An Energy-Efficient Frequency-Domain CMOS Temperature Sensor With Switched Vernier Time-to-Digital Conversion

This paper presents a CMOS smart temperature sensor using a switched Vernier time-to-digital converter to achieve an energy-efficient temperature sensing. The proposed temperature sensor employs two switched ring oscillators (SROs), of which the oscillation frequencies are slightly different from each other and varying linearly with temperature due to the deployment of a proportional-to-absolute-temperature current generator. The frequencies of the two SROs are measured by counting the rising edges with two counters, and hence, the frequency difference corresponding to temperature can be readily monitored with the digital numbers. A control strategy that switches OFF the ring oscillators after each sampling is developed, such that significant power saving is achieved. The proposed temperature sensor shows a measured resolution of 0.048 degrees C from -20 degrees C to 120 degrees C with 100-ms conversation time. With 1-kHz sampling rate, the power consumption is as small as 93.6 mu W, resulting in 93.6 nJ per sampling. The chip area is 0.118 mm(2) in a standard 0.18-mu m CMOS process.