Pressure transducer with error compensation from cross-coupling outputs of two sensors

A pressure transducer that includes at least two sensors having substantially similar or substantially identical error characteristics, wherein each sensor is arranged to be subjected to an applied pressure and the outputs of the sensors are electrically coupled so that errors associated with one sensor are compensated by errors associated with the other sensor. The sensors may be substantially identical silicon sensors formed in close proximity on the same wafer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a pressure transducer, and more 
particularly to a pressure transducer having error compensation. 
2. Discussion of the Related Art 
Pressure transducers using strain gauges in a Wheatstone bridge 
configuration are wellknown in the art. Such pressure transducers are 
sensitive to various disturbances, such as temperature changes, which, if 
uncompensated, will cause errors in the pressure reading. Temperature 
induced errors may be observed as a change in the output of the transducer 
as temperature varies with zero pressure applied, and as a change in the 
difference between the full-scale output and the zero pressure output as 
the temperature varies with full-scale pressure applied. These errors are 
known as "thermal effect on zero (or offset)" and "thermal effect on 
span", respectively. 
Methods are well-known in the art to compensate for such errors and 
initially require characterization of the transducer to define any errors. 
Typically, at least two points from the output signal of the transducer 
are recorded as temperature is varied over a desired range both with zero 
pressure applied and with some amount of pressure applied. The pressure 
applied is typically, but not necessarily, full-scale pressure, and the 
output is recorded at the same temperature points with zero pressure and 
with the applied pressure. Based on the output signals, the uncompensated 
thermal effects are calculated and used to derive the required amount of 
compensation. 
Any of several methods can be used to provide error compensation in a 
pressure transducer. One common method is to add resistors in series with 
the bridge supply voltage, and in series with and/or in parallel to the 
individual bridge resistors. The resistors are chosen based on the 
particular thermal properties necessary to negate the observed thermal 
effects, and their values are calculated based on the uncompensated 
thermal measurements. Error compensation may also be accomplished by laser 
trimming resistors or thermistors to force voltage changes at the sensor. 
Another method, known as digital compensation, uses stored data to 
generate error-correction signals which are added to or subtracted from 
the uncompensated output of the bridge. 
Error compensation to achieve accurate pressure measurements, however, can 
be a costly and time-consuming process. Frequently, the process of 
characterizing the transducer, adding compensation, re-characterizing and 
adjusting the compensation must be repeated several times to obtain the 
desired accuracy. This can be more difficult with particular transducer 
designs; for example, in micro-machined silicon sensors with full-scale 
pressures of 1 inch (1") H.sub.2 O or less. 
Acceleration and gravity are additional factors that can affect the 
sensitivity of pressure transducers, particularly for use in low pressure 
applications due to the relatively high mass of their diaphragms in 
relation to the small force necessary to deflect them. While acceleration 
forces may not be a factor in all applications, gravity is omnipresent and 
can cause transducers to be sensitive to their mounting position. Error 
compensation for acceleration and gravity typically requires using complex 
structures that are expensive and difficult to make. 
Warm-up errors and drift are also factors that affect the sensitivity of 
even a well-compensated transducer. Warm-up errors and drift occur when a 
transducer is first turned on due to a thermal lag between components. 
This cannot generally be reduced by existing compensation methods, but 
requires highly stable or closely matched components that can 
substantially increase the product cost. 
It is also known to make a pressure transducer using a thin, silicon chip 
on which have been formed a number of resistances that function as strain 
gauges. As the cost of these silicon strain gauges has decreased, it has 
been suggested to interconnect two of these silicon transistors so that 
errors associated with one sensor cancel the errors in the other sensor. 
In particular, U.S. Pat. No. 4,565,097 discloses a pair of interconnected 
wheatstone bridge sensors. In the '097 patent, the resistances of one 
sensor are connected in the same portion of the bridge with an opposing 
element of the other sensor so that offsets and drifts are opposed by and 
largely cancelled by those of the other sensor. Since the pressure of 
interest is applied to only one of the sensors in the pair of sensors, 
however, the pressure transducer produces an output that is not cancelled 
by the other sensor. 
