An area-efficient sub-10 nW temperature-to-digital converter for thermopile temperature sensors

Abstract

A 6.29 nW temperature-to-digital converter for IR radiation-based thermopile temperature sensors was designed and fabricated on the chip area of 0.068 mm2 using 90 nm CMOS technology. The sub-mV voltage produced by the sensor is nearly proportional to the temperature difference between the object and the environment and is converted to sub-nA current using the gate-leakage characteristics of PMOS transistors. The ratio of the sub-nA current and the reference current proportional to the sub-mV voltage difference is converted to the frequency ratio of the two current-to-frequency oscillators. Then, the digital output proportional to the tiny voltage difference can be obtained by the counters. After the two-point calibrated digital outputs are mapped to the object temperatures, the average resolution is 0.11 °C with a conversion time of 348 ms, which results in the figure of merit of 0.026 nJ × °C2. The inaccuracy of the object temperature is +0.26/−0.95 °C from −10 °C to 100 °C at the ambient temperature of 25 °C.