Patent Publication Number: US-8525722-B2

Title: Ad converting device, dial-type input device, and resistance-voltage conversion circuit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims priority from Japanese Patent Application Number 2011-058177, filed on Mar. 16, 2011, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an AD converting device having a double integral AD converter, a dial-type input device, and a resistance-voltage conversion circuit. 
     2. Description of the Related Art 
       FIG. 3  shows a configuration of a commonly-used double integral AD converter  10 . The double integral AD converter  10  has a switch  11 , an integrator  12 , a comparator  13  and a logic block  14 . The switch  11  selectively outputs one of an input first integrated voltage Vin and a second integrated voltage −Vref in accordance with a command from the logic block  14 . The integrator  12  has an operational amplifier  12   a , a resistance  12   b , and a capacitor  12   c . The operational amplifier  12   a  has a − terminal to which an output from the switch  11  via the resistance  12   b  is input and a + terminal which is grounded. The operational amplifier  12   a  integrates differences between a voltage input to the − terminal and a voltage input to the + terminal and outputs a voltage depending on a result of the integration according to a time constant determined by a resistance value R of the resistance  12   b  and a capacity C of the capacitor  12   c . The comparator  13  has a − terminal to which the output voltage of the integrator  12  is input and a + terminal which is grounded. When the output voltage of the integrator  12  is equal to the ground potential, the comparator  13  outputs a signal. The logic block  14  receives the output of the comparator  13  and a clock. The logic block  14  counts the input clocks and controls the switch  11  based on the counted value (hereinafter, referred to as integral time. The logic block  14  outputs the counted value (integral time) as a digital value when receiving the signal from the comparator  13 . 
     Referring to  FIG. 4 , an integrating operation of the double integral AD converter  10  will be explained.  FIG. 4  shows outputs of the integrator  12  of the double integral AD converter  10  over time. In the double integral AD converter  10 , when an AD converting operation is started, the logic block  14  switches the switch  11  such that the first integrated voltage Vin is input to the switch  11  and at the same time, starts to count the integral time. When the switch  11  is switched to a side of the first integral voltage Vin, the integrator  12  performs a first integration until the counted value (integral time) becomes a first counted value T 0  (hereinafter, referred to as first integral time T 0 ). During the first integration, the output of the integrator  12  constantly decreases with a slope of −Vin/RC as shown in  FIG. 4 . When the counted value (integral time) becomes the first counted value T 0  (first integral time T 0 ), the logic block  14  switches the switch  11  so as to receive the second integrated voltage −Vref, and, at the same time, starts to newly count an integral time. At the same time with the switching operation, the integrator  12  also starts to perform a second integration. The second integrated voltage −Vref is inversely polarized to the first integrated voltage Vin. Therefore, during the second integration, the output of the integrator  12  constantly increases with a slope of Vref/RC. When the output of the integrator  12  returns to 0, the comparator  13  outputs to the logic block  14  a signal indicating that the output of the integrator  12  returns to 0. When receiving the signal, the logic block  14  outputs the counted value T obtained when the logic block  14  receives the signal (hereinafter, referred to as second integral time T) as an output of the AD converter  10  as a digital value. Since the second integrated voltage −Vref is constant, the second integral time T is a value proportional to the first integrated voltage Vin. 
     The above double integral operation is shown by the following formula (1). The following formula (2) is obtained by solving the formula (1) and arranging the obtained formula as a formula about T. Since the second integrated voltage −Vref and the counted value T 0  are constant values, the second integral time T is proportional to the first integrated voltage Vin. The second integral time T does not depend on the resistance R and the capacity C as shown in the formula (2). 
     
