Abstract:
One embodiment provides an analog front-end circuit. When a chopping signal has a first logical value, a non-inverting instrumentation preamplifier subtracts a second input voltage from a first input voltage and generates a first output voltage by amplifying a subtraction voltage while outputting the second input voltage as a second output voltage. When the chopping signal has a second logical value, the non-inverting instrumentation preamplifier subtracts the first input voltage from the second input voltage and generates the first output voltage by amplifying and then inverting the polarity of a subtraction voltage while outputting the second input voltage as the second output voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims priority from Japanese Patent Application No. 2016-085016 filed on Apr. 21, 2016, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The present invention relates to an analog front-end circuit including an instrumentation preamplifier and an AD converter and, more particularly, to a technique for reducing an offset component, a temperature drift, low-frequency noise, etc. 
       BACKGROUND 
       [0003]    In analog front-end circuits, a very small analog signal obtained by detecting a temperature, for example, is amplified and AD-converted into data. In this operation, an offset component, a temperature drift, low-frequency noise, etc. (hereinafter referred to as “an offset component etc.”) occurring in an instrumentation preamplifier having a high amplification factor and an AD converter have great influences on the performance of the entire circuit. 
         [0004]    In view of the above, for example, an analog front-end circuit  20 A shown in  FIG. 3  is employed which is equipped with a configuration for reducing such offset component etc. (refer to Chopping on the AD7190, AD7192, AD7193, AD7194, and AD7195,” Mary McCarthy, AN-1131, APPLICATION NOTE, page 1 of 4, ANALOG DEVICES). The analog front-end circuit  20 A is equipped with switches SW 11  and SW 12  each of which switches between input voltages VINP and VINN that are input from a sensor  1 , a differential instrumentation preamplifier  21 A which performs differential amplification on the difference between the voltages VINP and VINN that are input via the switches SW 11  and SW 12 , an AD converter  22  for converting the difference between differential output signals VOUTP and VOUTN of the differential instrumentation preamplifier  21 A into a digital signal, a switch circuit  23  which passes the output data of the AD converter  22  as it is or after inverting its polarity, and a data calculator  24  for calculating average data of two data that are output from the switch circuit  23  successively. 
         [0005]    The differential instrumentation preamplifier  21 A is composed of operational amplifiers OP 11  and OP 12  and gain-setting resistors R 11 , R 12 , and R 13  (R 12 =R 13 ), and outputs the voltages VOUTP and VOUTN. The switch circuit  23  is composed of a switch SW 13  and a data inverter  23   a.    
         [0006]    A chopping signal CHOP is a signal that “0” and “1” occur repeatedly and alternately at a prescribed cycle. Each of the switches SW 11 , SW 12 , and SW 13  selects the input terminals “0” and “1” when the chopping signal CHOP is “0” and “1,” respectively. 
         [0007]    In  FIG. 3 , symbols Vos 11 , Vos 12 , and Vos 13  denote an offset component etc. occurring on the non-inverting input side of the operational amplifier OP 11 , an offset component etc. occurring on the non-inverting input side of the operational amplifier OP 12 , and an offset component etc. occurring on the data input side of the AD converter  22 , respectively. 
         [0008]    In the analog front-end circuit  20 A, an output voltage VO CHOP=0  of the switch circuit  23  when the chopping signal CHOP is “0” is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     VO 
                     
                       CHOP 
                       = 
                       0 
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           VINP 
                           - 
                           VINN 
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           1 
                           + 
                           
                             2 
                             × 
                             
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 12 
                               
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 11 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         ( 
                         
                           
                             Vos 
                              
                             
                                 
                             
                              
                             11 
                           
                           - 
                           
                             Vos 
                              
                             
                                 
                             
                              
                             12 
                           
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           1 
                           + 
                           
                             2 
                             × 
                             
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 12 
                               
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 11 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       Vos 
                        
                       
                           
                       
                        
                       13. 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0009]    On the other hand, an output voltage VO CHOP=1  of the switch circuit  23  when the chopping signal CHOP is “1” is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     VO 
                     
                       CHOP 
                       = 
                       1 
                     
                   
                   = 
                   
                     
                       
                         { 
                         
                           
                             
                               - 
                               
                                 ( 
                                 
                                   VINP 
                                   - 
                                   VINN 
                                 
                                 ) 
                               
                             
                             × 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   2 
                                   × 
                                   
