Abstract:
Differential pressure sensor systems and methods are disclosed, including an absolute pressure sensor composed of a plurality of pressure sense die for detecting sensed media, wherein each of the pressure sense die possess a back side and a front side and are respectively associated with a plurality of varying pressures. The sensed media are applied only at the backside of the pressure sense die, thereby preventing wire bonds and active regions associated with the front side of the pressure sense die from exposure to sensed media and attack from acids and abrasive chemicals associated with the sensed media.

Description:
REFERENCE TO RELATED APPLICATION 
   This patent application claims priority under 35 U.S.C. § 119(e) to provisional patent application Ser. No. 60/584,976 entitled “Pressure Sensor Methods and Systems,” which was filed on Jul. 2, 2004, the disclosure of which is incorporated herein by reference. 

   TECHNICAL FIELD 
   Embodiments are generally related to sensor methods and systems. Embodiments are also related to pressure sensors. Embodiments are additionally related to micromachined pressure sensors, including absolute pressure sensors that include sense die. Embodiments also relate to exhaust back pressure sensors. 
   BACKGROUND OF THE INVENTION 
   Many micromachined absolute pressure sensors function by mounting the material removal side of the silicon sense die into glass. Such a methodology can create an absolute pressure sensor. In general, an absolute pressure sensor can employ a sealed volume of gas or vacuum on one side of a diaphragm, with another side of the diaphragm being exposed to a sensed fluid. 
   An example of a conventional absolute pressure sensor is disclosed in International Patent Publication No. WO 03/064989 A1 to Gregory D. Parker, which published under the Patent Cooperation Treaty (PCT) on Aug. 7, 2003, and is entitled, “An Absolute Micromachined Silicon Pressure Sensor with Backside Hermetic Cover and Method of Making the Same”. International Patent Publication No. WO 03/064989 A1 is incorporated herein by reference and generally describes an absolute micromachined pressure sensory geometry that provides resistive or piezoresistive strain gauges, conductive traces, wire bond pads, and other electrical components on a micromachined silicon die. 
   The problem with such conventional geometric absolute pressure sensor arrangements and relationships is that the wire bond pads and active regions of the sense die end up facing the sensed media. Exhaust gas contains acids and other chemicals that can penetrate protective gels over time and attack the sense die and its wire bonds. Additionally, conventional absolute pressure sensor configurations lack the ability to process pressure and temperature signals from more than one absolute sense die. 
   A need therefore exists for improved pressure sensor systems and methods in which pressure and temperature sensing signals are converted into the digital realm and calibration functions are implemented to achieve the desired absolute and differential pressure values, followed by conversion back to analog to provide a final output. 
   BRIEF SUMMARY OF THE INVENTION 
   The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
   It is, therefore, one aspect of the present invention to provide for improved sensor methods and systems. 
   It is another aspect of the present invention to provide for improved pressure sensors. 
   It is a further aspect of the present invention to provide for an improved exhaust-back pressure sensor. 
   It is an additional aspect of the present invention to provide for an improved exhaust-back pressure sensor utilizing absolute micromachined pressure sense die. 
   It is also an aspect of the present invention to provide for an exhaust gas recirculation system module pressure sensor utilizing absolute micromachined pressure sense die. 
   It is a further aspect of the present invention to provide for a diesel particulate filter sensor using micromachined pressure sense die. 
   It is yet another aspect of the present invention to provide for differential pressure measurement using backside sensing and one or more application specific integrated circuit (ASIC) devices. 
   The aforementioned aspects of the invention and other objectives and advantages can now be achieved as described herein. Sensor systems and methods are disclosed herein. An absolute pressure sensor is therefore disclosed herein that includes isolation between the sensor&#39;s electronics and the sensed media. The sensor&#39;s electronic circuit can incorporate one or more application specific integrated circuit (ASIC) that processes and outputs the signal for both absolute and differential measurements. Such a sensor can be adapted for use in exhaust gas recirculation (EGR) systems utilized on automotive gasoline engines. Such a sensor can also be utilized f 0 r measuring differential pressure across diesel particulate filters and/or applications in which differential pressure is required for system control and/or monitoring purposes. The absolute pressure sensor disclosed herein can therefore sense the exhaust pressure on automotive engines and other mechanical and/or electro-mechanical devices and machines. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the present invention and, together with the detailed description of the invention, serve to explain the principles of the present invention. 
