Matched pair of sensor and amplifier circuits

Abstract: A low-cost, quickly prototyped sensor circuit assembly (140,160,240,260) characterized by a sensor integrated circuit (140,240) and an amplifier circuit (160,260), wherein all adjustable system resistors (141-149,151,153,155-158,241-249,251-255,257) are located on the sensor integrated circuit (140,240) in the form of adjustable thin-film resistive elements whose manufacture requires no new mask layers. An assembly method comprises trimming the sensor (140,240) and amplifier circuit (160,260) as a matched pair.

Description: Background of the Invention Technical Field of the Invention This invention relates generally to electronic sensor circuits, and, more particularly, to a sensor circuit assembly comprising a sensor integrated circuit and an amplifier circuit distinct from said sensor integrated circuit, wherein all system trim resistors are located on the sensor integrated circuit. Background Information The present invention has utility in electronic sensor circuits, especially in semiconductor pressure sensor circuits. Pressure responsive transducer circuits are utilized extensively in automobile engine control systems. For example, a manifold pressure sensor and amplifier circuit may provide an analog signal which varies with the engine manifold pressure. An analog-to-digital converter transforms the analog control signal into a digital control signal which is utilized by a microprocessor-controlled fuel injection system. US-A-4777826 describes a strain guage sensor with trimming and gain-setting resistors arranged in a thin film circuit for connection to an amplifier. The thin film circuit is formed of a single homogenous material and is inherently substantially unaffected by temperature. The operation of most semiconductor pressure transducers and op amps varies with temperature, requiring temperature-compensating techniques and circuitry. U.S. Patents 4,326,171 and 4,463,274 disclose temperature-compensating circuits for pressure transducers. Semiconductor transducer / amplifier circuits may be packaged in several ways. For example, they may be packaged on a printed circuit board. However, the customer is then responsible for performing the necessary trimming operations for temperature compensation and for adjusting the op amp gain. Customers typically may not be qualified or motivated to perform such work. Moreover, resistor temperature coefficients at the board level are not well matched, thus degrading system accuracy. Another known package is a two-chip system comprising a sensor chip and a second custom chip containing thin-film resistors for temperature compensation and for calibrating and amplifying the output. However, the disadvantages of the two-chip system are that the custom integrated circuit must be redesigned for each advance or alteration in circuit or process technology. Nor can a two-chip system provide necessary flexibility in circuit configuration. Yet another known package is a hybrid module, comprising a substrate on which is mounted a sensor chip, one or more amplifier chips, and suitable resistors and capacitors which are soldered to metal pads and wire-bonded to interconnecting metal traces. The resulting package may take the form of an encapsulated assembly with leads. One disadvantage of hybrid modules is that they tend to be very labor intensive because each component has to be individually placed, connected, calibrated, and temperature-compensated. It is also known to incorporate sensor circuits onto a single integrated circuit device. But even with a fully integrated circuit device it is necessary to adjust or "trim" on-chip passive elements for various purposes, such as setting the transducer offset voltage, the op amp input voltage offset, and resistive values. An advantage of a fully integrated device is that all trim is performed on a system-wide basis. That is, the op amp gain is set at the same time that the sensor element temperature calibration step is performed. All components on the chip, including the sensor and op amps, are matched with respect to values and temperature coefficients. At trim, all values and temperature coefficients are set. However, a disadvantage of such integrated circuit devices is that they have relatively long development cycles and are relatively expensive, especially in low-volume applications. There is therefore a significant need in the semiconductor transducer field to provide customers with a fully system-ready product, which requires no trimming or adjustment by the customer or user. There is also a strong demand by customers for relatively low cost sensor circuits. In addition, customers desire short development cycles for prototype sensor systems, so it would be desirable to provide a sensor assembly that is easy to modify to meet the needs of different customer applications. Therefore, there is a substantial need to provide a compact sensor assembly that is low in cost, high in quality, and which facilitates a short prototyping cycle and a short manufacturing cycle. Brief Summary of Invention In accordance with a first aspect of the invention, there is provided a matched sensor-amplifier assembly comprising an integrated circuit having a sensor, temperature compensating adjustable resistive means