                                     
                                       R 
                                        
                                       
                                           
                                       
                                        
                                       12 
                                     
                                     
                                       R 
                                        
                                       
                                           
                                       
                                        
                                       11 
                                     
                                   
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             
                               ( 
                               
                                 
                                   Vos 
                                    
                                   
                                       
                                   
                                    
                                   11 
                                 
                                 - 
                                 
                                   Vos 
                                    
                                   
                                       
                                   
                                    
                                   12 
                                 
                               
                               ) 
                             
                             × 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   2 
                                   × 
                                   
                                     
                                       R 
                                        
                                       
                                           
                                       
                                        
                                       12 
                                     
                                     
                                       R 
                                        
                                       
                                           
                                       
                                        
                                       11 
                                     
                                   
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             Vos 
                              
                             
                                 
                             
                              
                             13 
                           
                         
                         } 
                       
                       × 
                       
                         ( 
                         
                           - 
                           1 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         
                           ( 
                           
                             VINP 
                             - 
                             VINN 
                           
                           ) 
                         
                         × 
                         
                           ( 
                           
                             1 
                             + 
                             
                               2 
                               × 
                               
                                 
                                   R 
                                    
                                   
                                       
                                   
                                    
                                   12 
                                 
                                 
                                   R 
                                    
                                   
                                       
                                   
                                    
                                   11 
                                 
                               
                             
                           
                           ) 
                         
                       
                       - 
                       
                         
                           ( 
                           
                             
                               Vos 
                                
                               
                                   
                               
                                
                               11 
                             
                             - 
                             
                               Vos 
                                
                               
                                   
                               
                                
                               12 
                             
                           
                           ) 
                         
                         × 
                         
                           ( 
                           
                             1 
                             + 
                             
                               2 
                               × 
                               
                                 
                                   R 
                                    
                                   
                                       
                                   
                                    
                                   12 
                                 
                                 
                                   R 
                                    
                                   
                                       
                                   
                                    
                                   11 
                                 
                               
                             
                           
                           ) 
                         
                       
                       - 
                       
                         Vos 
                          
                         
                             
                         
                          
                         13. 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0010]    Thus, an output OUT of the data calculator  24 , that is, the average of two successive output voltages VO CHOP=0  and VO CHOP=1  of the switch circuit  23 , is given by 
         [0000]    
       
         
           
             
               
                 
                   OUT 
                   = 
                   
                     
                       
                         
                           VO 
                           
                             CHOP 
                             = 
                             0 
                           
                         
                         + 
                         
                           VO 
                           
                             CHOP 
                             = 
                             1 
                           
                         
                       
                       2 
                     
                     = 
                     
                       
                         ( 
                         
                           VINP 
                           - 
                           VINN 
                         
                         ) 
                       
                       × 
                       
                         
                           ( 
                           
                             1 
                             + 
                             
                               2 
                               × 
                               
                                 
                                   R 
                                    
                                   
                                       
                                   
                                    
                                   12 
                                 
                                 
                                   R 
                                    
                                   
                                       
                                   
                                    
                                   11 
                                 
                               
                             
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    In Equation (3), the offset components etc. Vos 11 , Vos 12 , and Vos 13  are eliminated. 
         [0011]    However, in the analog front-end circuit  20 A described above with reference to  FIG. 3  which employs the differential instrumentation preamplifier  21 A, output voltages VOUTP and VOUTN of the differential instrumentation preamplifier  21 A vary necessarily in opposite directions (differentially). Thus, where the bias points for input signals VINP and VINN are set at low potentials that are close to GND, the output voltages VOUTP and VOUTN are saturated immediately. 
         [0012]      FIG. 5C  shows example input voltages VINP and VINN and output voltages VOUTP and VOUTN of the differential instrumentation preamplifier  21 A of the analog front-end circuit  20 A shown in  FIG. 3  in which the input voltages VINP and VINN are 2 V and 1 V, respectively. When the chopping signal CHOP is “0,” the input voltages VINP and VINN pass the switches SW 11  and SW 12  as they are, respectively, and hence the output voltages VOUTP and VOUTN and the amplification factor GAIN are given by 
         [0000]    
       
         
           
             
               
                 
                   VOUTP 
                   = 
                   
                     VINP 
                     + 
                     
                       
                         
                           R 
                            
                           
                               
                           
                            
                           12 
                         
                         
                           R 
                            
                           
                               
                           
                            