       
         
           
             
               
                 
                   
                     
                       
                         - 
                         
                           1 
                           RC 
                         
                       
                       ⁢ 
                       
                         
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                             T 
                             0 
                           
                         
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                             V 
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                           ⁢ 
                           
                             ⅆ 
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                           T 
                         
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                             V 
                             ref 
                           
                           ⁢ 
                           
                               
                           
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                             ⅆ 
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                   = 
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                   ( 
                   1 
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                   T 
                   = 
                   
                     
                       
                         V 
                         in 
                       
                       
                         V 
                         ref 
                       
                     
                     ⁢ 
                     
                       T 
                       0 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In general, the double integral AD converter performs two integral operations, that is, the first integration in which the integration of the first integrated voltage Vin is performed and the second integration in which the integration of the second integrated voltage −Vref is performed. Therefore, a conversion rate of the double integral AD converter is lower than the other type AD converter but high conversion accuracy can be achieved. Accordingly, the double integral AD converter is used in a digital multi-meter, a digital temperature sensor, or the like. 
     Japanese Patent Application Publication No. H05-083135 discloses that a second integrated voltage is divided to form a reference voltage and the reference voltage is input to an integrator of a double integral AD converter and a comparator and the reference voltage. 
     In a dial-type tuning radio, and the like, a dial is operated to switch a received frequency. The dial is, for example, formed by a potentiometer. The potentiometer is, for example, formed by a variable resistance in which a resistance value varies according to a rotational angle. In order to convert a rotational amount of the dial of the potentiometer into a digital output by using the double integral AD converter, it is necessary to convert a resistance value obtained after being varied by the variable resistance of the potentiometer into a voltage and input the voltage to the double integral AD converter. 
     As shown in  FIG. 5 , a method for obtaining a first integrated voltage Vin by simply applying an electrical current from a current source  22  to the variable resistance  23  of the potentiometer  21  was considered. However, in this method, if the resistance value Rv of the variable resistance  23  or the current value I of the electrical current applied to the variable resistance varies according to a manufacturing variation of the potentiometer  21  and the current source  22  or a usage environment such as a temperature, the first integrated voltage Vin varies. Therefore, as shown by a broken line in  FIG. 4 , a slope of the first integration −Vin/RC changes and the output of the integrator  12  in the first integration after the first integral time T 0  changes. On the other hand, the second integrated voltage −Vref is substantially constant and a value of a slope of the second integration Vref/RC does not change. Therefore, time until the output from the integrator  12  in the second integration becomes zero, that is, the second integral time changes, for example, from T to T′ as shown in  FIG. 4 . That is, there is a problem in that, even when the rotational amount of the dial of the potentiometer  21  is the same, the second integral time varies according to the manufacturing variation or the usage environment. 
     This is shown by the following formula (3). That is, when Rv indicates the resistance value of the variable resistance  23  of the potentiometer  21 , x indicates the rotational amount (rate of the rotation) of the potentiometer  21 , and I indicates the electrical current applied from the current source  22 , the first integrated voltage Vin is shown by the formula (3). The formula (4) is obtained by substituting the formula (3) into the formula (1). That is, as shown in the formula (3), even if the value of x is constant, the value of the second integral time T (digital output) varies when the electrical current I applied from the current source  22  and/or the resistance value Rv of the variable resistance  23  of the potentiometer  21  varies. 
     
       
         
           
             
               
                 
                   
                     V 
                     in 
                   
                   = 
                   
                     x 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       R 
                       
                         v 
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                     ⁢ 
                     I 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       
                         0 
                         ≤ 
                         x 
                         ≤ 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   T 
                   = 
                   
                     
                       
                         x 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           R 
                           v 
                         
