A 3-stage Greinacher rectifier designed at 13.56 MHz for a 1 Vpp sinusoidal input and a 10 μA load current in 250 nm CMOS bulk technology is characterized. The measured output voltage is 1.923 V DC with an estimated 72% power conversion efficiency. This ultra low power and high efficiency AC/DC power converter with 0.13 mm2 chip area can be used with RF energy harvesters to power implantable or wearable biomedical devices in body sensor networks.