AN APPLICATION SPECIFIC INTEGRATED CIRCUIT FOR USE WITH AN IR THERMOMETER

Abstract: An application specific integrated circuit (ASIC) for use with an IR thermometer includes a voltage regulator, a power on reset, a current reference, a voltage reference, a bias voltage generator (resistor divider), a constant current source, an ambient sensor amplifier, a pyroelectric sensor amplifier, an analog multiplexer, and an analog to digital converter. A microprocessor is interconnected with the ASIC to process the electrical signals from the ASIC in order to calculate the absolute temperature of the object being measured. The electrical signals provided by the ASIC correspond to a signal generated by the pyroelectric sensor indicative of the temperature of the object being measured and a signal generated by the ambient temperature sensor indicative of the ambient temperature of the pyroelectric sensor.

Description: AN APPLICATION SPECIFIC INTEGRATED CIRCUIT FOR USE WITH AN IR THERMOMETER Background of the Invention The present invention generally relates to infrared (IR) thermometers for use in measuring patient temperature in a clinical setting. More particularly, the present invention relates to an enhanced infrared thermometer having simplified manufacturing with greater assurance of quality control and system calibration, greater ease in operation, more accurate and repeatable temperature readings and, most particularly, to a noncontacting infrared electronic thermometer having a new, novel application specific integrated circuit for measuring the temperature of an object. Infrared thermometry has been available for many applications based on a variety of designs. Typically, these early thermometer designs were developed for testing temperature of difficult to reach objects or objects operating at fairly high temperatures - such as the interior of an industrial oven. Such exotic environments were well suited to infrared detection due to the large temperature differences between ambient and the subject object and because there were few alternative measurement techniques. More recently, infrared temperature detection techniques have found application in clinical temperature measurement and specifically the diagnostic measurement of patient body temperature by detecting the infrared emissions radiating from the tympanic membrane and/or surrounding tissue of the ear. The temperature of this membrane and surrounding tissue has a high correlation to actual body temperature and its measurement with infrared detection techniques has become a highly accurate reading taken in a matter of seconds. The present invention application is directed to improvements to the prior IR thermometers disclosed in U.S. Patents 4,797,840 and its reissue RE. 34,789, and 5,368,038 both assigned to the assignee of the present invention, the disclosure of each is hereby incorporated by reference. As disclosed in the above mentioned patents, the temperature of an object, such as the human body, has been determined by using a contact thermosensor or by measuring the naturally radiated energy from the body such as the radiated energy in the far infrared range. The infrared radiation is directly related to temperature of the object and has been utilized to determine the temperature of the body. Medical thermometers are useful in the diagnosis and treatment of many diseases. In the past, measurement of a patient's body temperature had been most commonly performed by conventional mercury thermometers. Disadvantages of such thermometers included the amount of time required to obtain accurate readings, a minute or more, and that they needed to be disinfected before each use. Later, electronic thermometers became popular because they required far less time to obtain an accurate temperature reading. The probe of the electronic thermometer was commonly inserted into a protective disposable cover before use. Such electronic thermometers are rapidly reusable and are generally sanitary when used with sanitary sheaths. However, obtaining an accurate reading of a patient's temperature still required as much as 30 seconds, since the temperature is measured through the sanitary sheath which must equilibrate to the patient's temperature. Such thermometers were also generally used orally or rectally. The auditory canal and the tympanic membrane are also known to be useful for measurement of a patient ' s internal body temperature. Electronic thermometers for measuring the temperature of the tympanic membrane by directly contacting the tympanic membrane is well known and has been described in a number of U.S. Patents. However, such devices have proven to have certain disadvantages such as causing discomfort to the patient, inserting the probe without a sheath, thus requiring disinfection between uses in order to decrease the time required for an accurate temperature, or utilizing a sheath or speculum for sanitary purposes while generally increasing the time required to obtain an accurate measurement. More recently, infrared thermometers avoided the necessity of contacting the location at which temperature is actually being measured and were designed for use in measuring the patient ' s temperature from the auditory canal and/or from the tympanic membrane. As is described in the above mentioned patents, an infrared detector receives infrared radiation from the auditory canal through an internally polished truncated cone which serves as a shield and an insulator, so that temperature readings are only taken from the auditory canal. One of the major problems with infrared thermometers has been not only initially obtaining accurate readings but also maintaining the ability to obtain accurate readings over the life of the thermometer. One key to initially attaining accurate temperature readings and to maintaining the ability to reproduce accurate temperature readings over the useful life of the infrared thermometer has been calibration. One initial method of infrared thermometers calibration is disclosed in U.S. Patent 4,784,149 to Berman et al. Berman involved an automatic calibration circuit as a means for removing signal errors in the infrared thermometer caused by infrared energy which reaches the detector and is not associated with the target temperature. In this patent, an important feature of the thermometer is the mechanism and associated circuitry for automatic calibration. This mechanism and circuitry reduces temperature measurement errors which may result from temporal and thermal changes in the optical components and electronic circuitry. The calibration method utilizes an internal reference blackbody target, a comparator and an auto-zero circuit. At the initiation of the operation cycle, the "ON" switch is actuated with a probe while the probe views the internal reference target. The signal generated by the probe is compared to the temperature of the reference target. If the probe measurement differs from the temperature of the reference