                         ⁢ 
                         I 
                       
                       
                         V 
                         ref 
                       
                     
                     ⁢ 
                     
                       T 
                       0 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an AD converting device, a dial-type input device, and a resistance-voltage conversion circuit, in which, even when a resistance value of the variable resistance of the potentiometer or an electrical current applied from the current source varies according to a manufacturing variation, a usage environment, or the like, digital output without influence of the manufacturing variation, the usage environment, or the like can be obtained. 
     To achieve the above object, an AD converting device according to an embodiment of the present invention includes a double integral AD converter having an integrator which, when a first integrated voltage is input to the integrator, performs a first integration in which a difference between the first integrated voltage and a reference voltage is integrated, after the first integration is performed and when a second integrated voltage is input to the integrator, performs a second integration in which a difference between the second integrated voltage and the reference voltage is integrated, and outputs an integrated voltage according to at least the second integration; and an output part which receives the integrated voltage output from the integrator and the reference voltage, counts time from when the second integration is started to when the integrated voltage becomes equal to the reference voltage, and outputs the counted time as a digital value; a variable resistance; and a resistance-voltage conversion circuit which changes the first integrated voltage in proportion to a product of a resistance value of the variable resistance after being varied in the variable resistance and a value of an electrical current applied to the variable resistance and changes the second integrated voltage and the reference voltage in proportion to a product of a total resistance value of the variable resistance and the electrical current applied to the variable resistance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing a configuration of a dial-type input device according to an embodiment of the present invention. 
         FIG. 2  is a view showing a time change of output of an integrator of a double integral AD converter according to the embodiment of the present invention. 
         FIG. 3  is a view showing a configuration of a common double integral AD converter. 
         FIG. 4  is a view showing a time change of output of an integrator of a conventional double integral AD converter. 
         FIG. 5  is a view showing a configuration in case where a first integrated voltage which is an input voltage of an integrator of a common double integral AD converter is obtained by applying an electrical current to a variable resistance. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be explained with reference to the attached drawings. 
     1. Configuration 
       FIG. 1  is a configuration diagram of a dial-type input device according to an embodiment of the present invention. The dial-type input device has an AD converting device having a potentiometer  41 , a double integral AD converter  10 , and a resistance-voltage conversion circuit  30 . The configuration of the double integral AD converter  10  is the same as one explained with reference to  FIG. 3  and therefore detailed explanation will be omitted. The + terminals of the integrator  12  and the comparator  13  of the double integral AD converter  10  are not grounded but receive the later-described reference voltage Vcom. 
     The potentiometer  41  is configured by using a three-terminal type variable resistance  42 . The variable resistance  42  includes fixed terminals at both ends and an adjustment terminal between the fixed terminals. A rotary type potentiometer for detecting a rotational amount is used as the potentiometer  41 . A dial for operation is provided on a rotational shaft of the potentiometer  41  and a position of the adjustment terminal of the variable resistance  42  is changed by rotating the dial to change the resistance value between the adjustment terminal and the fixed terminals at both ends. 
     The resistance-voltage conversion circuit  30  is a circuit for converting the resistance value after being varied in the variable resistance  42  of the potentiometer  41  into a voltage. The resistance-voltage conversion circuit  30  has two current sources  31 ,  32 , an operational amplifier  33 , a p-type transistor  34 , and resistances  35 ,  36 ,  37 ,  38 ,  39 . The values of the resistances  35 ,  36 ,  37 ,  38 ,  39  are set to Rc, Rc, Rs, Rr, Rr, respectively. In this embodiment, Rs and Rr has the relationship of Rs=2Rr. The AD converter  10  and the resistance-voltage conversion circuit  30  are integrated in one integrated circuit. However, it is not limited thereto and the AD converter  10  and the resistance-voltage conversion circuit  30  may not be in a form of the integrated circuit. 
     Output of the current source  31  is connected to one fixed terminal of the variable resistance  42  and output of the current source  32  is connected to the other fixed terminal of the variable resistance  42 . The two current sources  31 ,  32  are formed by a current mirror circuit and generate the same electrical current I. 
     The resistances  35 ,  36  are connected in series between the fixed terminals of the variable resistance  42  of the potentiometer  41 . 
     The p-type transistor  34  has a source connected to a power source and a gate to which output from the operational amplifier  33  is input. 
     The resistances  37 ,  38 ,  39  are connected in series between drain and ground of the p-type transistor  34  in order from a side of the p-type transistor  34 . 
     The operational amplifier  33  has a − terminal connected to a connection part of the two resistances  35 ,  36  and a + terminal connected to a connection part of the resistances  37 ,  38 . 
     2. Operation 
     Hereinafter, an operation of the dial-type input device according to the embodiment of the present invention will be explained. 
     the AD converting device according to this embodiment includes the double integral AD converter ( 10 ), the variable resistance, and the resistance-voltage conversion circuit ( 30 ). The double integral AD converter has the integrator ( 12 ) which, when a first integrated voltage (Vin) is input to the integrator, performs a first integration in which a difference between the first integrated voltage (Vin) and a reference voltage (Vcom) is integrated, after the first integration is performed and when a second integrated voltage (Vref) is input to the integrator, performs a second integration in which a difference between the second integrated voltage (Vref) and the reference voltage (Vcom) is integrated, and outputs an integrated voltage according to at least the second integration. The double integral AD converter has an output part ( 13 ) which receives the integrated voltage output from the integrator and the reference voltage, counts time from when the second integration is started to when the integrated voltage becomes equal to the reference voltage, and outputs the counted time as a digital value. The resistance-voltage conversion circuit ( 30 ) changes the first integrated voltage in proportion to a product of a resistance value of the variable resistance after being varied in the variable resistance and a value of an electrical current (I) applied to the variable resistance and changes the second integrated voltage and the reference voltage in proportion to a product of a total resistance value of the variable resistance and the electrical current applied to the variable resistance. 
     The resistance-voltage conversion circuit  30  outputs a voltage of a connection part of the − terminal of the variable resistance  42  and an output of the current source  32  as the first integrated voltage Vin of the double integral AD converter  10 , outputs a voltage of a connection part of the drain of the p-type transistor  34  and the resistance  37  as the second integrated voltage Vref of the double integral AD converter  10 , and outputs a reference voltage Vcom of a connection part of the resistances  37 ,  38  to the + terminal of the operational amplifier  12   a  of the integrator  12  and the + terminal of the comparator  13  of the double integral AD converter  10 . The changes of the above voltages will be described later. 
     When the double integral AD converter  10  receives the first integrated voltage Vin, the second integrated voltage Vref, and the reference voltage Vcom from the resistance-voltage conversion circuit  30 , the double integral AD converter  10  operates in the following way. That is, the integrator  12  of the double integrated AD converter  10  performs the first integration in which difference between the first integrated voltage Vin and the reference voltage Vcom is integrated, when the first integrated voltage Vin is input from the resistance-voltage conversion circuit  30 . After the first integration is performed, the integrator  12  performs the second integration in which difference between the second integrated voltage Vref and the reference voltage Vcom is integrated, when the second integrated voltage Vref is input, and then outputs an integrated voltage according to a value of the integral. The comparator  13  receives the integrated voltage output from the integrator  12  and the reference voltage Vcom and compares the integrated voltage with the reference voltage Vcom. If the integrated voltage is equal to the reference voltage Vcom, the comparator  13  outputs a signal indicating that the integrated voltage is equal to the reference voltage Vcom to the logic block  14 . When the logic block  14  receives the signal, the logic block  14  outputs the counted value Y (the second integral time T) at that time as the output of the AD converter  10  as a digital value. 
     A voltage, and the like of each part in the dial-type input device will be explained by using the following formulas. In case that the rotational amount of the dial of the potentiometer  41  is x, the first integrated voltage Vin is shown by the above formula (3). The voltage V− input to the −terminal of the operational amplifier  33  is shown by the following formula (5). 
     
