SESSION XV: SOLID-STATE IMAGING AND BIOMEDICAL APPLICATIONS
T H P M 15.3: Integrated Signal Conditioning for Diaphragm
Pressure Sensors
John M. Borky
Kensall D. Wise
Air Force Institute of Technology
University of Michigan
Wright-Patterson AFB, OH
Ann Arbor, MI
THE HIGH SENSITIVITY, compact siAe and freedom from
mechanical instability which characterize integrated piezorcsistive
mechanical sensors have made them attractive for applications
ranging from automotive to biomedical instrumentation. By introducing signal-conditioning electronics on the monolithic sensor
chip along with the diffused piezoresistors, the strong temperature
sensitivity of the sensing mechanism can be compensated, and an
output signal format can be implemented which simplifies use of
the device. This report will discuss a catheter-tip cardiovascular
pressure sensor, although much of the design is applicable to a
variety of transducer types. Table 1 summarizes the design goals.
The scnsor has three main sections, the first of which is a
standard piezoresistive bridge, diffused in a thin silicon diaphragm
and producing an output in the range of 50 to 80pV/V-rnmHg1,2
The measured temperature coefficient of pressure sensitivity
(TCS) for these devices ranges from -1.500 t o -3500ppm/0 C,
depending on the diaphragm geometry and resistor doping levels.
This TCS presents the major temperature compensation task for
the on-chip circuitry.
The second section is a balanced amplifier which raises the
signal level, cancels the common-mode effectsof the temperature
coefficient of bridge resistance (TCR), and partially compensates
the TCS. The single-ended current output of this amplifier is
passed to the final section, a current-controlled oscillator (CCO),
which modulates a switching current onto the dc supply
leads.
The supply current variations can be sensed remotely via a small
resistor in the ground lead, eliminating the need for additional
chip connections. The CCO transfer function has a positive
temperature slope to compensate further the bridge TCS.
The first sensor designed is shown in Figures 1 and 2. The
chip incorporates a circular diaphragm formed by a novel twostep etching process to suit the device to catheter-tip installation3.
This sensor produces an output current attypically 1.5MHz with
a pressure soensitivity of 1.6kHz/mmHg and a TCS of about
1300ppm/ C. The chip meets all of the design goals listed in
Table 1 with the exception of quiescent frequency stability with
temperature, which requires off-chip compensation. To achieve
greater temperature stability, the more sophisticated circuit of
‘Wise, K.D., andClark,
S.K., “DiaphragmFormationand
Pressure Sensitivity in Bath-Fabricated Silicon Pressure Sensors”,
IEDM Digest of Technical Papers, p. 96-99; Dec., 1978.
’Sarnaun, Wise,K.D.,
and Angell, J.B.,“An
IC PiezoresistivePressureSensor
for BiomedicalInstrumentation”,
IEEE
T r a n s . B i o m e d . E n g . , vol. BME-PO, p . 101; March, 1973.
3Borky. J.M., “Silicon
Diaphragm
Pressure
Sensors
with
IntegratedElectronics”,
Ph.D.Dissertation,
TheUniversity
Michigan; 1977.
Grebene, A.B., “AnalogIntegratedCircuitDesign”,
N o s t r a n d - R e i n h o l d ; 1972.
of
Van
Figures 3 and 4 was evolved. This sensor exhibits trmperature
drift of less than 500ppm/0 C, which is adequate for the biomedical temperature range. At the same time, a square diaphragm
geometry was adopted t o achieve greater uniformity in diaphragm
thickness and to permit a comparison of the basic sensor propertics of the circular and squarc versions. Both circuits arc realized
in a triple-diffused, diaphragm-isolated structure which produces
both vertical PNP and vertical NPN transistors and requires no
epitaxial layers.
The circuit of Figure 4 incorporates several tcmpcraturcdependent bias and gain configurations. The lightly-doped
(around 1 0 0 a / 3 ) first diffusion of the triple-diffused process,
produced with a boron implant, yields resistors with a high TCR
(3500ppm/’ C) which is exploited in several ways. The series
combination of diodes D l - Dg, resistor Rg, and transistor Q7
produces a bias current which is nearly independent of temperature. In general, ndiodc-connected transistors in series with a
resistance R produce a stable current, I, if I = -n(dVb,/dT)/(dR/dT).
Elements Q g and R2 produce a scaled-down bias currcnt in thc
input stage with a positive temperature coefficient t o offsct the
thermal drop in transistor transconductance. Another current
sink formed by Qg and R4 establishes a control current to set the
CCO quiescent operating frequency, f,. The high-TCS resistor,
Rg, in the current mirror formed byQ l l and Q l 2 , gives a further
increase with tempcrature in this control current andcompletes
the stabilization off,.
Figure 5 is a simplified equivalent circuit of the input stage
indicating the several temperature-dependent elements which must
be considered to obtain the desired positive temperature coefficient
of gain for this stage and minimize the overall TCS. Feedback resistor R1 sets the gain for the desired sensor response, and its TCR
gives a gain increase with temperature. Rcferring t o Figure 4, Qg
buffers this input stage from the CCO, and its temperature coefficient of p gives a further gain increase. The CCO consists of
Q13 - 4 1 6 and R6 - R8, forming a Schmitt-trigger oscillator
whose basic thermal stability is well established4. The incremental response (frequency change vs input current) has a positive
temperature coefficient, as desired.
The quiescent frequency can be adjusted by the value of R and
is typically in the range of 1MHz. Thus, a series of sensors with
varying fo values can be multiplcxed on a single lead pair for pressure profile measurement. Power consumption is under 10mW,
and the high frequency output facilitates coupling across a
patient isolation interface for safety.
Acknowledgments
The authors are grateful to the staffs of the Electron Physics
Laboratory- at The University of Michigan and the Microelectronics
Laboratory of the Air Force Avionics Laboratory for their assistance in fabricating these devices.
Pressure Range :
Temperature Range:
Resolution:
Accuracy & Linearity:
Size:
General:
0-250 mm Hg
30-45 "C
1%
L1%
~ 1 . mm
5 dia.
2 external leads
Micropower operation
Batch fabrication
No off-chip components
TABLE 1-Sensor design goals.
FIGURE 3-Photomicrograph of modified activepressure
sensor, which incorporates a square diaphragm 24 mils on a
side in a chip 75 mils square; including test devices.
FIGURE 1-Photomicrograph of initial pressure sensor chip
design (chip is 70 mils square,with a circulardiaphragm
diameter of 40 mils); top illumination (a) and back illumination ( b ) with reduced top lighting.
FIGURE 4-Modified pressuresensor
thermal stability.
FIGURE 2-Circuit of initial pressure sensor.
[Right]
FIGURE 5-Simplified equivalent circuit of sensor bridge and
balanced input amplifier indicating temperature-dependent
elements.
circuitfor
improved