target, an error signal is generated and "ADDED" at the summing amplifier. Therefore, each time the probe is used, it is calibrated against a measured internal reference source. Utilization of the complex circuitry required for the automatic calibration of this patent adds excessive cost to the manufacture of the infrared thermometer. In U.S. Patent 5,150,969, to Goldberg et al . , a system and method for temperature determination and calibration of a biomedical thermometer is disclosed. This system and method was developed to avoid requiring the application of a heat source or a cold source to the detector during the temperature measurement operation. In this system and method, a microprocessor contains calibration data and combines the detector signal and the reference signal in a manner dependent upon whether the temperature of a blackbody calibration source is desired or the temperature of an anatomical target. The processor may also contain other calibration data and combines the detector signal and the reference signal in a manner dependent upon whether the temperature of the target is desired or the temperature of some other part of the anatomy. Further, means are provided for the user to recalibrate the system with a blackbody calibration source and to modify the processor calibration data to a limited extent. This re-calibration permits compensation for changes in the instrument due to aging, usage and other reasons. The system and method of the 5,150,969 patent did provide more accurate, reliable, and easier to use calibration, as well as enhanced calibration and measurement capabilities. However, the processor requirements and other electronic components of the infrared detector thermometer proved to be too expensive to manufacture in a mass produced consumer infrared thermometer. Other methods for calibrating the infrared thermometer include providing a chopper for automatic calibration of the thermometer. An example of this arrangement is shown in U.S. Patent 4,907,895 issued to Everest. As with the other calibration systems mentioned above, utilization of a chopper increases the complexity of the infrared thermometer, as well as the manufacturability and the cost, and is therefore undesirable. In the past, unit-to-unit manufacturing and assembly variances encountered in thermopiles, thermistors and other components precluded the use of a rigid set of equations describing the physical interactions of electronic and optical components to calculate a body temperature with sufficient accuracy. The errors introduced by each component are cumulative and affect each other component. Each component must be individually calibrated in prior art Tympanic thermometers. The relationships between all the inputs and the target temperature over a range of ambient temperatures are quite complex and difficult to specify. Some experiments have demonstrated that sufficient accuracy is not achievable by utilizing sensors to sense the temperature of the thermopile and waveguide and then processing the signals according to equations which subtract an amount from the measured temperature of the target which is attributable to temperature variations in the wave guide. In U.S. Patent 4,797,840 to Fraden, an optional calibration circuit is shown for calibrating the pyroelectric sensor signal to compensate for possible variations due to material aging, temperature drifts, and stability of electronic components etc. which may produce unacceptable error in the temperature measurements. The calibration operation is performed with a shutter in a closed position. Prior to opening the shutter a switch interconnects the electrode segment to the signal excitation circuit and the predetermined electrical signal is applied to the electrode. Due to the piezoelectric properties of the pyroelectric film, this causes mechanical stress and, in turn, the mechanical stress causes the piezoelectric film to generate a responsive electric signal which is conducted to the amplifier circuit via an electrode segment. Since the mechanical stress- induced electrode signal can be predetermined, deviation in the response signal is indicative in changes in the pyroelectric sensor and the degree of deviation from the predetermined standard provides the necessary calibration information for appropriate correction by the microprocessor. Immediately following the calibration operation, the switch interconnects the electrode segment to the amplifier circuit which thereby doubles the infrared sensitivity area of the film and the temperature measurement operation is performed. In this particular patent, calibration is preferably performed immediately prior to each measurement operation to ensure reliable and accurate absolute temperature measurement . A method for calibrating infrared thermometers during mass production upon completion of the assembly of the infrared thermometers is disclosed in U.S. Patent 5,293,877 to O'Hara et al. In the O'Hara patent, upon completion of the assembly process of the infrared thermometer, multiple completed units are simultaneously calibrated, preferably in groups of three, utilizing a calibration apparatus and an external IBM compatible personal computer to perform the calibration operation. This method suffers from the fact that the calibration is not completed until after all of the components have been assembled. Therefore, if any of the components when combined with other components produce a unit that is out of calibration, or is uncalibrateable, the entire unit must be scrapped or undergo a costly disassembly and rework. It would be preferable to calibrate as many individual components as possible prior to assembly. For those components which cannot be calibrated separately, it is preferable to calibrate as early as possible in the assembly process, and prior to final assembly. Calibrating based on these principles will reduce scrap and rework costs, and ultimately the unit cost of the infrared thermometer. While the above described thermometers were clear advances in the art, they had disadvantages in that they included a large number of individual components, were difficult to manufacture, were less reliable than desired due to their complexity, and were more expensive to manufacture than believed necessary. Concerning calibration, the above described thermometers required extensive calibration equipment, and this calibration can be very time consuming. And since infrared thermometers are precise optical/electronic systems that require sophisticated calibration significant accuracy problems can result if there are any errors in the calibration process. Thus, there is a need for an improved noncontacting infrared electronic thermometer. Such improved infrared thermometer should significantly reduce the number of individual components by incorporating many of them into a single chip; should be more readily manufacturable; should require less calibration resources; should be capable of undergoing final