       
         
           
             
               
                 
                   
                     V 
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                   = 
                   
                     
                       
                         
                           R 
                           v 
                         
                         ⁢ 
                         I 
                       
                       + 
                       
                         2 
                         ⁢ 
                         
                           ( 
                           
                             1 
                             - 
                             x 
                           
                           ) 
                         
                         ⁢ 
                         x 
                         ⁢ 
                         
                           
                             R 
                             v 
                             2 
                           
                           
                             R 
                             c 
                           
                         
                         ⁢ 
                         I 
                       
                     
                     
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                       + 
                       
                         
                           R 
                           v 
                         
                         
                           R 
                           c 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In this embodiment, the resistance value Rc of the resistances  35 ,  36  is set to be substantially larger than the resistance value Rv of the variable resistance  42  of the potentiometer  41 . Therefore, the voltage of the connection part of the resistances  35 ,  36 , that is, the voltage V− to be input to the −terminal of the operational amplifier  33  can be approximately shown by the following formula (6). Furthermore, the resistance-voltage conversion circuit  30  according to this embodiment of the present invention has a configuration in that virtual short between the −terminal and the +terminal of the operational amplifier  33  occurs. Therefore, the voltage V+ input to the +terminal of the operational amplifier  33  is equal to the voltage V− of the −terminal as shown in the following formula (7). 
     