       FIG. 1  illustrates an exhaust back pressure sensor system in a piezoelectric sense die configuration, which can be implemented in accordance with a preferred embodiment; 
       FIG. 2  illustrates additional features of the system depicted in  FIG. 1 , including a sense die mechanical/electrical interface thereof, in accordance with a preferred embodiment; 
       FIG. 3  illustrates a general packaging configuration for the system depicted in  FIGS. 1–2 , in accordance with a preferred embodiment; 
       FIG. 4  illustrates an over-all exhaust back pressure sensor configuration for the system depicted in  FIGS. 1–3 , in accordance with a preferred embodiment; 
       FIG. 5  illustrates a high-level block diagram of a system, which can be implemented in accordance with a preferred embodiment; 
       FIG. 6  illustrates graphs depicting voltage output and ratio metric outputs in accordance with one embodiment; 
       FIG. 7  illustrates a first level assembly and a second level assembly of the absolute pressure sensor system depicted in  FIGS. 1–5 , in accordance with an alternative embodiment; 
       FIG. 8  illustrates a high-level flow chart of a first level assembly flow sequence, which can be implemented in accordance with one embodiment of the present invention; 
       FIG. 9  illustrates a high-level flow chart of a second level assembly flow sequence, which can be implemented in accordance with another embodiment of the present invention; 
       FIG. 10  illustrates a conventional pressure sensing system that utilizes one die to sense differential pressure across two sides of the die; 
       FIG. 11  illustrates an improved pressure sensing system for measuring differential pressure in accordance with an alternative embodiment; 
       FIG. 12  illustrates a block diagram of a two-ASIC circuit system, which can be implemented in accordance with an alternative embodiment; and 
       FIG. 13  illustrates a block diagram of a single-ASIC circuit system, which can be implemented in accordance with an alternative embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment of the present invention and are not intended to limit the scope of the invention. 
   An exhaust back pressure sensor using absolute micromachined pressure sense die is disclosed herein. In such a device, an absolute pressure sensor with high isolation between the sensed media and the sensor&#39;s electronics can be implemented. Such a sensor can be utilized, for example, to sense the exhaust pressure on automotive engines. 
     FIG. 1  illustrates an exhaust back pressure sensor system  100  in a piezoelectric sense die configuration, which can be implemented in accordance with a preferred embodiment. Note that in  FIGS. 1–5 , identical or analogous parts or elements are generally indicated by identical reference numerals. A portion  101  of system  100  is depicted in  FIG. 1 , as indicated by arrow  103 . Portion  101  indicates that in general, a circuit board (e.g., PCB—Printed Circuit Board)  120  can be co-located proximate to a carrier portion  124  from which a pressure inlet  102  is formed. Carrier portion  124  and carrier portion  122  together can form a single carrier that surrounds pressure inlet  102 . System  100  can be implemented as an exhaust back pressure sensor. 
   A sensor  116  can be configured generally above pressure inlet  102 , as indicated by arrow  105 . Sensor  116  generally includes a hermetically sealed glass cover  114  for absolute pressure reference and a backside reference pressure cavity  112  for absolute pressure sensor capabilities. One or more piezoresistors  110  can be located on micromachined silicon die  104  below the backside reference pressure cavity  112 . Micromachined silicon die  104  can function as absolute pressure sensor die. A diaphragm  106  can be formed from micromachined silicon die such that piezoresistor(s)  110  is incorporated into diaphragm  106 . Additionally, one or more wire bond pads, such as wire bond pad  108  can be provided, which link the micromachined silicon die  104  to circuit board  120 . 
