Design of mixed analog/digital interface circuits for sensors

140 Pages·2007·5.48 MB·English

UNIVERSITA’ DEGLI STUDI DI PAVIA
FACOLTA’ DI INGEGNERIA
DOTTORATO DI RICERCA IN
INGEGNERIA ELETTRONICA, INFORMATICA ED ELETTRICA

XIX CICLO

Design of mixed analog/digital
interface circuits for sensors
and micro-systems

Relatore:
Chiar.mo Prof Piero Malcovati

Tesi di dottorato
di Andrea Rossini

Anno accademico 2005-2006

A mio papà Domenico e
a mia mamma Tilde…

…una sola parola:
grazie per tutto

Index
Index...............................................................................................4

Abstract ..........................................................................................7

Chapter 1 Introduction .............................................9

Introduction................................................................................................................9
The light...................................................................................................................12
Gamma rays.............................................................................................................16
X rays.......................................................................................................................18
Photographic plate.....................................................................................................................20
Photostimulable Phosphors (PSPs)...........................................................................................20
Geiger counter...........................................................................................................................20
Scintillators................................................................................................................................21
Image Intensification..................................................................................................................21
Direct Semiconductor Detectors................................................................................................21
Radio astronomy......................................................................................................22
The acquisition chain...............................................................................................22
Design overview ......................................................................................................23

Chapter 2 The Read-Out Integrated Circuit...............25

Introduction..............................................................................................................25
The LFDR project....................................................................................................29
The ROIC chip.........................................................................................................31
The analog acquisition chain.....................................................................................................32
The digital processing chain......................................................................................................34
The pipeline converter...............................................................................................................35
The Wilkinson converter............................................................................................................36
Layout of the test chip..............................................................................................37

Chapter 3 The Wilkinson A/D Converter ..................41

Introduction..............................................................................................................41
The basic blocks.......................................................................................................43
Ramp generator..........................................................................................................................43
Voltage Comparator..................................................................................................................44
The clock generator...................................................................................................................47
The counter and the output register...........................................................................................47
Measurement Setup..................................................................................................47
Experimental results.................................................................................................49
Conclusions..............................................................................................................54

Chapter 4 Clock-less Pipeline-Like A/D Converter....56

Introduction..............................................................................................................56
Error on the gain of the amplifier..............................................................................................57
Error on the DAC level..............................................................................................................59
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Error in the comparator threshold............................................................................................59
Basic blocks.............................................................................................................63
The ADC.....................................................................................................................................64
The residue amplifier.................................................................................................................65
The DAC.....................................................................................................................................68
The EOC generator....................................................................................................................69
Simulation Results...................................................................................................70
Experimental results.................................................................................................76
Conclusions..............................................................................................................79

Chapter 5 Magnetic Sensor Models ..........................81

Introduction..............................................................................................................81
Types of magnetic sensors.......................................................................................82
Squid..........................................................................................................................................83
Search-coil.................................................................................................................................83
Magneto-inductive sensor..........................................................................................................83
Magneto-resistance....................................................................................................................84
Hall sensor.................................................................................................................................88
Fluxgate.....................................................................................................................................90
The magnetic sensor model .....................................................................................94
Magneto-resistance....................................................................................................................96
Fluxgate.....................................................................................................................................97
Hall sensor.................................................................................................................................98
Magneto-inductance...................................................................................................................99
Temperature drift.....................................................................................................................101
Parasitic effects........................................................................................................................102
Dispersed field.........................................................................................................................103
Hysteresis.................................................................................................................................104
Experimental results...............................................................................................106
Conclusions............................................................................................................109

Chapter 6 Interface Circuit for Fluxgate Magnetic

Sensor .................................................................................112

The interface circuit design....................................................................................112
Timing Block............................................................................................................................114
Excitation Block.......................................................................................................................115
Read-Out Block........................................................................................................................117
The ADC...................................................................................................................................118
Simulation Results.................................................................................................118
Layout....................................................................................................................121
Experimental results...............................................................................................122
Conclusions............................................................................................................126
Bibliography...............................................................................129