and pressure compensating adjustable resistive means, and a separate amplifier circuit having an amplifier, wherein the integrated circuit and the amplifier circuit are coupled together, characterized in that: the temperature compensating adjustable resistive means and the pressure compensating adjustable resistive means are contained in the integrated circuit, and the temperature compensating adjustive resistive means and the pressure compensating adjustive means are adjusted under various conditions so that the integrated circuit and the amplifier circuit form a matched pair, wherein the matched pair is formed during manufacture of the assembly. In a preferred embodiment, the temperature compensating adjustable resistive means are set to minimize temperature variation of an output of the matched sensor-amplifier assembly when together, so as to provide the matched pair. Furthermore, the temperature compensating adjustable resistive means are set to provide a predetermined output off-set for the matched sensor-amplifier assembly when together, so as to provide the matched pair. In the preferred embodiment, the matched sensor-amplifier assembly may be mounted in apparatus, wherein the integrated circuit is located in a position on the apparatus which is subject to temperature variation when the apparatus is in use and the amplifier circuit is located remote from the integrated circuit in a position on the apparatus which is relatively stable in temperature when in use. In a further aspect of the invention, there is provided a method of manufacturing a matched sensor-amplifier assembly having a predetermined temperature characteristic comprising the steps of: providing an integrated circuit comprising a sensor and temperature compensating adjustable resistive means; providing an amplifier circuit, separate from the integrated circuit, comprising an amplifier; coupling the sensor and the temperature compensating resistive means to the amplifier circuit such that the temperature compensating resistive means are arranged to adjust gain and/or offset of the amplifier, the method characterized by the steps of: placing the temperature compensating adjustable resistive means within the integrated circuit; subjecting the sensor to a first predetermined temperature and adjusting the temperature compensating adjustable resistive means; and subjecting the sensor to a second, different predetermined temperature and adjusting the temperature compensating adjustable resistive means, thereby forming a matched sensor-amplifier assembly with predetermined temperature characteristics, wherein the matched sensor-amplifier assembly is formed during manufacture. A preferred method of the further aspect includes the additional steps of: providing pressure compensating adjustable resistive means in the integrated circuit; coupling the pressure compensating adjustable resistive means to the amplifier circuit (160, 260) such that the pressure compensating adjustable resistive means (153, 255) are arranged to adjust the gain and/or offset of the amplifier; performing the steps of subjecting the sensor to the first predetermined temperature and adjusting the temperature compensating adjustable resistive means, and subjecting the sensor to the second, different predetermined temperature and adjusting the temperature compensating adjustable resistive means; subjecting the sensor to a change of pressure, and adjusting the pressure compensating adjustable resistive means, thereby forming a matched sensor-amplifier assembly with predetermined temperature and output characteristics, wherein the matched sensor-amplifier assembly is formed during manufacture. Furthermore, the preferred method may also include the step of providing a compensation element in the integrated circuit for determining datum pressure off-set and adjusting the compensation element to provide a predetermined datum pressure off-set for the matched sensor-amplifier assembly. In summary, the present invention fulfills the above-mentioned requirements by providing a sensor chip containing the desired trim resistors and a separate module containing only the amplifier(s). The amplifier module may take the form of a board or chip. All trim operations are performed at the factory on the combination integrated circuit / amplifier module. Exemplary embodiments of the invention will now be described with reference to the accompanying drawings. Brief Description of the Drawings The invention is pointed out with particularity in the appended claims. However, other features of the intention will become more apparent and the invention will be best understood by referring to the following detailed description in conjunction with the accompanying drawings in which: FIG. 1 shows a prior art circuit board sensor assembly comprising a sensor integrated circuit 40 and other electrical components including an op amp 15. FIG. 2 shows a matched pair of sensor and amplifier circuits according to a preferred embodiment of the present invention, including sensor circuit 140 and amplifier circuit 160. FIG. 3 shows a matched pair of sensor and amplifier circuits according to an alternative embodiment of the present invention, including sensor circuit 240 and amplifier circuit 260. Description of Preferred Embodiment FIG. 1 shows