                           11 
                         
                       
                       × 
                       
                         ( 
                         
                           VINP 
                           - 
                           VINN 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   VOUTN 
                   = 
                   
                     VINN 
                     + 
                     
                       
                         
                           R 
                            
                           
                               
                           
                            
                           12 
                         
                         
                           R 
                            
                           
                               
                           
                            
                           11 
                         
                       
                       × 
                       
                         ( 
                         
                           VINN 
                           - 
                           VINP 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   GAIN 
                   = 
                   
                     
                       
                         VOUTP 
                         - 
                         VOUTN 
                       
                       
                         VINP 
                         - 
                         VINN 
                       
                     
                     = 
                     
                       1 
                       + 
                       
                         2 
                         × 
                         
                           
                             R 
                              
                             
                                 
                             
                              
                             12 
                           
                           
                             R 
                              
                             
                                 
                             
                              
                             11 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0013]    Where the amplification factor GAIN is 10, R 12 /R 11  is calculated to be 4.5 from Equation (6). Thus, the output voltages VOUTP and VOUTN are calculated as 
         [0000]        V OUT P= 2+4.5×1=6.5( V )
 
         [0000]        V OUT N= 1+4.5×(−1)=−3.5( V ).
 
         [0014]    On the other hand, when the chopping signal CHOP is “1,” the input voltages VINP and VINN are inverted by the switches SW 11  and SW 12  and hence the output voltages VOUTP and VOUTN are given by 
         [0000]    
       
         
           
             
               
                 
                   VOUTP 
                   = 
                   
                     VINN 
                     + 
                     
                       
                         
                           R 
                            
                           
                               
                           
                            
                           12 
                         
                         
                           R 
                            
                           
                               
                           
                            
                           11 
                         
                       
                       × 
                       
                         ( 
                         
                           VINN 
                           - 
                           VINP 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
             
               
                 
                   VOUTN 
                   = 
                   
                     VINP 
                     + 
                     
                       
                         
                           R 
                            
                           
                               
                           
                            
                           12 
                         
                         
                           R 
                            
                           
                               
                           
                            
                           11 
                         
                       
                       × 
                       
                         
                           ( 
                           
                             VINP 
                             - 
                             VINN 
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0015]    Thus, 
         [0000]        V OUT P= 1+4.5×(−1)=−3.5( V )
 
         [0000]        V OUT N= 2+4.5×1=6.5( V ).
 
         [0016]    As described above, in the analog front-end circuit  20 A shown in  FIG. 3 , theoretically, the output voltages VOUTP and VOUTN can become negative. As a result, where a single power source is employed for the operational amplifiers OP 11  and OP 12 , the negative side is saturated to 0 V. Thus, to handle a single-ended signal in which only a signal VINP that is output from the sensor  1  varies on the positive side and a voltage VINN that is output from the sensor  1  is fixed, it is necessary to set high bias points for the voltages VINP and VINN in advance, which results in a problem that the signal range of the output voltage VOUTP is narrowed. 
         [0017]    One measure for widening the signal range is an analog front-end circuit  20 B shown in  FIG. 4  which employs a non-inverting instrumentation preamplifier  21 B which uses a single operational amplifier OP 11 .  FIG. 5D  shows example input voltages VINP and VINN and output voltages VOUTP and VOUTN of the differential instrumentation preamplifier  21 B of the analog front-end circuit  20 B shown in  FIG. 4 . 
         [0018]    When the chopping signal CHOP is “0”. 
         [0000]    
       
         
           
             
               
                 
                   VOUTP 
                   = 
                   
                     VINP 
                     + 
                     
                       
                         
                           R 
                            
                           
                               
                           
                            
                           12 
                         
                         
                           R 
                            
                           
                               
                           
                            
                           11 
                         
                       
                       × 
                       
                         ( 
                         
                           VINP 
                           - 
                           VINN 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
             
               
                 
                   VOUTN 
                   = 
                   VINN 
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
             
               
                 
                   GAIN 
                   = 
                   
                     
                       
                         VOUTP 
                         - 
                         VOUTN 
                       
                       
                         VINP 
                         - 
                         VINN 
                       
                     
                     = 
                     
                       1 
                       + 
                       
                         
                           R 
                            
                           
                               
                           
                            
                           12 
                         
                         
                           R 
                            
                           
                               
                           
                            