Acknowledgments/Ringraziamenti............................................134

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6
Abstract

Scope of the work described in this thesis is the design of integrated interface circuits for microsensors.
This work can be divided into two parts. The first part describes the interface circuit for an array of X-
ray detectors, which is part of the acquisition chain for an X-ray spectrometer, that will be one of the
instruments used for the space mission towards Mercury “Bepi Columbus” of the European Space
Agency (ESA). More specifically a 10 bit clock-less pipeline-like A/D converter and a 10 bit
Wilkinson A/D converter have been designed, implemented and characterized. Moreover, we
implemented the complete interface circuit for a 16x16 array of X-ray detectors based on the Wilkinson
A/D converter. The complete acquisition chain achieves ±0.2 LSB of differential non-linearity, ±3.3
LSB of integral non-linearity and 34 electrons rms of input referred noise, consuming less than 1 mW
per channel.
The second part of the thesis deals with the interface circuit for an integrated fluxgate magnetic sensor
for electronic compass applications. The circuit can provide a widely programmable excitation current
to the fluxgate sensor and read-out the sensor signal, providing digital output. The interface circuit,
together with an integrated fluxgate sensor, achieves an angular error in detecting the Earth magnetic
field as low as 4 degrees.
This thesis is structured in the following way:

Chapter 1 Introduction

This chapter provides an overview of the integrated interface circuits for microsensors as well
as all the background information about X-ray detectors and detection systems.

Chapter 2 The Read Out Integrated Circuit

This chapter describes an interface circuit for an array of X-ray detectors. The project, named
Large Format Detector Readout, is funded by ESA (European Space Agency) and is carried-out
by University of Pavia, Polytechnic of Milano and Alcatel Alenia Space LABEN.

Chapter 3 The Wilkinson A/D converter

Ph.D thesis, Andrea Rossini 7
This chapter describes the first solution for implementing the A/D conversion in the read-out
integrated circuit described in Chapter 2: a 10-bit Wilkinson A/D converter. All the design and
implementation details are presented together with experimental results.

Chapter 4 Clock-less Pipeline- like A/D converter

This chapter describes the second solution for implementing the A/D conversion in the read-out
integrated circuit described in Chapter 2: a 10-bit clock-less pipeline-like A/D converter. The
peculiarity of this converter is the absence of the clock. The circuit, indeed, acts as a
combinatory logic, thus avoiding the presence of a synchronization. All the design and
implementation details are presented together with experimental results.

Chapter 5 Magnetic sensor models

This chapter provides the background information about magnetic sensors. Moreover, it
describes the magnetic sensors model that have been developed to simulate the behaviour of
four different types of sensors: an hall devices, a magneto-resistance, a flux-gate and a magneto-
inductance. These models have been used to design the interface circuit for fluxgate magnetic
sensors described in Chapter 6.

Chapter 6 Interface circuit for fluxgate magnetic sensor

This chapter describes the interface circuit for fluxgate magnetic sensors. The circuit can
provide a widely programmable excitation current to the fluxgate sensor and read-out the sensor
signal with variable gain. Moreover, the circuit provides digital output. All the design and
implementation details are presented together with experimental results.

Ph.D thesis, Andrea Rossini 8
Chapter 1 Interface circuits

Chapter 1
Introduction

1.1 Introduction

Computers can only handle zeros and ones and if we want to automate the procedure for sensing
any physical quantity in a repetitive mode, we need sensors to translate temperature, pressure,
vehicle speed or any other parameter into a form manageable by computers. This can be done with
appropriate interface circuits that translate the analog output of the sensors into the digital domain
of the computer.
Over the years, sensors have been fabricated by using many different technologies. At the
beginning, sensors were realized in a mechanical way, but, because of the success of the integrated
silicon technologies, also the sensor technology received renewed interest. Around 1965, Prof.
James Angell started a silicon sensor research group at Stanford University. One of his students,
Kensall Wise, started performing selective etching of structures and this marks the start of the
electronic sensors and the interface circuits as we know today. Many internationally well-known
universities in the USA, Europe and Japan were quick to follow.
Obviously, first applications of this sensors were bulky, costly and consuming a lot of power. But
following the development of the integrated circuit technology, new markets for these sensors were
found and even more new applications developed, thus increasing the effort to obtain even better
and cheaper sensors.
The main advantages of an electronic sensor are the lack of any mechanical part that can break, the
possibility to obtain a large number of pieces at the same price and with the same characteristics,
and, more importantly, the possibility to miniaturize the component itself step by step with the
evolution of the electronic integrated technologies.
Thus the need for sensors has grown rapidly in recent years for industrial control, automation and
consumer applications. In modern measurement and control systems, a large number of sensors
collect the information about the process variable in the system being monitored. They usually
provide analog information on the system being monitored through signal conditioning circuits
connected to a processor. The latter interprets the information, makes appropriate decisions most

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Ph.D thesis, Andrea Rossini