Multi-purpose interface for sensor systems fabricated by CMOS technology with post-processing

17

Sensors and Actuators A, 37-38 (1993) 77-81

Multi-purpose interface for sensor systems fabricated by CMOS technology with post-processing Carlos Azeredo Leme and Henry Baltes* Physical Electronrcs Laboratory, ETH-Zurich,

CH-8093 Zunch (Switzerland)

Abstract We propose an interface design strategy for CMOS sensor systems The objective IS to provide a microprocessor compatible sensor on a smgle chip Great flexlblllty and robustness are achieved by the use of a sigma-delta modulator m the A/D converter and by mcludmg the sensor element m the modulator first stage Implementations for three different

classes of sensors,

namely,

a magnetic,

1. IutrcNluction

Sensors are becommg increasingly important in many apphcatlon areas such as industrial automatlon, home appliances, car electrotucs and secunty systems A user fnendly output mterface 1s essential, ideally provldmg a microprocessor compatible dlgtal output On-chip clrcultry for signal processmg is, therefore, mdlspensable [l] This can be achieved by fabricating the sensors m an mdustnal CMOS technology as has already been demonstrated for magnetic field [2], temperature [3] and radiation [4] measurands Moreover, certain humidity, mechanical and chemical sensors can be obtamed by combmmg estabhshed IC technologes vvlth a limited number of additional post-processmg steps specific to the pertinent sensor function and compatible with the IC process [5,6] Figure 1 shows the block diagram of a conventional approach for reahsmg the sensor interface The functions of signal ampldicatlon, antlahasmg filtenng and analog-digital conversion are camed out mdependently and mostly in the analog domain Tlus approach has the disadvantage that it contains many preclslon analog components To Integrate these together wth

a humidity

and a thermal

sensor,

are presented

the sensor at the same substrate 1s a serious problem, as the arcmt has to function under the environmental condltlons to which the sensors are usually exposed, such as temperature vatlatlons and mechamcal vlbratlons Moreover, the clrcmt 1s exposed to the post-processing steps required for the creation of the mlcrosensor element itself Though conceived as a relatively ‘mild procedure, the post-processing may somewhat degrade the performance of both the analog devices and the component matching provided by the standard process Therefore a high degree of robustness 1s required from the clrcmtry which renders unpractical most clrcult techniques that rely on precise matching and/or complex analog functions Moreover, it 1s desirable to keep high flexlblhty so that, by simple reconfiguration of the basic structure, different classes of sensors can be handled Thus 1s very important m view of the vast vmety of sensors which may have, for example, voltage or current output slgnals, or a capacltlve vanatlon In addltlon, sensltlvlty and dynamic range can vary widely A flexible interface that can be easily reconfigured m order to accommodate a reasonable range of different sensors can save design tune and simphfy system updates associated with the evolution m the sensor design itself

2. System architechne

I

1 Fig

1 Conventmnal

*Author

archttecture

to whom correspondence

0924-4247/93/$6 00

of the sensor interface

should be addressed

In view of the above considerations, a system archltecture rmmrmsmg analog content, where most of the signal processing 1s done diptally, 1s highly desirable, as dlustrated m Fig 2 An optnnal coupling of the arcmt to the sensor can be achieved by mcludmg the sensor element as an appropnate component of the interface circuit Itself, and not Just as an external slgnal source

@ 1993 - Elsevler Sequoia All nghts reserved

78

[I3

AD DtgttiIl code

Rg 2 Sensor mterface where signal processmg and with optimum couphng to the sensor

1s mostly dlgltal

Most of these requirements can be met by using a sigma-delta modulator for the sensor interface This modulator, together with an output digital decimator constitutes a tigh performance A/D converter [7] It 1s a preferred technique for high-resolution moderatespeed A/D conversion [S] In sensor apphcatlons the resolution and speed speclficatlons are usually less demandmg, but it 1s the robustness and flexibility of the sigma-delta modulator that make it very interesting for sensor interfaces In this paper we describe some apphcatlon examples lllustratmg how the same sigma-delta modulator structure can best be Interfaced with sensors dehvermg quite different signals, such as capaatlve vanatlon or current signal, or a sensor that inherently performs an mtegratmg function The schematic diagram of the basic structure of the modulator 1s shown m Fig 3 A second-order modulator has been chosen which has several advantages over a first-order solution firstly, m-band noise tones, which are a problem with first-order modulators, are more easily suppressed Next, oversamphng ratios are lower enabling a faster speed of operation reqwred by some sensors such as the lateral magnetic transistor Finally, when a high gam 1s needed m the loop for high sensltlv-

lty, the capacitance spread can be greatly reduced by distributing the gam between the two integrators For achieving a good PSRR (power supply rejection ratlo), interference suppression and clock-feedthrough compensation, a fully-dlfferentlal arcmt has been selected Addltlonally, for effective ehmmatlon of clockfeedthrough, dummy switches are used extensively for charge inJection compensation, and also delayed clock phases are apphed for ehmmatlon of signal dependent charge mJectlon These measures are essential since, even when low resolution 1s required, the signal level may be very low Low input level also means that an amphficatlon of the signal will have to be carried out together urlth the sigmadelta modulation This 1s achieved by scaling the value of the voltage reference fed to each of the mtegrators together with their gains The gain, g, from the input to the output of the second integrator 1s simply given by

g,+&

(1)

c

with C,, C,, C,, and C,, as m Fig 3 Nevertheless, the transfer function of the loop must be preserved which leads to the condltlon

