A Fully-Integrated 71 nW CMOS Temperature Sensor for Low Power Wireless Sensor Nodes

We propose a fully-integrated temperature sensor for battery-operated, ultra-low power microsystems. Sensor operation is based on temperature independent/dependent current sources that are used with oscillators and counters to generate a digital temperature code. A conventional approach to generate these currents is to drop a temperature sensitive voltage across a resistor. Since a large resistance is required to achieve nWs of power consumption with typical voltage levels (100 s of mV to 1 V), we introduce a new sensing element that outputs only 75 mV to save both power and area. The sensor is implemented in 0.18 mu m CMOS and occupies 0.09 mm(2) while consuming 71 nW. After 2-point calibration, an inaccuracy of +1.5 degrees C/-1.4 degrees C is achieved across 0 degrees C to 100 degrees C. With a conversion time of 30 ms, 0.3 degrees C (rms) resolution is achieved. The sensor does not require any external references and consumes 2.2 nJ per conversion. The sensor is integrated into a wireless sensor node to demonstrate its operation at a system level.