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Über dieses Buch

This book describes the design and implementation of energy-efficient smart (digital output) temperature sensors in CMOS technology. To accomplish this, a new readout topology, namely the zoom-ADC, is presented. It combines a coarse SAR-ADC with a fine Sigma-Delta (SD) ADC. The digital result obtained from the coarse ADC is used to set the reference levels of the SD-ADC, thereby zooming its full-scale range into a small region around the input signal. This technique considerably reduces the SD-ADC’s full-scale range, and notably relaxes the number of clock cycles needed for a given resolution, as well as the DC-gain and swing of the loop-filter. Both conversion time and power-efficiency can be improved, which results in a substantial improvement in energy-efficiency. Two BJT-based sensor prototypes based on 1st-order and 2nd-order zoom-ADCs are presented. They both achieve inaccuracies of less than ±0.2°C over the military temperature range (-55°C to 125°C). A prototype capable of sensing temperatures up to 200°C is also presented. As an alternative to BJTs, sensors based on dynamic threshold MOSTs (DTMOSTs) are also presented. It is shown that DTMOSTs are capable of achieving low inaccuracy (±0.4°C over the military temperature range) as well as sub-1V operation, making them well suited for use in modern CMOS processes.



Chapter 1. Introduction

Temperature is the most often-measured environmental quantity [1]. This is because nearly all physical, chemical, mechanical, and biological systems exhibit some sort of temperature dependence. Temperature measurement and control are therefore critical tasks in many applications. Traditionally, temperature sensors have been implemented with discrete components such as resistance temperature detectors (RTDs), thermistors, or thermocouples. In the last three decades, integrated temperature sensors, particularly in CMOS technology, have become a promising alternative. A sustained research effort has been devoted to the development of compact, low-cost temperature sensors with co-integrated readout circuitry, thus providing temperature information in a digital format. Such smart temperature sensors (see Fig. 1.1) are conventional products nowadays [3–7].
Kamran Souri, Kofi A. A. Makinwa

Chapter 2. Readout Methods for BJT-Based Temperature Sensors

As discussed in the previous chapter, BJT-based temperature sensors are promising candidates for use in wireless temperature sensing applications. In this chapter, we first describe the operating principle of BJT-based sensors, followed by an overview of various readout methods. The energy-efficiency of these methods is then discussed, and compared to the ultimate achievable efficiency of BJT-based sensors.
Kamran Souri, Kofi A. A. Makinwa

Chapter 3. Energy-Efficient BJT Readout

In analog circuit design, speed and precision are typically achieved at the expense of power dissipation. Therefore, the design of energy-efficient, high-resolution smart temperature sensors, as targeted in this work, is not trivial and involves fundamental trade-offs. Assuming that an ADC’s power dissipation scales linearly with sampling frequency, then simply decreasing its conversion time will translate into a proportional increase in power dissipation; i.e. the ADC’s energy consumption per conversion remains unchanged. Improving an ADC’s energy-efficiency, thus, calls for architecture-level solutions.
Kamran Souri, Kofi A. A. Makinwa

Chapter 4. BJT-Based, Energy-Efficient Temperature Sensors

As was shown in the previous chapter, the zoom-ADC is well suited for use in energy-efficient temperature sensors. It combines the strengths of SAR- and ΔΣ-ADCs to realize an accurate, and energy-efficient temperature to digital conversion. In this chapter, a sensor prototype that employs a 1 st -order zoom-ADC is described. It is compact and power efficient, requiring only a few μW to operate. Its energy-efficiency, however, is limited, due to the use of an inherently slow 1 st -order ΔΣ modulator. To improve energy-efficiency, a second prototype is presented, which requires less power to operate, while employing a 2 nd -order zoom-ADC and a faster sampling scheme. Finally, a third prototype for sensing very high temperatures ( > 150C) is presented, which uses robust techniques to overcome the different sources of temperature sensing errors at such temperatures.
Kamran Souri, Kofi A. A. Makinwa

Chapter 5. All-CMOS Precision Temperature Sensors

In CMOS technology, BJT-based sensors are usually the temperature sensors of choice due to their decent accuracy after a single temperature trimming, e.g. ± 0. 2C (3σ) over the military temperature range: − 55C to 125C [1–3]. They also achieve low supply-sensitivity, typically in the order of 0. 1C/V. However, BJT-based sensors typically require supply voltages above 1V, since V BE will be about 0.8V at − 55C and some headroom is required for the current source (often cascoded) that biases the BJT. This restricts the use of such sensors in battery-powered systems, and also restricts the temperature range of implementations in nanometer CMOS [4]. However, from an energy-efficiency perspective, a lower V DD value is preferred.
Kamran Souri, Kofi A. A. Makinwa

Chapter 6. Conclusions

In this thesis, the development of energy-efficient, accurate smart temperature sensors for wireless temperature sensing applications has been investigated. It has been shown that the existing temperature sensors prior to the start of this research were ill suited for use in such applications, where energy efficiency and low cost are critical requirements. In the following, first the main findings of this research are discussed. The other applications of the developed techniques are then presented, followed by some proposals for the future improvements.
Kamran Souri, Kofi A. A. Makinwa


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