Abstract:
One aspect of the embodiments relates to a programmable gain circuit including an amplification unit amplifying an input signal, an input resistor coupled to an input terminal of the amplification unit, a feedback resistor coupled between an output terminal of the amplification unit and the input terminal of the amplification unit, a first switch switching a resistance value of the feedback resistor, a second switch switching a resistance value of the input resistor, and a control unit controlling the second switch such that the second switch switches the resistance value of the input resistor when the first switch switches the resistance value of the feedback resistor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority of Japanese Patent Application No. 2007-247673 filed on Sep. 25, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The present application relates to a programmable gain circuit and an amplification circuit. 
         [0004]    2. Description of the Related Art 
         [0005]      FIG. 1  illustrates a typical programmable gain circuit. A gain of the typical programmable gain circuit may be adjusted by switching operations of switching circuits  200 . 
         [0006]    As shown in  FIG. 1 , an input signal IN is inputted to one input terminal of an amplifier  100 . An output signal OUT is outputted from an output terminal of the amplifier  100 . A reference voltage VS is supplied to the output terminal of the amplifier  100  via a plurality of resistors Rs. Electric potentials of electrical connection points between respective resistors Rs become electric potentials obtained by dividing an electric potential difference between a voltage of the output signal OUT and the reference voltage Vs based on resistance values of respective resistors Rs. 
         [0007]    As further shown in  FIG. 1 , the other input terminal of the amplifier  100  is coupled to the electrical connection points between respective resistors Rs via a plurality of switching circuits  200 . The plurality of switching circuits  200  are controlled such that any one of the switching circuits  200  becomes conductive. When any one of the plurality of switching circuits  200  selectively becomes conductive, a voltage inputted to the other input terminal of the amplifier  100  may be adjusted. A gain of the amplifier  100  may be adjusted based on the adjustment of the voltage inputted to the other input terminal of the amplifier  100   
         [0008]    For example, if the switching operations of the plurality of switching circuits  200  are controlled with a three-bit control signal in the typical programmable gain circuit in  FIG. 1 , there may be three selectable gains. 
         [0009]    Consequently, if a fine adjustment of the gain is required, the requirement causes an increase in the number of resistors and switching circuits. Moreover, the requirement causes an increase in the number of bits of the control signal that controls the plurality of switching circuits. 
         [0010]    There are an N number of gains that may be selected with an N-bit control signal, in the typical programmable gain circuit in  FIG. 1 . The typical programmable gain circuit in  FIG. 1  requires, for the fine adjustment of the gain, a control circuit that generates a multi-bit control signal, the switching circuits, and the resistors, wherein the number of the switching circuits and the number of the resistors are equal to the number of bits of the control signal. Consequently, there is a problem that an increase in circuit size is caused in the typical programmable gain circuit in  FIG. 1 . 
         [0011]      FIG. 2  illustrates another typical programmable gain circuit. 
         [0012]    As shown in  FIG. 2 , an input signal IN is inputted to one input terminal of an amplifier  300  via an input resistor R 100 . A reference voltage Vs is inputted to the other input terminal of the amplifier  300 . An output signal OUT is outputted from an output terminal of the amplifier  300 . The output terminal of the amplifier  300  is coupled to the one input terminal via a plurality of feedback resistors R 200  to R 500 . For example, resistance values of the resistors R 300  to R 500  are set in the ratio of 1 to 2 to 4 (1:2:4). 
         [0013]    As further shown in  FIG. 2 , switching circuits SW 100  to SW 300  are coupled in parallel to each of the resistor R 300  to R 500 . The switching circuits SW 100  to SW 300  are switching-controlled based on a three-bit control signal. Since resistance values of the feedback resistors R 200  to R 500  may be adjusted in eight levels by the switching-control of the switching circuits SW 100  to SW 300 , a gain of the amplifier  300  may be adjusted in eight levels with the three-bit control signal. 
         [0014]    The gain is adjusted in 2N levels with an N-bit control signal in another typical programmable gain circuit in  FIG. 2 . In another typical programmable gain circuit in  FIG. 2 , a precise adjustment of the gain is very difficult due to ON-resistances of the respective switching circuits that affect the gain. 
         [0015]      FIG. 3  illustrates a typical inverting amplification circuit. A gain G of the inverting amplification circuit is represented by Equation (1) if an input resistor is R 600  and a feedback resistor is R 700 . 