                           11 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0019]    Where the amplification factor GAIN is 10, R 12 /R 11  is calculated to be 9 from Equation (11). Thus, the output voltages VOUTP and VOUTN are calculated as 
         [0000]        V OUT P= 2+9×1=11( V )
 
         [0000]        V OUT N= 1( V ). 
         [0020]    On the other hand, when the chopping signal CHOP is “1.” the output voltages VOUTP and VOUTN are given by 
         [0000]    
       
         
           
             
               
                 
                   VOUTP 
                   = 
                   
                     VINN 
                     + 
                     
                       
                         
                           R 
                            
                           
                               
                           
                            
                           12 
                         
                         
                           R 
                            
                           
                               
                           
                            
                           11 
                         
                       
                       × 
                       
                         ( 
                         
                           VINN 
                           - 
                           VINP 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
             
               
                 
                   VOUTN 
                   = 
                   
                     VINP 
                     . 
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
         [0021]    Thus, 
         [0000]        V OUT P= 1+9×(−1)=−8( V )
 
         [0000]        V OUT N= 2( V ). 
         [0022]    In this case, when the chopping signal CHOP is “0,” neither of the output voltages VOUTP and VOUTN is saturated and hence a wide signal range can be obtained. However, when the chopping signal CHOP is “1,” the output voltage VOUTP becomes negative, which raises the same problem as in the analog front-end circuit  20 A shown in  FIG. 3 . 
         [0023]    One object of the invention is to solve the above problems and thereby provide an analog front-end circuit capable of eliminating offset components etc. by a chopper operation and, in addition, preventing saturation of output voltages of an instrumentation preamplifier in a case of handling a single-ended signal having a wide signal range. 
       SUMMARY 
       [0024]    The invention provides following Aspects 1-5. 
         [0000]    1. An analog front-end circuit including: 
         [0025]    an instrumentation preamplifier which receives a first input voltage and a second input voltage and outputs a first output voltage and a second output voltage; 
         [0026]    a switch unit which outputs the first output voltage and the second output voltage of the instrumentation preamplifier by alternately switching them according to a logical transition of a chopping signal; 
         [0027]    an AD converter which converts an analog difference voltage between the first output voltage and the second output voltage that are output from the switch unit into a digital signal; and 
         [0028]    a data calculator which performs an averaging processing on two successive data that are output from the AD converter every time the logic transition of the chopping signal occurs, 
         [0029]    wherein,
       when the chopping signal has a first logical value, the instrumentation preamplifier performs calculations       
 
         [0000]        V OUT P=V IN P+G ×( V IN P−V IN N )
 
         [0000]        V OUT N=V IN N , and       when the chopping signal has a second logical value, the instrumentation preamplifier performs calculations         
         [0000]        V OUT P=V IN P+G ×( V IN N−V IN P )×(−1)
 
         [0000]        V OUT N=V IN N,          where
           VINP is the first input voltage,   VINN is the second input voltage,   VOUTP is the first output voltage,   VOUTN is the second output voltage, and   G is a gain of the instrumentation preamplifier.
 
2. The analog front-end circuit of Aspect 1,
   
                 
         [0038]    wherein the instrumentation preamplifier includes:
       a first switch having a second input terminal and a first input terminal which receives the first input voltage;   a second switch having a first input terminal and a second input terminal which receives the first input voltage;   a first operational amplifier having a non-inverting input terminal to which an output terminal of the first switch is connected and an inverting input terminal to which an output terminal of the second switch is connected;   a third switch having a first input terminal to which an output terminal of the first operational amplifier is connected and a second input terminal to which the output terminal of the first operational amplifier is connected via an inversion circuit;   a first resistor having one end which is connected to the second input terminal of the first switch and the first input terminal of the second switch and the other end which receives the second input voltage; and   a second resistor connected between an output terminal of the third switch and a connecting point of the second input terminal of the first switch and the first input terminal of the second switch,       
 
