High-Accuracy CMOS Smart Temperature Sensors

121 Pages·2000·10.57 MB·English

by  Anton Bakker

HIGH-ACCURACY CMOS SMART TEMPERATURE SENSORS
HIGH-ACCURACY
CMOS SMART
TEMPERATURE SENSORS
by
Anton Bakker
Philips Semiconductors
Tempe, AZ, U.S.A.
and
Johan Huijsing
Delft University of Technology
Delft, The Netherlands
Springer Science+Business Media, LLC
A C.I.P. Catalogue record for this book is available from the Library of Congress.
ISBN 978-1-4419-4862-5 ISBN 978-1-4757-3190-3 (eBook)
DOI 10.1007/978-1-4757-3190-3
Printed on acid-free paper
All Rights Reserved
© 2000 Springer Science+Business Media New York
Originally published by Kluwer Academic Publishers, Boston in 2000.
Softcover reprint of the hardcover 1s t edition 2000
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic or mechanical,
including photocopying, recording or by any information storage and
retrieval system, without written permission from the copyright owner.
Table of contents
.
Preface
lX
1
Introduction 1
1.1 Temperature sensing .................................................. l
1.2 CMOS smart temperature sensors ............................. 3
1.3 Motivation and objectives ........................................ .4
1.4 Organization of the work ........................................... 5
References ................................................................. 7
2 Dynamic offset-cancellation 9
techniques
2.1 Introduction ............................................................... 9
2.1.1 Offset and noise in CMOS amplifiers ....................................... 10
2.1.2 Naming conventions and classification .................................... 13
2.2 Autozero techniques ................................................ 14
2.2.1 Principle .................................................................................... 14
2.2.2 Residual noise ........................................................................... 15
2.2.3 Self-calibrating opamp .............................................................. 16
2.2.4 Correlated double sampling ...................................................... 17
2.2.5 Ping-pong opamp ...................................................................... 17
2.2.6 Chopper-stabilization ................................................................ 18
2.2.7 Three-signal approach ............................................................... 19
2.3 Chopper techniques ................................................. 20
2.3.1 Principle .................................................................................... 20
2.3.2 Residual noise ........................................................................... 22
v
HIGH-ACCURACY CMOS SMART TEMPERATURE SENSORS
Table of contents
2.3.3 Residual offset .......................................................................... 23
2.3.4 Gain accuracy ........................................................................... 25
2.3.5 Chopper opamp ......................................................................... 26
2.4 Nested chopper technique ....................................... 28
2.4.1 Principle .................................................................................... 28
2.4.2 Analysis .................................................................................... 29
2.4.3 Realization ................................................................................ 30
2.4.4 Measurement results ................................................................. 32
2.5 Conclusions ............................................................. 33
References ............................................................... 34
3
CMOS bandgap references 37
3.1 Introduction ............................................................. 37
3.2 Temperature curves of the bipolar transistor .......... 40
3.3 Design ..................................................................... 43
3.3.1 Non-idealities in practical CMOS bandgap references ............ 43
3.3.2 Curvature correction techniques ............................................... 45
3.4 Realization .............................................................. 48
3.4.1 Filtering of modulated offset .................................................... 48
3.4.2 Nested chopper technique ......................................................... 52
3.4.3 Piece-wise-linear circuit implementation ................................. 54
3.4.4 Measurement results ................................................................. 55
3.5 Conclusions ............................................................. 58
References ............................................................... 60
4 Design of CMOS Smart 63
Temperature Sensors
4.1 Introduction ............................................................. 63
4.1.1 Accuracy ................................................................................... 64
4.1.2 Power consumption .................................................................. 65
4.2 Analog-to-Digital conversion ................................. 66
4.2.1 Frequency conversion ............................................................... 67
4.2.2 Duty-cycle modulation ............................................................. 68
4.2.3 Sigma-delta A-to-D conversion ................................................ 69
VI HIGH-ACCURACY CMOS SMART TEMPERATURE SENSORS
Table of contents
4.3 Kelvin-to-Celsius conversion .................................. 71
4.4 Curvature correction ................................................ 73
4.5 Single transistor temperature sensors ...................... 74
4.6 Bus interfaces .......................................................... 77
References ............................................................... 78
5 Realizations of CMOS Smart 79
Temperature Sensors
5.1 Tyre monitoring system ........................................... 79
5.1.1 Motivation ................................................................................. 79
5.1.2 Specification ............................................................................. 82
5.1.3 Design of the tyre temperature sensor ...................................... 83
5.1.4 Detailed design of the tyre temperature sensor. ........................ 85
5.1.5 Measurement results ................................................................. 90
5.2 High-accuracy temperature sensor .......................... 93
5.2.1 Motivation ................................................................................. 93
5.2.2 Specification ............................................................................. 95
5.2.3 Design ....................................................................................... 98
5.2.4 Measurement results ............................................................... 105
5.3 Remote microprocessor temperature sensof. ......... 106
5.3.1 Motivation ............................................................................... 106
5.3.2 Specification ........................................................................... 108
5.3.3 Design of the single-transistor temperature sensor interface .. 110
5.3.4 Detailed design of the remote temperature sensOf. ................. 111
5.3.5 Measurement results ............................................................... 113