         [0000]        G =OUT/IN= R 700 /R 600   (1) 
         [0016]    That is to say, the gain G is represented as a ratio of a resistance value of the input resistor R 600  to a resistance value of the feedback resistor R 700 . 
         [0017]    As shown in  FIG. 4 , a feedback resistor R 800 , to which a switching circuit SW 400  is coupled in parallel, is coupled in series to a feedback resistor R 700  for an adjustment of a gain G. If the switching circuit SW 400  becomes conductive and an ON-resistance in the aforementioned conductive state is represented as RSW 400 , the gain G is represented by Equation (2). 
         [0000]        G =OUT/IN=( R 700+ RSW 400)/ R 6   (2) 
         [0018]    That is to say, the ON-resistance RSW 400  of the switching circuit SW 400  affects the gain G. 
         [0019]    An amplification circuit discussed in Japanese Laid-open Patent Publication No. 1985-236509 is the amplification circuit which selects one of N level(s) in gain with an N-bit control signal. An amplifier discussed in Japanese Laid-open Patent Publication No. 1981-28524 corresponds to multiple input signals by switching feedback resistors and input resistors of the amplifier with analog switches. However, the number of the selectable feedback resistors and input resistors is equal to the number of the analog switches. 
         [0020]    Consequently, it is very difficult to perform the fine adjustment of the gain even if the gain is selected by selection of the feedback resistor and the input resistor. 
       SUMMARY 
       [0021]    One aspect of the embodiments relates to a programmable gain circuit including an amplification unit amplifying an input signal, an input resistor coupled to an input terminal of the amplification unit, a feedback resistor coupled between an output terminal of the amplification unit and the input terminal of the amplification unit, a first switch switching a resistance value of the feedback resistor, a second switch switching a resistance value of the input resistor, and a control unit controlling the second switch such that the second switch switches the resistance value of the input resistor when the first switch switches the resistance value of the feedback resistor. 
         [0022]    These together with other aspects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  illustrates a circuit diagram of a conventional embodiment; 
           [0024]      FIG. 2  illustrates a typical programmable gain circuit; 
           [0025]      FIG. 3  illustrates a typical inverting amplification circuit; 
           [0026]      FIG. 4  illustrates another typical programmable gain circuit; 
           [0027]      FIG. 5  illustrates a first embodiment; 
           [0028]      FIG. 6  illustrates a control circuit in  FIG. 5 ; 
           [0029]      FIG. 7  illustrates an operation of the control circuit in  FIG. 6 ; 
           [0030]      FIG. 8  illustrates one aspect of the first embodiment of  FIG.5 ; 
           [0031]      FIG. 9  illustrates a second embodiment; 
           [0032]      FIG. 10  illustrates a third embodiment; 
           [0033]      FIG. 11  illustrates a fourth embodiment; and 
           [0034]      FIG. 12  illustrates a fifth embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0035]      FIG. 5  illustrates a first embodiment. 
         [0036]    As shown in  FIG. 5 , an input signal IN is input to one input terminal of an amplifier  11  via an input resistor R 11  and a group of switching circuits  12 . A reference voltage Vs is input to the other input terminal of the amplifier  11 . 
         [0037]    As further shown in  FIG. 5 , an output signal OUT is output from an output terminal of the amplifier  11 . The output terminal of the amplifier  11  is coupled to the one input terminal via a feedback resistor unit Rf. The feedback resistor unit Rf includes a plurality of feedback resistors R 12  to R 15  coupled in series and switching circuits (first switching circuits) SW 11  to SW 13  which may be used for adjusting a gain and be respectively coupled in parallel to the resistors R 12  to R 14 . 
         [0038]    For example, resistance values of the resistors R 12  to R 14  that are set are in the ratio of 4 to 2 to 1 (4:2:1). The switching circuits SW 11  to SW 13  are respectively and independently switching-controlled based on three-bit control signals B 1  to B 3  output from a control circuit  13 . The switching circuits SW 11  to SW 13  become conductive when the control signals B 1  to B 3  that are inputted are in an H level, and, on the other hand, the switching circuits SW 11  to SW 13  become non-conductive when the control signals B 1  to B 3  that are inputted are in an L level. 
