Abstract:
There is provided a reference voltage generating circuit generating a reference voltage to be applied to a current-to-voltage converting circuit in order to compensate for an offset voltage of the current-to-voltage converting circuit converting an input current into a voltage and outputting the voltage, the reference voltage generating circuit including: a sampling conversion circuit having the same circuit characteristics as the current-to-voltage converting circuit and adding a predetermined offset to the reference voltage to generate an output voltage; and a comparator controlling the reference voltage so that the output voltage of the sampling conversion circuit is equal to a predetermined voltage, wherein the reference voltage is applied as an input to the sampling conversion circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application No. 2007-0073997 filed on Jul. 24, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an offset compensation circuit, and more particularly, to a circuit that can compensate for an offset value by inputting to the current-to-voltage converting circuit, a corrected reference voltage in which an offset value that may occur in the current-to-voltage converting circuit is reflected. 
         [0004]    2. Description of the Related Art 
         [0005]    In a photo diode integrated circuit (PDIC), an offset voltage is generated due to a physical device size mismatch, a current flowing through both input terminals of a current-to-voltage converter, and a leakage current occurring in a feedback resistor. 
         [0006]    In the related art, in order to compensate for the offset voltage, a method of compensating the offset voltage by using a compensation resistor is used to reduce the amount of offset current flowing through the input terminals of the current-to-voltage converter. 
         [0007]      FIG. 1  is a schematic view illustrating a current-to-voltage converting circuit according to the related art. 
         [0008]    Referring to  FIG. 1 , a current-to-voltage converting circuit  100  according to the related art may include a photo diode integrated circuit  140 , an amplifier  130 , and a compensation resistor  110 . 
         [0009]    An output voltage V 0  of the circuit is obtained by the following Equation. 
         [0000]        V   0   =i   p   R   f   +V   REF +[( I   B− ) R   f −( I   B+ ) R   C ] 
         [0010]    The output voltage V 0  may be calculated on the basis of a voltage i p R f  obtained by using photocurrent ip generated by an external light source and a feedback resistor R f , a reference voltage V ref , and an offset voltage I b   −R   f  generated by leakage current generated from the feedback resistor R f . 
         [0011]    In order to compensate for the offset voltage, in the related art, the compensation resistor R c  is used to reduce the amount of offset current flowing through input terminals of the current-to-voltage converting circuit. In this way, the offset voltage is compensated. 
         [0012]    However, according to the related art, the size of the compensation resistor R c  may be changed during a semiconductor process, and the voltage between both terminals of the photo diode may be changed to generate a new offset error. 
       SUMMARY OF THE INVENTION 
       [0013]    An aspect of the present invention provides a circuit that can compensate for offset by controlling a reference voltage despite variation in physical device size and a change in environment. 
         [0014]    According to an aspect of the present invention, there is provided a reference voltage generating circuit generating a reference voltage to be applied to a current-to-voltage converting circuit in order to compensate for an offset voltage of the current-to-voltage converting circuit converting an input current into a voltage and outputting the voltage, the reference voltage generating circuit including: a sampling conversion circuit having the same circuit characteristics as the current-to-voltage converting circuit and adding a predetermined offset to the reference voltage to generate an output voltage; and a comparator controlling the reference voltage so that the output voltage of the sampling conversion circuit is equal to a predetermined voltage, wherein the reference voltage is applied as an input to the sampling conversion circuit. 
         [0015]    The sampling conversion circuit may include an operational amplifier having the same offset as the current-to-voltage converting circuit. 
         [0016]    The sampling conversion circuit may include an operational amplifier having a non-inverted input terminal to which the reference voltage is input and an inverted input terminal to which the output voltage of the sampling conversion circuit is fed back. 
         [0017]    The comparator may include an operational amplifier having an inverted input terminal to which the output voltage of the sampling conversion circuit is input and a non-inverted input terminal to which the predetermined voltage is input. 
         [0018]    The comparator may further include filters. 
         [0019]    According to another aspect of the present invention, there is provided a current-to-voltage converting circuit including: a current-to-voltage converting circuit converting an input current into a voltage and outputting the voltage; a sampling conversion circuit having the same circuit characteristics as the current-to-voltage converting circuit and adding a predetermined offset to a reference voltage to generate an output voltage; and a comparator controlling the reference voltage so that the output voltage of the sampling conversion circuit is equal to the predetermined voltage, wherein the reference voltage is applied as an input to the current-to-voltage converting circuit and the sampling conversion circuit to remove offset in the current-to-voltage converting circuit. 
         [0020]    The current input to the current-to-voltage converting circuit may be a current generated by a photo diode. 
         [0021]    The current-to-voltage converting circuit may include an operational amplifier having a non-inverted input terminal to which the reference voltage is input and an inverted input terminal to which the input current and the fed-back output voltage of the current-to-voltage converting circuit are input. 