5.4 Conclusions ........................................................... 114
References ............................................................. 116
Appendix 117
Index 119
HIGH-ACCURACY CMOS SMART TEMPERATURE SENSORS VII
Preface
This book describes the theory and design of high-accuracy CMOS smart
temperature sensors. The major topic of the work is the realization of a smart
temperature sensor that has an accuracy that is so high that it can be applied
without any form of calibration. Integrated in a low-cost CMOS technology, this
yields at the publication date of this book one of the most inexpensive intelligent
general purpose temperature sensors in the world.
The first thermometers could only be read by the human eye. The industrial
revolution and the following computerization asked for more intelligent sensors,
which could easily communicate to digital computers. This led to· the
development of integrated temperature sensors that combine a bipolar
temperature sensor and an A-to-D converter on the same chip. The
implementation in CMOS technology reduces the processing costs to a minimum
while having the best-suited technology to increase the (digital) intelligence.
The accuracy of conventional CMOS smart temperature sensors is degraded by
the offset of the read-out electronics. Calibration of these errors is quite
expensive, however, dynamic offset-cancellation techniques can reduce the offset
of amplifiers by a factor 100 to 1000 and do not need trimming.
Chapter two gives an elaborate description of the different kinds of dynamic
offset-cancellation techniques. Also a new technique is introduced called the
nested chopper technique. An implementation of a CMOS nested-chopper
instrumentation amplifier shows a residual offset of less than lOOn V, which is the
best result reported to date.
Chapter three describes the most important part of a smart temperature sensor,
which is the bandgap voltage reference. It is shown that although this reference is
based on a bipolar transistor, it can be implemented in standard CMOS
technology by utilizing the (parasitic) substrate PNP transistor. This chapter also
describes different kinds of curvature correction techniques, which are very
important to further improve the accuracy.
HIGH-ACCURACY CMOS SMART TEMPERATURE SENSORS IX
Preface
Chapter four describes the general design of CMOS smart temperature sensors.
This includes Analog-to-Digital conversion, Kelvin-to-Celsius conversion,
curvature correction and bus interfaces. Also single-transistor temperature
sensors are discussed, which are very important nowadays for thermal
management in laptops.
The last chapter describes three different smart temperature sensors that have
been realized by the authors. The first one is an ultra-low power version for
application in a tyre monitoring system. The second is a high-accuracy
temperature sensor that meets the industry specifications without any form of
calibration. The last one is a remote microprocessor temperature sensor, which
can be found in all laptops.
The authors wish the reader a pleasant time in investigating the interesting aspects
of CMOS smart temperature sensor design.
Anton Bakker
lohan H. Huijsing
Sunnyvale, August 2000
x
HIGH-ACCURACY CMOS SMART TEMPERATURE SENSORS
Introduction
This work describes the theory and design of high-accuracy CMOS smart
temperature sensors. The major topic of the work is the realization of a smart
temperature sensor that has an accuracy that is so high that it can be applied
without any form of calibration. Integrated in a low-cost CMOS technology, this
yields one of the most inexpensive intelligent general purpose temperature
sensors in the world. This chapter introduces the reader to the general aspects of
the design of CMOS smart temperature sensors. It explores the possibilities and
detect the bottle-necks. It ends with the motivation and the organization of the
book.
1.1 Temperature sensing
Even many centuries ago, people already had a great desire to know the
temperature. Reference points could be found, such as the freezing point of water
or the body temperature. However, to determine temperatures between those
fixed points a temperature meter or thermometer was needed. This led to the
invention of the mercury-in-glass thermometer, which is still widely used today.
Another issue was the definition of the scale. In 1714, Fahrenheit proposed a
scale that takes the minimal possible temperature he could imagine to appear in
Central Europe as OOf" (-18°C) and the body temperature as 960f". Celsius
proposed a few years later in 1742 a similar scale but he took the freezing point of
water as OOC and its boiling point as 100°C. Both scales are still in use, but the
Celsius scale is the most widely used scale. More than a century later, in 1851,
Kelvin proposed his law of thermodynamics in which he proved that the
HIGH-ACCURACY CMOS SMART TEMPERATURE SENSORS 1
A. Bakker et al., High-Accuracy CMOS Smart Temperature Sensors
© Springer Science+Business Media Dordrecht 2000
Introduction
Analog-to-Digital
digital
converter
Fig. 1-1 Communication between a temperature sensor and a
computer through an analog-to-digital converter.
theoretically minimal possible temperature is -273.l5°C. Since that time,
scientists have often used the Kelvin scale, which is a Celsius scale that is shifted
by -273.15 degrees.
The early function of thermometers was to literally read the temperature.
However, the industrial revolution asked for automatic temperature controllers.
These so-called thermostats were used in central heating systems, ovens and
engines. The thermometer needed to be adapted to control heaters, fuel valves or
other so-called actuators. We still use these thermometers with mechanical output
in thermostat taps and local temperature control in central heating systems.
In the second half of the 20th century, the temperature controllers became more
and more intelligent. Switching a heater or ventilator on and off was in many
applications not good enough anymore. For example, to reduce fuel consumption
in central heating systems, it is necessary to monitor temperature changes and
adapt the heating power to reduce unwanted temperature overshoots. The
electronic controller was introduced and the thermometers had to be adapted to
communicate with them. Thermometers needed to have an electrical output,
which resulted in the development of resistance thermometers. The most widely
used material for resistance thermometers nowadays is platinum because of its
stability and reproducible temperature characteristics. The Pt-100, which is a
platinum resistor with a nominal value of 100Q, is now the standard resistance
thermometer. Other widely used materials for resistance thermometers are
(lightly-doped) semiconductors, such as silicon. These resistance thermometers
are called thermistors and have the advantage of a much higher sensitivity than
metal-based resistance thermometers.
To communicate with a computer, the resistance changes have to be converted
into a digital signal. This is done with an analog-to-digital converter. Such a
system is shown in figure 1-1. These kind of temperature sensor systems are still
widely used. To reduce the cost of a system that consists of both a temperature
sensor and a computer interface, integration of the temperature sensor on the
2 HIGH-ACCURACY CMOS SMART TEMPERATURE SENSORS