Although the pressure transducer illustrated in the '097 patent may result 
in the cancellation of temperature effects, drifts, and offsets, it still 
requires that the errors of each of the sensors that make up the pressure 
transducer be characterized so that sensors having opposite error effects 
are paired together. For example, if the first sensor in the pressure 
transducer has a positive temperature coefficient, the second sensor to be 
used for cancellation of the positive temperature coefficient should have 
a negative coefficient so that when the sensors are connected together to 
form the pressure transducer, the positive and negative temperature 
coefficients will cancel each other out. 
In addition, the '097 patent wires the two sensors together so that the 
resistors that make up each leg of the wheatstone bridge are placed in 
series with each other. Due to this series connection of the resistances, 
the '097 patent requires that the connections between the resistances of 
the bridge be externally accessible. This can be somewhat inconvenient 
when working with silicon strain gauges that have already been completely 
constructed in a particular die. Additionally, the series connection of 
resistances in the '097 patent requires that the two sensors to be paired 
together have opposite error characteristics so that the errors will 
cancel when the bridges are wired together. 
SUMMARY OF THE INVENTION 
The present invention improves upon the state of the art by providing a 
pressure transducer that includes at least two sensors having 
substantially similar or substantially identical error characteristics, 
wherein each sensor is arranged to be subjected to an applied pressure and 
the sensors are electrically cross-coupled so that errors associated with 
one sensor are compensated or substantially cancelled by errors associated 
with the other sensor. 
In an illustrative embodiment, the pressure transducer comprises a pair of 
sensors having substantially similar or substantially identical error 
characteristics. Each sensor is arranged to be subjected to a first 
pressure and a second pressure. The sensors are electrically cross-coupled 
so that an error in one of the sensors is compensated with a substantially 
similar or substantially identical error in the other of the sensors. 
In another illustrative embodiment, the pressure transducer comprises a 
first base member, a second base member, and a first sensor and a second 
sensor having substantially identical error characteristics. The first 
sensor is disposed on the first base member and the second sensor is 
disposed on the second base member. The transducer also includes an 
interface member disposed between the first base member and the second 
base member to define a first cavity about the first sensor, and a cover 
disposed on the second base member to define a second cavity about the 
second sensor. The first and second sensors are fluidly and electrically 
coupled so that an error in the first sensor is compensated with a 
substantially identical error in the second sensor. 
In a further illustrative embodiment, a method of forming a pressure 
transducer with error compensation comprises steps of providing a pair of 
sensors having substantially identical error characteristics, fluidly 
coupling the sensors so that each sensor is arranged to be subjected to a 
first pressure and a second pressure, and interconnecting the sensors so 
that an error in one sensor is compensated with a substantially identical 
error in the other sensor.

DETAILED DESCRIPTION 
A pressure transducer may include two or more pressure sensors having 
substantially similar or substantially identical error characteristics. 
The sensors may be electrically interconnected in such a manner that the 
errors associated with each sensor compensate or cancel one another to 
provide accurate pressure measurements. Although the error compensation 
techniques of the invention may theoretically be accomplished with any 
type of sensor, the pressure transducer preferably uses micro-machined 
silicon sensors or dies formed on a silicon wafer. 
Silicon sensors are small, inexpensive and can be closely matched to 
provide a high degree of compensation with minimal additional processing. 
Since error characteristics are predominantly process related, errors tend 
to be very similar for sensors processed on the same silicon wafer which 
may contain from several hundred to several thousand sensor die. As the 
relative position of the sensor die on the wafer become closer to each 
other, the error characteristics of each die tend to become even more 
similar. Accordingly, the transducer may use sensors having substantially 
identical error characteristics simply by choosing sensors that are formed 
in close proximity to each other on the same wafer. Preferably, sensor die 
that have been formed adjacent or next to each other on the wafer are 
chosen to substantially increase the likelihood that they have essentially 
the same error characteristics. By using sensors that have been formed in 
close proximity to each other in the arrangement of the invention, error 
characterization and matching may be advantageously avoided thereby 
providing an accurate pressure transducer at relatively low cost. 