         [0039]    When the switching circuits SW 11  to SW 13  are switching-controlled based on the three-bit control signals, resistance values of the feedback resistors R 12  to R 15  may be adjusted in eight levels. That is to say, when the switching circuits SW 11  to SW 13  are switching-controlled based on the three-bit control signals, a gain of the amplifier  11  may be adjusted. 
         [0040]    As further shown in  FIG. 5 , a switching circuit SW 14 , switching circuits SW 15  and SW 16  coupled in series, and switching circuits SW 17  to SW 19  coupled in series are coupled in parallel in the group of switching circuits  12 . 
         [0041]    As further shown in  FIG. 5 , the switching circuit SW 14  is switching-controlled based on a control signal C 1  output from the control circuit  13 . The switching circuits SW 15  and SW 16  are switching-controlled based on a control signal C 2  output from the control circuit  13 . The switching circuits SW 17  to SW 19  are switching-controlled based on a control signal C 3  output from the control circuit  13 . Respective switching circuits SW 14  to SW 19  become conductive when the control signals C 1  to C 3  are in an H level, and on the other hand, the switching circuits SW 14  to SW 19  become non-conductive when the control signals C 1  to C 3  are in an L level. 
         [0042]    The control circuit  13  in  FIG. 5  generates the control signals BE to B 3  and C 1  to C 3  based on three-bit input signals A 1  to A 3  that are inputted. 
         [0043]      FIG. 6  illustrates the control circuit in  FIG. 5 . The control circuit  13  in  FIG. 6  includes a feedback resistor selection unit  14 , which generates the control signals B 1  to B 3 , and an input resistor selection unit  15 , which generates the control signals C 1  to C 3 . The feedback resistor selection unit  14  outputs the control signals B 1  to B 3  by inverting each of the input signals A 1  to A 3  by inverter circuits  16   a  to  16   c.    
         [0044]    As shown in  FIG. 6 , the input signals A 1  to A 3  are inputted to a NAND circuit  17   a , and the NAND circuit  17   a  outputs the control signal C 3 . 
         [0045]    The input signals A 1  and A 2 , and a signal obtained by inverting the input signal A 3  by an inverter circuit  18   a  are inputted to a NAND circuit  17   b . The input signals A 1  and A 3 , and a signal obtained by inverting the input signal A 2  by an inverter circuit  18   b  are inputted to a NAND circuit  17   c . The input signals A 2  and A 3 , and a signal obtained by inverting the input signal A 1  by an inverter circuit  18   c  are inputted to a NAND circuit  17   d.    
         [0046]    As further shown in  FIG. 6 , output signals from the NAND circuits  17   b  to  17   d  are inputted to an OR circuit  19   a , and the OR circuit  19   a  outputs the control signal C 2 . The input signal A 1 , a signal obtained by inverting the input signal A 2  by an inverter circuit  18   d , and a signal obtained by inverting the input signal A 3  by an inverter circuit  18   e , are inputted to a NAND circuit  17   e.    
         [0047]    The input signal A 2 , a signal obtained by inverting the input signal A 1  by an inverter circuit  18   f , and a signal obtained by inverting the input signal A 3  by an inverter circuit  18   g , are inputted to a NAND circuit  17   f.    
         [0048]    The input signal A 3 , a signal obtained by inverting the input signal A 1  by an inverter circuit  18   h , and a signal obtained by inverting the input signal A 2  by an inverter circuit  18   i , are inputted to a NAND circuit  17   g.    
         [0049]    As further shown in  FIG. 6 , output signals from the NAND circuits  17   e  to  17   g  are inputted to an OR circuit  19   b , and the OR circuit  19   b  outputs the control signal C 1 .  FIG. 7  illustrates operations of the control circuit  13  in  FIG. 6 . Since the control signals B 1  to B 3  become the H level when the input signals A 1  to A 3  are at an L level, all the switching circuits SW 11  to SW 13  become conductive. Since the control signal C 3  becomes the H level when the input signals A 1  to A 3  are at the L level, the switching circuits SW 17  to SW 19  become conductive. Since the control signals C 1  and C 2  become the L level when the input signals A 1  to A 3  are at the L level, the switching circuits SW 14  to SW 16  become non-conductive. 