calibration (or having calibration checked) after final assembly; should be more reliable; and should be more economical to manufacture. Summary of the Invention It is accordingly one object of the present application to provide a new and improved noncontacting electronic thermometer which is accurate, reliable and economical to manufacture. Another object of the invention is to provide a noncontacting electronic thermometer for medical use which is compact, inexpensive and convenient and easy to use. A still further object of the invention is to provide a new and improved infrared thermometer which significantly reduces the number of individual components by incorporating them into a single chip thereby reducing the complexity and increasing reliability. Another object of the invention is to provide a new and improved infrared thermometer that is more readily manufactured. The above and other objects of the present invention are realized in a novel thermometer design and manufacturing process therefor, that act in concert to enhance both production and operating characteristics after production. The new design begins with the integration of many discrete analog functions on a single application specific semiconductor "chip". This new chip controls key operation functions such as voltage regulation, ambient sensor amplifier, pyroelectric sensor amplifier, current and voltage reference and analog to digital conversion. On board the chip, special topographic elements are etched to insure isolation of sensitive sensor input channels. The thermometer with this new semiconductor chip is specifically tailored for ease of manufacturing. The reduction in parts count reduces the number of components that must be installed on the printed circuit board. Also, this new chip can be hermetically sealed after mounting on the printed circuit board. This sealing lends itself to performing the ambient channel calibration of the thermometer in a precision fluid bath (by immersing the thermometer) , which is faster and more accurate than alternate calibration technologies such as an air circulating oven. Other objects and advantages of the present application will become apparent from the following description, the accompanying drawings and the appended claims. Brief Description of the Drawings FIG. 1 is an exploded schematic view of a representative prior medical infrared thermometer; FIG. 2 is a side view of the medical infrared thermometer of the present invention in the closed or stored position; FIG. 3 is a view of the thermometer of Figure 2 with parts in phantom and parts cut away; FIG. 4 is a side view of the thermometer of Figure 1 in the open, ready to use position; FIG. 5 is a front view of the thermometer of Figure 4 in the open, ready to use position; FIG. 6 is a top view of the thermometer of Figure 4 in the open, ready to use position with the protective cover removed from the probe; FIG. 7 is a block diagram showing the application specific integrated circuit useful with the present invention interfacing with a microprocessor; FIG. 8 is a block diagram showing the inputs and outputs of the voltage regulator; FIG. 9 is a schematic showing the connection of an external resistor used to generate an ASIC reference current; FIG. 10 is a block diagram of the voltage Reference converting source voltage Vdd to reference voltage Vref; FIG. 11 is an illustration of the bias voltage generator function used in one embodiment of the present invention; FIG. 12 is an illustration of a constant current source used to provide bias to a transistor in one embodiment of the present invention; FIG. 13 is a graph showing temperature versus Voltage for an average PN100; FIG. 14 is an illustration of the amplifier circuit used in one embodiment of the present invention; FIG. 15 is a graph showing ambient channel Voltage (Vamb) versus Temperature for a thermometer with a nominal Vref and a nominal PN100; FIG. 16 is an illustration showing the ASIC configured for use with an external Pyro channel amplifier; FIG. 17 is an illustration showing the ASIC configured for use with its internal amplifier; FIG. 18 is an illustration of a typical output signal on the pyro channel when the target temperature exceeds the ambient temperature; FIG. 19 is a graphical representation of the Pyrobias and the ADCbias derived from possible pyro voltage swings ; FIG. 20 is a diagram of an Analog multiplexer used with the present invention including a description of the signals; FIG. 21 is an illustration of the analog to digital converter (ADC) utilized in one embodiment of the ASIC of the present invention; FIG. 22 is a table illustrating the parameters used to scale the ADC components and bias voltages for one specific embodiment of the present invention; FIG. 23 is a table illustrating further optimization of the parameters used to scale the ADC components and bias voltages for one specific embodiment of the present invention; FIG. 24 is an illustration of the pad layout of the IR Thermometer of the present invention; FIG. 25 is an illustration of the pad layout of the ASCI of the IR Thermometer of the present invention; FIG. 26 is a plan view of the circuit board of the present invention showing the location of the ASIC and the pyro sensor positioned thereon; and FIG. 27 is a side view of the circuit board of Fig. 26. Detailed Description of the Invention As shown in FIG. 1, a representative drawing of a prior medical infrared thermometer 30 is shown in an exploded view. This thermometer comprises a self-contained, battery powered unit 32 that has probe 34 adapted for insertion into an ear canal, short of the tympanic membrane. Housing member 36 of thermometer is shaped for convenient handling by a user. The thermometer has an actuation button 38 that when depressed triggers the device to take a reading of the infrared radiation from within the ear canal . Probe 34 at the front of the thermometer is of a shape and dimension that is compatible with the profile of a human ear canal. Before insertion into the canal, probe is covered by protective probe cover 40 conventionally fabricated of a thin polymer material that is substantially transparent to light in the near and far infrared spectral ranges. As shown, electronic circuity 42 is connected to the IR assembly via a cable 46. A power supply in the form of a nine (9) volt battery is connected to the circuitry 42 and is typically contained in housing. Other conventional components are typically used to complete the encasement of the IR Thermometers' components inside housing 36. The purpose of the front portion of the probe is to gather infrared light from the tympanic membrane and surrounding tissue. An infrared sensor assembly is positioned remote from the end of the probe, being positioned inside housing member of thermometer. As shown in Figures 2-6, one specific embodiment of the electronic thermometer of the present invention is generally designated by the numeral 50. Thermometer 50 