Gain-adjusting circuitry for combining two sensors to form a media isolated differential pressure sensor

A media-isolated differential pressure sensor apparatus and corresponding method combines a first signal (207, 209) provided by a first pressure sensor (101), indicative of a difference between a first pressure and second pressure applied across the first pressure sensor, and a second signal (213, 215) provided by a second pressure sensor, indicative of a difference between the second pressure and a third pressure applied across the second pressure sensor (103) to form a differential pressure sensor. Responsive to a pressure span the first signal (207, 209) responds with a slope response different than a slope response of the second signal (213, 215). A slope adjustment circuit (217) enables an adjustment of the slope response of the first signal (207, 209) to correspond to the slope response of the second signal (213, 215), and provides a slope adjusted first signal (221) dependent on the adjusted slope response. A difference circuit (225) provides an output signal (227) dependent on a difference between the slope adjusted first signal (221) and the second signal (223), where the output signal (227) is indicative of a pressure differential sensed between the first pressure sensor (101) and the second pressure sensor (103).

FIELD OF THE INVENTION 
This invention is generally directed to the field of pressure sensors, and 
specifically for media-isolated differential pressure sensors. 
BACKGROUND OF THE INVENTION 
In contemporary automotive systems it is often desirable to measure a 
pressure difference between two locations. For instance, it is desirable 
to measure a pressure difference across a sharp edge oriface in an EGR 
(exhaust gas reflow) system. Often, as in this case, the media can be very 
harsh. Because of this averse environment, isolation from the medium, here 
exhaust gas, is desirable to ensure that the sensor, typically 
semiconductor based, survives and functions properly over a long period of 
time. 
Prior art approaches solved this challenge by using stainless steel 
diaphragms for sensing a pressure coupled by oil to a conventional 
semiconductor based pressure sensor. The stainless steel diaphragm 
provides the necessary isolation between the harsh media and the pressure 
sensor, and the oil provides the transfer of pressure to the pressure 
sensor. The oil medium used in this approach adds error to a pressure 
measurement because in the manufacturing process is difficult if not 
impractical to eliminate all air pockets. These air pockets add error to 
the pressure transfer between the stainless steel diaphragm sensing the 
media harsh pressure and the actual pressure sensor. Also, the oil 
pressure transfer performance is degraded with increasing temperature and 
time because of changes in oil viscosity and leakage of oil. Furthermore, 
using the oil filled approach is difficult to manufacture because the oil 
needs to be hermetically sealed between the stainless steel diaphragm and 
the pressure sensor. 
What is needed is an improved media-isolated differential pressure sensor, 
that is more accurate, easier to manufacture, and has better field 
performance over time and temperature variations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A media-isolated differential pressure sensor apparatus and corresponding 
method combines a first signal provided by a first pressure sensor, 
indicative of a difference between a first pressure and second pressure 
applied across the first pressure sensor, and a second signal provided by 
a second pressure sensor, indicative of a difference between the second 
pressure and a third pressure applied across the second pressure sensor, 
to form a differential pressure sensor. Responsive to a pressure span, or 
a range of pressures, the first signal has a slope response different than 
a slope response of the second. signal. A slope adjustment circuit enables 
an adjustment of the slope response of the first signal to correspond to 
the slope response of the second signal, and provides a slope adjusted 
first signal dependent on the adjusted slope response. A difference 
circuit provides an output signal dependent on a difference between the 
slope adjusted first signal and the second signal, where the output signal 
is indicative of a pressure differential sensed between the first pressure 
sensor and the second pressure sensor. 
Features of the present invention include providing a structure that 
enables the combining of two pressure sensors to form a differential 
pressure sensor that has its critical elements isolated from harsh media. 
Furthermore, the structure enables parallel compensation of span and 
offset errors associated with each sensor. Given this teaching this 
structure can be easily expanded to include more than two sensors. These 
and other benefits of the present invention will be better appreciated 
with a review of the accompanying figures. 
FIG. 1 is a schematic diagram of a first and second pressure sensor both 
mounted on a common assembly. A first pressure sensor 101 and second 
pressure sensor 103 are both hard mounted in a common sensor assembly 100. 
Preferably, these pressure sensors 101 and 103 are constructed of silicon 
and are of the piezoresistive type. The first pressure sensor 101 is 
electrostatically bonded to a glass pedestal 102. The glass pedestal 102 
is bonded to an alumina ceramic substrate 105 using a hard-mount 
technique, such as soldering, or alternatively using an adhesive 107. A 
bond wire 111 electrically connects the sensor 101 to a main substrate 
113. Note that there are actually several bond wires but one is shown for 
clarity. The second pressure sensor 103 has a construction corresponding 
to the construction of the first pressure sensor 101. 