(2) wth V,,, and Wrenas m Fig 3 An offset compensation topology was chosen for the first integrator which has the additional advantage of ehmmatmg the low frequency flicker noise generated by its amplifier The

--

N channel Switch

-/-

P channel

-I-

CMOS

Swrtcb

--

Swtch

wth

Smtch

dummy

kll k2a x1=Nt2 x2 = NXOR(Q y1=

y2 = NXOR(Q

Rg

3 Schematic diagram of the base structure of the modulator

kt)

ny2 k2)

79 BlWllg

+“. ._-.- ------w-4

Currentsubtractor +_ ~~~__~~___~~__*

l3g 5 Schematic diagram for the magnetic sensor

Rg

4 Layout

of base modulator

of the auxdiaq

cwcmt

-cihmln

lmplementatlon

second mtegrator, on the other hand, has a conventional topology which enables the operation of the clrcmt urlth only a positive voltage reference and a two phase clock Thus modulator has been mtegrated and its layout 1s shown m Fig 4 It contains the clock phases generator and, since this 1s a prototype design, auxiliary testing clrcmts enabling a non-tisturbmg momtormg of several internal clrcmt nodes have also been included

condltlonmg

Output

Rg 6 Block diagram sensors

of the reconfigured

be applied to this stage directly at referred OTA This 1s illustrated by m Fig 6 TUB has the same effect as signal at the input of the modulator

modulator

for current

the output of the the block diagram applymg a voltage sven by

3. Practwal implementations The sigma-delta modulator described above can already be drectly applied to classical voltage output sensors A wide range of sensltlvltles can be accommodated by appropnately scaling the voltage references and the capacitor ratios More mterestmg 1s the apphcatlon of this same structure to different types of sensors This can be achieved by simple reconfigurations of the first integrator m the loop 3 1 Magnetic tramstor As a first example, a current type sensor such as a CMOS compatible lateral dual-collector bipolar magnetic transistor [9] 1s used This sensor 1s sensltlve to magnetic fields transversal to the clup surface which produce an imbalance of the two output collector currents After some snnple condltlomng arcmtry for blasmg the magnetic transistor and for ehmmatmg of the strong common mode component, a differential current signal 1s produced TUB auxlhary condltiomng circuit 1s shown m Fig 5 Since the first integrator on the sigma-delta modulator 1s actually built around an operational transconductance amplifier (OTA), this current signal can actually

where GM,, 1s the transconductance of the OTA and C,, 1s the parallel combmahon of C,, and the parasltlc capacitance to ground at the OTA mput node Actually this 1s equivalent to generating a signal-dependent offset voltage at the input of the OTA Therefore, and since this stage 1s offset compensated, the input current must be conveniently chopped To this end, output switches are included m the auxlhary condltlomng clrcmt m Fig 5 A more efficient way of applying the input current signal to the modulator 1s to use the dram nodes of the OTA input dtierentlal pair, when they are available The reason IS that these nodes present both a lower impedance and a lower signal swing than the output nodes This 1s the solution that we have nnplemented The resulting layout 1s shown in Fig 7 The magnetic sensor 1s identified by the arrow at the bottom nght comer As expected, this layout looks very smular to the previous modulator m Fig 4, thereby coniirmmg the flexlblhty of our approach 3 2 Capacztrve humzdzty sensor Another important class of sensors produces as output signal a capacitance vanatlon In this case, since the

second

Rg 7 Layout sensor (arrow)

of the modulator

vREFI Fig 8 Block diagram Mve sensors

contauung

a magnetic

current Rg 9 Schematic diagram mclude the humldlty

reconfigured

to

VREF2 of the reconfigured

modulator

for capac-

modulator first stage IS a conventional swltched-capaator clrcmt, the capacitance sensor can be optimally mtroduced m one of the input switched capacitor branches Thus IS Illustrated as a block diagram in Ag 8 The equivalent input sIgna IS gven by VIN(squ,v)= V,, AC,

of the first Integrator

(4)

The design of the capacitor ratios and voltage ratios can then be carried out as before for the desired senslt1v1ty This reconfigured structure has been Implemented together vvlth a humldlty capacitive sensor [5] The reconfigured mput stage IS shown m Fig 9 The capacstance of the sensltlve element vanes from 0 22 to 0 30 pF over the usable hurmdlty range from 20 to 90% A non-sensitive reference element was used as C,, thereby compensatmg the 0 22 pF offset The layout of the mtegrated clrcult IS shown m Fig 10 3 3 Thermoconverter A different class of sensors IS capable of unplementmg an integrating function Mechanical and thermal sensors are examples By judlclous design of such sensors, the whole first mtegratmg stage of the sigma-delta modulator can be ehmmated, the sensor mherently performs its fun&on as illustrated m Fig 11

