A Fully Digital Low Cost Time Domain Smart Temperature Sensor with Extremely Tiny Size

To explore the possibility of soft IP implementation, a fully digital smart temperature sensor without any full-custom device is proposed for painless VLSI or SOC on-chip integrations. The signal is processed thoroughly in time domain instead of conventional voltage or current domain. A cyclic delay line is used to generate the thermally sensitive pulse with a width proportional to the measured temperature. The timing reference is just the input clock, and a counter instead of voltage or current analog-to-digital converter is utilized for digital output coding. The circuit is realized by FPGA chips for functionality verification and performance evaluation. Implemented with as few as 140 Logic Elements, the proposed smart sensor was measured to have an error of -0.7degC~0.9degC over a wide temperature range of -40degC~130degC. The effective resolution is better than 0.1degC, and the power consumption is 8.42 muW at a sample rate of 2 samples/s. The performance is as good as those of most full-custom predecessors. The longest conversion time is around 260 mus, and a conversion rate of 3 kHz at least is promised.

I. Introduction

UNDER the fast evolution of science and technology, various kinds of high performance but low cost systems are produced in the market. As a result of the remarkable market growth, the need of small chip size but high accuracy smart temperature sensors is becoming stronger than ever. The main applications are requested as follows: 1) the thermal compensation in single chip systems and micro systems with built-in sensors; 2) the ambient temperature monitoring for home or office electronics; 3) the environment temperature control in automatic fabrication factories; 4) the overheat warning in portable or mobile consumer electronics products, such as: personal computers, personal digital assistant, and cell phones. Low power consumption is also required for smart temperature sensors in battery-powered systems to enhance the battery performance and reduce the error caused by self-heating of the sensor when integrated. To greatly reduce the cost and to measure the on-chip temperature directly, standard digital CMOS processes should be adopted so that the temperature sensors can be fully integrated with the VLSI chips. Fig. 1. Typical block diagram of conventional smart temperature sensor Table I Common Specifications for Smart Temperature Sensors

	Min	Max	Unit
Accuracy	0.3	3	°C
Resolution	0.05	1	°C
Supply Voltage	2.5	5	V
Supply Current	50	500
Speed	1	50	Samples/s