FIGS. 1-3 illustrate one embodiment of a pressure transducer 20 in 
accordance with the present invention. The pressure transducer 20 may 
include a first sensor module 22 and a second sensor module 24 that are 
interconnected in a manner so as to provide an accurate measurement device 
having error compensation. The second sensor module 24 may be mounted on 
top of the first sensor module 22 to provide a compact pressure transducer 
that is easily assembled to establish both the fluid and electrical 
interconnections. 
The first sensor module 22 may include a first base member 26, a first 
sensor 28, and a first cover 30. Similarly, the second sensor module 24 
may include a second base member 32, a second sensor 34, and a second 
cover 36. Each sensor 28, 34 is mounted on its base member 26, 32 and 
enclosed with its cover 30, 36 to protect the sensor and also to establish 
fluid interconnections between the modules. The pressure transducer 20 may 
also include a plurality of electrical contacts 38 for electrically 
interconnecting the first and second modules 22, 24 and consequently the 
first and second sensors 28, 34 which may be electrically coupled to the 
contacts 38 in a manner apparent to one of skill in the art. 
In one embodiment, each base member 26, 32 may be an interconnection device 
that conveniently establishes the electrical interconnections between the 
sensors 28, 34 and the electrical contacts 38. As shown in FIG. 2, each 
base member 26, 32 is preferably a printed circuit board that includes 
conductive circuitry 40 formed on insulative material using manufacturing 
methods well known in the art. Each circuit board 26, 32 may include an 
insulating layer (not shown) for protecting the circuitry 40 and reducing 
the possibility of electrical shorts between adjacent circuit traces. The 
use of printed circuit boards significantly enhances the assembly of the 
transducer at a relatively low cost. 
The assembly process of the transducer 20 may be further enhanced by 
combining the individual circuit boards 26, 32 for the first and second 
sensor modules 22, 24 in one printed circuit board that includes the 
circuitry for both the first and second sensor modules. As shown in FIGS. 
2 and 3, the first and second sensor modules 22, 24 may be advantageously 
assembled as a pair, thereby ensuring that the substantially identical 
first and second sensors 28, 34 remain together during the assembly 
process. At the final stages of the transducer assembly process, the first 
and second sensor modules 22, 24 may be separated from each other by 
splitting the circuit board along a separation line 42 so that the second 
sensor module 24 may then be mounted to the first sensor module 22 to form 
the pressure transducer 20 as shown in FIG. 1. 
The electrical contacts 38 may be elongated, conductive members having a 
lower end connected to the circuitry on the first base member 26 and an 
upper end connected to the circuitry on the second base member 32. In one 
embodiment, the lower end of each contact 38 may be electrically connected 
to a circuit pad 44 (FIG. 4) along an edge of the first base member 26. 
The second base member 32 may include corresponding electrical 
feedthroughs, such as plated holes 46, that receive the upper end of the 
electrical contacts 38 therethrough so that the contacts may be 
electrically connected to the feedthroughs. The electrical contacts 38 may 
be soldered to the first and second base members 26, 32 to establish the 
electrical connections between the sensor modules. Each sensor 28, 34 may 
be electrically connected to the circuitry 40 using conventional 
techniques, such as wire bonds 48 (FIG. 2) that may be ultrasonically 
welded between pads 50 on the sensor and pads 52 on the circuitry. It 
should be understood that other electrical interconnection techniques may 
be used as would be apparent to one of skill in the art. 
As will be explained in greater detail later on, the wheatstone bridges 
that comprise sensors 28 and 34 are connected together in parallel. Thus, 
the illustrated arrangement of electrical contacts 38, through holes 46 
and circuitry 40 is particularly advantageous because it allows the 
parallel electrical connection between sensors 28 and 34 to be made 
quickly and easily. 
The first and second covers 30, 36 are mounted to their respective base 
members 26, 32 to enclose and protect the sensors 28, 34. Further, the 
covers are constructed so as to fluidly interconnect the first and second 
sensor modules 22, 24 to each other in a desired manner to compensate for 
process errors in the sensors 28, 34. The covers 30, 36 may include 
cavities and orifices that communicate with each other and also with 
orifices in the base members to selectively channel pressure media to the 
upper and lower sides of the sensors. The covers should be mounted to the 
base members in a manner that seals the sensor modules against leakage to 
maintain pressure therein. The covers may be formed from a material such 
as plastic, metal or the like as would be apparent to one of skill in the 
art. 