generally comprises a first housing member 52 having an interior chamber, a waveguide 54 for directing infrared radiation into the chamber, a shutter assembly 56 for controlling the passage of infrared radiation through the waveguide, a pyroelectric sensor assembly 58, an ambient temperature sensor 60, and an electronic circuit 62 (see Figures 26 and 27) and a second housing member 64 connected to the first housing member by a hinge 66 and rotatable thereabout. The first housing member having the hinge at one end forms in combination with the hollow matching second housing member a pistol grip type handle of convenient size for one hand operation when the two housing members are separated from each other, as shown in Figure 4. When the housing members are rotated about one hundred eighty (180) degrees toward each other, the two housings form a closed unit with the waveguide and speculum 68 being housed inside the hollow interior of the second housing member 64. The second housing member is generally hollow and is operated connected to the first housing member by the hinge 66. Retainer means 70 for storing a plurality of probe covers 72 for future use is formed in the cavity 74 of the second housing 64. The upper end of the first housing member includes the interior chamber (not shown) for mounting the pyroelectric sensor assembly and the ambient temperature sensor and provides a shield to exterior infrared radiation other than that received through the waveguide positioned inside a probe 76. The waveguide is mounted to the forward side of first housing member in alignment with the pyroelectric sensor so as to direct or aim infrared radiation from the object to be measured to the pyroelectric sensor mounted within the chamber. The waveguide is preferably made of metal and is interconnected in the assembly so as to be in thermal equilibrium with the shutter. A typical medical infrared thermometer is a self- contained, battery powered unit which has probe adapted for insertion into an ear canal, short of tympanic membrane. An actuation button which when depressed triggers the device to take a reading of the infrared radiation from within canal is positioned on the first housing member. The probe at the front of the thermometer is of a shape and dimension that is compatible with the profile of a human ear canal. Before insertion into the canal, probe is covered by protective probe cover 72 which is fabricated of a thin polymer material that is substantially transparent to light in the near and far infrared spectral ranges. The purpose of the front portion of the probe is to gather infrared light from the tympanic membrane and surrounding tissue. The infrared sensor is remote from the end of the probe, being positioned inside housing of thermometer. Referring to FIG. 4, the front end of the probe and the infrared sensor are optically coupled through a waveguide which is typically a metal tube with a highly reflective coating on the inside. Waveguide is preferably internally plated with high-purity gold. Infrared ray (IR) entering front end of waveguide at almost any angle is successfully, totally internally reflected within the waveguide, and propagated, or conducted along its length with minimal loss. The position of shutter is controlled by mechanism which is triggered by activation button. The optical assembly comprising waveguide and shutter is positioned within elongated speculum 68 which forms the outer surface of probe. Thin front end 88 of probe cover 72 is substantially transparent to IR radiation. In a typical embodiment, the infrared sensor and ambient sensor are connected to first and second signal conditioners and which are in turn connected to signal multiplexer (MUX) . MUX is a gate, intended to conduct an appropriate signal from the conditioners, one at a time, to microprocessor. Microprocessor has a built-in analog to digital converter and a driver to control display which displays the calculated temperature of the target such as ear canal . Operation of the thermometer is as follows. Infrared ray from the target ear canal passes through front end of probe cover and enters waveguide. IR ray propagates along waveguide to back end with only slight absorption. The rays normal to front end go directly toward shutter, while rays entering front end from other angles are reflected from the inner walls of the waveguide. As long as shutter is closed, no rays reach sensor. When mechanism opens shutter, infrared rays reach the sensor which responds with output signal V. That signal is typically treated by first signal conditioner and passed to microprocessor by way of multiplexer. Microprocessor converted the signal into a digital format. At a specific moment, either before or after shutter activation, signal T a was taken from ambient sensor, through second signal conditioner, to microprocessor. When both signals were received, microprocessor calculates T b according to an algorithm based on equation 2 , and sends the result of the calculation to display. The above described prior art thermometers, while clear advances in the art, included a large number of individual components, were difficult to manufacture, required excessive calibration resources, were less reliable than desired due to their complexity and were more expensive to manufacture than believed necessary. The new improved IR thermometer of the present invention includes a new application specific integrated circuit (ASIC) 80 which significantly reduces the number of individual components by incorporating them into a single chip (see Figure 26) thereby reducing the complexity and increasing reliability. The new design begins with the integration of many discrete analog functions on a single application specific semiconductor. The new chip controls key operation functions such as, for example, voltage regulation, ambient sensor amplifier, pyroelectric sensor amplifier, current and voltage reference and analog to digital conversion. Special topographic elements are etched on the chip to ensure isolation of sensitive sensor input channels. The new ASIC in one single embodiment includes a voltage regulator 82, a power on reset 84, a current reference 86, a voltage reference 88, a bias voltage generator 90 (resistor divider) , a constant current source 92, on ambient channel amplifier 94, Pyro channel amplifier 96, an analog multiplexer 98 and an analog to digital convertor 100 (minus counters, registers) . The ASIC 80 interfaces with a microprocessor 102, as shown in the functional block diagram of Figure 7. The microprocessor 102 is interconnected to the application specific integrated circuit 80 to receive electrical input signals indicative of the internal ambient temperature of the thermometer housing and the temperature differential between the pyroelectric sensor and the object to be measured. The microprocessor 102 is of conventional design with the presently preferred microprocessor being a Model KS57C2408, available from Samsung Corporation, having suitable data and program memory and being programmed to process the electrical signal from the application specific internal circuit 80, in accordance with the following description, to calculate the absolute temperature of the subject to be measured. Based upon the calculated temperature of the subject, the microprocessor 102 generates control signals to drive the display device to visually indicate the calculated temperature as a number on a liquid crystal display (LCD) 104 (See Figure 5) . More specifically, the infrared thermometers of the present invention measures the infrared radiation emitted by a body or subject at which they are pointed and use that result to calculate the temperature of the subject or body. The infrared sensor used is a pyroelectric sensor 58, which is a AC device which responds to a change in the IR radiation incident on the sensor element. By placing a shutter 56 in front of the sensor 58 and quickly opening that shutter, a transient output wave form is generated. Performing an analog to digital conversion over a given period of that wave form produces a result that is proportionately the difference between the temperature of the shutter, T a and the temperature of the body or target, T b . The relationship between these three variables is described by Botlzman's equation which states that, in a varied reduced form, as follows: θ = C*(T b 4 - T a 4 ) Each of the components of the application specific integrated circuit (ASIC) 80 will now be individually described. The voltage regulator 82 provides Vdd to all functions in the ASIC 80 as well as all requirements external to the integrated circuit. The voltage regulator 82 does not regulate the Vdd out. The voltage regulator functions as a simple pass transistor with a minimum V drop "on" state. The voltage regulator 82 is responsible for power up and power down of the thermometer 50. The battery voltage 106 is directly connected or wired to the input of the voltage regulator 82 with a minimum input leakage current when the unit is powered down. Powering up the thermometer will be accomplished by an input 108 connected to a momentary switch. The input is preferably a Schmitt input, having an internal pull-up resistor and must be latched. When toggled from "no connect" to GND or ground, the unit is powered on. Repeated "on's" while the system is powered up will not have any effect. Power down will occur on command from the microprocessor through an additional input 110. The signal will be active high and should be latched by the ASIC. The Vdd output 112 of the cell must be short circuit protected from external short circuit conditions. While the microprocessor 102 and the ASIC 80 will provide the means for measuring the battery voltage and determine when the voltage is too low for operation, the voltage regulator 82 will also sense a "Vbat lower limit". On sensing the lower limit, the ASIC 80 will allow the system to power up but the "RESET" line from the power on reset (POR) 84 will be held low to avoid situations where the ASIC 80, microprocessor 102 and EEPROM is into a position where erroneous results are displayed on the Liquid Crystal Display (LCD) 104 or the EEPROM could get corrupted by, for example, someone installing batteries 114 with total voltage below 3V. The microprocessor 102 will have a flag and display a "dead battery" condition at a voltage high enough above the ASIC "Vbat lower limit" to ensure that the user will have ample time to buy new batteries before the ASIC holds the system in the reset condition. The ASIC shutdown condition will appear as if completely dead batteries were used, since the RESET line will be held low. The voltage regulator 82 also implements an internal POR (power up reset) off the Vbat line to ensure that when batteries are installed, the ASIC and the unit will remain in the off state. A block diagram illustrating the inputs and outputs to the voltage regulator 82 is shown in Figure 8. The power on reset (POR) 84 function will be provided by the ASIC for the microprocessor 102. The ASIC 80 will hold this line low for a specified time and then drive the line high. The output will be compatible with the reset input on the presently preferred microprocessor or other microprocessor used. The current reference 86 function of the ASIC 80 of the present invention is provided by a pad used to generate a reference current 116. The current will be used to set biases within the ASIC 80 as well as the bias current for the ambient temperature sensor 118. A diagram of the current reference 86 is shown in Figure 9 with the Riref value having a resistor 120 of preferably 10. OK, a temperature coefficient of 100 ppm/°C maximum and a maximum tolerance of +/- one (1%) percent. As illustrated in Figure 10, the voltage reference 88 function of the ASIC 80 in one embodiment of the present invention is based on a 1.2 volt band gap reference but is gained up to provide the desired voltage. The voltage reference 88 provides the means for generating the bias voltage required by the other functions in the ASIC of the present invention. Both the tolerance and end temperature coefficients of the output voltage are important to the accurate performance of the application specific integrated circuit. The voltage reference 88 is designed to have the ability to adjust the output voltage via a metal mask change. As illustrated in Figure 11, the bias voltage generator 90 function in one embodiment is present preferably provided by a simple resistor divider that divides down the output of the voltage reference into the voltages required by the other functions in the ASIC 80. In one preferred embodiment, two voltage taps 122, 124 are required. The voltage reference 88 supplies the bias generator 90. The voltage taps 122, 124 provide the pyro circuit bias 126 and the analog to digital converter integrator comparator bias 128. The absolute value of these resistors is not critical, however, the ratio between them is important to the proper functioning of the ASIC 80 in the illustrated thermometer 50. In like fashion, the absolute value of the temperature coefficient is not critical but tracking the temperature coefficients from resistor to resistor is also important for the proper functioning of the ASIC 80 in the illustrated thermometer. The bias network is designed to provide adjustability to both the tap voltages via a metal mask change. The Pyrobias voltage 126 requires a buffer 130 as shown in figure 11. The bias voltage is used externally to the ASIC for shielding the high impedance input on the pyro circuit. To adequately drive the shield, the bias voltage must be low impedance. The ratios listed below in table produce the following bias voltages: Voltage (V) at Vref at Vref at Vref Bias = 1.828 = 1.966 = 2.104 Pyro-bias .723 .778 .832 ADC-bias 1.102 1.185 1.268 Resistor ratios Rl/Rt 0.396 R2/Rt 0.207 R3/Rt 0.397 RT = Rl + R2 + R3 As illustrated in Figure 12 , a constant current source 92 is used in one embodiment to provide bias to a transistor 132, specifically a PN100 transistor, available from National Semiconductor, which senses the ambient temperature. A forward biased PN junction in silicone produces a very linear voltage versus temperature characteristic. Key to the linearity reality is the application of the relatively constant current through the junction. The constant currents source is generated in the ASIC 80 by mirroring the current reference 86 discussed earlier. The connection to the transistor 132 is shown in Figure 12. The ambient channel amplifier 94 provides the interface between the presently preferred PN100 ambient temperature sensor 118 and the analog to digital converter (ADC) 100. The configuration of the ambient channel is driven by the ADC 100 and the signal from the ambient temperature sensor 118. Through testing of more than ten different lots of PNlOOs ambient temperature sensors 132, provided by National Semiconductor, an average of the range of outputs for the PN100 ambient temperatures sensors was determined. Using the configuration in the constant current source 92 section described earlier with a current of about 100 uA, the following range of "slopes" "offsets" for the Vbe versus temperature relationships in the PNlOOs was determined. This relationship is shown below. Specification Slope (mV/°C. Offset (Volt, at 0°C. Minimum -2.37878 0.601763 Maximum -2.27107 0.633119 Average -2.32167 0.618273 In absolute voltage, over the ambient temperature range (10 ? C to 40°C) the PN100 ambient temperature sensors generate a range of Vbe's shown in the table below: Vbe (volts. Temperature °C Minimum Maximum 40 .507 .542 10 0.578 .610 An average PN100 ambient temperature sensor is displayed graphically in Figure 13. As illustrated in Figure 14, the amplifier circuit 94 used for the PN100 132 is configured to provide both gain and level shifting. The circuit generates, Vamb = Vbe*(R7/R5 + R7/R6 + 1) - Vref* (R7/R6) The ASIC bias voltages 126, 128 listed in the bias generator 90 discussion above was derived when scaling the pyro portion of the ASIC design. To simplify the design of the specific ASIC illustrated, the ambient circuit 94 was scaled to work with the same ADC integrator/comparator bias 128 and AdC-deinteqrate bias 126. In calculating the values for resistors shown above, ± 10 mV was added to the minimum Vbe's (10°C to 40°C) to produce the following worse case range for the ambient channel. maximum ambient input range: 0.497V to 0.620V. Using the upper and lower limits on Vref, minimum Vbat, and worse case ADC components, the ratio shown in the equation for Vamb were derived to be as follows: R7/R5 = 2.499 and R7/R6 = .885. These values produced: Vamb = 4.384 x Vbe - .885 x Vref. The nominal Vref and nominal PN100 are graphically displayed in Figure 15. The voltage span from 0.869V to the ADCbias and from ,563V to 0V represent the headroom for the part to part variation of the PN100, the ± lO V of additional headroom on that range and the effect of the Vref variability. Working all the way through the worse case ADC conditions, the output of the ambient channel will have a worst case resolution of- 3.02 bits/0.01°C at the ADC.bits/O .01 Considering the transfer function for the ambient channel, all of the terms are represented as resistor ratios. The method of using resistor ratios has proven effective for canceling out tolerances and temperature coefficients. In order to compensate out the adjustment that may be made to the Vref cell through the metal mask change, R6 of the ambient circuit has metal mask adjustment taps. The ASIC was designed to use an external 150 or internal (IW ASIC) pyro sensor amplifier 94. Because of the high impedance of the pyro sensor configuration in prior thermometers, the connection from the sensor to the input of the amplifier was not allowed to touch the Printed Circuit Board (PCB) . In one prior thermometer model, the pyro channel amplifier was wired direct from the sensor to the OP amp input pin on the integrated circuit. Configurations of the pyro channel amplifier in conjunction with the ASIC of the present invention utilizes two options. In the first option, the amplifier 150 is external to the ASIC 80 and the on chip (ASIC) amplifier 94 is not used. The external amplifier output is bussed directly to the ASIC multiplexer. In the second option, the amplifier 94 in the ASIC is enabled and the ASIC is interfaced to the pyro sensor 58. The amplifier 150 external to the application specific circuit 80 is illustrated by Figure 16. The amplifier as included on the ASIC 80 is illustrated by Figure 17. The pyro channel amplifier 94 interfaces with the pyroelectric sensor 88 which senses the IR radiation to or from the body at which the thermometer is aimed. The pyroelectric sensor, as described earlier, is an AC device which responds to a change in IR energy incident on the pyroelectric element 132. The output is an extremely small current which looks like a ramp function with a relatively fast ramp up and a slow exponential ramp back to zero. The magnitude of the signal depends on the difference between the. ambient temperature (temperature of the thermometer) and the target temperature (temperature of the object in view) . The thermometer ambient temperature operating range and target temperature sensoring range for the thermometer of one representative embodiment of the present invention is as follows: Ambient Temperature Range 10°C to 40°C Target Temperature Range 20°C to 42.2°C The overlap of these ranges means that the pyro output is bipolar. Over the range of possible ambient and target temperature combinations, the peak output current of the pyro sensor used with the representative embodiment of the present invention will vary from about -60 picoamps to about 93 picoamps. Implementing a current to voltage converter with a CMOSop amp and a 10 GOHM resistor supplied in the pyro sensor package, resulted in an output of about 0.60V to about -0.93V peak. A typical output signal 152 is illustrated in Figure 18. As illustrated in Figure 18, the typical output signal 152 is an exponential decay with Vpeak defined by the pyro sensitivity, ambient temperature, and target temperature. For example, in the ADC 100, using the standard pyro with the nominal 8.5 GOHM resistor and a .1 uf capacitor, the integrate resistor became a 3.25 Mohm resistor. In an effort to reduce the ASIC voltage headroom consumed by the pyro signal and, thus, the size of the integrate resistor, the output of the pyro sensor 58 was attenuated by using a 4.5 GOHM resistor instead of an 8.5 GOHM resistor inside the sensor. Using an amplifier to interface to the pyro sensor, as described in option 1 above, the maximum pyro output for both the 4.5 GOHM and the 8.5 GOHM parts can be seen below. Temperature Combination Pyro Output using resistor shown Ambient (°C) Target (°C) 8.5Gohm 4.5Gohm 10 42.2 -1.05V -0.556V 40 20 0.677V 0.358V In one specific embodiment, the Pyrobias 126 and the ADCbias 128 were derived from the pyro voltages listed above. Over the targeted ambient ranges for the pyro sensor 58, the pyro output is bidirectional. To ensure that the ADC 100 results all have the same polarity, the ADC 100 was biased above the full range of the pyro sensor 58. Accounting for the Vref variance, pyro amplitude input offset voltage and input bias current and other factors, the biases became 0.778V for the Pyrobias 116 and 1.185V for the ADCbias 118. The above described relationship is shown in Figure 19. As illustrated, the integrate voltage in the ADC 100 is always positive. Considering the worst case situation for a minimum strength pyro, the analog to digital converter (ADC) produced about 1.5 bits/.01°C. During the output and measurement functions of the representative embodiment of the ASIC 80 included in the thermometer 50 of the present invention, the pyro channel is zeroed. The pyro channel output is read once a second and stored as the pyro offset. When the shutter opens, signaling the start of pyro measurement, the microprocessor will delay for about lOOms and then execute a standard ADC cycle, integrating for about 200ms and deintegrating to the result. The offset stored earlier, is then subtracted from the measured pyro signal, resulting in the final reading. The zeroing of the pyro channel is utilized to eliminate the temperature dependent variables that affect the offset. In the present application, a very low input current amplifier is utilized as the pyro sensor amplifier 96. The automatic zeroing function described earlier reduced the requirements on the op amp, in that the offset voltage and any temperature coefficients of that voltage are zeroed out. The same is true of the input bias current except that the bias current through the 4.5 gigohm resistor has a much greater ability to eat up headroom and other functions in the rest of the circuit. Additionally in the present embodiment, the PYROEN 154 input must be an internal pull-up resistor. Grounding the input disables the amplifier and leaving the input floating enables the amplifier. The analog multiplexer 98 utilized in one embodiment of the ASIC of the present invention is preferably a seven (7) in one (1) out multiplexer controlled by the microprocessor. This configuration provides for three (3) integrate channels, ambient 160, pyro 162 and battery 164; two (2) deintegrate channels, pyro/ambient 166 and battery 168; one (1) zero channel 170 for the ADC and one test channel (ADC bias) 172. In the present application only the integrate and deintegrate voltages are multiplexed. The bias for the ADC 100 will not be multiplexed. Three of the channels will be extremely important in meeting the performance requirements for the ADC sub system of the ASIC. The three important channels are the ambient integrate 160, the pyro integrate 162 and the ambient/pyro deintegrate 166. To maximize the accuracy of these three channels, the "ON" resistances are kept as small as possible (typically 50 ohms) . The remaining channels are relatively less important, requiring less accuracy in the signals from the ADC 100. These other channels are not critical to the accuracy of the thermometer. The three control lines (A, B, C) are always driven by the microprocessor and will not need to be latched into the ASIC 80 because the microprocessor 120 holds these lines high or low. Figure 20 is a diagram of a representative Analog Multiplexer 98 useful with the present invention and includes a description of the signals. As illustrated in Figure 21, the analog to digital converter (ADC) 100 utilized in one embodiment of the ASIC 80 of the present application is a presently preferred dual slope ADC providing fifteen (15) bits of accuracy. In one embodiment, the ADC contains only the analog components, the integrator and comparator required to build the ADC. In one specific embodiment, the timing and counting functions are performed by the microprocessor 102. However, this function could also be performed by circuits on the ASIC 80. The pyro sensor 58 has considerable influence on the design of the ADC. The integrating function of the dual slope ADC greatly reduces the effect of the highly noise sensitive high impedance pyro interface. The integration duration is chosen to eliminate the effects of both 50 and 60 cycle interference. A . luF capacitor and a 1M resistor reside external to the ADC. The ADC in signal 174 comes from the output of the multiplexer and the ADC - bias signal 128 comes directly from the bias generator 90. The compare 176 is connected to an external pad for connection to the microprocessor 102. The ADC-zero signal 170 is connected back to the input of the multiplexer 98 and to the output of the integrator allowing for the zeroing of and fast deintegration of the ADC 100. The ADC-zero cntrl signal 172 will come from the multiplexer control line decoder and during the zeroing phase, will control a switch that will zero the integrate capacitor. Not shown in Figure 21 is the control line Int-cntrl. This digital signal is "anded" with ADC zero-cntrl 172 and is used to disable the comparator during all ADC phases except for the integrate phases which are Ambient, Pyro, and Pyro-bias. In one specific embodiment, the ADC-bias signal 128 level was defined by the pyro sensor/ADC interface. With a nominal bias of about 1.185V (Vref equal to 1.966V) about IV of signal room for the amplified pyro output and about 2.1V of room for the integrate voltage was available. Requiring 15 bits of resolution, the worst case was about 58uV/bit. The deintegrate voltage of about 1.83V minimum and about 2.1V maximum along with a 125kHz nominal clock produces about 32,768 counts, best case, and about 28,500 bits, worst case, in the microprocessor for a maximum input signal. At 125kHz, 32,768 counts takes 262ms, which when added to the 200ms integrate time produces a maximum ADC cycle time of 462ms. The battery voltage ADC measurement uses the pyrobias 164 to charge the integrate capacitor up to a nominal- voltage of around 0.81V. Using Vdd-int 168 for the deintegrate, produces deintegrate times of 15.7ms (1970 counts) to 32.9ms (4113 counts) for Vdd-int equal to 5.5V to 3.6V respectively. Using Vdd-int signal 168 for the deintegrate, produces deintegrate times of about 15.7ms (about 1970 counts) to about 32.9ms (about 4113 counts) for Vdd int equal to about 5.5V to 3.6V respectively. The fast deintegrate that was performed as a separate function in the prior thermometer is performed as part of the ADC zero control 172 (an output from the multiplexer control line decoder that, during the zero phase, will control a switch that zeroes the integrate capacitor) function in the present invention. In summary, the parameters used to scale the ADC components and bias voltages for one specific embodiment are shown in Figure 22. It is important to the proper function of the thermometer of the present invention that the ADC performance must not degrade when modified. During further optimization, the parameters of the integrate resistor, the integrate capacitor and the integrate time were modified, as shown in Figure 23. The key performance parameter in the ADC of the present invention is linearity and the stability of that linearity. Zero crossings are not an issue, since the system was designed to never require a zero result from the analog to digital converter. Roll over area is also not an issue, since the ADC is unipolar. The calibration scheme also helps to reduce the requirements on the ADC. The pad layout of the IR thermometer 50 of the present invention is shown in Figure 24 and for the ASIC 80 in Figure 25. One of the important features of the present infrared thermometer is the calibration of the thermometer during the production process. Three components of the application specific integrated circuit system will be. calibrated during the production process. Those components will be the ambient channel, the pyro channel and the battery limits. During calibration, a system will be assembled to the level that the ASIC, the microprocessor, the ambient sensor and the pyro sensor are integrated. The interaction of the microprocessor and the ASIC will be critical to calibration. In accordance with the varying aspects of the present invention, the new design provides for an enhanced ambient temperature calibration process, involving an inert bath at two known and well controlled temperatures, using a fluid environment adapted to rapidly change temperature for the specific range necessary for the thermometer to operate in a clinical application. First we will discuss the calibration of the ambient channel. The ambient channel calibration involves accumulating data at two temperatures and then using the accumulated data to calculate the slope and bias of the straight line that best fits the actual data. The ambient channel calibration is accomplished as follows: (1) The thermometer is submerged in an inert fluid bath and brought to a stable "low" limit temperature of approximately 15°C. (2) The thermometer ' s microprocessor 102 continuously performs analog-to-digital conversions of the ambient channel and transmits this data to the calibration computer system (not shown?) . Simultaneously, the computer system measures the temperature of the fluid bath via precalibrated temperature reference "probes" . (3) Once the temperature of the bath and the analog-to- digital values from the thermometer have stabilized at this "low" temperature, the recorded data values are stored to the calibration system's data base. (4) The thermometer's are then submerged in an inert fluid bath and brought to a stable "high" limit temperature of approximately 35°C. (5) The thermometer's microprocessor continuously. performs analog-to-digital conversions on the ambient channel and transmits this data to the calibration computer system. Simultaneously, the computer system measures the temperature of the fluid bath via pre-calibrated temperature reference probes. (6) Once the temperature of the bath and the analogto- digital values from the thermometer have stabilized at this "high" temperature, the recorded data values are stored to the calibration system's data base. (7) A slope and bias are calculated from the low and high data points. This slope and bias are then written to the thermometer's non-volatile memory and to the system's data base. (8) The calculated calibration data is then verified by comparing the temperature calculated by the thermometer to the temperature of the reference probes. Next, we will discuss the calibration of the pyro channel. The pyro channel calibration is accomplished by taking data at one combination of ambient temperature and target temperature. The pyro calibration is accomplished as follows: (1) the thermometer's microprocessor continuously performs analog-to-digital conversions on the ambient channel and transmits this data to the calibration computer system, until stability of the ambient channel has been reached. (2) The thermometer's temperature is then verified against a known temperature reference. (3) The ambient temperature of the thermometer is stored in the system's data base. (4) The thermometer will then take the temperature of a known temperature reference "target" . From this measurement, the calibration computer will calculate a compensated pyro channel gain. (5) The pyro channel gain is written to the thermometer's non-volatile memory and to the calibration. system's data base. (6) The calibration is then verified by taking an additional target reading and then comparing the thermometer's calculated temperature with the actual target temperature. Finally we will discuss the calibration of the battery limits during the assembly process. To calibrate the battery limits, two limits need to be established during the battery tests, the low battery limit and the dead battery limit. The calibration of these limits for the system will require generation of data at two voltages. The battery limits calibration is accomplished as follows: (1) the thermometer is powered up at a battery voltage equal to the low voltage value; (2) an analog to digital conversion on the battery sense input is used to establish the analog to digital count for that limit; (3) the thermometer voltage will then be changed to a battery voltage equal to the dead battery voltage value; (4) an analog to digital conversion on the battery sense input is used to establish the analog to digital conversion counts for the dead battery voltage limit; (5) the low battery limit and the dead battery limit are then loaded into the unit and the calibration data base. Thus, it can be seen that the new improved IR thermometer of the present invention includes a new ASIC which significantly reduces the number of individual components, is easier to manufacture, requires less calibration resources, is more reliable and is less expensive to manufacture when compared to prior IR thermometers. Changes and modifications in this specifically described embodiment can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.

Claims: What is claimed is: 1. An application specific integrated circuit for use with an IR thermometer comprising: a multiplexer; voltage supply means operatively connected to the multiplexer; an analog to digital converter, operatively connected to both the multiplexer and the voltage supply means, for communication with a microprocessor means; an ambient sensor amplifier, operatively connected to the multiplexer and to an ambient sensor, for providing an ambient temperature measurement; and a pyro sensor amplifier, operatively connected to the multiplexer and a pyro sensor, for providing a pyro sensor temperature measurement.

Inventors: BOHL ROBERT HOWARD, WHITE TIMOTHY ERIN, BOHL, ROBERT, HOWARD, WHITE, TIMOTHY, ERIN
Applicants: THERMOSCAN INC, THERMOSCAN, INC.
Application year: 1998
Main classification: G01J5
IPC codes: G01J5