a prior art circuit board sensor assembly comprising a sensor integrated circuit 40, including transducer 50, and other electrical components including an op amp 15 and trim resistors 12 and 14. Resistor 14 is used to adjust the gain of op amp 15, while resistor 12 is used to adjust the offset of op amp 15. As mentioned above, the sensor assembly shows in FIG. 1 has the disadvantage of requiring the customer or user to trim resistor 12 and 14. This can be a fairly complicated procedure because the assembly must be subjected to at least two different pressures at each of at least two different temperatures. Because many users are not experienced circuit designers, they may encounter difficulties, since the prior art circuit board sensor assembly requires the user to design and characterize a specific circuit. In addition, the user must obtain and assemble the various components of the circuit board assembly. FIG. 2 shows a matched pair of sensor and amplifier circuits according to a preferred embodiment of the present invention, including sensor circuit 140 and amplifier circuit 160. Sensor circuit 140 is implemented as an integrated circuit having external terminals 201-208. Sensor circuit 140 comprises a sensor transducer 150 having a positive output 134, a negative output 133, and excitation terminals 132 and 135. As is well known, an increase in pressure on transducer 150 creates a voltage differential across output terminals 133 and 134, the differential increasing proportional to the applied pressure. Sensor circuit 140 also comprises temperature-compensating resistors, such as resistors 152 and 154. Resistors 152 and 154 are conveniently but not essentially provided as thin-film resistors. Any trimmable monolithic form of resistors may be used. All of the trimmable, passive elements in the sensor assembly, including resistors 141-149, 151, 153, and 155-158 are provided on IC 140. Resistors 141-149, 151, 153, and 155-158 may also be thin-film resistors. Amplifier circuit 160 comprises a pair of op amps 180 and 190, each having an inverting input (181, 191), a non-inverting input (182, 192), and an output (185, 195). The negative output 133 of sensor 150 is coupled to the non-inverting input 182 of op amp 180 through terminal 206. The positive output 134 of sensor 150 is coupled to the non-inverting input 192 of op amp 190 through terminal 207. A resistor network comprises resistors 149, 151, 153, 155, and 156. Resistors 149, 151, and 156 form a resistive divider network which is used to set the temperature coefficient of the offset of op amp 180. Resistor 149 is coupled between positive power supply terminal 130′ and junction 172, and resistor 151 is coupled between junction 172 and ground. One side of resistor 153 is coupled via output terminal 202 to the output 195 of op amp 190, and the other side of resistor 153 is coupled to junction 174. One side of resistor 155 is coupled to junction 174, and the other side of resistor 155 is coupled to junction 175. Junction 174 is coupled through output terminal 203 to the inverting input 191 of op amp 190. One side of resistor 156 is coupled to junction 173 which is also coupled to junction 172, and the other side of resistor 156 is coupled to junction 175, which is coupled through output terminal 204 to the output of op amp 180. Junction 173 is coupled through output terminal 205 to the inverting input 181 of op amp 180. A positive supply voltage is applied to IC 140 at terminal 201. Terminal 208 is coupled to ground. And the circuit output voltage is produced at output terminal 220. It should be understood that amplifier circuit 160 may be implemented either as a circuit board or integrated circuit. The matched sensor-amplifier arrangement of FIG. 2 can be provided as two die in a single package, as a two-chip assembly, or as two packages marketed together in a single envelope. It should also be understood that the trimmable passive components are located on sensor IC 140. Operation of Preferred Embodiment The operation of the circuit shown in FIG. 2 as pressure is applied to sensor 150 will now be explained. After the circuit-trimming operation has been completed (see below), with no pressure applied the outputs at terminal 206 and 207 are nominally equal and remain so over the temperature range. As the temperature is raised, the networks comprising resistors 152 and 154 draw more or less current, causing the positive sensor output 134 to track the negative sensor output 133. Again at room temperature, as pressure is applied, positive sensor output 134 increases in voltage causing the output of op amp 190 to increase. As the temperature is raised, the input resistance of sensor 150 increases, the resistance of thin-film resistors 141 and 146 remain nominally unchanged, and as a result the excitation voltage at 132 and 135 increases, compensating for the decrease in output per volt of sensor 150. Manufacturing the Preferred Embodiment The various steps that occur in trimming the sensor IC 140 after assembly and before it is shipped to the customer will now be described. First, the thin-film resistors 141 and 146 are trimmed to compensate the sensor span for temperature. The span can be compensated for temperature, for example, over a range of between -40° Centigrade to 125° Centigrade, with