   The core technology of such an exhaust back pressure sensor system is the absolute pressure sensor die (i.e., Micromachined silicon die  104 ). The sense or sensor die  104  can be micromachined from silicon. The die  104  can be, for example, approximately 2.7 mm long, 2.2. mm wide, and 0.39 mm thick. The die  104  can be configured to include a sense cavity (e.g., backside reference pressure cavity  112 ) that is, for example, approximately 1.3 mm square at the base of the die  104 . A conventional silicon etching process can form the cavity angle of cavity  112 . The resulting thickness of diaphragm  106  can be, for example, approximately 20 to 50 microns for a 4 bar full scale absolute pressure sensor, depending of course, upon design considerations. The sensor die can also be configured to incorporate a Wheatstone bridge circuit configuration, referred to simply as a “Wheatstone bridge”. 
   One or more piezoresistors (e.g., 4 piezoresistors) such as piezoresistor  110  can be embedded in diaphragm  106  at locations that maximize the output of the sensor&#39;s Wheatstone bridge (not shown in  FIG. 1 ). A diode can be created as a die temperature reference device. Conductive traces can be embedded in the die  104  to connect the piezoresistors in a full Wheatstone bridge configuration and to connect the bridge corners and the diode to, for example, six wire bond pads at one of the long edges of the die. Four of the six wire bond pads, for example, can be attached to the four corners of the Wheatstone bridge. At least two of the wire bond pads can be attached to the diode. The P diode connection pad may be connected to one of the Wheatstone bridge outputs (voltage supply) to eliminate a wire bond. The wire bonds can then be metalized. The glass cover  114  can then be attached by anodic bond to the top surface of the silicon die  104 , thereby entrapping a reference volume of gas. 
     FIG. 2  illustrates additional features of system  100  of  FIG. 1 , including the sense die mechanical/electrical interface of system  100 , which can be implemented in accordance with a preferred embodiment. System  100  is generally configured so that the absolute sense die  104  with hermetic glass cover  114  can then be attached to a plastic carrier (e.g., carrier portions  124  and  122 ) using, for example, a silicone adhesive  132 . The adhesive  132  can be cured. A protective fluorsilicone gel  126  can be inserted into the carrier&#39;s hole under vacuum in order to protect the silicone adhesive  132  and the diaphragm  106  of the silicon sense die  104 . 
   Circuit board  120  can be implemented as a flexible PCB (Printed Circuit Board) having two stiffened lobes populated using standard surface mount components and techniques. The surface mount components can be utilized to provide electromagnetic interference and electrostatic discharge protection for the circuit. Circuit board  120  can then be coupled to the carrier assembly formed from carrier portions  122 ,  124  using an epoxy adhesive  136 . The adhesive can then be cured. An application specific integrate circuit (ASIC) can then adhesively be connected to the circuit board  120  on the same lobe as the hole for the silicon sense die  104 . An example of such an ASIC is shown in  FIG. 5  as ASIC  502 . The ASIC adhesive can then be cured. The wire bond pads  108  of the sense die  104  and the ASIC generally face the same direction. The ASIC and the sense die  104  can then be connected to the printed circuit board  120  using aluminum and/or gold wire bonds  130 . A plastic cover can be installed over the ASIC and the sense die  104  using an adhesive in order to protect the wire bonds  130  from being subjected to damage during additionally assembly steps. 
   The printed circuit board  120  assembly can then be connected to pressure and electrical connections at several temperatures in order to calibrate the ASIC. At each temperature the uncalibrated output of the sensor system  100  can be verified at two different pressures within the standard output pressure range of the sensor system  100 . The ASIC can then be calibrated using EEPROM trim techniques so that it has the expected output offset and span/slope characteristics at the temperature. Note that as utilized herein, the acronym EEPROM refers generally to Electrically Erasable Programmable Read-Only Memory, which is a user-modifiable read-only memory (ROM) that can be erased and reprogrammed (i.e., written to) repeatedly through the application of higher than normal electrical voltage. 
   The correction values at each temperature of calibration can be recorded utilizing a computer. A calculation can then be utilized to fabricate a multi-order equation that corrects the sensor&#39;s output over temperature. The coefficients for that equation can be loaded into the unit&#39;s EEPROM after the final calibration temperature data is taken. After the correction equation coefficients are loaded into the unit&#39;s EEPROM, it is fully calibrated. An example of such an EEPROM capability is illustrated herein with respect to  FIG. 5  (i.e., see EEPROM calibration  506  of  FIG. 5 ). 