Rg 10 Layout of the modulator contammg Ity sensor (right side of the picture)

VREFL

a capacltlve

humld-

%EF2

Fig 11 Block diagram gratmg sensors

of the reconfigured

modulator

for mte-

A specific example IS the rmcromachmed a c power sensor or thermoconverter [lo] illustrated m Fig 12 The mput signal drives a reslstlve heater at the hot Junctions of a poly/ahummum thermoplle while the cold Junctions are kept at room temperature Thermal

81

References I H Bakes, Microtransducers c, L

Rg

3

J

+ 12 Structure

4

and symbol of the power bridge devxe

5

6

I Fig 13 Diagram of the first integrator the power bndge devices

reconfigured

to Include 8

msulatlon between both Junctions 1s achieved by a mlcromachmmg step that removes the slhcon under the thermoplle hot Junctions embedded in silicon oxide Through the Seebeck effect, a voltage proportional to the mean square value of the input signal (heating voltage V,,) is produced at the output Smce the thermal time constant of the device, m the order of 1 ms, IS much longer than the period of the clock to be applied to the modulator, its transfer charactenstlc can be modelled by

9

10

by mdustnal IC technology and mlcromachmmg, f 0th Sensor Symp , I&??,Tokyo,Japan,May 30-31, 1991 T Nakamura and K Maenaka, Integrated magnetic sensors, Sensors and Actuators, A21 -A23 (19%) 762-769 P Krummenacher and H Oguey, Smart temperature sensors m CMOS technology, Sensors and Acb&ors, A21 -A23 (1990) 636-638 I Kramer, P Seltz and H Bakes, Photo-ASICS Integrated cmxuts for optrcal measurements usmg mdustnal CMOS technology, Tech Dzgest, 6th Int Conf Sohd-State Sensors and Actuators (Transducers ‘91), San Francwo, CA, USA, June 24-28, 1991 T Boltshauser and H Bakes, Capacitive hmmdlty sensors m SACMOS technology arlth mmsture absorbmg photosensltlve polymnde, Sensors and Actuaiors A, 25-27 (1990) 509512 D Moser, R Lenggenhager and H Bakes, Sdlcon gas flow sensors using mdustnal CMOS and bipolar IC technology, Sensors and Actuators A, 25-27 (1991) 577-581 B Baser and B Wooley, The design of sigma-delta modulatlon analog-to-d@al converters, IEEE J Sohd-State Czrcutts, SC-23 (1988) 1298-1308 E DiJkmans and P Naus, Sigma-delta versus binary weighted AD/DA converslon, what IS the most pronusmg?, Proc 15th European Solid-State Cvctat Conf, Vienna, Austrza, Sept 1989, pp 35-63 R Castagnettl, H Bakes and A Nathan, Nmse correlations and operating condltlons of dual-collector magnetotrannstors, Sensors and Actuators A, 25-27 (1990) 363-367 D Jaeggl, D Moser and H Baltes, Thermoelectnc AC power sensor by CMOS technology, IEEE Electron Deozce Lett , EDL-13 (1992) 366-368

Biographies VOUT=~

s

Vhdt

(5)

denotmg the proportlonahty constant Two such devices are then combined, one dnven by the input signal and the other by the feedback signal of a slgmadelta modulator, as illustrated m Fig 13 This combmatlon completely substitutes the first mtegratmg stage of the modulator The output bit stream represents the mean square value, or power, of the input signal

with

l

4. Conclusions In this paper we have demonstrated, with example lmplementatlons, the advantage of sigma-delta modulators for sensor interfaces By very simple reconfiguratlons of a standard structure, different classes of sensors can be applied The robustness and flexlblhty of this techmque are exploited, rather than its potential of high resolution

Carlos Azeredo Leme graduated m Electrical Engneenng at IST, Lisbon, m 1986 In 1990, he recewed the M SC degree m Electronics Engmeermg at the same university In 1991 he Jomed the Physical Electronics Laboratory where he is working towards a Ph D degree His main areas of interest are high resolution A/D converSlon and analog interfaces for nucrosensors

Henry Babes received the D Sc degree from ETH Zurich m 1971 Smce 1988 he 1s Professor of Physical Electronics at ETH Zurich, where he dmzcts the Physical Electronics Laboratory active m &con rmcrosensors He IS also on the Board of Ascom MIcroelectromcs, Director of the SWSS Federal Pnonty Program LESIT, and associate editor of the Journal Sensors and Materials He has authored over 250 pubhcations m sclentdic or techmcalJoumals, 15 patents, and 4 books