slightly less accuracy outside of that range. Next resistor 143 or resistor 145 is trimmed to provide the absolute offset of sensor 150 at room temperature. Then resistor 149 or 151 is trimmed to match the voltage at junction 172 to junction 175 at room temperature. Next, the temperature is elevated to a higher temperature. Resistor 156 is then trimmed until output 220 returns to the room temperature absolute offset. Finally, pressure is increased to a higher pressure. Then resistor 153 is trimmed until the correct output at 220 is produced. Description of Alternative Embodiment FIG. 3 shows a matched pair of sensor and amplifier circuits according to an alternative embodiment of the present invention, including sensor circuit 240 and amplifier circuit 260. The basic concept seen in FIG. 2 also applies to the circuit of FIG. 3, wherein the trim elements are located on the sensor IC 240. But the sensor IC 240 is implemented slightly differently from sensor IC 140, and the amplifier circuit 260 comprises only one op amp 290 in this implementation. Sensor IC circuit 240 comprises sensor transducer 250 having a positive output 234, a negative output 233, and excitation terminals 232 and 235. Circuit 240 also comprises monolithic temperature-compensating resistors 252 and 254 of, for example, thin-film resistive material. Sensor circuit 240 also comprises trimmable resistors 241-249, 251, and 253. Also included are temperature-compensating resistors 252 and 254 and adjustable resistive elements 255 and 257. Thin-film resistors which may be laser-trimmed are preferred, but other trimmable resistor materials and other trimming techniques well known in the art may also be used. Adjustable resistive element 255 is used to set the gain of op amp 290, and resistive element 257 is used to adjust the offset of op amp 290. The output 275 of op amp 290 is fed back to the inverting input 291 via terminal 302, resistive element 255, junction 272 and terminal 303. The non-inverting input 292 of op amp 290 is coupled to one junction 273 of resistive element 257 via terminal 304. Junction 273 is also coupled to the positive output 234 of sensor 250 through junction 268 and resistor 247. Sensor IC 240 has external terminals 301-305. A positive supply voltage is applied to IC 240 at terminal 301. Terminal 305 is coupled to ground. The circuit output voltage is produced at output terminal 320 of amplifier circuit 260. Manufacturing the Alternative Embodiment The steps that occur in trimming the sensor IC 240 after assembly and before it is shipped to the customer will now be described. First, the thin-film resistors 241 and 249 are trimmed to compensate the sensor span for temperature. Next resistor 244 or resistor 248 is trimmed to provide the absolute offset of sensor 250 at room temperature. Next, the temperature is elevated to a higher temperature, and resistors 245 and 251 are trimmed until output 320 returns to the room temperature absolute offset. Finally, pressure is increased to a higher pressure. Then resistor 255 is trimmed until the correct output at 320 is produced. As an option, resistor 257 can be adjusted to provide the absolute offset instead of trimming resistors 244 and 248. It will be apparent that the present invention provides a straight-forward, inexpensive solution to the requirement for short development cycles, relatively easy modification, short production cycles. Only seven mask layers are required, and no epitaxial or isolation diffusions are required, since there are no transistors on the sensor IC. Another advantage of the present invention is that by keeping the amplifier circuit off the sensor chip, the amplifier circuit can be mounted at a suitable distance from harsh environments to which the sensor IC may be exposed, e.g. the sensor may be mounted on an automotive engine manifold and the amplifier on the firewall. It will also be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than the preferred form specifically set out and described above. For example, the present invention can be extended to any type of transducer, such as accelerometers, flow meters, force sensors, load cells, chemical sensors, humidity sensors, Hall effect sensors, industrial microphones, etc. It will be apparent that other signal-conditioning circuits could be used to supplement or substitute for the amplifier circuit (i.e. circuit 160 or 260). Also, other temperature-compensating means could be used on the sensor IC in place of temperature-compensating resistors, such as diodes, active circuitry, or other temperature-compensating techniques. Moreover, it will be understood that other resistor-trimming methods may be used, such as pulse-trimming. In addition, the invention could be carried out by manufacturing the sensor IC, including trimming the resistors on the sensor IC, and putting the sensor IC's into inventory. At a later date, a sensor IC could be retrieved and matched with an amplifying circuit. Moreover, the matching could be carried out by trimming the op amp(s) on the amplifying circuit, for example, by pulse-trimming the op amps. It will be apparent that implant resistors could be used in addition to or in place of thin-film resistors on the sensor IC, and that such resistors could be trimmed using metal migration techniques.

