Resistor-based Temperature Sensors in CMOS Technology

Table of Contents

1. Frontmatter

2. Chapter 1. Introduction

   Sining Pan, Kofi A. A. Makinwa

   Abstract

   This chapter is an introduction to the book. It first discusses some general aspects of integrated temperature sensors, including their applications and specifications. Temperature compensation of MEMS/crystal frequency references is a demanding application: high temperature sensing resolution is needed to prevent the sensor’s noise from increasing the frequency reference’s jitter, while high energy efficiency is required to minimize the sensor’s contribution to the reference’s total energy budget. Moreover, high stability is required to suppress the reference’s error under various conditions. After comparing various temperature sensing elements available in CMOS technology, resistor-based temperature sensors are chosen, which, compared to traditional BJT-based sensors, are in theory over 5× more energy-efficient.

3. Chapter 2. Sensor and Readout Topologies

   Sining Pan, Kofi A. A. Makinwa

   Abstract

   In this chapter discusses some general concerns involved in the design of resistor-based temperature sensors. After comparing the characteristics of the different sensing resistors available in standard CMOS technology, silicided resistors are chosen due to their large temperature sensitivity and high stability. Apart from the sensing resistor, a reference impedance is required to convert resistance changes into digital information. Two possible sensor structures are dual-R (with a resistor reference) and RC (with a capacitor reference). The former is more energy efficient, while the latter is more accurate. Among popular front-end topologies, Wien-bridge (RC) and Wheatstone bridge (dual-R) are preferred. To achieve high-resolution, they should be digitized by Delta-Sigma ADCs (ΔΣ-ADCs).

4. Chapter 3. Wien Bridge–Based Temperature Sensors

   Sining Pan, Kofi A. A. Makinwa

   Abstract

   In this chapter presents the design of Wien-bridge sensors focusing on accuracy. To improve its noise and efficiency, the phase demodulator of the continuous time (CT) ΔΣ-ADC is achieved by multiplexers made from switches. Due to the use of square-wave inputs and phase references, the ADC output (μ) has a strong nonlinearity, which cannot be eliminated after a two-point trim. To suppress this effect, a μ-to-RC nonlinearity correction should be applied before trimming. Three Wien-bridge sensor prototypes have been implemented. After trimming, the first prototype achieves a 3σ inaccuracy of 0.03°C over an industrial temperature range from −40°C to 85°C, but occupies 0.72 mm². With improved readout circuits, the third prototype achieves a similar inaccuracy over a wider military temperature range from −55°C to 125°C, while the area is only 0.12 mm².

5. Chapter 4. Wheatstone Bridge–Based Temperature Sensors

   Sining Pan, Kofi A. A. Makinwa

   Abstract

   In this chapter discusses the design of Wheatstone bridge sensors focusing on energy efficiency. Compared to traditional Wheatstone bridge readout circuits using instrumentation amplifiers, sensors presented in this chapter are directly readout by DAC resistors and CTΔΣ-ADCs. The first prototype uses a single-bit CTΔΣ-ADC and achieves a resolution figure-of-merit (FoM) of 65 fJ·K². By systematically improving the design, the FoM of the fourth design reaches 10 fJ·K², which is only six times worse compared to the theoretical value defined by the thermal noise of the Wheatstone bridge. This is enabled by design techniques including multi-bit DAC, return-to-CM switching, and linearized OTA. It is worth mentioning that the OTA linearization is achieved by simply replacing the current source of its differential pair with a resistor, and the linearity of its transconductance is improved by at least an order of magnitude.

6. Chapter 5. Application-Driven Designs

   Sining Pan, Kofi A. A. Makinwa

   Abstract

   This chapter presents two designs to broaden the application of resistor-based temperature sensors. The first is aimed for biomedical applications. With power scaling and properly-designed parallel DAC resistors, this sensor consumes only 6.6 μW, and achieves a 0.2 °C inaccuracy from 27.5 °C to 47.5 °C after an on-chip trimming, which greatly simplifies its digital backend. The second design is a sensor embedded in an RC-based frequency reference. By extensive circuit reusing, the additional area required by the Wheatstone bridge temperature sensor is negligible. Also, as the RC filter nonlinearity is similar to that of the Wheatstone bridge sensor output, the RC frequency reference error can be mostly removed by simple offset trims to the outputs of both the RC filter and the temperature sensor. Its remaining error is less than ±400 ppm from −45 °C to 85 °C.

7. Chapter 6. Conclusions and Outlook

   Sining Pan, Kofi A. A. Makinwa

   Abstract

   This chapter summarises this book. Resistor-based temperature sensors are especially competitive in applications that require both high resolution and high energy-efficiency. Compared to the state-of-the-art in 2016, the work presented in this book improves the energy-efficiency of CMOS temperature sensors is by 65×. This chapter also contains some suggestions for future directions, including a systematic design approach based on sensor accuracy, area- and power- efficient digital backend, background calibration, long-term-stability measurement, application of the tail-resistor-linearized OTA, etc.

8. Backmatter