In one embodiment, the covers 30, 36 may be bonded to their respective base 
members 26, 32 using an adhesive material that mechanically secures and 
fluidly seals the covers to the base members. As shown in FIG. 2, a bead 
of adhesive 54, 56 may be applied to each base member 26, 32 in a pattern 
that bonds the lower surfaces of each cover 30, 36 to the base member. 
Similarly, as shown in FIG. 3, a bead of adhesive 58 may be applied to the 
upper surface of the first cover 30 to bond the second sensor module 24 to 
the first sensor module 22 so that the first cover acts as an interface 
member between the first and second sensor modules. In a like manner, the 
perimeter of each sensor 28, 34 may be bonded to its respective base 
member 26, 32 so as to mechanically secure the sensor and fluidly isolate 
the opposing sides of the sensor from each other. The covers and sensors 
may be bonded to the base members using an adhesive such as RTV silicone, 
an epoxy or similar adhesive material as would be apparent to one of skill 
in the art. 
As illustrated in FIGS. 4, 5 and 6, the pressure transducer 20 may be 
mounted in a housing 60 that protects the transducer and allows the 
transducer to be mounted in a system to be coupled to one or more pressure 
sources for measuring pressure. The housing 60 may include a first inlet 
port 62 that may be coupled to a first pressure source and a second inlet 
port 64 that may be coupled to a second pressure source. The inlet ports 
62, 64 may be configured as would be readily apparent to those skilled in 
the art to provide a fluid connection to the pressure sources. The 
pressure transducer 20 may be supported on a bottom portion of the housing 
and secured using an adhesive material that also fluidly seals the 
transducer to the housing. The bottom portion of the housing may include a 
first plenum 66 fluidly coupled to the first inlet 62 and a second plenum 
68 fluidly coupled to the second inlet 64 to distribute pressurized media 
to the pressure transducer. The electrical contacts 38 of the transducer 
may be interconnected to monitoring equipment or the like using wire, a 
connector or other interconnection device that would be apparent to one of 
skill in the art. 
The covers 30, 36 and the base members 26, 32 of the sensor modules may 
include cavities and orifices that are configured to distribute the 
pressurized media to particular sides of the sensors. Each of the silicon 
die that comprises a sensor has a "inert" side and a "circuitry" side. 
Since the sensors are formed on silicon substrates, one side, the 
"circuitry" side will have the various resistors and electrical components 
formed thereon. The other side, the "inert" or substrate side, will not 
have any components formed on it. Therefore, the covers 30, 36 and the 
base members 26, 32 can be arranged so that a particular pressure medium 
is directed to the inert or circuitry side of the sensor. This can be 
advantageous in the case of, for example, corrosive fluids that would 
adversely effect the "circuitry" side of the sensor. The corrosive fluid 
could instead be directed to the inert side of the sensor thus allowing 
the pressure to be measured without damaging the sensor itself. This will 
be explained in more detail in conjunction with the discussion of the 
embodiments illustrated in FIGS. 5 and 6. 
FIG. 4 is a schematic cross-sectional view of the pressure transducer 20 
illustrated in FIGS. 1-3 which is configured so that pressure is applied 
to opposite sides of the sensors and the sensor output signals are 
subtracted (as will be explained in greater detail later on) to compensate 
for the errors. As illustrated, the first pressure P.sub.1, which is 
present in the first plenum 66 of the housing, may be distributed to the 
lower side 70 of the first sensor 28 and the upper side 72 of the second 
sensor 34 through a combination of orifices and cavities in the base 
members and first cover. The first base member 26 may include a first 
orifice 74 disposed below the first sensor 28, which is enclosed by a 
first cavity 76 in the first cover 30, and a second orifice 78 spaced from 
the first orifice 74 so that it is not obstructed by the sensor and 
communicates with a second cavity 80 in the first cover 30. A third 
orifice 82 may couple the second cavity 80 to a third cavity 84 in the 
first cover which in turn may be coupled to a fourth cavity 86 in the 
second cover 36 by a fourth orifice 88 extending through the second base 
member 32. The fourth orifice 88 is spaced from the second sensor 34, 
which is enclosed by the fourth cavity 86, so that the first pressure 
P.sub.1 is present in the fourth cavity 86. Accordingly, the first 
pressure P.sub.1 is directed from the first plenum 66 to the lower side 70 
of the first sensor 28 and the upper side 72 of the second sensor 34. 
