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

Abstract

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 -0.7°C40.9°C over a wide temperature range of -40°C-130°C. The effective resolution is better than 0.1°C, and the power consumption is 8.42pW 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 260ps, and a conversion rate of 3 kHz at least is promised.Index Terms-temperature sensor, smart sensor, fully digital smart sensor, cyclic delay line, time domain, on-chip integration, low cost.

A field programmable gate array-based digital temperature sensor's response to supply voltage variation

“…The pulse-based design measures the time for a delay line to reach a variable number of circulations, rather than count the number of circulations in a given time. It is also formulated based on the fact that longer delay is needed to minimize loss of linearity in a ring oscillator [3].…”

“…Since low voltage VLSI chips lose linearity at its frequency/temperature relationship, sensing accuracy was improved using a time amplifier to connect the delay line with a counter that circulates the delay line in a given time [3]. This sensing technique retained good area utilization at the expense of self-heat generated from circulation on short delay line.…”

Ultra-miniature wireless temperature sensor for thermal medicine applications

“…As a result, the temperature reading is proportional to the number of delay stages locked. Using standard cells commonly available in foundry CMOS technologies, the same concept has been demonstrated but with a different circuit topology [3,4]. …”