The above-described structure is encapsulated in a housing 115. Reference 
number 110 indicates an unprotected side of the sensor assembly 100, and 
reference number 120 indicates a protected, or media-isolated side of the 
sensor assembly 100. The unprotected side 110 is considered unprotected 
because if the harsh media present on the protected side 120 was exposed 
to the bond wire 111 it would chemically attack it and the bond wire would 
rapidly deteriorate and fail. The protected side 120 of the sensor 
assembly is considered protected because of the mostly hermetic seal 
resulting from the bonding of the elements 101, 102, 105, and 107. The 
unprotected side 110 of the both pressure sensors is not exposed to the 
harsh media. 
A first pressure P1 117, is applied to pressure port 109 on the protected 
side 120 of the first pressure sensor 101. A second, typically ambient, 
pressure Pa 119, is provided on the unprotected side 110 of the sensor 
assembly and is common to both the first and second pressure sensors 101 
and 103. A third pressure P3 121, is applied to pressure port 112 on the 
protected side 120 of the second pressure sensor 103. Next, an electrical 
circuit used to combine outputs from the two pressure sensors 101 and 103 
is detailed in FIG. 2. 
FIG. 2 is a system block diagram illustrating a structural relationship 
used to convert signals provided by the pressure sensors 101 and 103 shown 
in FIG. 1 to form a differential pressure sensor that provides an output 
signal 227 whose response will be dependent on a difference between the 
first pressure P1 111 and the third pressure P3 121 and independent of the 
second pressure Pa 119. Note that circuitry representing the structure 
shown in FIG. 2 is shown in FIG. 3 and is physically located on the main 
substrate 113. 
As pressure P1 117 and Pa 119 are applied to the sensor assembly 100, the 
first pressure sensor 101 outputs a first signal 207, 209 indicative of a 
difference between the first pressure P1 117 and the second pressure Pa 
119 applied across the first pressure sensor 101. The second pressure 
sensor 103 provides a second signal 213, 215 indicative of a difference 
between the second pressure Pa 119 and the third pressure P3 121 applied 
across the second pressure sensor 103. Ordinarily these two signals 207, 
209 and 213, 215 have different slopes over a pressure span because the 
present state of the art manufacturing processes do not allow the 
manufacture of sensors that have perfectly the same slopes. Thus over a 
range of pressure the first signal 207, 209 responds with a slope and over 
the same range of pressure the second signal 213, 215 provided by the 
second pressure sensor 103 responds with a slope different than the slope 
provided by the first pressure sensor 101. An important step in 
synthesizing the output signal 227 is the matching of the slope response 
of the two signals 207, 209 and 213, 215 provided by the two pressure 
sensors 101, 103. 
A slope adjustment circuit 217 conditions the first pressure sensor signal 
207, 209 and provides a slope adjusted first signal 221. Another slope 
adjustment circuit 219 conditions the second pressure sensor signal 213, 
215 and provides a slope adjusted second signal 223. In a minimal 
implementation only the first slope adjustment circuit 217 is necessary 
because the slope of the first pressure sensor signal 207, 209 needs to be 
adjusted to the slope of the second pressure sensor signal 213, 215, so 
the second pressure sensor signal 213, 215 slope can be fixed. In this 
case the slope response of the slope adjusted first signal 221 is adjusted 
to correspond to the slope response of the second pressure sensor signal 
213, 215. In this embodiment the slope of the second pressure sensor 
signal 213, 215 is first adjusted and a slope adjusted second signal 223 
is provided from the slope adjustment circuit 219. Then the first slope 
adjustment circuit 217 provides the slope adjusted first signal 221 after 
the adjustment of the slope of the first pressure sensor signal 207, 209. 
Next the slope adjusted first signal 221 is subtracted from the slope 
adjusted signal 223 by a difference amplifier 225. To understand the 
relevant aspects of combining the slope adjusted first signal 221 and the 
slope adjusted second signal 223, a brief review of equations determining 
the combination will be reviewed as follows. 
The response of the slope adjusted first signal 221 can be expressed using 
the following equation. 
EQUATION 1 
EQU S1=Offset1+m1(P1-Pa) 
Where Offset 1 is a pressure independent (constant) term of the slope 
adjusted first signal 221 derived from the first pressure sensor 101, and 
m1 is a pressure slope of the slope adjusted first signal 221 derived from 
the first pressure sensor 101. P1-Pa is a differential pressure applied 
across the first pressure sensor 101 with P1 111 applied from the 
protected side 120 and Pa 119 applied from the unprotected side 110. 
The response of the slope adjusted second signal 223 is described using the 
following equation. 
EQUATION 2 
EQU S2=Offset2+m2(P3-Pa) 
Where Offset2 is a pressure independent (constant) term of the slope 
adjusted second signal 223 derived from the second pressure sensor 103, 
and m2 is a pressure slope of the slope adjusted second signal 223 derived 
from the second pressure sensor 103. P3-Pa is a differential pressure 
applied across the second pressure sensor 103 with P3 121 applied from the 
protected side 120 and Pa 119 applied from the unprotected side 110. 
