Abstract:
An IR sensing device including a sensing element configured as a three terminal device, a differential signal processing circuit and an electrical connection to the conductive housing.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates in general to infrared (IR) sensing devices and more particularly to IR sensing devices including circuitry configured to protect the operation of the IR sensing device in an electrically hostile environment. 
     IR sensors are used to measure temperatures of remote objects by detecting the infrared radiation emitted from the target object. In a typical configuration, IR radiation enters the housing in which the sensor element is located through an IR transparent window and impinges upon the sensing element. The temperature is typically measured by detecting the IR radiation and determining its effect on a thermally isolated radiation absorbing area of the sensing element. The sensing element produces an electrical signal representing the temperature of the target object and which varies with the impinging radiation. This electrical signal is processed and amplified by electronic processing circuitry. The signals are small in amplitude and the sensor is therefore susceptible to the effects of EMC and ESD and also to leakage in protection circuitry. 
     A typical arrangement of the electronic processing and protection circuitry coupled to a sensing element is shown in FIG. 1. A thermal sensing element  101  is connected to processing circuit  110  via connection lines  112  and  113  as shown. Processing circuit  110  amplifies and processes the signal from sensing element  101  so as to produce a temperature readout signal  107 . Sensing element  101  is also connected to protection circuit elements  102 ,  103 ,  104  and  105  via connection lines  112  and  113  as shown. Typically, sensing element  101  has one end connected to a ground, or earth, terminal shown here as connection  106 . Sensing element  101  typically includes a number of thermopile elements in series with a combined series impedance on the order of tens of [Kohm] kΩ. The connections  112  and  113  between the two ends of the series of thermopile elements and the processing circuit are normally protected at one or even both ends against static damage and incoming EMC radiation by protection circuits  102 ,  103 ,  104 , and  105 , which are arranged such as to limit the voltage excursions of the connections using diode structures connected to the power supply rails. The diodes are typically configured to conduct when the voltage on the pins exceeds the power rail voltage by the diode voltage. 
     Such protection structures, even when not operating, have an inherent leakage current, and when the external impedance is high, e.g., on the order of tens of [Kohm] kΩ, the leakage current generates an error voltage superimposed on the desired signal voltage generated by the sensor. 
     It is therefore desirable to provide IR sensing devices including circuitry that protects the sensing device from electrical interference in an electrically hostile environment and which has reduced leakage. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides infrared (IR) sensing devices including circuitry configured to protect the sensing device from electrical interference in an electrically hostile environment and which has reduced leakage. 
     According to the invention, an IR sensing device includes a sensing element configured as a three terminal device, a differential signal processing circuit and an electrical connection to the conductive housing. 
     According to an aspect of the present invention, an Infra Red (IR) sensing device is provided that typically comprises an IR sensing element for detecting IR radiation, wherein the IR sensing element includes a plurality of thermopile elements with connections to both ends and to its center point, a processing circuit coupled to the IR sensing element configured to receive and analyze the electrical signals generated by the IR sensing element, wherein the processor connects the center point of the sensing element to a first potential and analyzes the signals from the ends of the sensing element as a differential pair of signals. The device also typically includes protection circuitry associated with the inputs to the processing circuit from the sensing element that connect only to the inputs and the first potential, and an electrically conducting housing connected to the first potential. 
     According to another aspect of the present invention, an Infra Red (IR) sensing device is provided that typically comprises an IR sensing element for detecting IR radiation, wherein the IR sensing element includes a plurality of serially connected thermopile elements, a processing circuit configured to receive and process the electrical signals generated by the thermopile elements, the processing circuit having first and second inputs coupled to the two ends of the series of thermopile elements and a third input coupled to a center point of the series of thermopile elements and to a first potential, and protection circuitry coupled to the two inputs and the first potential. In operation, the processing circuit typically processes the signals at the first and second inputs as a differential pair of signals relative to the first potential so as to produce a temperature readout signal. 
     Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a typical IR sensor arrangement; 
     FIG. 2 is a sectional schematic diagram showing an arrangement of elements of an IR sensing device according to an embodiment of the present invention; and 
     FIG. 3 is a schematic diagram showing a sensor arrangement according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     IR sensors are used to measure temperatures of remote objects by detecting the infrared radiation emitted from the target object. FIG. 2 illustrates an IR sensing device arrangement according to one embodiment of the present invention. As shown, IR radiation  204  enters the housing  202  in which a sensor element  201  is located through an IR transparent window  203 , and impinges upon the sensing element  201 . The temperature is measured by determining the effect of the IR radiation on a thermally isolated radiation absorbing area of the sensing element. The sensing element typically produces an electrical signal that is proportional to and varies with the impinging radiation. This electrical signal is processed by electronic circuitry  206  shown mounted on the same baseplate  205  of the housing  202 . The signals are typically small in amplitude and the sensor is therefore susceptible to the effects of EMC and ESD and also to leakage in the protection circuitry. 
     According to one embodiment of the present invention, as shown schematically in FIG. 3, a sensing element  301  generates a voltage signal across the extremities  312  and  313 , which is a function of the received radiant energy. Preferably sensing element  201  includes a plurality of serially connected thermopile elements as is well known, with lines  312  and  313  connecting to opposite ends of the serially connected thermopile elements. The voltage signal is received over lines  312  and  313  and amplified and processed by electronic processing circuit  310  so as to produce a digital temperature readout signal on line  314 . There is also an additional connection  307  to a point substantially in the center of the string of serially connected thermopile elements of sensing element  301 . Connection  307  is electrically connected to the substrate of the silicon chip (e.g., element  201  of FIG. 2) on which the sensing element  301  is fabricated. Connection  307  is further electrically connected to the substrate of the silicon chip  306  (e.g., element  206  of FIG. 2) on which the electronic processing circuit  310  is implemented as a single integrated circuit. Connection  307  is further electrically connected to the protection circuitry elements  303  and  304  on the silicon chip  306  on which the electronic processing circuit  310  is implemented. Connection  307  is farther electrically connected to the base of the housing (e.g., base  205  of the housing  202  in FIG.  2 ). Preferably, as shown in FIG. 2, a bond wire  208  makes the connection between the sensing element  201  and a pin  209  in intimate contact with the base  205 . Similarly, pin  210  provides a connection to ground for processing circuit silicon chip  206  via bond wire  211 . Other techniques for making these connections are well known and can include, for example, a bond wire direct from the sensing element  201  to the base  205 , the use of conducting adhesives when fixing the sensing element chip  201  and/or the processing circuit silicon chip  206  down to the base  201 . 
     The signals generated by the sensing element  301  are small in magnitude, e.g., typically between about 10 microvolts/° C. and about 100 microvolts/° C., and more typically approximately 50 microvolts/° C. In a conventional arrangement, the leakage currents associated with the protection circuit elements at the inputs to the processing circuitry would have leakage currents of between 1 and 10 nA. These leakage currents combined with the impedance of the thermopile elements at typically 50 kΩ would give unwanted error voltages between about 50 and about 500 microvolts, equivalent to an error of about 1 to about 10° C. In one embodiment of the present invention, the protection circuit elements  303  and  304  are advantageously connected to the reference connection  307 , and not the power supply. Protection circuit elements  303  and  304  each preferably include a diode element. The magnitude of the signals from the radiation sensing element  301  is small and the diodes in this region of their operating curve have a leakage that is much smaller and substantially linear with voltage and is preferably compensated for by adjustments to the gain in the ensuing electronic processing circuitry (e.g., element  310  or other element). The desired signal is also differentially distributed around the reference line  307  that is connected to the local zero volts or ground line, thereby providing improved immunity to EMC interference. The conducting housing is also connected to the reference or ground pin, which provides improved immunity to ESD interference. 
     The combination of the center tapped sensing element, the fully differential zero voltage centered processing circuitry and the screening effects provided by the connection to conducting parts of the housing advantageously provide an IR sensing device with enhanced performance and protection from the electrical environment. 
     Additional processing circuitry for any of the above implementations can include analogue and/or digital circuitry for configuring, calibrating and/or controlling operation of the sensor. Memory, such as non-volatile memory is also preferably included. Such non-volatile memory is used in some embodiments as storage for look up tables or other constants and configuration parameters that are used for optimization and calibration of the sensing devices. In one digital implementation, a microprocessor is used as both a control and processing circuit, and can further be integrated with a memory, such as a non volatile memory unit. Memory may be programmed before manufacture, after manufacture of the sensor, or after installation of the sensor in an operating location. 
     While the invention has been described by way of example and in terms of the specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.