Abstract:
Compensated pressure sensor includes a MEMS pressure sensor die having resistors RA and RD connected in series in a first leg of a Wheatstone bridge and resistors RB and RC connected in series in a second leg of the Wheatstone bridge; a first and second fuse; and a first, second third, fourth, fifth and sixth resistor; wherein: a first end of the first resistor is connected in series with the first leg of the bridge and a first end of the second resistor is connected in series with the second leg of the bridge; the first fuse is connected, at a first end, to a first output of the bridge, and at a second end, to a second end of the third resistor and to a first end of the second fuse; the second fuse is connected, at a second end, to a second output of the bridge; a first end of the third resistor is connected to an input to the bridge and to a first end of the fourth resistor; a second end of the fourth resistor is connected to a second end of the first resistor, a second end of the second resistor and a first end of the sixth resistor; and the fifth resistor is connected, at a first end, to the input to the bridge.

Description:
TECHNICAL FIELD 
     One or more embodiments relate to compensated pressure sensors and, in particular, to compensated differential pressure sensors, and, more particularly, to a differential pressure transducer using two temperature compensated pressure sensors with output coupling and signal conditioning for high performance. 
     BACKGROUND 
     There are a variety of sensing devices on the market that are capable of providing an indication of stimuli. Many incorporate electrical elements that are subjected to some form of manipulation caused by the physical quantity being sensed, thereby causing a change in their electrical characteristics. One example of such sensing devices is a MEMS (“Micro-Electro-Mechanical Systems”) pressure sensor. A typical such MEMS pressure sensor includes a small, thin silicon chip or diaphragm onto which a number of resistances that function as strain gauges (for example, piezoresistors) are formed by well-known processes in a Wheatstone bridge. In operation, stresses caused by pressure applied to the chip or diaphragm change the resistance values of the piezoresistors in the Wheatstone bridge (the applied pressure causes the chip or diaphragm to deflect, which deflection creates compressive and tensile forces in the resistances thereby causing a change in their electrical values). An electronic circuit detects the changes in resistance values, and outputs an electrical signal representative of the applied pressure. 
     Differential pressure sensors are used to measure pressure differences between two pressure sources. It is known to use separate Wheatstone bridge arrangements of interconnected resistances as pressure sensors for measuring each of the two pressure sources. Ideally, in order to provide an accurate differential pressure measurement, the output voltage versus pressure characteristics for each of the bridge pressure sensors should be similar and should remain similar despite factors such as changing temperature. 
     The above described pressure sensors 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, for example, as a change in the output of the sensor 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. 
     SUMMARY 
     One or more embodiments provide improved compensated pressure sensors. Further, an improved compensated differential pressure sensor is formed by a pair of pressure sensors of similar construction in such a manner as to compensate for unwanted effects in the sensors such as effects due to temperature. In particular, one or more such embodiments relate to compensated differential pressure sensor devices that include MEMS (“Micro-Electro-Mechanical Systems”) pressure sensors. In accordance with one or more such embodiments, two compensated MEMS pressure sensors are combined so that effects from the two sensors would be in opposition to one another. As a result, if they were both subjected to variations of the same polarity, their respective signals would cancel each other, and thus, no output signal would result. If, for example, each of the pair of sensors was subjected to temperature, they would drift in the same way (since they are adjusted to have the same general characteristics) so that one temperature signal would cancel the other temperature signal. In other words, unwanted effects due, for example, to temperature of one sensor compensates for the effects of the other sensor, and a signal is obtained that is more purely responsive to the pressure stimuli. 
     Specifically, in accordance with one embodiment, a compensated pressure sensor comprises: a MEMS pressure sensor die having resistors RA and RD connected in series in a first leg of a Wheatstone bridge and resistors RB and RC connected in series in a second leg of the Wheatstone bridge; a first and second fuse; and a first, a second, a third, a fourth, a fifth and a sixth resistor; wherein: a first end of the first resistor is connected in series with the first leg of the bridge and a first end of the second resistor is connected in series with the second leg of the bridge; the first fuse is connected, at a first end, to a first output of the bridge, and at a second end, to a second end of the third resistor and to a first end of the second fuse; the second fuse is connected, at a second end, to a second output of the bridge; a first end of the third resistor is connected to an input to the bridge and to a first end of the fourth resistor; a second end of the fourth resistor is connected to a second end of the first resistor, a second end of the second resistor and a first end of the sixth resistor; and the fifth resistor is connected, at a first end, to the input to the bridge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic circuit diagram of a compensated differential pressure sensor that is fabricated in accordance with one or more embodiments; 
         FIG. 2  shows the schematic circuit diagram of the compensated differential pressure sensor shown in  FIG. 1  with the addition of an ASIC (“Application-Specific Integrated Circuit”) for compensation and amplification of output from the compensated differential pressure sensor shown in  FIG. 1 ; 
         FIG. 3  shows a cross section of a differential pressure package where pressure ports are on different sides of the package; and 
         FIG. 4  shows a cross section of a differential pressure package where pressure ports are on the same side of the package. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will be described with reference to the figures, wherein like components, parts, and so forth are designated by like reference numerals throughout the various figures. Further, specific parameters such as pressure values, materials, size, dimensions, shapes, and the like are provided herein, and are intended to be explanatory rather than limiting. 
     One or more embodiments are directed to a differential pressure sensor that comprises two compensated MEMS (“Micro-Electro-Mechanical Systems”) pressure sensors, each being in the form of a MEMS sensor die having a Wheatstone bridge which is compensated, as described below, to eliminate unwanted effects when measuring pressure such as, for example and without limitation, temperature sensitivity. MEMS pressure sensor dies used to fabricate embodiments are well known to those of ordinary skill in the art and may readily be obtained commercially from any one of a number of companies such as, among others, companies having readily accessible websites on the Internet. Note that, the compensated differential pressure sensors described herein are not limited to the use of MEMS pressure sensor dies. 
       FIG. 1  is a schematic circuit diagram of compensated differential pressure sensor  1000  that is fabricated in accordance with one or more embodiments, which compensated differential pressure sensor  1000  is typically used to sense and measure the differential pressure of fluids or liquids. As shown in  FIG. 1 , compensated differential pressure sensor  1000  includes compensated MEMS pressure sensor  100  that is cross-coupled, and in parallel, to compensated MEMS pressure sensor  200 . 
     In accordance with one or more embodiments, compensated MEMS sensor  100  shown in  FIG. 1  comprises a MEMS sensor die that includes Wheatstone bridge  110  and compensated MEMS sensor  200  shown in  FIG. 1  comprises a MEMS sensor die that includes Wheatstone bridge  210 . As shown in  FIG. 1 , the positive output of a power supply (not shown in  FIG. 1 ), for example, a voltage supply, is applied to a first end of resistor R s11  and a first end of resistor R s21 , respectively. As further shown in  FIG. 1 , a second end of resistor R s11  is connected to Wheatstone bridge  110  of compensated MEMS sensor  100 , to a first end of resistor R ts1 , and to a first end of resistor R tz1 . Wheatstone bridge  110  comprises resistors R A1 , R B1 , R C1  and R D1 , hence, the second end of resistor R s11  is connected to a first end of resistor R A1 , to a first end of resistor R B1 , to a first end of resistor R ts1 , and to a first end of resistor R tz1 . As further shown in  FIG. 1 , a second end of resistor R s21  is connected to Wheatstone bridge  210  of compensated MEMS sensor  200 , to a first end of resistor R ts2 , and to a first end of resistor R tz2 . Wheatstone bridge  210  comprises resistors R A2 , R B2 , R C2  and R D2 , hence, the second end of resistor R s21  is connected to a first end of resistor R A2 , to a first end of resistor R B2 , to a first end of resistor R ts2 , and to a first end of resistor R tz2 . 
     Resistive elements R A1 , R B1 , R C1  and R D1  of Wheatstone bridge  110  are formed on a planar substrate (not shown) and mounted in a support mechanism that allows the substrate to be subjected to pressure to be measured. The pressure causes the substrate to deform, thereby inducing compressive and tensile forces in the resistive elements, and effecting a change in their electrical characteristics. Typically, placement of resistive elements R A1  and R C1  on the substrate is such that their response to the pressure is in a similar fashion to each other, while resistive elements R B1  and R D1  are placed to respond similarly to the pressure, but in an opposite sense to that of resistances R A1  and R C1 . The placement of the resistances for Wheatstone bridge  210  is the same as that for Wheatstone bridge  110 . 
     As shown in  FIG. 1 , a second end of R D1  of Wheatstone bridge  110  is connected to a first end of resistor R z11 , and a second end of resistor R C1  of Wheatstone bridge  110  is connected to a first end of resistor R z12 . As further shown in  FIG. 1 , a second end of resistor R ts1  is connected to a second end of resistor R z11 , a second end of resistor R z12 , and a first end of resistor R s12 . As further shown in  FIG. 1 , (a) a first end of fuse F 11  is connected to a second end of resistor R A1  of Wheatstone bridge  110  and to a first end of resistor R D1  of Wheatstone bridge  110 ; (b) a second end of fuse F 11  is connected to a second end of resistor R tz1  and to a second end of fuse F 12 ; and (c) a first end of fuse F 12  is connected to a second end of resistor R B1  of Wheatstone bridge  110  and to a first end of resistor R C1  of Wheatstone bridge  110 . 
     As shown in  FIG. 1 , a second end of R C2  of Wheatstone bridge  210  is connected to a first end of resistor R z22  and a second end of resistor R D2  of Wheatstone bridge  210  is connected to a first end of resistor R z21 . As further shown in  FIG. 1 , a second end of resistor R ts2  is connected to a second end of resistor R z21 , a second end of resistor R z22 , and a first end of resistor R s22 . As further shown in  FIG. 1 , (a) a first end of fuse F 21  is connected to a second end of resistor R A2  of Wheatstone bridge  210  and to a first end of resistor R D2  of Wheatstone bridge  210 ; (b) a second end of fuse F 21  is connected to a second end of resistor R tz2  and to a second end of fuse F 22 ; and (c) a first end of fuse F 22  is connected to a second end of resistor R B2  of Wheatstone bridge  210 , to a first end of resistor R C2  of Wheatstone bridge  210 , and to a first end of fuse F 11 . 
     As further shown in  FIG. 1 , a second end of resistor R s12  is connected to a second end of resistor R s22 , and to a negative output of the power supply. 
     Lastly, compensated MEMS sensor  100  and compensated MEMS sensor  200  are cross-coupled. This is accomplished, for the polarities of voltages shown in  FIG. 1 , by connecting: (a) negative terminal  120  of compensated MEMS sensor  100  to positive terminal  230  of compensated MEMS sensor  200 ; and (b) positive terminal  130  of compensated MEMS sensor  100  to negative terminal  220  of compensated MEMS sensor  200 . In accordance with one or more embodiments, output from differential pressure sensor  1000  is taken from positive terminal  230  of compensated MEMS sensor  200  and positive terminal  130  of compensated MEMS sensor  100 . 
     Since the output nodes of Wheatstone bridges  110  and  210  are cross-coupled so that the outputs subtract from each other, and since Wheatstone bridges  110  and  210  will respond to thermal variations so that these errors will tend to cancel each other, the output will be the differential pressure substantially free of these errors. 
     In accordance with one or more embodiments, resistors R z11 , R z12 , R s11 , R s12 , R tz1 , and R ts1  are used to provide compensation for compensated MEMS sensor  100  as follows (resistors R z11 , R z12 , R s11 , R s12 , R tz1 , and R ts1  are typically external to Wheatstone bridges  110  and  210 ) (i.e., this provides a compensated pressure sensor). 
     “Type 1” Compensation for Compensated MEMS Sensor  110 : 
     “Type 1” compensation refers to the following: (a) to provide compensation for compensated MEMS sensor  100  to bring the output voltage at zero pressure desirably close to zero (for example and without limitation, within about +/−0.25 mV or +/−0.1 mV); and (b) to provide compensation for compensated MEMS sensor  100  if the output voltage at zero pressure, over a range of temperatures (typically, this range goes from a cold temperature such as, for example, −25° C., includes room temperature, i.e., about 20° C., and ends at a hot temperature such as 85° C.) is not equal, or suitably close, to zero (for example and without limitation, within about +/−0.25 mV or +/−0.1 mV)—ideally the output from compensated MEMS sensor  100  should be the same or suitably close to being the same over the range of temperatures. Resistors R tz1 , R z11  and R z12  are used to provide Type 1 compensation, and the values of R tz1 , R z11  and R z12  are determined by considering two cases. Case 1 is used when the voltage output of compensated MEMS sensor  100  at zero pressure is negative at room temperature, i.e., in the middle of the range. For case 1, fuse F 12  is cut. Case two is used when the voltage output of compensated MEMS sensor  100  at zero pressure is positive at room temperature, i.e., in the middle of the range. For case 2, fuse F 11  is cut. 
     For Case 1:
 
