that, storing and delivering such kind of harvested energy to the device in an appropriate manner is still a challenge. This paper presents the design of a harvester able to efficiently collect energy from both low and high power levels of the RF field. The objective is to correctly power supply an ultra-low power consumption device requiring a regulated voltage. The starting point is the characterization of a commercial Power Management Integrated Circuit which operates with ultra- low power levels. Then, a RF harvester circuit, consisting in the association of a low and a high level power rectifier, is presented. By the means of a circulator, the RF power is absorbed by the low power rectifier or reflected to the high power rectifier. The rectifiers have their outputs associated in series. At a frequency of 868 MHz, the efficiencies of the association of rectifiers are 43 % and 76 % and were obtained for input powers of -20 dBm and 0 dBm respectively.