       
         
           
             
               
                 
                   
                     V 
                     - 
                   
                   = 
                   
                     
                       
                         R 
                         v 
                       
                       ⁢ 
                       I 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
             
               
                 
                   
                     V 
                     + 
                   
                   = 
                   
                     
                       V 
                       - 
                     
                     = 
                     
                       
                         
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                           v 
                         
                         ⁢ 
                         I 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   7 
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     Input impedances of the +terminal and the −terminal of the operational amplifier  12   a  of the integrator  12  and the +terminal of the comparator  13  become high and therefore, the reference voltage Vcom and the second integrated voltage Vref are shown by the following formula (8), (9), respectively.
 
 V   com =2 V   +   R   v   I   (8)
 
 V   ref =2 V   com =2 R   v   I   (9)
 
     The +terminal of the integrator  12  and the +terminal of the comparator  13  in the conventional circuit as shown in  FIGS. 3 and 5  are grounded. On the other hand, the +terminal of the comparator  13  in the circuit of this embodiment shown in  FIG. 1  receives the reference voltage Vcom. Therefore, the first integrated voltage is (Vcom−Vin), and the second integrated voltage is (Vref−Vcom). In this embodiment, the second integral time T is shown by the following formula (10) by substituting Vin in the formula (2) with Vcom−Vin, and Vref with Vref−Vcom. 
     
       
         
           
             
               
                 
                   T 
                   = 
                   
                     
                       
                         
                           V 
                           com 
                         
                         - 
                         
                           V 
                           in 
                         
                       
                       
                         
                           V 
                           ref 
                         
                         - 
                         
                           V 
                           com 
                         
                       
                     
                     ⁢ 
                     
                       T 
                       0 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     By substituting the formulas (3), (8), (9) into the formula (10) and arrange the formula, the following formula (11) is obtained. As shown in the formula (11), the second integral time T does not depend on the resistance value Rv of the variable resistance  42  of the potentiometer  41  and the current value I of the current sources  31 ,  32  and only depends on the rate of the rotation x of the potentiometer  41 . That is, even when the resistance value Rv of the variable resistance  42  and the value I of the electrical current applied to the variable resistance  42  vary according to a manufacturing variation and a usage environment, influence on the second integral time T (digital output) is canceled. Therefore, the digital output without influence of the manufacturing variation, the usage environment, and the like can be obtained.
 
 T= (1− x ) T   0   (11)
 
     The first integrated voltage Vin varies in proportion to a product of the resistance value after being varied in the variable resistance  42  and the value I of the electrical current applied to the variable resistance  42  and therefore is shown by Vin=α×x×Rv×I, where α is a constant. On the other hand, the second integrated voltage Vref and the reference voltage Vcom vary in proportion to a product of a total resistance value (resistance value between both ends) of the variable resistance  42  and the value I of the electrical current applied to the variable resistance  42  and therefore are respectively shown by Vref=β×Rv×I and Vcom=γ×Rv×I, where each of β and γ is a constant. By substituting these formulas into the above formula (10) and arranging the formula, the following formula (12) is obtained.
 