         [0050]    As shown in  FIG. 7 , when control signal A 1  is an H level, control signal A 2  is an L level, and control signal A 3  is an L level, control signal B 1  becomes an L level, control signal B 2  becomes an H level, and control signal B 3  becomes an H level. Thus, switching circuit SW 13  becomes non-conductive and switching circuits SW 12  and SW 11  become conductive. When control signal A 1  is an H level, control signal A 2  is an L level, and control signal A 3  is an L level, control signal C 2  becomes an H level, control signal C 1  becomes an L level, and control signal C 3  becomes an L level. Thus, switching circuits SW 15  and SW 16  become conductive and switching circuits SW 14  and SW 17  to SW 19  become non-conductive. 
         [0051]    As further shown in  FIG. 7 , when control signal A 1  is an H level, control signal A 2  is an H level, and control signal A 3  is an L level, control signal B 1  becomes an L level, control signal B 2  becomes an L level, and control signal B 3  becomes an H level. Thus, switching circuit SW 11  becomes conductive and switching circuits SW 12  and SW 13  become non-conductive. When control signal A 1  is an H level, control signal A 2  is an H level, and control signal A 3  is an L level, control signal C 1  becomes an H level, control signal C 2  becomes an L level, and control signal C 3  becomes an L level. Thus, switching circuit SW 14  becomes conductive and switching circuits SW 15  to SW 19  become non-conductive. 
         [0052]    As further shown in  FIG. 7 , since the control signal C 1  is an H level when one of the switching circuits SW 11  to SW 13  becomes conductive, the one switching circuit SW 14  becomes conductive. Since the control signal C 2  is an H level when two of the switching circuits SW 11  to SW 13  become conductive, the two switching circuits SW 15  and SW 16  become conductive. 
         [0053]    Since the control signal C 3  is an H level when all the switching circuits SW 11  to SW 13  become conductive, the three switching circuits SW 17  to SW 19  become conductive. 
         [0054]    Consequently, in the group of switching circuits  12  in  FIG. 5 , the number of switching circuits that become conductive among the switching circuits SW 11  to SW 13  becomes equal to the number of switching circuits coupled in series to one input terminal of the amplifier  11 . As further shown in  FIG. 7 , when all the control signals B 1  to B 3  are an L level, all the control signals C 1  to C 3  are an L level. Since all the switching circuits SW 14  to SW 19  in the group of switching circuits  12  become non-conductive when all the control signals B 1  to B 3  are an L level, a state is entered in which the input signal IN is not inputted to the amplifier  11 . Consequently, the number of levels in adjustable gain range is seven levels in the first embodiment in  FIG. 5 , except for the case where all the switching circuits SW 11  to SW 13  become non-conductive. 
         [0055]      FIG. 7  illustrates a substantial gain corresponding to respective control signals when a resistance value of the resistor R 14  is X, a resistance value of the resistor R 13  is 2X, and a resistance value of the resistor R 12  is 4X. Hereinafter, the gain of an inverting amplification circuit related to the first embodiment when a switching circuit is added to the group of switching circuits  12  of  FIG. 5  will be disclosed according to  FIG. 8 . As shown in  FIG. 8 , a switching circuit S 1  is coupled between an input resistor R 1  and the amplifier  11 , and a switching circuit S 2  is coupled to a resistor R 21  coupled in series to a feedback resistor R 2 . If ON-resistances of the respective switching circuits S 1  and S 2  are represented as Ron 1  and Ron 2  respectively, a gain of the inverting amplification circuit is represented by Equation (3). 
         [0000]        G =( R 2 +R on2)/( R 1 +R on1)   (3) 
         [0056]    Here, a ratio of the ON-resistance Ron 1  of the switching circuit S 1  to ON-resistance Ron 2  of the switching circuit S 2  is set as follows: 
         [0000]      Ron1/Ron2≈R2/R1   (4) 
         [0057]    Under Equation (4), the gain of the inverting amplification circuit is represented by Equation (5). 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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         [0058]    Consequently, the ON-resistance Ron 1  cancels an influence of the ON-resistance Ron 2  when setting the ratio of the ON-resistance Ron 1  and the ON-resistance Ron 2  as disclosed above. 
         [0059]    In the programmable gain circuit in  FIG. 5 , the ON-resistances of the switching circuits SW 11  to SW 13  are set such that the ON-resistances of the switching circuits SW 11  to SW 13  have the same resistance values. Furthermore, a ratio of the ON-resistances of the switching circuits SW 11  to SW 13  to ON-resistances of switching circuits (second switching circuits) SW 14  to SW 19  for adjusting ON-resistances of the group of switching circuits  12  is set such that the ratio is substantially equal to a ratio of the feedback resistor R 15  to the input resistor R 11 . 
