A 10 nW, 10 mV Signal Detector Using a 2 pA Standby Voltage Reference, for Always-on Sensors and Receivers

An RF energy harvesting circuit is usually designed to maximise efficiency and therefore output power, while a passive wake-up radio is usually optimised for a high open-circuit output voltage resulting in high sensitivity. These two functions have conflicting design requirements, but are generally both needed in Internet-of-Things devices. This paper presents a new approach to holding almost the entire system fully powered down whilst listening, whilst also obtaining an effective wake-up and energy harvesting circuit using the same rectenna (rectifying antenna). The topology uses a rectenna that is optimised for efficiency, and two signal detector circuits that draw up to 3.5 nA from the battery. One detector is configured to trigger at 85 mV, to start up the boost converter when enough power is available to obtain netpositive energy harvesting. The other detector is set to be more sensitive, to wake up subsystems when the rectenna output reaches 10 mV. The detector architecture and transistor-level design are presented, and the detection threshold and power levels experimentally verified. The circuit draws 10 nW at a sensitivity of 10 mV, and 3.9 nW at 85 mV. This detection system is the first reported circuit with a configurable detection threshold that draws only nW from the battery, and that, in addition to RF signals, can be used with any transient signals, such as outputs from piezoelectric sensors, microphones, or energy harvesters that produce in excess of around 10 mV. The low power consumption of this circuit is largely due to use of the UB20M voltage detector, whose internal on-demand voltage reference generator is also reported here. It has the lowest reported standby current of 2 pA, and a sub-microsecond-scale turn-on response time.