 T =(γ− x·α )/(β−γ)· T 0  (12)
 
     As shown in the above formula, the second integral time T does not depend on the resistance value Rv of the variable resistance  42  and the value I of the electrical current applied to the variable resistance  42  and only depends on the constants α, β, γ, the rotational amount x of the variable resistance  42  and the first integral time T 0 . Since the constants α, β, γ and the first integral time T 0  are constant, the second integral time T varies based on only the rotational amount x. That is, even when the resistance value Rv of the variable resistance  42  and the value I of the electrical current applied to the variable resistance  42  vary according to the manufacturing variation and the usage environment, influence on the digital output for the second integral time T is canceled. Therefore, the digital output without the influence of the manufacturing variation, the usage environment, and the like can be obtained. 
     In this embodiment, the values of α, β, γ are α=1, β=2, γ=1 in the formula (12). By substituting these values into the formula (12), the above formula (11) can be obtained. 
       FIG. 2  is a view showing a change of the output of the integrator  12 . The output of the integrator  12  constantly increases during the first integration (until the first integral time T 0  passes) with a slope (Vcom−Vin)/RC. After the first integral time T 0  passes, the second integration starts and the output of the integrator  12  constantly decreases with a slope −(Vref−Vcom)/RC. Even if the slope during the first integration changes as shown by the broken line from the solid line according to the manufacturing variation of the variable resistance  42  and the current sources  31 ,  32  or the usage environment, the slope during the second integration also changes from the solid line to the broken line and therefore the same second integral time T is obtained. Accordingly, the constant digital output can be obtained. That is, in case where the rotated angle of the dial of the potentiometer  41  is the same, the constant second integral time T (digital output) only based on the rotational amount x without depending on the manufacturing variation and the usage environment can be obtained. 
     3. Summary 
     As described above, in the dial-type input device (AD converter) according to the embodiment of the present invention, the first integrated voltage Vin is changed in proportion to the product of the resistance value after being varied in the variable resistance  42  and the value I of the electrical current applied to the variable resistance  42  and the second integrated voltage Vref and the reference voltage Vcom are changed in proportion to a product of the total resistance value Rv (resistance value between both ends) of the variable resistance  42  and the electrical current I applied to the variable resistance  42 . Thereby, the second integral time T changes only based on the rotational amount x. That is, even when the resistance value Rv of the variable resistance  42  or the value I of the electrical current applied to the variable resistance  42  changes according to the manufacturing variation or the usage environment, influence on the digital output for the second integral time T is canceled. Therefore, the digital output without influence of the manufacturing variation or the usage environment can be obtained. 
     The resistance-voltage conversion circuit  30  and the double integral AD converter  10  are integrated on one integrated circuit and therefore only the variable resistance is necessary as an external component so that the cost can be reduced. 
     The two current sources  31 ,  32  are formed by a current mirror. Accordingly, the two current sources which apply the same current can be configured with a simple circuit. 
     Since the dial input device has the above described resistance-voltage conversion circuit  30 , digital output with high accuracy for a scale of the dial can be obtained. 
     In this embodiment, although a rotary-type potentiometer in which the rotational amount is detected is used as the potentiometer, for example, a linear-type potentiometer in which a linear movement is detected may be used. 
     In this embodiment, the case where α=1, β=2, γ=1 is explained as an example but it is not limited thereto and therefore α, β, γ may be other values. 
     According to the AD converting device, the dial-type input device, and the resistance-voltage conversion circuit according to an embodiment of the present invention, digital output without influence of a manufacturing variation, usage environment, or the like can be obtained. The AD converting device according to an embodiment of the present invention can be widely applied to an AD converting device using a double integral AD converter. 
     According to an embodiment of the present invention, in case where an first integrated voltage which is a product of a resistance value after being varied in a variable resistance and a value of an electrical current applied to the variable resistance is changed when the resistance value of the variable resistance or the value of the electrical value applied to the variable resistance changes according to influence of the manufacturing variation, the usage environment, and the like, the second integrated voltage and the reference voltage input to the double integral AD converter together with the first integrated voltage changes in proportion to a product of a total resistance value of the variable resistance and the electrical current applied to the variable resistance. Thereby, in case where the resistance value of the variable resistance or the value of the electrical current applied to the variable resistance changes according to the manufacturing variation, the usage environment, or the like, the influence on the output of the digital value (that is, digital output) on the time from when the second integration is started to when the integrated voltage becomes equal to the reference voltage is canceled. Thereby, the digital output without influence of the manufacturing variation, the usage environment, or the like can be obtained. 
     Although the present invention has been described in terms of exemplary embodiments, it is not limited thereto. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims.