Abstract:
A circuit is provided that can provide, in a single package, a means to monitor a sensing element which uses a variable resistor. The circuit (also known as a signal conditioning circuit) may contain resistor input terminals to which a reference set resistor and a resistive sensor can be attached. A reference voltage signal can be applied to both terminals. There are also means for sensing the resulting current flowing through both the set resistor and the resistive sensor. The difference of the currents flowing through each element can then be monitored as being indicative of the difference in resistance between the set resistor and the resistive sensor. The current difference signal can be used to generate a voltage difference signal indicative of the difference in resistance between the set resistor and the resistive sensor. The signal conditioning circuit may be used to adjust the temperature of various devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/790,951, entitled “Instrumentation Amplifier,” filed Feb. 22, 2001. 
     
    
     
       FIELD OF INVENTION  
         [0002]    The present invention relates to electronic circuits. More particularly, the present invention relates to a signal conditioning circuit for determining the resistance of resistive sensors, including thermistors.  
         BACKGROUND OF THE INVENTION  
         [0003]    There are types of resistors that have a resistance characteristic that varies with respect to changes to a certain property. The resistance can then be used to measure that property. For example, a thermistor has a resistance that varies with temperature. Thus, the thermistor can be used to measure temperature by measuring the resistance of the thermistor. There are other types of resistors available that are sensitive to different variables, such as pressure or light.  
           [0004]    One prior art system for measuring a resistance is illustrated in circuit  100  of FIG. 1. Circuit  100  features an excitation circuit and an amplifier section. The excitation circuit is configured to excite a resistive sensor and a reference resistor, while the amplifier is configured to output a result that is proportional to the difference in resistance between the reference resistance and the resistance of the sensor.  
           [0005]    The excitation circuit comprises a voltage source  102 , a resistor  110  and a resistor  112 , a set resistor (or reference resistor)  114 , and a resistive sensor, e.g., a thermistor  116 . Resistors  110  and  112  may be identical in configuration, i.e., matched resistors, such that known biases are applied to set resistor  114  and thermistor  116 . This bias of set resistor  114  creates a voltage that is propagated to an input terminal  125  of instrumentation amplifier  120 . The current flowing across thermistor  116  creates a voltage that propagates to an input terminal  123  of instrumentation amplifier  120 .  
           [0006]    The amplification section comprises an instrumentation amplifier  120 . Instrumentation amplifier  120  is typically configured as a differential amplifier that amplifies the difference in voltage between the voltage at input terminal  123  and the voltage at input terminal  125  and generates a signal at the output terminal  124  of instrumentation amplifier  120 . This voltage difference is proportional to the difference in resistance between thermistor  116  and set resistor  114 . A typical instrumentation amplifier may have a gain of approximately  100 . Set resistor  114  has a known resistance, while the temperature/resistance characteristics of thermistor  116  and the gain of instrumentation amplifier  120  are also known. Due to these known characteristics, the temperature being sensed by thermistor  116  can be calculated. However, a significant drawback of circuit  100  is that it is important for resistors  110  and  112  to be matched to provide a known bias, often requiring expensive precision resistors to be included.  
           [0007]    An alternative layout for a prior art circuit  400  of measuring a resistance is shown in FIG. 4, where the excitation of set resistor  414  and thermistor  416  is accomplished through the use of current sources  410  and  412 , i.e., voltage source  102 , and resistors  110  and  112  are replaced with current sources  410  and  412 . However, there may be difficulty in matching current sources  410  and  412  to provide known, equal currents.  
           [0008]    The measurement of temperature can be important in a variety of applications. For example, one use of thermistors is in the field of optical networking. An optical network system may use lasers to transmit light through a fiber optic cable. The lasers are typically kept at a predetermined temperature, in order to have the laser transmit light of a predetermined wavelength. One method that can be used to control the temperature is to use a thermoelectric cooler and a thermistor mounted on the laser diode. The thermistor will change in resistance when there is a change in temperature. The thermistor may be coupled to the thermoelectric cooler in such a way that the amount of cooling increases when the temperature becomes too high and decreases when the temperature lowers to a desired level. However, prior art measurements systems for such applications can be quite complex.  
           [0009]    There is a desire for a simpler and more compact system and method for testing and/or measuring the resistance in resistive sensors. In addition, to determine the difference between a set resistor and a thermistor or other resistive sensor, it would be desirable to have the currents exciting the set resistor and the resistive sensor be as closely matched as possible, i.e., to minimize the difference in excitation sources, without requiring precision resistors, matched resistors, or the difficult matching of current sources.  
         SUMMARY OF THE INVENTION  
         [0010]    The method and circuit according to the present invention addresses many of the shortcomings of the prior art. In accordance with one aspect of the present invention, a circuit is provided that can facilitate accurate resistance measurements.  
           [0011]    In accordance with an exemplary embodiment of the present invention, a self-contained signal conditioning circuit can be provided that contains a mechanism for testing and/or measuring resistance in a resistive sensor by connecting the resistive sensor and a reference resistor, e.g., a set resistor, to the self-contained signal conditioning, circuit. Such a signal conditioning circuit may contain an amplification stage coupled to the set resistor, with a similarly configured amplification stage coupled to the resistive sensor. The current being supplied to the set resistor and to the resistive sensor can be monitored and the difference between the amount of current being supplied to the set resistor and the amount of current being supplied to the resistive sensor can be sensed. This difference in current is proportional to the difference in resistance between the set resistor and the resistive sensor. This difference in current may be converted to a voltage signal, if so desired. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, where like reference numbers refer to similar elements throughout the Figures, and:  
         [0013]    [0013]FIG. 1 illustrates a schematic diagram of a prior art system for measuring resistance;  
         [0014]    [0014]FIG. 2 illustrates a schematic diagram of an exemplary embodiment of the present invention for measuring resistance;  
         [0015]    [0015]FIG. 3 illustrates a schematic diagram another exemplary embodiment of the present invention for measuring resistance; and  
         [0016]    [0016]FIG. 4 illustrates a schematic diagram of another prior art system for measuring resistance. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The present invention may be described herein in terms of various functional components and various processing steps. It should be appreciated that such functional components may be realized by any number of hardware or structural components configured to perform the specified functions. For example, the present invention may employ various integrated components comprised of various electrical devices, e.g., resistors, transistors, capacitors, diodes and the like, whose values may be suitably configured for various intended purposes. In addition, the present invention may be practiced in a variety of integrated circuit applications. It should be noted that while various components may be suitably coupled or connected to other components within exemplary circuits, such connections and couplings can be realized by direct connection between components, or by connection through other components and devices.  
         [0018]    As discussed above, prior art resistance measurement systems are more complex than desired. In accordance with various aspects of the present invention, a signal conditioning circuit that can facilitate accurate resistance measurements is provided.  
         [0019]    In accordance with an exemplary embodiment of the present invention, a self-contained signal conditioning circuit is provided that contains a mechanism for testing and/or measuring resistance in a resistive sensor by connecting a resistive sensor and a reference resistor, e.g., a set resistor, to the self-contained signal conditioning circuit. Such a signal conditioning circuit may contain an amplification stage coupled to the set resistor, with a similarly configured amplification stage coupled to the resistive sensor. The current being supplied to the set resistor and to the resistive sensor can be monitored and the difference in amount of current can be sensed. This difference is proportional to the difference in resistance between the set resistor and the resistive sensor.  
         [0020]    For example, an exemplary measurement system  200  is illustrated in FIG. 2. Measurement system  200  is generally configured as a self-contained circuit to measure the resistance of a resistive sensor, where the same circuit provides both excitation and measurement of the resistive sensor. System  200  comprises a signal conditioning circuit  202 , to which can be attached a set resistor  214  and a thermistor  216 .  
         [0021]    Signal conditioning circuit  202  includes two voltage input terminals  204  and  206 , and two output terminals  230  and  232 . Voltage input terminals  204  and  206  can be coupled to a voltage supply or other power source to provide excitation to the resistors being tested. In one embodiment, output terminal  230  is configured to provide a current signal, to which can be attached a load resistor  234 , and output terminal  232  is configured to provide a voltage signal. Terminals  211  and  213  can be used to connect a set resistor  214  and a thermistor  216 , respectively.  
         [0022]    Signal conditioning circuit  202  further comprises buffers  222  and  224 . Buffers  222  and  224  can comprise various types of buffers, amplifiers or other like devices. Buffer  222  is coupled to voltage input terminal  204 , while buffer  224  is coupled to voltage input terminal  206 . Buffer  222  is configured to convert the voltage signal from voltage input terminal  204  to a current signal at terminal  211 . In a similar manner, buffer  224  is configured to convert the voltage signal from voltage input terminal  206  to a current signal at terminal  213 .  
         [0023]    Between the input terminals and the output terminals, there is a device configured to sense the difference in the current flowing between a set resistor  214  and a thermistor  216 . This sensed difference signal, as provided by the difference sensing device, is represented in FIG. 2 by a current source  226 . The signal from current source  226  (indicative of the difference between the current flowing through set resistor  214  and the current flowing through thermistor  216 ) can be accessed directly via terminal  230 . Alone or in addition, an amplifier  228  can be configured to provide a voltage signal based on current source  226  at output terminal  232 .  
         [0024]    After coupling a voltage source to voltage input terminals  204  and  206 , and coupling set resistor  214  and resistive sensor  216  to system  200 , the difference in resistance between set resistor  214  and resistive sensor  216  can be determined by monitoring the output signal at output terminal  230  or output terminal  232 : the output signal is proportional to the difference in current flowing in set resistor  214  and thermistor  216 , which is proportional to the difference in the resistance between set resistor  214  and resistive sensor  216 .  
         [0025]    A detailed schematic diagram of another exemplary embodiment of a measurement system  300  is presented in FIG. 3. System  300  comprises a voltage input circuit for application of voltage signals; input terminals for connecting a set resistor and a thermistor; a circuit for generating a differential current signal, and an output circuit. An example of operation for the various circuits and components can be found in U.S. application Ser. No. 09/790,951, entitled “Instrumentation Amplifier”, having a common inventor and common assignee, and hereby incorporated by reference. The voltage input circuit comprises voltage input terminals  301  and  303  configured for connection to one or more voltage sources. It should be noted that terminal  303  and terminal  301  may also be coupled together such that they are driven by the same input voltage signal. An exemplary input voltage signal may be approximately 1.25 volts, however, other voltages can be utilized.  
         [0026]    The voltage signal from terminal  301  propagates to a buffer  302  comprising an op-amp  304  and an output stage  305 . In a similar manner, the voltage signal from terminal  303  propagates to a buffer  308 , comprising an op-amp  310  and an output stage  311 .  
         [0027]    As discussed more fully in U.S. application Ser. No. 09/790,951, buffers  302  and  308  can be configured to create a current-mode signal at terminals  391  and  393 , respectively. These current-mode signals result from a voltage applied to a set resistor  314  and a thermistor  316 . A difference sensing device comprising current mirrors  312 ,  315 ,  317 , and  318  may be configured to supply bias current to output stages  305  and  311 . Current mirrors  312  and  317  are coupled to a power supply  377  to generate the bias current. In a similar manner, current mirrors  315  and  318  are coupled to a power supply  374  to generate a bias current. The current that flows through set resistor  314 , i.e., from output stage  305 , can be sensed by determining the current flowing through current mirrors  312  and  315 . More particularly, current mirrors  312  and  315  are coupled to output stage  305  through current mirror input terminals  382  and  384 , respectively. Current mirror output terminals  386  and  388  are coupled to thermistor  316 . Current mirrors  317  and  318  are coupled to output stage  311  via current mirror input terminals  392  and  394 . Because output stage  311  is also coupled to current mirrors  312  and  315 , as described above, current mirrors  317  and  318  are thus sensing, not only the current being supplied to thermistor  316 , but also the current being supplied to set resistor  314 . Current mirror output terminals  396  and  398  are then coupled to node  355 . Because of the above-described configuration of current mirrors  312 ,  315 ,  317 , and  318 , the current signal present at node  355  is equal to the difference between the current flowing through set resistor  314  and the current flowing through thermistor  316 . Such a signal can be propagated through a buffer amplifier  320  to result in an output voltage at output terminal  360 .  
         [0028]    In accordance with another exemplary embodiment, the difference current signal can be accessed at terminal  355  in various manners. One possible use of such a current signal would be to allow the connection of an output resistor  341  to produce an output voltage signal. Other embodiments can include the use of an adjustable reference, an output filtering configuration, or an offsetting output configuration, such as discussed more fully in U.S. application Ser. No. 09/790,951 with respect to FIGS.  4 A- 4 C.  
         [0029]    Set resistor  314  can be attached to a terminal  391  and thermistor  316  can be attached to terminal  393 . A reference voltage can be applied to input terminals  301  and  303 . The same reference voltage can be applied to both input terminals  301  and  303 . In another embodiment, different voltages can be applied, e.g., in a situation where the set resistance is very different from the resistance of the thermistor. For example, one may use a 10 KΩ set resistor and a thermistor that measures 10 kΩ in nominal conditions. When using such a configuration, it may be desirable to apply the same excitation voltage to input terminals  301  and  303 .  
         [0030]    However, if one uses a 1 kΩ set resistor and a thermistor that is 10 kΩ in nominal conditions, it may be desirable to apply different voltages to input terminals  301  and  303 .  
         [0031]    The operation of circuit  300  begins by providing an excitation signal to set resistor  314  and thermistor  316 . The excitation signal is provided by the connection of a voltage source to input terminals  301  and  303 . The current flowing through both resistor  314  and thermistor  316  can then be compared with the difference in current being available at output terminal  355 . Output terminal  355  can be accessed if a current signal is desired, while an output terminal  360  can be accessed if a voltage signal is desired.  
         [0032]    In accordance with another aspect of the present invention, while buffers  302  and  308  can comprise various configurations, measurement system  300  may be suitably configured with chopper stabilized amplifiers for buffers  302  and  308  and/or for buffer  320 , if desired. As a result, buffers  302  and  308  and/or output buffer  320  can be configured to address offset and drift errors. In accordance with this aspect of the present invention, the chopper-stabilized amplifiers can comprise any conventional chopper amplifier or any auto-zero configuration or dynamic element matching configuration and the like now known or hereinafter developed.  
         [0033]    In addition, current mirrors  312  and  317 , and current mirrors  315  and  318  may also be suitably configured to use chopper stabilization in accordance with various exemplary embodiments. For example, chopper stabilization may be suitably implemented to correct for various errors that may be present in the current mirrors. In addition, chopper stabilization may be implemented to reduce gain error and drifting problems, as well as improve the linearity of the current mirrors. Further, the chopper stabilized current mirrors can comprise any chopper stabilized, auto-zero, or dynamic element matching configuration and the like now known or hereinafter developed.  
         [0034]    The present invention has been described above with reference to various exemplary embodiments. However, those skilled in the art will recognize that changes and modifications may be made to the exemplary embodiments without departing from the scope of the present invention. For example, the various components may be implemented in alternate ways, such as, for example, by providing other pin layouts or arrangements, and/or additional or fewer current mirrors. Further, while the invention has been described in reference to a thermistor, it should be understood that various forms of resistive sensors may also be used in various embodiments of this invention. In addition, the invention described above may also be used to measure any unknown resistance. In addition, for embodiments including chopper stabilized buffers or current mirrors, the devices can be configured to operate at various frequencies and other operating parameters. Moreover, the instrumentation amplifiers can be configured to aid the summation of multiple channels or for mixing, current-mode referencing, or signal processing applications and the like. These alternatives can be suitably selected depending upon the particular application or in consideration of any number of factors associated with the operation of the system. Moreover, these and other changes or modifications are intended to be included within the scope of the present invention, as expressed in the following claims.