     FIG. 3  illustrates a general packaging configuration for the system  100  depicted in  FIGS. 1–2 , in accordance with a preferred embodiment. In general, one or more tin or gold plated brass terminals  152  can be inserted into a plastic connector  150 . For example, three tin or gold plated brass terminals can be inserted into the plastic connector  150  (e.g., a current connector). A terminal  160  may also be provided, which passes through a section  148 . A patch  162  formed from a thin, porous membrane such as Gore-Tex®, for example, can be crimped into a pocket in the plastic connector  150 . It can be appreciated that the use of Gore-Tex®, for example, is not a limiting feature of the invention described herein, but is presented for exemplary and illustrative purposes only. Other porous membranes or porous materials can be implemented in accordance with embodiments disclosed herein. The pocket leads to a vent hole  166  in the plastic connector  150 , so that the electronics may be vented during assembly through the vent patch  162 . The printed circuit board  120  includes a surface  154  that is connected to the hermetically sealed glass cover  114 . The three terminals of the connector  150  assembly, for example, can then be soldered to an output lobe of the printed circuit board  120 . 
     FIG. 4  illustrates an over-all exhaust back pressure sensor configuration for system  100  depicted in  FIGS. 1–3 , in accordance with a preferred embodiment. A silicone sealant  156  can be inserted onto the base ledge of a housing  168 , which can be configured to incorporate a pressure fitting  170 . The housing  168  can be fabricated from aluminum, but may be implemented as brass or another material, depending upon design considerations. The printed circuit board  120  assembly, which includes the soldered connector  150  and the attached carrier formed by carrier portions  122 ,  124  with sense die  104  can then be inserted into the housing  168  having a surface  172 . A sponge can be inserted between the two folded circuit lobes to prevent shorting and add height to the overall assembly of sensor  100 . Printed circuit board  120  can be configured as a 2-lobe flexible circuit board with stiffeners, an ASIC and electrical components thereof. 
   The additional height allows the carrier formed by carrier portions  122 ,  124  to mate properly with the hole in the housing  168 . The base of the carrier formed by carrier portions  122 ,  124  generally touches a dispensed adhesive  156  in the housing  168 . Such a carrier base can rest on a gap feature in the housing  168  that sets the adhesive height at, for example, approximately 0.5 mm. This 0.5 mm gap can create stress isolation between the housing  168  and the sense die carrier (i.e., carrier portions  122 ,  124 ). The connector  150  possesses an edge that rests on a mating edge in the housing  168 . The carrier-to-housing adhesive  156  can then be cured. 
   The housing  168  can be crimped around the connector  150 , thereby locking the connector  150  in place vertically. The housing  168  crimp also prevents rotation. A sealant  164  can then be installed around the top of the crimp to seal the connector  150  to the housing  168  and provide additional anti-rotation support. That sealant can then be cured. 
   The sensors can be pressured beyond twice their full scale pressure to ensure adhesive integrity. The sensors can then be final-tested to ensure that they possess the proper output when pressure is connected. The units can then be bar-coded and/or labeled for identification and tracking purposes. 
   As explained previously, many conventional micromachined absolute pressure sensors function by mounting the material removal side of the silicon sense die into glass. Such a methodology can create a conventional absolute sensor. The problem with this geometric relationship is that the wire bond pads and active regions of the sense die end up facing the sensed media. Exhaust gas contains acids and other chemicals that can penetrate protective gels over time and attack the sense die and its wire bonds. 
   The embodiments described herein, solve this problem by implementing an absolute sense die concept that creates a necessary reference cavity by hermetically sealing glass or another appropriate material to the backside of the sense die. This arrangement creates a geometry wherein the electrically sensitive portions of the sense die are not exposed to the sensed media. Additionally, protective gel can be inserted into the sensed media side of the pressure sensor. Such a gel can provide an additional layer of protection for both the sense die and the adhesives that attach the sense die to the carrier. 