         [0045]    wherein, when the chopping signal has the first logical value, the first input terminal is selected in each of the first switch, the second switch and the third switch, 
         [0046]    wherein, when the chopping signal has the second logical value, the second input terminal is selected in each of the first switch, the second switch and the third switch, and 
         [0047]    wherein the first output voltage is output from the output terminal of the third switch, and the first input voltage is output as the second output voltage. 
         [0000]    3. The analog front-end circuit of Aspect 1, 
         [0048]    wherein the instrumentation preamplifier includes:
       a first switch having a second input terminal and a first input terminal which receives the first input voltage;   a second switch having a first input terminal and a second input terminal which receives the first input voltage;   a first operational amplifier having a non-inverting input terminal to which an output terminal of the first switch is connected and an inverting input terminal to which an output terminal of the second switch is connected;   a third switch having a first input terminal to which an output terminal of the first operational amplifier is connected and a second input terminal to which the output terminal of the first operational amplifier is connected via a first inversion circuit;   a seventh switch having a first input terminal and a second input terminal which receives the second input voltage;   an eighth switch having a second input terminal and a first input terminal which receives the second input voltage;   a second operational amplifier having an inverting input terminal to which an output terminal of the seventh switch is connected and a non-inverting input terminal to which an output terminal of the eighth switch is connected;   a ninth switch having a first input terminal to which an output terminal of the second operational amplifier is connected and a second input terminal to which the output terminal of the second operational amplifier is connected via a second inversion circuit;   a first resistor connected between a connecting point of the second input terminal of the first switch and the first input terminal of the second switch and a connecting point of the first input terminal of the seventh switch and the second input terminal of the eighth switch; and   a second resistor connected between an output terminal of the third switch and a connecting point of the second input terminal of the first switch and the first input terminal of the second switch,       
 
         [0059]    wherein an output terminal of the ninth switch is connected to the first input terminal of the seventh switch and the second input terminal of the eighth switch, 
         [0060]    wherein, when the chopping signal has the first logical value, the first input terminal is selected in each of the first switch, the second switch, the third switch, the seventh switch, the eighth switch and the ninth switch, 
         [0061]    wherein, when the chopping signal has the second logical value, the second input terminal is selected in each of the first switch, the second switch, the third switch, the seventh switch, the eighth switch and the ninth switch, and wherein the first output voltage is output from the output terminal of the third switch, and the second output voltage is output from the output terminal of the ninth switch. 
         [0000]    4. The analog front-end circuit of any one of Aspect 1 to 3, further including: 
         [0062]    a sixth switch which is connected between the AD converter and the data calculator, and has a first input terminal to which an output terminal of the AD converter is connected and a second input terminal to which the output terminal of the AD converter is connected via a data inverter, 
         [0063]    wherein, when the chopping signal has the first logical value, the first input terminal is selected in the sixth switch, 
         [0064]    wherein, when the chopping signal has the second logical value, the second input terminal is selected in the sixth switch, and 
         [0065]    wherein the data calculator performs addition processing on data that are received from the sixth switch, as the averaging processing. 
         [0000]    5. The analog front-end circuit of any one of Aspect 1 to 3, 
         [0066]    wherein the data calculator performs subtraction processing on data that are received from the AD converter, as the averaging processing. 
         [0067]    The above-mentioned Aspects 1-5 provide an advantage that offset components etc. can be eliminated by the chopper operation and output voltages of the instrumentation preamplifier are not saturated in a case of handling a single-ended signal having a wide signal range. Furthermore, Aspect 3 provides an advantage of preventing the bias voltage of the AD converter from being affected by the impedance on its input side. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0068]      FIG. 1  is a circuit diagram of an analog front-end circuit according to a first embodiment. 
           [0069]      FIG. 2  is a circuit diagram of an analog front-end circuit according to a second embodiment 
           [0070]      FIG. 3  is a circuit diagram of an analog front-end circuit of a first comparison example. 
           [0071]      FIG. 4  is a circuit diagram of an analog front-end circuit of a second comparison example. 
           [0072]      FIG. 5A  is a table showing input/output states of an instrumentation preamplifier of the analog front-end circuit shown in  FIG. 1 .  FIG. 5B  is a table showing input/output states of an instrumentation preamplifier of the analog front-end circuit shown in  FIG. 2 .  FIG. 5C  is a table showing input/output states of an instrumentation preamplifier of the analog front-end circuit shown in  FIG. 3 .  FIG. 5D  is a table showing input/output states of an instrumentation preamplifier of the analog front-end circuit shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
     Embodiment 1 
       [0073]      FIG. 1  shows an analog front-end circuit  10 A according to a first embodiment. The analog front-end circuit  10 A is equipped with a non-inverting instrumentation preamplifier  11 A which amplifies the difference between input voltages VINP and VINN that are input from a sensor  1  and outputs output voltages VOUTP and VOUTN, switches SW 4  and SW 5  (switch unit) each of which switches between the output voltages VOUTP and VOUTN, an AD converter  22  (AD converter) for converting the difference between the output voltages VOUTP and VOUTN that are output from the switches SW 4  and SW 5  into a digital signal, a switch circuit  13  which passes the output data of the AD converter  12  as it is or after inverting its polarity, and a data calculator  14  (data calculator) for calculating average data of two data that are output from the switch circuit  13  successively. 
         [0074]    The non-inverting instrumentation preamplifier  11 A is composed of switches SW and SW 2  each of which switches between input voltages VINP and VINN, an operational amplifier OP 1  for amplifying the difference between the input voltages VINP and VINN that have passed the switches SW 1  and SW 2 , gain-setting resistors R 1  and R 2 , and a switch circuit  11   a  which has an inversion circuit  11   a   1  and a switch SW 3  and passes an output voltage of the operational amplifier OP 1  as it is or after inverting its polarity. 
         [0075]    The switch circuit  13  has a switch SW 6  which passes input data as it is or outputs the input data as polarity-inverted by a data inverter  13   a.    
         [0076]    A chopping signal CHOP is a signal that “0” and “1” occur repeatedly and alternately at a prescribed cycle. Each of the switches SW 1 -SW 6  selects the input terminals “0” and “1” when the chopping signal CHOP is “0” and “1,” respectively. 
         [0077]    In  FIG. 1 , symbols Vos 1  and Vos 3  denote an offset component etc. occurring on the non-inverting input side of the operational amplifier OP 1  and an offset component etc. occurring on the data input side of the AD converter  12 , respectively. 
         [0078]    In the analog front-end circuit  10 A, an output voltage VO CHOP=0  of the switch circuit  13  when the chopping signal CHOP is “0” is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     VO 
                     