         [0060]    In the programmable gain circuit in  FIG. 5 , since the number of switching circuits that become conductive among the switching circuits SW 11  to SW 13  becomes equal to the number of switching circuits coupled in series to the one input terminal of the amplifier  11 , the group of switching circuits  12  cancels an influence of the ON-resistances of the switching circuits SW 11  to SW 13 . 
         [0061]    The programmable gain circuit in  FIG. 5  is summarized as follows: 
         [0062]    (1) The switching circuits SW 11  to SW 13  are respectively coupled in parallel to the three resistors R 12  to R 14  among the four feedback resistors R 12  to R 15  that are coupled in series. Since respective switching circuits SW 11  to SW 13  are switchable, for example, seven levels of gains may be selected. 
         [0063]    (2) Since the resistance values of the feedback resistors R 12  to R 14  may be set in the ratio of 4 to 2 to 1 (4:2:1) for example, the gain changes substantially in a linear manner by selection of switching operations of respective switching circuits SW 11  to SW 13 . 
         [0064]    (3) The group of switching circuits  12  coupled in series to the input resistor R 11  cancels the influence of the ON-resistances of the switching circuits SW 11  to SW 13  that select the resistance values of the feedback resistors R 12  to R 15 . Consequently, the influence of the ON-resistances of the switching circuits SW 11  to SW 13  on the gain may be prevented, whereby the gain may be precisely adjusted. 
         [0065]    (4) The ON-resistances of the switching circuits SW 14  to SW 19  cancel the influence of the ON-resistances of the switching circuits SW 14  to SW 19  by setting ON-resistance values of the respective switching circuits SW 14  to SW 19  in the group of switching circuits  12  to a value obtained by multiplying ON-resistance values of the switching circuits SW 11  to SW 13 , which select feedback resistance values, by a reciprocal of the gain. 
         [0066]    (5) The influence of the ON-resistances of the switching circuits SW 11  to SW 13  is cancelled by selecting from the group of switching circuits  12  the same number of switching circuits as the number of switching circuits that become conductive among the switching circuits SW 11  to SW 13 . 
         [0067]      FIG. 9  illustrates a second embodiment. A switching circuit SW 20  is coupled in series to a feedback resistor R 16  and the switching circuit SW 20  is in a constant conductive state. 
         [0068]    As shown in  FIG. 9 , an input signal IN is input to three input resistors R 17  to R 19  coupled in parallel. The respective input resistors R 17  to R 19  are coupled to an input terminal of an amplifier  11  via each of switching circuits SW 21  to SW 23  included in a group of switching circuits. 
         [0069]    Different resistance values are set for the respective input resistors R 17  to R 19 . Any one of the switching circuits SW 21  to SW 23  is selected and the selected switching circuit becomes conductive. ON-resistance values of the respective switching circuits SW 21  to SW 23  are set to a value obtained by multiplying a reciprocal of a gain, which is set based on the input resistor coupled to the selected switching circuit and a feedback resistor R 16 , by a resistance value of the switching circuit SW 20 . 
         [0070]    A programmable gain circuit in  FIG. 9  selects any one of the switching circuits SW 21  to SW 23  and causes one of the selected switching circuits SW 21  to SW 23  to become conductive, whereby any one of the input resistors R 17  to R 19  is selected and coupled to the amplifier  11 . As a result, the gain is set based on the selected input resistor and the feedback resistor R 16 . 
         [0071]    An ON-resistance of the selected switching circuit among the switching circuits SW 21  to SW 23  cancels an influence of an ON-resistance of the switching circuit SW 20  coupled to the feedback resistor R 16 . 
         [0072]    Any one of the input resistors R 17  to R 19  is selected by selecting any one of the switching circuits SW 21  to SW 23  in the programmable gain circuit in  FIG. 9 . The gain may be switched in three levels by switching the input resistors and this allows an adjustable range of the gain to be increased based on settings of resistance values of the input resistors R 17  to R 19 . 
         [0073]    ON-resistances of the switching circuits SW 21  to SW 23  coupled to the input resistors R 17  to R 19  cancel the influence of the ON-resistance of the switching circuit SW 20  coupled to the feedback resistor R 16 . Consequently, the gain may be precisely adjusted.  FIG. 10  illustrates a third embodiment. In the third embodiment, a configuration selecting an input resistor is added to the first embodiment. The same reference symbols are given to the same elements as those shown in the first embodiment in  FIG. 5 . 
         [0074]    As shown in  FIG. 10 , a configuration of a feedback resistor unit Rf is the same as that of the first embodiment. An input signal IN is input to two input resistors R 20  and R 21 . Each of the input resistors R 20  and R 21  is coupled to an amplifier  11  via groups of switching circuits (third switching circuits)  20   a  and  20   b , respectively. Each of the groups of switching circuits  20   a  and  20   b  has the same configuration as the group of the switching circuits  12  in the first embodiment. Any one of the input resistors R 20  and R 21 , that is to say, any one of the groups of the switching circuits  20   a  and  20   b , is selected so as to be controlled in the same manner as the group of switching circuits  12  in the first embodiment. Respective switching circuits in the group of switching circuits that are not selected become non-conductive. 
         [0075]    When input resistor R 20  is selected, switch circuit group  20   a  is controlled so that the number of switch circuits in a conductive state becomes the same number of switch circuits in a conductive state in the feedback resistor unit Rf. 
         [0076]    When input resistor R 21  is selected, switch circuit group  20   b  is controlled so that the number of switch circuits in a conductive state becomes the same number of switch circuits in a conductive state in the feedback resistor unit Rf. In the third embodiment in  FIG. 10 , an adjustable range of a gain may further be increased by switching the input resistors R 20  and R 21 . 
         [0077]      FIG. 11  illustrates a fourth embodiment. The fourth embodiment is an example in which a programmable gain circuit of the aforementioned embodiments may be applied to a sensor detection circuit. 
         [0078]    As shown in  FIG. 11 , the sensor detection circuit includes a sensor element  21 , a differential amplification stage  22 , and an output stage  23 . 
         [0079]    An output signal from the sensor element  21  is amplified by the differential amplification stage  22  and the output stage  23 , and the output signal is outputted. For example, the output stage  23  may basically have the same configuration as the programmable gain circuit in the first embodiment. The programmable gain circuit in  FIG. 11  has a configuration in which switching circuits are coupled in parallel to two of the three resistors included in feedback resistors. Note that the aforementioned embodiments may be applied to the output stage  23 . 
         [0080]    The output stage  23  of the sensor detection circuit in  FIG. 11  has the same functions and advantages as those of the aforementioned embodiments. 
         [0081]      FIG. 12  illustrates a fifth embodiment. In the fifth embodiment, a programmable gain circuit is used for a differential amplification stage  22  in the sensor detection circuit that does not require an output stage. 
         [0082]    As shown in  FIG. 12 , a differential output signal from a sensor element  21  is inputted to a pair of differential inputs  24   a  and  24   b  of the differential amplification stage  22 , respectively. 
         [0083]    An output circuit  25  amplifies output signals from each of the pairs of differential inputs  24   a  and  24   b , and the output signals are outputted. 
         [0084]    As further shown in  FIG. 12 , the programmable gain circuit having the same configuration as the fourth embodiment is used for the pair of differential inputs  24   a  and  24   b . The aforementioned embodiments may be applied to the pair of differential inputs  24   a  and  24   b . The differential amplification stage  22  of the sensor detection circuit in  FIG. 12  has the same functions and advantages as those of the aforementioned embodiments. 
         [0085]    The aforementioned embodiments may be modified in view of the following aspects. For example, the number of feedback resistors to which the switching circuits may be coupled in parallel may be four or more. 
         [0086]    For example, the ratios of resistance values of the feedback resistors to which switching circuits are coupled in parallel may be arbitrary ratios other than 1 to 2 to 4 (1:2:4). 
         [0087]    At least one of the aforementioned embodiments provides a programmable gain circuit capable of increasing the number of selectable choices of the gain and capable of precisely adjusting the gain. 
         [0088]    According to at least one of the aforementioned embodiments, a programmable gain circuit capable of increasing the number of selectable choices of the gain and capable of precisely adjusting the gain may be provided. 
         [0089]    The many features and advantages of the embodiments are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the embodiments that fall within the true spirit and scope thereof. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the inventive embodiments to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope thereof. 
         [0090]    Although aforementioned embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents. 
         [0091]    Numbers applying to embodiments (first, second or third etc.) do not show priorities of the embodiments. Many variations and modifications will be apparent to those skilled in the art.