     FIG. 5  illustrates a high-level block diagram of a system  500 , which can be implemented in accordance with an alternative embodiment. System  500  generally incorporates an ASIC  502  connected to protection circuitry  504 . An unamplified small voltage signal can be provided to ASIC  502  from sensor  116 . Additionally, EEPROM calibration data can be provided by EEPROM calibration as indicated at block  506 , including digital rail limits and software flexibility. Over voltage, reverse polarity, EMC and voltage transient features can be provided to protection circuitry  504 , as indicated by block  508  of  FIG. 5 . 
   ASIC  502  depicted in  FIG. 5  generally converts Wheatstone resistance delta data in to output voltage and corrects the output digitally by an internal 2 nd  order curve fitting equation. The equation coefficients can be calculated and programmed during EEPROM calibration as indicated by block  506 . Temperature can be determined by a diode located on sense die  104 . A signal path from die  104  to the output of sensor  116  is generally analog-digital-analog for isolation and compensation flexibility. 
   The second embodiment relates to an absolute and differential pressure sensor with isolation between the sensor&#39;s electronics and the sensed media. The sensor&#39;s electronic circuitry can incorporate a specific integrated circuit (ASIC) that processes and outputs the signal for both absolute and differential measurements. Such a sensor can be utilized for exhaust gas recirculation (EGR) systems on automotive gasoline engines. 
   The embodiment of  FIG. 5  is based upon the use of absolute pressure sense die  104 . The sense die  104  can be configured via micromachined silicon with piezoresistors in a Wheatstone bridge configuration and a topside glass cap. Wire bond pads on the topside allow for electrical connection to the Wheatstone bridge. The sense die  104  can be mounted to a plastic carrier using, for example, a silicone adhesive. The adhesive can be cured and the assembly can be leak checked. 
   Two sense die/carrier assemblies can be attached to a printed circuit board (PCB) using an epoxy adhesive. The epoxy can be cured and the sense dies can be gold ball wire bonded to the PCB. The PCB can incorporate ASIC  504 ,  502 , along with over-voltage and reverse polarity protection circuitry, EMC protection circuitry as indicated by block  508  and output protection circuitry  504 . A steel EMI shield, for example, can be crimped onto the PCB to protect the wire bonds from damage and also provide EMI protection. Such an assembly can be referred to as the “Board Assembly”. 
   The Board Assembly can be calibrated by applying various pressures at various temperatures and measuring the sense die response via the ASIC  502 . Coefficients for a correction function can be calculated and programmed into EEPROM locations in the ASIC  502 . During calibration, software can be utilized to control desired output transfer functions, diagnostic limits, output modes, time response, and resolution, which are indicated by block  506  in  FIG. 5 . 
   The calibrated Board Assembly can then be attached to a housing using silicone sealant on the bottom of the carriers and soldering terminals to the PCB. The silicone sealant can then be cured. A lid can be pressed into the housing and sealed with a silicone adhesive. The silicone adhesive can then be cured. A vent hole on the lid can be sealed with an epoxy and cured. Such an assembly can be referred to as the “Final Assembly”. The Final Assembly can then be final-tested to ensure proper output. The Final Assembly can then be labeled for identification and tracking purposes. 
   The ASIC  502  is a key aspect of the aforementioned second embodiment. The ASIC  502  can allow for two sense dies to be digitally calibrated with temperature compensation. An advantage of such a configuration is that the ASIC  502  permits the ability to control output modes via software during calibration. Possible output modes are Pressure  1 , Pressure  2 , Temperature  1 , Temperature  2  Differential Pressure and Flow. Whereas conventional sensor configurations must be re-designed to change from a DP/MAP output configuration to MAP/MAP configuration, the ASIC  504502  allows for this change to occur in-process. Note that the acronym DP represents differential pressure, while the acronym MAP represents manifold absolute pressure. Thus, DP/MAP represents differential pressure/manifold absolute pressure. 