         [0022]    The sampling conversion circuit may include an operational amplifier having the same offset as the current-to-voltage converting circuit. 
         [0023]    The sampling conversion circuit may include an operational amplifier having a non-inverted input terminal to which the reference voltage is input and an inverted input terminal to which the output voltage of the sampling conversion circuit is fed back. 
         [0024]    The comparator may include an operational amplifier having an inverted input terminal to which the output voltage of the sampling conversion circuit is input and a non-inverted input terminal to which the predetermined voltage is input. 
         [0025]    The comparator may further include filters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0027]      FIG. 1  is a configuration view illustrating a current-to-voltage converting circuit according to the related art; 
           [0028]      FIG. 2  is a configuration view illustrating a reference voltage generating circuit according to an exemplary embodiment of the present invention; 
           [0029]      FIG. 3  is a configuration view illustrating an offset compensated current-to-voltage converting circuit according to another exemplary embodiment of the present invention; and 
           [0030]      FIGS. 4A to 4D  are graphs illustrating changes in offset values according to a temperature change and processes in a current-to-voltage converting circuit according to the related art and a current-to-voltage converting circuit according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0031]    Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
         [0032]      FIG. 2  is a schematic view illustrating a reference voltage generating circuit according to an exemplary embodiment of the present invention. 
         [0033]    Referring to  FIG. 2 , a reference voltage generating circuit  200  according to this embodiment may include a sampling conversion circuit  210  and a comparator  220 . 
         [0034]    The sampling conversion circuit  210  may include an operational amplifier  211  and a feedback resistor  212 . The sampling conversion circuit  210  may have the same circuit characteristics as a current-to-voltage converting circuit to which the reference voltage generating circuit  200  according to this embodiment is connected. 
         [0035]    The comparator  220  may include an operational amplifier  221  and filters  222  and  223 . 
         [0036]    The operational amplifier  211  of the sampling conversion circuit  210  may receive a corrected reference voltage V REF1  through a non-inverted terminal thereof and provide an output voltage V 01  of the operational amplifier  211  as inversion input to the operational amplifier  221  of the comparator  220 . The output voltage V 01  of the operational amplifier  211  of the sampling conversion circuit maybe fed back through the feedback resistor  212  into an inverted input terminal of the operational amplifier  211 . 
         [0037]    In the operational amplifier  221  of the comparator  220 , a predetermined voltage V REF  is input to a non-inverted input terminal, and the output voltage V 01  of the sampling conversion circuit may be input to a non-inverted input terminal. 
         [0038]    Filters formed of a resistor  222  and a capacitor  223  may be connected to an output terminal of the operational amplifier  221  of the comparator  220 . The filters can filter a signal that is output from the operational amplifier  221  of the comparator  220 . 
         [0039]    The output signal of the comparator  220  becomes the corrected reference voltage V REF1 , and the corrected reference voltage V REF1  may be provided to the current-to-voltage converting circuit. 
         [0040]    At this time, when an offset voltage V os  is generated in the operational amplifier  211  of the sampling conversion circuit  210 , the output voltage V 01  of the sampling conversion circuit  210  is represented by the following Equation 1. 
         [0000]        V   01   =V   OS   +V   REF1    [Equation 1] 
         [0041]    The output voltage V 01  of the sampling conversion circuit  210  may be fed back to an inverted input terminal of the operational amplifier  221  of the comparator  220 . The comparator  220  makes the output voltage V o1  input from the sampling conversion circuit  210  equal to the predetermined voltage V ref  to thereby output the reference voltage V ref1 . Therefore, when characteristics of the operational amplifier  221  of the comparator  220  are used, the fed-back output voltage V 01  of the sampling conversion circuit  210  can be represented by the following Equation 2. 
         [0000]      V 01 =V REF    [Equation 2] 
         [0042]    By using the above-described Equations 1 and 2, the following equation can be obtained. 
         [0000]    
       
      
       V 
       REF1 
       =V 
       REF 
       −V 
       os  
      
     
         [0043]    That is, the reference voltage V REF1  that is input to the current-to-voltage converting circuit is obtained by subtracting the offset voltage V os  of the sampling conversion circuit  210  from the predetermined voltage V REF . 
         [0044]      FIG. 3  is a schematic view illustrating an offset-compensated current-to-voltage converting circuit according to another exemplary embodiment of the present invention. 
         [0045]    Referring to  FIG. 3 , an offset-compensated current-to-voltage converting circuit  300  according to this embodiment may include a current-to-voltage converting circuit  330 , a sampling conversion circuit  310 , and a comparator  320 . 
         [0046]    The current-to-voltage converting circuit  330  may include an operational amplifier  331  that receives a reference voltage V REF1  through a non-inverted input terminal thereof and an input current and an output voltage V o  through an inverted input terminal thereof. 
         [0047]    The input current that is input to the current-to-voltage converting circuit  330  may be a current generated by a photo diode  341 . The photo diode  341  is a kind of photoelectric converter that can generate a current in response to light irradiation by using a p-n junction formed between p-type and n-type semiconductors or a metal having a rectifying effect and a semiconductor. 
         [0048]    The sampling conversion circuit  310  may include an operational amplifier  311  and a feedback resistor R f . The sampling conversion circuit  310  may have the same circuit characteristics as the current-to-voltage converting circuit  330 . 
         [0049]    The comparator  320  may include an operational amplifier  321  and filters  322  and  323 . 
         [0050]    The operational amplifier  311  of the sampling conversion circuit  310  may receive the reference voltage V REF1  through a non-inverted input terminal and provide an output voltage V o1  as an input to an inverted terminal of the operational amplifier  321  of the comparator  320 . The output voltage V o1  of the operational amplifier  311  of the sampling conversion circuit  310  may be fed back through the feedback resistor  312  into an inverted input terminal of the operational amplifier  311 . 
         [0051]    The operational amplifier  321  of the comparator  320  has the non-inverted input terminal to which the predetermined voltage V REF  is input and the inverted input terminal to which the output voltage V o1  of the sampling conversion circuit  310  is input. 
         [0052]    The filters formed of the resistor  322  and the capacitor  323  may be connected to an output terminal of the operational amplifier  321  of the comparator  320 . The filters can filter a signal that is output from the operational amplifier  321  of the comparator  320 . 
         [0053]    The output signal of the comparator  320  becomes the reference voltage V REF1 . The reference voltage V REF1  may be provided to the current-to-voltage converting circuit  330 . 
         [0054]    Here, when an offset voltage V os  is generated in the operational amplifier  311  of the sampling conversion circuit  310 , the output voltage V o1  of the sampling conversion circuit  310  can be represented by the following Equation 3. 
         [0000]        V   01   =V   os   +V   REF1    [Equation 3] 
         [0055]    The output voltage V o1  of the sampling conversion circuit  310  is fed back into the inverted input terminal of the operational amplifier  321  of the comparator  320 . Therefore, when characteristics of the operational amplifier  321  of the comparator  320  are used, the fed-back output voltage V o1  of the sampling conversion circuit  310  can be represented by the following Equation 4. 
         [0000]      V 01 =V REF    [Equation 4] 
         [0056]    By using the above-described Equations 3 and 4, the following equation can be obtained. 
         [0000]    
       
      
       V 
       REF1 
       =V 
       REF 
       −V 
       os  
      
     
         [0057]    That is, the reference voltage V REF1  that is input to the current-to-voltage converting circuit  330  and the sampling conversion circuit  310  is obtained by subtracting the offset voltage V os  of the sampling conversion circuit  310  from the predetermined voltage V REF . 
         [0058]    In this embodiment, an offset value that has the same size as an offset value that may be generated in the current-to-voltage converting circuit  330  is obtained in advance by the sampling conversion circuit  310 . The reference voltage V REF1  obtained by subtracting the offset value, obtained by the sampling conversion circuit  310 , from the predetermined voltage V REF  is provided to the current-to-voltage converting circuit  330 . In this way, it is possible to compensate for offset in the current-to-voltage converting circuit  330 . 
         [0059]      FIGS. 4A to 4D  are graphs each illustrating a comparison between an offset value of a current-to-voltage converting circuit according to the related art and an offset value of an offset-compensated current-to-voltage converting circuit according to an exemplary embodiment of the present invention. 
         [0060]    In  FIGS. 4A and 4B , a change in offset value according to a change in temperature is shown. In  FIGS. 4C and 4D , offset value according to a manufacturing process is shown. 
         [0061]    Referring to  FIGS. 4A and 4B , while the temperature increases from −30 to 120 degrees, the offset value of the current-to-voltage converting circuit according to the related art changes from approximately 181 mV to approximately 94.4 mV. That is, the change in the offset value is approximately 86 mV. On the other hand, the offset-compensated current-to-voltage converting circuit according to the embodiment of the invention changes from approximately −167 μN to approximately −133 μN. That is, the change in the offset value is approximately 34 μN. 
         [0062]    Referring to  FIGS. 4C and 4D , a transistor that forms the current-to-voltage converting circuit is manufactured according to index  1  to  3 . Here, in the related art, the offset value changes from approximately 58.4 mV to approximately 265 mV, and thus the change is approximately 206 mV. On the other hand, in the embodiment of the invention, the offset value changes from approximately −179 μN to approximately −119 μN, and thus the change is approximately 60 μN. 
         [0063]    Therefore, as compared when the current-to-voltage converting circuit according to the related art controls the offset by using a resistor, the offset value is significantly compensated by using the reference voltage generating circuit according to the embodiment of the invention. 
         [0064]    As set forth above, according to the exemplary embodiments of the invention, it is possible to reduce an offset that may occur due to a change in temperature during a manufacturing process in a current-to-voltage converting circuit. 
         [0065]    While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.