Claims: A matched sensor-amplifier assembly comprising an integrated circuit (140, 240) having a sensor (150, 250), temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257) and pressure compensating adjustable resistive means (153, 255), and a separate amplifier circuit (160, 260) having an amplifier (180, 190, 290), wherein the integrated circuit (140, 240) and the amplifier circuit (160, 260) are coupled together, characterized in that: the temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257) and the pressure compensating adjustable resistive means (153, 255) are contained in the integrated circuit (140, 240); and the temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257) and the pressure compensating adjustable resistive means (153, 255) are adjusted under various conditions so that the integrated circuit (140, 240) and the amplifier circuit (160, 260) form a matched pair, wherein the matched pair is formed during manufacture of the assembly. A matched sensor-amplifier assembly according to claim 1, wherein the temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257) are set to minimize temperature variation of an output (220, 320) of the matched sensor-amplifier assembly when together, so as to provide the matched pair. A matched sensor-amplifier assembly according to claim 1 or 2, wherein the temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257) are set to provide a predetermined output off-set for the matched sensor-amplifier assembly when together, so as to provide the matched pair. A matched sensor-amplifier assembly according to claim 1, 2 or 3, wherein the integrated circuit (240) further comprises adjustable resistive means (257) for connection between an inverting (291) input and a non-inverting (292) input of the amplifier (290). A matched sensor-amplifier assembly according to any one of the preceding claims, mounted in an apparatus, wherein the integrated circuit (140, 240) is located in a position on the apparatus which is subject to temperature variation when the apparatus is in use and the amplifier circuit (160, 260) is located remote from the integrated circuit (140, 240) in a position on the apparatus which is relatively stable in temperature when in use. A method of manufacturing a matched sensor-amplifier assembly having a predetermined temperature characteristic, comprising the steps of: providing an integrated circuit (140, 240) comprising a sensor (150, 250) and temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257); providing an amplifier circuit (160, 260), separate from the integrated circuit (140, 240), comprising an amplifier (180, 190, 290); coupling the sensor (150, 250) and the temperature compensating resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257) to the amplifier circuit (160, 260) such that the temperature compensating resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257) are arranged to adjust gain and/or offset of the amplifier (180, 190, 290); the method characterized by the steps of: locating the temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257) within the integrated circuit (140, 240); subjecting the sensor (150, 250) to a first predetermined temperature and adjusting the temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257); and subjecting the sensor to a second, different predetermined temperature and adjusting the temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 248, 245, 249, 251, 252, 254, 257), thereby forming a matched sensor-amplifier assembly with predetermined temperature characteristics, wherein the matched sensor-amplifier assembly is formed during manufacture. A method according to claim 6, comprising the further steps of: providing pressure compensating adjustable resistive means (153, 255) in the integrated circuit (140, 240); coupling the pressure compensating adjustable resistive means (153, 155) to the amplifier circuit (160, 260) such that the pressure compensating adjustable resistive means (153, 255) are arranged to adjust the gain and/or offset of the amplifier (180, 190, 290); performing the steps of subjecting the sensor to the first predetermined temperature and adjusting the temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 245, 248, 249, 251, 252, 254, 257), and subjecting the sensor (150, 250) to the second, different predetermined temperature and adjusting the temperature compensating adjustable resistive means (141, 143, 145, 146, 149, 151, 155, 156, 241, 244, 245, 248, 249, 251, 252, 254, 257); subjecting the sensor (150, 250) to a change of pressure, and adjusting the pressure compensating adjustable resistive means (153, 255), thereby forming a matched sensor-amplifier assembly with predetermined temperature and output characteristics, wherein the matched sensor-amplifier assembly is formed during manufacture. A method according to claim 6 or 7, comprising the further steps of providing a compensation element (257) in the integrated circuit (240) for determining datum pressure off-set and adjusting the compensation element (257) to provide a predetermined datum pressure off-set for the matched sensor-amplifier assembly.

Inventors: BASKETT IRA E, BASKETT, IRA E.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1991
Main classification: G01L9
IPC codes: G01L9, H05K1, G01D18, G01L1, H01L29, H03F1, G01D3, G01B7