   ASIC  502  can be powered, for example, by a 5 DC voltage source and output two 0.100 DC to 4.900 DC voltage ratio metric analog signals. An internal current source can drive each sense die with approximately 1 mA of constant current excitation. A multiplexer switches the excitation while measuring the pressure and temperature signal of the each Wheatstone bridge thereof. The pressure signal can be measured by reading the differential voltage across the powered Wheatstone bridge. The temperature signal can be measured by reading the bridge resistance of the sense die. The input signals can be conditioned utilizing a chopper pre-amplifier stage. 
   The amplified signals can then be converted to digital values via, for example, a 13-bit Analog-to-Digital Converter (ADC). Such values can then be used along with coefficients from the EEPROM in a second-order curve fit function. The mathematical result can then be passed through a digital averaging filter and compared against diagnostic limitations. The final value can be converted to an analog voltage via an 11-bit Digital-to-Analog converter (DAC). A second order low-pass filter controlled by internal resistors and external capacitors can provide another means of controlling time response of the output signal. 
   As indicated earlier, many conventional micromachined absolute pressure sensors function by mounting the material removal side of the silicon sense die onto glass. The opposite side of the sensor, which contains the wire bond pads and active regions, can be directed toward the sensed media. Various chemicals and environmental elements can corrode and break down the sense die and wire bonds. The embodiment disclosed herein therefore solves these problems by creating the necessary reference cavity by hermetically sealing glass to the active side of the sense die. The passive side of the sense die can be directed towards the sensed media, isolating all of the sensor electronics. 
   Such an embodiment can also provide improved measurement accuracies over conventional sensor configurations, along with improved resolution and speed. The sensor circuit incorporates an ASIC, as indicated above, which can process pressure and temperature signals from one or more absolute sense dies, but preferably, at least two absolute sensor or sense dies. The signals can be converted into the digital realm where calibration functions are implemented to achieve the desired absolute and differential pressure values. Conversion back to analog can provide a final output signal. 
     FIG. 6  illustrates graphs  602  and  604  depicting voltage output and ratio metric outputs in accordance with one embodiment. Graph  602  indicates voltage output, while graph  604  depicts ratio metric output. Graph  602  tracks voltage output versus absolute pressure in kPaA. Graph  604 , on the other hand tracks voltage output ratios versus absolute pressure in kPaA. Graphs  602  and  604  indicate that sensor output may be adjusted in either Span (Slope) or Offset. The upper clamping rail is adjustable from 95% to 97.5%. Additionally, a lower clamping rail is adjustable from 2.5% to 5%. The graphs  602  and  604  are depicted with a lower clamping rail of 2.5% and an upper clamping rail of 97.5%. 
     FIG. 7  illustrates a first level assembly  702  and a second level assembly  704  of the absolute pressure sensor system  100  depicted in  FIGS. 1–5 , in accordance with an alternative embodiment. Note that in  FIGS. 1–7 , identical or similar parts or elements are generally indicated by identical reference numerals. The first level assembly  702  configuration provides a fully functional sub-assembly for flexibility. Calibration (e.g., see block  506  of  FIG. 5 ) generally occurs at the first level assembly  702 . The second level assembly  704  generally features that isolate housing and connector stresses from the calibrated 1 st  level assembly. 
     FIG. 8  illustrates a high-level flow chart  800  of a first level assembly flow sequence, which can be implemented in accordance with one embodiment of the present invention. A carrier such as a carrier formed from carrier portions  122 ,  124  can be loaded to a pallet, as indicated at blocks  801  and  802 . An RTV silicone sealant can be screen printed on the carrier as indicated at block  803  and  804 . Sense die (e.g., sense die  104 ) can be assembled to the carrier as depicted in block  805  and  806 . Thereafter, as indicated at block  808 , in-line cure processes can be implemented upon the devices assembled according to prior blocks  801 ,  802 ,  803 ,  804 ,  805  and  806 . 
   Next, as indicated at block  810 , a flip/leak check operation can be implemented, followed by dispensing of a Sifel gel material on the back side of the sense die (e.g., backside of sense die  104 ). The assembled components can then be moved for curing, as indicated at block  814 , followed by an actual batch vacuum cure, as indicated at block  816 . Thereafter, as indicated at block  818 , the assembly can be moved for epoxy dispensing. Thereafter, as indicated at blocks  824 ,  826  and  820 , an epoxy and a populated, tested, serialized PCB (e.g., circuit board  120 ) can be configured such that the epoxy is dispensed on the PCB followed by assembly of the carrier (e.g., carrier portions  122 ,  124 ) to the PCB (e.g., circuit board  120 ). 
   An in-line cure operation can then be implemented, as indicated at block  822 , followed by connecting the wire bond sense die to the PCB, as depicted at block  828 . Thereafter, as indicated at blocks  831  and  830 , an EMI shield can be assembled (e.g., crimped) to the PCB. Next, as indicated at block  832 , the PCB can undergo a singulate operation. Thereafter, as depicted at block  834 , characterization, calibration and temperature and pressure verification operations can be performed. Finally, as indicated at block  836 , the 2 nd  level assembly can be initiated, the process of which is described in greater detail in  FIG. 9 . 
     FIG. 9  illustrates a high-level flow chart  900  of a second level assembly flow sequence, which can be implemented in accordance with another embodiment of the present invention. Flow chart  900  represents a continuation of the operations depicted in  FIG. 8 . As indicated at block  902 , the PCB (e.g., circuit board  120 ) can be subjected to a singulate operation. Thereafter, as depicted at block  904 , the assembly can be characterized, calibrated and verified with respect to temperature and pressure sensing. Next, as depicted at blocks  905  and  906 , the PCB assembly can be loaded and the ASIC read. Next, as depicted at blocks  907  and  908 , the housing (e.g., housing  168 ) can be loaded. 
   Thereafter, as indicated at blocks  909  and  910 , an RTV sealant can be dispensed into the housing for load sealing thereof. Next, as indicated at blocks  911  and  912 , an RTV sealant can be dispensed for a housing-to-carrier seal and for placing the PCB into the second level assembly. Next, as depicted at block  914 , a load cure pallet operation can be implemented. Thereafter, as depicted at block  916 , the RTV sealant or adhesive can be cured. Thereafter, an unload cure pallet operation can be implemented, as indicated at block  918 , followed by an operation in which solder performs are placed into the second level assembly, as depicted at blocks  919  and  920 . 
   Thereafter, as indicated at block  922 , the preform solder can be checked and used to solder the PCB (e.g., circuit board  120 ) to the housing (e.g., housing  168 ). Following the operation depicted at block  924 , a null check/solder vision check operation can be implemented, followed thereafter by an epoxy and leak check, as depicted at blocks  925  and  926 . Next, as depicted at blocks  927  and  928 , a lid may be located in place in the second level assembly, followed by an operation in which epoxy is dispensed at particular assembly stations, as indicated by blocks  929  and  930 . 
   Thereafter, the epoxy can be dispensed and inspected, as depicted at block  932 , followed by a load cure pallet operation, as indicated by block  934 . Next, as indicated at block  935  an epoxy cure operation can be implemented, followed by an unload cure pallet operation, as depicted at block  936 . Thereafter, as depicted at block  938 , a final test can be performed using four assembly stations, followed by an impact mark operation, as indicated at block  940 . Next, as indicated at block  942 , the second level assembly can be unloaded, sorted and packed into the final product. Finally, as indicated at block  944 , the final product implementing system  100  can be inspected and labeled. 
     FIG. 10  illustrates a conventional pressure sensing system  1000  that utilizes one die  1004  to sense differential pressure across two sides of the die  1004 . System  1000  is configured such that the sense die  1004  detects sensed media  1002 . Media flow is indicated by arrows, such as arrow  1008 . A filter  1006  functions to filter media and is located opposite a containment  1012  in which sense die  1004  is located. In the conventional system  1000 , the differential pressure is transmitted to a control module for controlling and/or monitoring purposes. The problem with such a design is that the wire bonds and active regions of the sense die  1004  end up facing the sensed media  1002 . Because the sensed media  1002  contains acids and other chemicals, the active regions and the wire bonds can be attacked by such chemicals, causing the sense die  1004  to fail. 
     FIG. 11  illustrates an improved pressure sensing system  1100  for measuring differential pressure in accordance with an alternative embodiment. The alternative embodiment of system  1000  solves the problems associated with system  1100  of  FIG. 10  by implementing two separate dies  1107 ,  1109 , one for each pressure P 1  and P 2 , thereby creating a geometry wherein the sensed media is applied only at the backside of the sense dies  1107 ,  1109 . Therefore, the wire bonds and the active regions on one side of the sense die  1107 ,  1109  are not exposed to the sensed media and thus, are not subjected to acids and/or chemical attack. The sensed media is not present on side  1110  of the die. The backside of the dies or the sensing dies, through exposure to the sensed media, can be coated with a layer of protective gel  1112 ,  1114 . Media flow is indicated in  FIG. 11  generally by arrows such as arrow  1108 . The media may flow through a filter  1106 . 
   Because known acids and chemicals contained in the sensed media do not attack this gel  1112 ,  1114 , this side of the die  1107 ,  1109  can also be protected from acids and chemical attacks. Products implemented via the third embodiment discussed herein sense pressure from the backside of two dies  1107 ,  1109 . Associated circuitries can provide EMC protection, over-voltage and reverse polarity protection and other algorithms utilizing a single ASIC. Products based on conventional configurations, however, sense pressure on both sides of a single die. 
     FIG. 12  illustrates a block diagram of a two-ASIC circuit system  1200 , which can be implemented in accordance with an alternative embodiment. System  1200  generally includes an over-voltage and polarity protection circuit  1202  whose output is coupled to a high port ASIC  1206  and a reference port ASIC  1204 . The Output from the high port ASIC  1206  can be coupled to a differential circuit  1208 , which in turn is coupled to a pressure pulsation circuit  1210 . Output from the differential circuit  1208  and the pressure pulsation circuit  1210  can be fed as input to an output protection circuit  1212 . Output from the reference port ASIC  1204  can also be fed to the output protection circuit  1212 . Output from the output protection circuit  1212  can in turn be fed to EMC protection circuitry  1214 . Output from the EMC protection circuitry  1214  can in turn be fed back to the over-voltage and reverse polarity protection circuit  1202 . 
     FIG. 13  illustrates a block diagram of a single-ASIC circuit system  1300 , which can be implemented in accordance with an alternative embodiment. System  1300  generally includes an over-voltage and reverse polarity protection circuit  1302 , whose output is coupled and fed as input to a two-input ASIC  1304 . Output from the two-input ASIC  1304  is then fed to an output protection circuit  1306 . An output signal from the output protection circuit  1306  can then be fed as input to EMC protection circuit  1308 . Finally, output from the EMC protection circuit  1308  can be fed as input to the over-voltage and reverse polarity protection circuit  1302 . 
   Because a need exists for measuring differential pressure across a diesel particulate filter and/or applications in which differential pressure is required for system control and/or monitoring purposes, the embodiments of FIGS.  10  and  12 – 13  can be utilized. Using backside sensing techniques and a single ASIC  1304 , such an embodiment can be implemented as a pressure sensor for measuring the differential pressure across a diesel particulate filter, and/or applications in which differential pressure is used for system control and/or monitoring purposes. 
   Such an embodiment can transmit this differential pressure to a control module in the form of an analog output voltage signal. This output voltage signal is low when the filter  1106  is clean or the media flow is high. Over time, minute particles can build up in the filter  1106 , thereby restricting the media flow. As the media flow decreases, differential pressure across the filter  1106  and the output voltage signal can increase. The engine control module can then use this output voltage signal, together with other parameters to determine whether or not the filter  1106  should be cleaned. 
   The embodiments and examples set forth herein are presented to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. Those skilled in the art, however, will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only. Other variations and modifications of the present invention will be apparent to those of skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. 
   The description as set forth is not intended to be exhaustive or to limit the scope of the invention. Many modifications and variations are possible in light of the above teaching without departing from the scope of the following claims. It is contemplated that the use of the present invention can involve components having different characteristics. It is intended that the scope of the present invention be defined by the claims appended hereto, giving full cognizance to equivalents in all respects.