If an adjustment is made in such a way that m1=m2=m, then the subtraction 
done by the difference amplifier 225 of the slope adjusted first signal 
221 from the slope adjusted second signal 223 will produce resultant 
signal S2-S1 that is dependent on the difference of third pressure P3 121 
and first pressure P1 111. In addition, signal S2-S1 is completely 
independent to the second pressure Pa 119 which is common to the first 
pressure sensor 101 and second pressure sensor 103. Thus a differential 
pressure sensor that responds to the protected side 120 pressures only is 
created. This deterministic result can be expressed in the following 
equation. 
EQUATION 3 
EQU S2-S1=(Offset2-Offset1)+m(P3-P1)+m(Pa-Pa) 
As is indicated above the difference amplifier 225, provides necessary 
subtraction, of the slope adjusted first signal 221 and the slope adjusted 
second signal 223. In addition, the difference amplifier 225 is coupled to 
an offset-temperature-compensation circuit 237 which allows temperature 
compensation of the offset term of both pressure sensors 101 and 103 at 
the same time. Furthermore, the difference amplifier 225 possesses means 
for adjustment of the total circuit gain and offset at a reference 
temperature. 
An additional component of the block diagram shown in FIG. 2 is a 
span-temperature compensation circuit comprising a network 229 coupled to 
each of the first and second pressure sensors 101, 103, and one side of a 
power supply signal 203, and a network 233 coupled to each of the first 
and second pressure sensors 101, 103, and another side of the power supply 
signal shown at reference number 205. The span-temperature compensation 
circuit 203, 229, 233, and 205 provides span-temperature compensation for 
the first and second pressure sensors 101, 103 at the same time. Signals 
present at reference numbers 231 and 235 derived from the span-temperature 
compensation circuit 203, 229, 233, and 205 are provided to the 
offset-temperature-compensation circuit 237 which in turn provides an 
offset-span-temperature-compensation signal 239. 
FIG. 3 is a schematic diagram illustrating details of the system block 
diagram introduced in FIG. 2. Referring to FIG. 3 the span-temperature 
compensation circuit introduced in FIG. 2 is repeated here and includes 
elements 203, 229, 233, and 205. Additionally, thermistor elements 230 and 
232 are part of the networks 229 and 233. The first adjustment will be to 
trim resistors in the networks 229, 233 to set a correct temperature 
coefficient of this network for span-temperature compensation. Here 
networks 229 and 233 are shown as variable resistors. In the preferred 
embodiment these networks 229 and 233 are fixed resistive ink resistors 
printed on a thick film network. Adjustment is achieved by laser trimming. 
The next step is to adjust the slopes of the signals 221 and 223 to be the 
same. The slope adjusted first signal 221 is an output of the differential 
pair composed of operational amplifiers 307, 309 and gain determining 
resistors 329, 321 which produces a positive differential voltage from the 
output voltage 207, 209 of the first pressure sensor 101. The slope 
adjusted second signal 223 is an output of the differential pair composed 
of operational amplifiers 303, 305 and gain determining resistors 323, 331 
which produces a negative differential voltage from the output voltage 
213, 215 of the second pressure sensor 103. These two signals 221 and 223 
are then summed together using an operational amplifier 301 and equal 
input resistors 317 and 319. Thus summation of the positive differential 
voltage and negative differential voltage creates the virtual subtraction 
of the signal from the first pressure sensor 101 from signal from the 
second pressure sensor 103. The slope of either voltage 221 or 223 can be 
adjusted but only one has to be actually trimmed to match the other slope. 
Thus, for example, the slope of the slope adjusted first signal 221 can be 
adjusted by trimming resistor 321 or the slope of the voltage 223 can be 
adjusted by trimming resistor 323. As was mentioned before resistors 317 
and 319 have to equal for perfect subtraction of the voltages 221 and 223. 
After the slope adjustment step the circuit gain is trimmed using resistor 
311. Circuit offset is increased by trimming resistor 327 and decreased by 
trimming resistor 325. Finally, the entire sensor assembly 100 is exposed 
to two different temperatures to determine temperature drift of offset. 
Measured temperature drift of the offset voltage is then trimmed using 
either resistor 315 or 313 depending on the direction of the drift. In 
conclusion, an improved media-isolated differential pressure sensor, that 
is more accurate, easier to manufacture, and has better. field performance 
over time and temperature variations has been detailed. It overcomes the 
deficiencies of prior art approaches by replacing the media isolation 
technique of a stainless steel diaphragm and oil with an electrostatically 
bonded semiconductor structure. Furthermore, a simplified approach for 
calibrating multiple sensors and combining their outputs to electronically 
form a differential signal provides a substantial manufacturability and 
field performance advantage.