As illustrated in FIG. 4, the second pressure P.sub.2, which is present in 
the second plenum 68 of the housing, may be distributed to the upper side 
90 of the first sensor 28 and the lower side 92 of the second sensor 34 
through a similar combination of orifices and cavities in the base members 
and first cover. The second plenum 68 may be coupled to the first cavity 
76 by a fifth orifice 94 extending through the first base member 26 so 
that the upper side 90 of the first sensor is subjected to the second 
pressure P.sub.2. A sixth orifice 96 may extend through the first cover 30 
to couple the first cavity 76 to a fifth cavity 98 in the first cover. The 
second base member 32 may include a seventh orifice 100 disposed below the 
second sensor 34 to couple the lower side 92 of the second sensor to the 
second pressure P.sub.2 that is present in the fifth cavity 98. 
Accordingly, the second pressure P.sub.2 is directed from the second 
plenum 68 to the upper side 90 of the first sensor 28 and the lower side 
92 of the second sensor 34. 
FIG. 5 is a schematic cross-sectional view of the pressure transducer 20 
illustrated in FIGS. 1-3 which is configured so that sensor 28 acts as a 
reference sensor and the pressure to be measured is applied to sensor 34. 
The sensor output signals are subtracted (as will be explained in greater 
detail later on) to compensate for the errors. As illustrated, this 
arrangement may be readily achieved by configuring the second sensor 
module 24 so that the second sensor 34 is mounted to the second base 
member 32 over the fourth orifice 88 and the fourth cavity 86 is coupled 
to the fifth cavity 98 by the seventh orifice 100. In addition, housing 60 
is reconfigured so that the second pressure P.sub.2 (which acts as the 
reference pressure) in plenum 68 passes through the first orifice 74 to 
the lower side 70 of sensor 28 and through the fifth orifice 94 into the 
first cavity 76 and the upper side 90 of sensor 28. In addition, reference 
pressure P.sub.2 passes through the sixth orifice 96 into the fifth cavity 
98, through the seventh orifice 100 into the fourth cavity 86 and impinges 
upon the upper side 72 of sensor 34. Reference pressure P.sub.2 may be any 
pressure but is typically atmospheric pressure. 
Pressure P.sub.1, the pressure to be measured, is present in plenum 66 and 
passes through the second orifice 78 into the second cavity 80, through 
the third orifice 82 into the third cavity 84 and through the fourth 
orifice 88 to impinge upon the lower side 92 of sensor 34. 
FIG. 6 is a schematic cross-sectional view similar to FIG. 4 of a pressure 
transducer 20 that is configured to provide a differential pressure 
transducer. As illustrated, this arrangement may be readily achieved by 
reconfiguring housing 60 and base member 26 from the configuration 
illustrated in FIG. 5. In the embodiment of FIG. 6, the fifth orifice 94 
in base member 26 is closed off. A reference pressure P.sub.3 (which may 
be any pressure but is typically atmospheric pressure) is applied through 
the eighth orifice 110 into the fourth cavity 86. Reference pressure 
P.sub.3 impinges upon upper side 72 of sensor 34. In addition, reference 
pressure P.sub.3 passes through the seventh orifice 100 into the fifth 
cavity 98, through the sixth orifice 96 into the first cavity 76 to 
impinge upon upper side 90 of sensor 28. One pressure to be measured, 
pressure P.sub.1, is introduced into plenum 66 and passes through the 
second orifice 78 into the second cavity 80. From the second cavity 80, 
pressure P.sub.1 passes through the third orifice 82 into the third cavity 
84, through the fourth orifice 88 and impinges on the lower side 92 of 
sensor 34. Another pressure to be measured, pressure P.sub.2, is present 
in plenum 68 and passes through the first orifice 74 to impinge the lower 
surface 70 of sensor 28. As will be explained in detail later on, when the 
outputs of sensors 28 and 34 are subtracted from each other, since 
reference pressure P.sub.3 impinges upon one side of sensors 28 and 34 
respectively, the resulting output is the difference between pressures 
P.sub.1 and P.sub.2 with the errors between the two sensors being 
cancelled out. 