                       CHOP 
                       = 
                       0 
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           VINP 
                           - 
                           VINN 
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           1 
                           + 
                           
                             
                               R 
                                
                               
                                   
                               
                                
                               2 
                             
                             
                               R 
                                
                               
                                   
                               
                                
                               1 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       Vos 
                        
                       
                           
                       
                        
                       1 
                       × 
                       
                         ( 
                         
                           1 
                           + 
                           
                             
                               R 
                                
                               
                                   
                               
                                
                               2 
                             
                             
                               R 
                                
                               
                                   
                               
                                
                               1 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       Vos 
                        
                       
                           
                       
                        
                       3. 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
         [0079]    On the other hand, an output voltage VO CHOP=1  of the switch circuit  13  when the chopping signal CHOP is “1” is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     VO 
                     
                       CHOP 
                       = 
                       1 
                     
                   
                   = 
                   
                     
                       
                         { 
                         
                           
                             
                               - 
                               
                                 ( 
                                 
                                   VINP 
                                   - 
                                   VINN 
                                 
                                 ) 
                               
                             
                             × 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     R 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   
                                     R 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             Vos 
                              
                             
                                 
                             
                              
                             1 
                             × 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     R 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   
                                     R 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             Vos 
                              
                             
                                 
                             
                              
                             3 
                           
                         
                         } 
                       
                       × 
                       
                         ( 
                         
                           - 
                           1 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         
                           ( 
                           
                             VINP 
                             - 
                             VINN 
                           
                           ) 
                         
                         × 
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 2 
                               
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                           
                           ) 
                         
                       
                       - 
                       
                         Vos 
                          
                         
                             
                         
                          
                         1 
                         × 
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 2 
                               
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                           
                           ) 
                         
                       
                       - 
                       
                         Vos 
                          
                         
                             
                         
                          
                         3. 
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
         [0080]    Thus, an output OUT of the data calculator  14 , that is, the average of two successive output voltages VO CHOP=0  and VO CHOP=1  of the switch circuit  13 , is given by 
         [0000]    
       
         
           
             
               
                 
                   OUT 
                   = 
                   
                     
                       
                         
                           VO 
                           
                             CHOP 
                             = 
                             0 
                           
                         
                         + 
                         
                           VO 
                           
                             CHOP 
                             = 
                             1 
                           
                         
                       
                       2 
                     
                     = 
                     
                       
                         ( 
                         
                           VINP 
                           - 
                           VINN 
                         
                         ) 
                       
                       × 
                       
                         
                           ( 
                           
                             1 
                             + 
                             
                               2 
                               × 
                               
                                 
                                   R 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 
                                   R 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                             
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
         [0000]    In Equation (16), the offset components etc. Vos 1  and Vos 3  are eliminated. 
         [0081]      FIG. 5A  shows example input voltages VINP and VINN and output voltages VOUTP and VOUTN of the non-inverting instrumentation preamplifier  11 A of the analog front-end circuit  10 A shown in  FIG. 1  in which the input voltages VINP and VINN are 2 V and 1 V, respectively. When the chopping signal CHOP is “0,” the input voltages VINP and VINN pass the switches SW 1  and SW 2  as they are, respectively, and hence the output voltages VOUTP and VOUTN and the amplification factor GAIN are given by 
         [0000]    
       
         
           
             
               
                 
                   VOUTP 
                   = 
                   
                     VINP 
                     + 
                     
                       
                         
                           R 
                            
                           
                               
                           
                            
                           2 
                         
                         
                           R 
                            
                           
                               
                           
                            
                           1 
                         
                       
                       × 
                       
                         ( 
                         
                           VINP 
                           - 
                           VINN 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
             
               
                 
                   VOUTN 
                   = 
                   VINN 
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
             
               
                 
                   GAIN 
                   = 
                   
                     
                       
                         VOUTP 
                         - 
                         VOUTN 
                       
                       
                         VINP 
                         - 
                         VINN 
                       
                     
                     = 
                     
                       1 
                       + 
                       
                         
                           
                             R 
                              
                             
                                 
                             
                              
                             2 
                           
                           
                             R 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   19 
                   ) 
                 
               
             
           
         
       
     
         [0082]    Where the amplification factor GAIN is 10, R 2 /R 1  is calculated to be 9 from Equation (19). Thus, the output voltages VOUTP and VOUTN are calculated as 
         [0000]        V OUT P= 2+9×1=11( V )
 
         [0000]        V OUT N= 1( V ). 
         [0083]    On the other hand, when the chopping signal CHOP is “1”, the output voltages VOUTP and VOUTN are given by 
         [0000]    
       
         
           
             
               
                 
                   VOUTP 
                   = 
                   
                     VINP 
                     + 
                     
                       
                         
                           R 
                            
                           
                               
                           
                            
                           2 
                         
                         
                           R 
                            
                           
                               
                           
                            
                           1 
                         
                       
                       × 
                       
                         ( 
                         
                           VINN 
                           - 
                           VINP 
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           - 
                           1 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   20 
                   ) 
                 
               
             
             
               
                 
                   VOUTN 
                   = 
                   
                     VINN 
                     . 
                   
                 
               
               
                 
                   ( 
                   21 
                   ) 
                 
               
             
           
         
       
     
         [0084]    Thus, 
         [0000]        V OUT P= 2+9×(−1)×(−1)=11( V )
 
         [0000]        V OUT N= 1( V ). 
         [0085]    As described above, in the analog front-end circuit  10 A shown in  FIG. 1 , the output voltages VOUTP and VOUTN do not become negative. That is, even if a single-ended signal in which the input voltage VINN is fixed and the input voltage VINP varies, the output voltages VOUTP and VOUTN are not saturated and their signal ranges are widened to a large extent. 
       Embodiment 2 
       [0086]      FIG. 2  shows an analog front-end circuit  10 B according to a second embodiment. The analog front-end circuit  10 B is different from the analog front-end circuit  10 A shown in  FIG. 1  in a non-inverting instrumentation preamplifier  11 B. The non-inverting instrumentation preamplifier  11 B is different from the non-inverting instrumentation preamplifier  11 A shown in  FIG. 1  in being added with, as a voltage follower circuit, an operational amplifier OP 2 , switches SW 7  and SW 8 , and a switch circuit  11   b  which has an inversion circuit  11   b   1  and a switch SW 9  and passes an output voltage of the operational amplifier OP 2  as it is or after inverting its polarity. 
         [0087]    Each of the switches SW 1  and SW 2  switches between input voltages for the operational amplifier OP 1 , and each of the switches SW 7  and SW 8  switches between input voltages for the operational amplifier OP 2 . 
         [0088]    Each of the switches SW 1 -SW 6  selects the input terminals “0” and “1” when the chopping signal CHOP is “0” and “1,” respectively. In  FIG. 2 , symbols Vos 1 , Vos 2 , and Vos 3  denote an offset component etc. occurring on the non-inverting input side of the operational amplifier OP 1 , an offset component etc. occurring on the non-inverting input side of the operational amplifier OP 2 , and an offset component etc. occurring on the input side of the AD converter  12 , respectively. 
         [0089]    In the analog front-end circuit  10 B, an output voltage VO CHOP=0  of the switch circuit  13  when the chopping signal CHOP is “0” is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     VO 
                     