 R   tz1 =( D*B−H*F )/( H*F/E−D*B/A−G+C )  (1)
 
 R   z11   =R   z12 =( A*C/B )− D   (2)
 
     For Case Two:
 
 R   tz1   =−b ±(SQRT( b   2 −4* a*c )/2* a (the positive value)  (3)
 
 R   z11   =R   z12 =( B*D/A )− C   (4)
 
where:
 
 a=E *( B*D−A*C )− A *( F*H−E*G )  (5)
 
 b=F*E *( BD−AC+AG )− AB ( C*E+F*H−E*G )   (6)
 
 c=A*B*E*F *( G−C )  (7)
 
     where: A is R A1  (as measured at zero pressure at the coldest temperature in the temperature range), B is R B1  (as measured at zero pressure at the coldest temperature in the temperature range), C is R C1  (as measured at zero pressure at the coldest temperature in the temperature range), D is R D1  (as measured at zero pressure at the coldest temperature in the temperature range), E is R A1  (as measured at zero pressure at the hottest temperature in the temperature range), F is R B1  (as measured at zero pressure at the hottest temperature in the temperature range), G is R C1  (as measured at zero pressure at the hottest temperature in the temperature range), and H is R D1  (as measured at zero pressure at the hottest temperature in the temperature range). 
     “Type 2” Compensation for Compensated MEMS Sensor  110 : 
     “Type 2” compensation refers to the following: (a) to provide compensation for compensated MEMS sensor  100  to bring the FSO voltage, i.e., the “full scale output” voltage at FSO (“full scale output”) pressure desirably close to a predetermined value, V pred  (for example and without limitation, within about +/−1% of V pred ) (where V pred  is the predetermined FSO voltage at FSO pressure at a middle temperature of the temperature range (for example, and without limitation, room temperature); and (b) to provide compensation for compensated MEMS sensor  100  if the output voltage at the FSO pressure over a range of temperatures (typically, this range goes from a cold temperature such as, for example, −25° C., includes room temperature, i.e., about 20° C., and ends at a hot temperature such as 85° C.) is not equal, or suitably close, to a predetermined value (for example and without limitation, within about +/−1% of the predetermined FSO voltage—ideally the output from compensated MEMS sensor  100  should be the same or suitably close to being the same over the range of temperatures. R s11 , R s12 , and R ts1  are used to provide Type 2 compensation, and the values of R s11 , R s12 , and R ts1  are determined as follows. 
     Define S (the “span”) as the change in output voltage from zero pressure to FSO pressure.
 
 R   ts1 =( R   brg-h   *S   c   −R   brg-c   *S   h )/( S   h   −S   c )  (8)
 
     where: (a) R brg-c  is the resistance of bridge  110  (i.e., the resistance of the combination of resistors R A1 , R B1 , R C1 , R D1 , R z11  and R z12 ; as measured at the full scale output pressure at the coldest temperature in the temperature range (e.g., −25° C.)); (b) R brg-h  is the resistance of bridge  110  (i.e., the resistance of the combination of resistors R A1 , R B1 , R C1 , R D1 , R z11  and R z12 ; as measured at the full scale output pressure at the hottest temperature in the temperature range (e.g., 85° C.)); (c) S c  is the span of MEMS sensor  100  as measured at the coldest temperature in the temperature range (e.g., −25° C.); and (d) S h  is the span of compensated MEMS sensor  100  as measured at the hottest temperature in the temperature range (e.g., 85° C.).
 
 R   s =( R   b   *V   fso )/( V   fso   −V   pred )− R   b   (9)
 
     where: (a) R s  is the sum of R s11  and R s12  (R s11  and R s12  should be equally divided in value to adjust the bridge resistance symmetrically, i.e., R s11 =R s12 )—in other words, resistors R s11  and R s12  could be replaced by a single resistor and R s  placed in the position of resistor R s11 ; (b) V fso  equals the FSO voltage at FSO pressure at a middle temperature of the temperature range (for example, and without limitation, room temperature); and (c) R b  is the resistance of bridge  110  as measured at full scale output pressure at a middle temperature of the temperature range (for example, and without limitation, R b  is the resistance of the combination of resistors R A1 , R B1 , R C1 , R D1 , R z11  and R z12  as measured at full scale output pressure at room temperature). 
     In accordance with one or more embodiments, resistors R z21 , R z22 , R s21 , R s22 , R tz2 , and R ts2  are used to provide compensation for compensated MEMS sensor  200  in the same manner in which resistors R z11 , R z12 , R s11 , R s12 , R tz1 , and R ts1  are used to provide compensation for compensated MEMS sensor  100  (as described above). 
       FIG. 2  shows the schematic circuit diagram of differential pressure sensor  1000  shown in  FIG. 1  with the addition of ASIC  1500  (“Application-Specific Integrated Circuit”) for compensation and amplification of the output signal from differential pressure sensor  1000 . For example, ASIC  1500  will compensate for sensor errors, using its internal circuitry and algorithms, so that there will be a second order compensation (i.e., compensation in addition to that described above) that is useful when there are tight specifications on the output signal from differential pressure sensor  1000 . Such compensation will correct the voltage output from differential pressure sensor  1000 : (a) if the voltage output at zero pressure differs from zero, ASIC  1500  will change the value to be closer to zero; (b) if the FSO voltage at FSO pressure differs from a predetermined value, ASIC  1500  will change the value to be closer to the predetermined value; (c) if the voltage output at zero pressure over the temperature range is not the same, ASIC  1500  will correct the voltage outputs to make them closer to each other over the range; and (d) if the FSO voltage output at FSO pressure over the temperature range is not the same, ASIC  1500  will correct the voltage outputs to make them closer to each other over the range. In addition, ASIC  1500  will amplify and customize the output signal from differential pressure sensor  1000  to meet a customer&#39;s specification as to voltage output or to current output (for example, so that the voltage output is in a range from about 0.5V to about 4.5V or in a range from about 0.5V to about 3.5V or so that the current output is in a range from about 4 mA to about 20 mA). Such ASICs are well known to those of ordinary skill in the art, and as such, a suitable ASIC may readily be obtained from any one of a number of companies such as, among others, companies having readily accessible websites on the Internet. The manner in which to operate ASIC  1500  to accomplish the compensation, amplification and customization of the output signal from differential pressure sensor  1000  is provided by the manufacturers&#39; specifications. As shown in  FIG. 2 , voltage supply  1400  is applied to differential pressure sensor  1000  in such a fashion that the negative terminal is ground. As further shown in  FIG. 2 , the positive output from voltage supply  1400  is connected to terminal V DD  of ASIC  1500 , the ground terminal from voltage supply  1400  is connected to terminal V SS  of ASIC  1500  to have a common ground, the positive output from differential pressure sensor  1000  is connected to terminal V BP , and the negative output from differential pressure sensor  1000  is connected to terminal V BN . Lastly, a capacitor is connected across the positive output of voltage supply  1400  and ground. In accordance with one or more embodiments, the output from ASIC  1500  is in a range from about 0.5 v to about 4.5 v or in a range from about 4 mA to about 20 mA over a pressure differential, for example and without limitation, from about 0 to about 100 PSI. 
     One skilled in the art will appreciate that although particular polarities of the power supply and output signal are illustrated in  FIGS. 1 and 2 , differential pressure sensor  1000  would function in the same manner if all of the polarities were reversed. 
     As one of ordinary skill in the art would readily appreciate, a differential pressure may be measured using differential pressure sensor  1000  when one of a first and a second pressures is an applied pressure from a first pressure source and the second of the first and second pressures is a different applied pressure from a second pressure source. Further, as one of ordinary skill in the art would readily appreciate, 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 second of the first and second pressures is a vacuum or a reference pressure such as, for example and without limitation, atmospheric pressure. Still further, as one of ordinary skill in the art would readily appreciate, differential pressure sensor  1000  may be used to measure the pressure of various media such as fluids, including gases and liquids. For example and without limitation, the medium may be air, a refrigerant, or oil. 
     In operation of differential pressure transducer  1000 , when both dies are compensated properly, and are coupled together, the differential pressure transducer is virtually error free. The differential pressure transducer measures the difference in pressure between pressure P 1  (which may be any pressure, for example and without limitation) impinging on the first side of compensated MEMS sensor  100  and on the second side of compensated MEMS sensor  200  and pressure P 2  (which may be any pressure, for example and without limitation) impinging on the second side of compensated MEMS sensor  100  and on the first side of compensated MEMS sensor  200 . The resulting output is the difference between pressures P 1  and P 2 . 
       FIG. 3  shows a cross section of differential pressure package  4000  where pressure ports  2000  and  2001  are on different sides of package  4000 . As seen in  FIG. 3 , pressure sources P 1  and P 2  are applied as input to ports  2000  and  2001 , respectively, where ports  2000  and  2001  are disposed on opposite sides of package  4000 . As further seen in  FIG. 3 , pressure P 1  is applied to the back and the front of sensors  2010  and  2020 , respectively, and pressure P 2  is applied to the front and the back of sensors  2010  and  2020 , respectively. 
       FIG. 4  shows a cross section of differential pressure package  4001  where pressure ports  2100  and  2101  are on the same side of package  4001 . As seen in  FIG. 4 , pressure sources P 1  and P 2  are applied as input to ports  2100  and  2101 , respectively, where ports  2100  and  2101  are disposed on the same side of package  4001 . As further seen in  FIG. 4 , pressure P 1  is applied to back and front of sensors  2060  and  2070 , respectively, and pressure P 2  is applied to front and back of sensors  2060  and  2070 , respectively. 
     Embodiments described above are exemplary. For example, numerous specific details are set forth such as parts, dimensions, temperature ranges, materials, mechanical design, etc. to provide a thorough understanding of the present invention. However, as one having ordinary skill in the art would recognize, the present invention can be practiced without resorting to the details specifically set forth. As such, many changes and modifications may be made to the description set forth above by those of ordinary skill in the art (i.e., various refinements and substitutions of the various embodiments are possible based on the principles and teachings herein) while remaining within the scope of the invention. In addition, materials, methods, and mechanisms suitable for fabricating embodiments have been described above by providing specific, non-limiting examples and/or by relying on the knowledge of one of ordinary skill in the art. Materials, methods, and mechanisms suitable for fabricating various embodiments or portions of various embodiments described above have not been repeated, for sake of brevity, wherever it should be well understood by those of ordinary skill in the art that the various embodiments or portions of the various embodiments could be fabricated utilizing the same or similar previously described materials, methods or mechanisms. As such, the scope of the invention should be determined with reference to the appended claims along with their full scope of equivalents.