In order that the errors in sensors 28 and 34 cancel when the outputs are 
subtracted, the sensors should be oriented so that the pressure of 
interest impinges upon an inert side of one sensor and a circuitry side of 
the other sensor. For example, in the embodiment illustrated in FIG. 4, 
pressure P.sub.1 impinges upon the lower side 70 of sensor 28 and the 
upper side 72 of sensor 34. Therefore, sensors 28 and 34 should be 
oriented so that side 70 of sensor 28 is the inert side and side 72 of 
sensor 34 is the circuitry side. Alternatively, sensor 34 could be 
oriented so that the upper side 72 is the inert side and sensor 28 could 
be oriented so that the lower side 70 is the circuitry side. The same 
orientation of sensors as discussed in connection with the embodiment of 
FIG. 4 is also applicable to the embodiment illustrated in FIG. 5. 
In the embodiment illustrated in FIG. 6, wherein a reference pressure 
P.sub.3 (which can be any pressure but is typically atmospheric pressure) 
is applied to both sensors 28 and 34, the sensors need to be oriented so 
that reference pressure P.sub.3 is applied to an inert side of one sensor 
and a circuitry side of the second sensor. Therefore, sensor 34 could be 
oriented so that upper side 72 is the circuitry side and sensor 28 could 
be oriented so that upper side 90 is the inert side. Alternatively, sensor 
34 could be oriented so that the upper side 72 is the inert side and 
sensor 28 could be oriented so that the upper side 90 is the circuitry 
side. 
The embodiments of the pressure transducer illustrated in FIGS. 5 and 6 are 
particularly useful for measuring pressure of corrosive fluids. For 
example, in the embodiment of FIG. 5, if the reference pressure P.sub.2 is 
atmospheric pressure and pressure P.sub.1 to be measured is provided by a 
corrosive fluid, then the inert side of sensor 34 can be oriented so that 
it is in contact with orifice 88. Thus, the corrosive fluid, since it 
would impinge upon the inert side of sensor 34, would not adversely affect 
the circuitry of sensor 34. Furthermore, since sensors 28 and 34 are 
fluidly isolated from each other than the corrosive fluid would not come 
in contact with sensor 28. 
In a similar manner, with respect to FIG. 6, reference pressure P.sub.3 can 
simply be atmospheric pressure. The circuitry side of sensor 28 can be 
oriented so that it receives the reference pressure P.sub.3. The inert 
side of sensor 34 can be oriented so that it receives pressure P.sub.1. As 
a result, pressure P.sub.1 could be a corrosive fluid and pressure P.sub.2 
could be a noncorrosive fluid. Thus, the embodiment illustrated in FIG. 6 
advantageously allows differential pressure measurement for two fluids 
where one of the fluids may be a corrosive fluid. Alternatively, the 
positions of the circuitry side of sensors 28 and 34 could be reversed and 
pressure P.sub.2 could be a corrosive fluid and pressure P.sub.1 could be 
a noncorrosive fluid. 
Reference is now made to FIG. 7, which figure illustrates an electrical 
schematic diagram illustrating how the output and inputs of wheatstone 
bridge sensors 28 and 34 would be wired together to provide error 
cancellation. The circuit 120 illustrated in FIG. 7 is the same for all 
embodiments of the pressure transducer 20 illustrated in FIGS. 1-6. One 
skilled in the art will appreciate that although particular polarities of 
the power supply and output signal are illustrated in FIG. 7, the circuit 
120 would function in the same manner if all of the polarities were 
reversed. 
