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This paper presents both an integrated energy supply and detection circuits for an RFID localization system. The equations to calculate the expected AC antenna voltage are presented as well as a fast method to calculate the expected output voltage from a rectifier. An ultra low power voltage detection circuit is described which monitors the output voltage of the rectifier and consumes only 3 nW. Also the input of the rectifier is monitored by an input power dip detection circuit, which provides a pulse whenever the amplitude of the AC antenna signal drops to zero. This circuit too, is ultra low power and consumes only 5 nW from a 0.5 V supply. It can even serve as an OOK demodulator for bitrates up to 6.7 kbps. All circuits have been designed in a 130 nm CMOS process. Measurement results are presented for all circuits.