In FIG. 7, resistors R.sub.1, R.sub.2, R.sub.3, and R.sub.4 form a first 
wheatstone bridge that comprises sensor 28. Resistors R.sub.5, R.sub.6, 
R.sub.7, and R.sub.8 form a second wheatstone bridge that comprises sensor 
34. A voltage or current supply source for circuit 120 is provided at node 
122 which supplies nodes 124 and 126 of sensors 28 and 34, respectively. 
Nodes 128 and 130 are coupled, through node 132, to a reference voltage, 
which is typically ground. Nodes 134 and 136 are coupled together to 
provide a -V.sub.out output at node 138. Nodes 140 and 142 are connected 
together at node 144 to provide a +V.sub.out output. 
As is evident from FIG. 7, the wheatstone bridges that comprise sensors 28 
and 34 are connected in a cross-coupled fashion. That is, for the 
polarities of voltages illustrated, positive output node 140 and negative 
output node 142 are connected together and negative output node 134 and 
positive output node 136 are connected together. Thus, since sensors 28 
and 34 are selected from, in a preferred embodiment, adjacent sensor die, 
they will have substantially similar or substantially identical 
characteristics with respect to changes in their offsets and spans as a 
result of thermal variations. Since the output nodes of the bridges are 
cross-coupled so that the outputs will subtract from each other and since 
both bridges will respond in substantially the same way to thermal 
variations these errors will tend to cancel each other and the output at 
V.sub.out will be the applied or differential pressure substantially free 
of these errors. 
Reference is now made to FIG. 8, which figure illustrates a more detailed 
version of the circuit of FIG. 7. In the circuit of FIG. 8, resistors 
R.sub.S1, R.sub.S2, and R.sub.S3 have been added to allow for adjustment 
of the sensitivity of the pressure transducer. Resistors R.sub.S1, 
R.sub.S2, and R.sub.S3 are typically resistors external to the wheatstone 
bridges themselves. In addition, resistors R.sub.9, R.sub.10, R.sub.11, 
and R.sub.12 have been added to allow for adjustment of the zero point of 
the transducer. Resistors R.sub.9, R.sub.10, R.sub.11, and R.sub.12 are 
typically external to the wheatstone bridges. Although the circuit 
illustrated in FIG. 7 does compensate for the vast majority of the error 
between the two transducers, the circuit of FIG. 8 may be useful in 
applications where additional accuracy in the transducer output may be 
necessary. 
Since the wheatstone bridges illustrated in FIGS. 7 and 8 are connected so 
that the outputs are subtracted, an additional benefit of this circuit 
configuration is that the effects of gravity are substantially eliminated. 
Thus, any of the embodiments of the present invention can be used for a 
particular application without requiring compensation for the orientation 
of the transducer. This allows additional flexibility when incorporating 
the pressure transducer into a particular installation. 
The pressure transducer 20 may be used to measure gauge pressure, 
differential pressure or absolute pressure as defined by the type of 
pressure being applied to each side of the sensors. A gauge pressure may 
be measured when one of the first and second pressures is an applied 
pressure from a pressure source that is to be measured and the other of 
the first and second pressures is a reference pressure, typically 
atmospheric pressure. A differential pressure may be measured when one of 
the first and second pressures is an applied pressure from a pressure 
source and the other of the first and second pressures is a different 
applied pressure from another pressure source. An absolute pressure may be 
measured when one of the first and second pressures is an applied pressure 
from a pressure source that is to be measured and the other of the first 
and second pressures is a vacuum or sealed reference pressure. 
The pressure transducer 20 may be useful for measuring pressures from 
approximately 0.1" (inches) H.sub.2 O to approximately 15 psi, and may be 
particularly suited for measuring pressures of approximately 5 psi or less 
where pressure transducers tend to be more susceptible to acceleration and 
gravitational effects. The actual pressure range of the pressure 
transducer may be limited by the strength of the particular components or 
material such as the adhesive material used to mount and seal the 
components to each other. However, it should be understood that the 
present invention is not to be limited to any particular pressure. 
The pressure transducer 20 may be used to measure the pressure of various 
media such as fluids including gases and liquids as would be apparent to 
one of skill in the art. For example, the medium may be air, a 
refrigerant, oil or the like. In some applications, such as with 
noncorrosive fluids, the sensors may be directly exposed to the medium. 
However, when the medium is corrosive, it may be additionally desirable to 
isolate the sensors from direct contact with the medium, particularly the 
sides of the sensors containing the wiring and circuitry which is 
generally more susceptible to damage. For example, it may be advantageous 
to apply a coating of material, such as RTV silicone or the like, to the 
sensors so that the fluid does not damage the devices. This type of 
protection may also be desirable for keeping moisture, such as may be 
present in air, away from the sensors. For more severe applications, it 
may be desirable to fill the sensor cavities with a nonrigid material, 
such as RTV silicone or the like, which seals the cavity and will transmit 
pressure to the sensor. Further, the sensors may be isolated from the 
pressure medium using bladders, diaphragms or the like. 
The pressure transducer 20 of the present invention may be used in a wide 
range of applications. For example, the transducer may be used to measure 
pressure, including differential pressure, of coolants in refrigeration 
systems such as air conditioners, chillers and the like. The transducers 
may be used to measure oil and hydraulic fluid pressures and the like. The 
transducers may also be used to monitor various processes. One such 
application would be to measure the pressure differential across a filter 
as a means of monitoring when the filter should be replaced as indicated 
by an increase in the pressure differential. The flow rates of fluids 
through a system can also be monitored by measuring the pressure 
differential across the system. It should be understood that these 
applications are exemplary, and numerous other applications for the 
transducer are possible and will readily occur to those skilled in the 
art. 
By using sensors in this manner, offset errors that are intrinsic to the 
basic sensing clement including, but not limited to: offset temperature 
errors, offset warm-up, offset stability, offset thermal hysteresis, 
offset error due to gravity sensitivity may be compensated. The process 
involves building devices using at least two sensors in a single device 
with sensor die from the same silicon wafer, particularly sensor die 
formed in close proximity to each other. The cost of silicon pressure 
sensors has become low enough to justify placing two sensor die in a 
package in lieu of the cost associated with other forms of compensation. 
This is especially true for applications where the present approach 
provides accuracies that do not require any temperature testing. The cost 
associated with having to temperature test and then do other forms of 
compensation is more expensive than the cost of the additional sensor die. 
In those instances where greater accuracy is required, the sensors used in 
the pressure transducer can be presorted. For example, a simple 
temperature test can be done on each sensor to determine the direction (or 
sign) of the thermal effect on zero variations. Zero pressure is applied 
to the sensor and the temperature is increased. Thereafter, sensors with 
thermal effect on zero variations that change in the same direction (i.e., 
have the same sign) can be used as a pair in a pressure transducer since 
their outputs will be cross-coupled so that the variations in the zero 
balance subtract. Thus, a very simple and gross temperature sort can be 
used to provide a very accurate pressure transducer while still avoiding 
the need for exacting and accurate error characterization of each 
individual sensor. 
From the foregoing description, it will be appreciated that the present 
invention provides a relatively low cost pressure transducer having error 
compensation that substantially reduces the effects of process related and 
other types of errors while substantially eliminating the need for sensor 
characterization and matching. Error compensation is enhanced using 
silicon, micro machined pressure sensors from silicon wafers, and is most 
effective for offset errors associated with silicon wafer processing and 
the adjunct micro machining technology typically employed to manufacture 
pressure sensors. Error compensation is also enhanced for offset errors 
associated with variables such as acceleration or gravity effects, warm-up 
drift and long term instability that are not easily compensated by 
characterization and have not had adequate forms of compensation in prior 
technology, thus limiting the use of silicon sensors. 
Having thus described at least one illustrative embodiment of the 
invention, various alterations, modifications, and improvements will 
readily occur to those skilled in the art. Such alterations, 
modifications, and improvements are intended to be within the spirit and 
scope of the invention. Accordingly, the foregoing description is by way 
of example only and is not intended as limiting. The invention is limited 
only as defined in the following claims and the equivalents thereto.