                       CHOP 
                       = 
                       0 
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           VINP 
                           - 
                           VINN 
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           1 
                           + 
                           
                             
                               R 
                                
                               
                                   
                               
                                
                               2 
                             
                             
                               R 
                                
                               
                                   
                               
                                
                               1 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         ( 
                         
                           
                             Vos 
                              
                             
                                 
                             
                              
                             1 
                           
                           - 
                           
                             Vos 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           1 
                           + 
                           
                             
                               R 
                                
                               
                                   
                               
                                
                               2 
                             
                             
                               R 
                                
                               
                                   
                               
                                
                               1 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       Vos 
                        
                       
                           
                       
                        
                       3. 
                     
                   
                 
               
               
                 
                   ( 
                   22 
                   ) 
                 
               
             
           
         
       
     
         [0090]    On the other hand, an output voltage VO CHOP=1  of the switch circuit  13  when the chopping signal CHOP is “1” is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     VO 
                     
                       CHOP 
                       = 
                       1 
                     
                   
                   = 
                   
                     
                       
                         { 
                         
                           
                             
                               - 
                               
                                 ( 
                                 
                                   VINP 
                                   - 
                                   VINN 
                                 
                                 ) 
                               
                             
                             × 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     R 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   
                                     R 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             
                               ( 
                               
                                 
                                   Vos 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                                 - 
                                 
                                   Vos 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                               
                               ) 
                             
                             × 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     R 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   
                                     R 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             Vos 
                              
                             
                                 
                             
                              
                             3 
                           
                         
                         } 
                       
                       × 
                       
                         ( 
                         
                           - 
                           1 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         
                           ( 
                           
                             VINP 
                             - 
                             VINN 
                           
                           ) 
                         
                         × 
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 2 
                               
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                           
                           ) 
                         
                       
                       - 
                       
                         
                           ( 
                           
                             
                               Vos 
                                
                               
                                   
                               
                                
                               1 
                             
                             - 
                             
                               Vos 
                                
                               
                                   
                               
                                
                               2 
                             
                           
                           ) 
                         
                         × 
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 2 
                               
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                           
                           ) 
                         
                       
                       - 
                       
                         Vos 
                          
                         
                             
                         
                          
                         3. 
                       
                     
                   
                 
               
               
                 
                   ( 
                   23 
                   ) 
                 
               
             
           
         
       
     
         [0091]    Thus, an output OUT of the data calculator  14 , that is, the average of two successive output voltages VO CHOP=0  and VO CHOP=1  of the switch circuit  13 , is given by 
         [0000]    
       
         
           
             
               
                 
                   OUT 
                   = 
                   
                     
                       
                         
                           VO 
                           
                             CHOP 
                             = 
                             0 
                           
                         
                         + 
                         
                           VO 
                           
                             CHOP 
                             = 
                             1 
                           
                         
                       
                       2 
                     
                     = 
                     
                       
                         ( 
                         
                           VINP 
                           - 
                           VINN 
                         
                         ) 
                       
                       × 
                       
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 2 
                               
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   24 
                   ) 
                 
               
             
           
         
       
     
         [0000]    In Equation (24), the offset components etc. Vos 1 , Vos 2  and Vos 3  are eliminated. 
         [0092]      FIG. 5B  shows example input voltages VINP and VINN and output voltages VOUTP and VOUTN of the non-inverting instrumentation preamplifier  11 B of the analog front-end circuit  10 B shown in  FIG. 2 . Since the switches SW 7  and SW 8 , the switch circuit  11   b , and the operational amplifier OP 2  serve as a voltage follower, the input voltages VINP and VINN and the output voltages VOUTP and VOUTN of the non-inverting instrumentation preamplifier  11 B are the same as those of the non-inverting instrumentation preamplifier  11 B shown in  FIG. 1 . 
         [0093]    In the analog front-end circuit  10 B shown in  FIG. 2 , since the impedance relating to the output voltage VOUTN is isolated from the impedance relating to the input voltage VINN, a phenomenon can be prevented that the bias voltage of the AD converter  12  is affected by the impedance on its input side. 
       Other Embodiments 
       [0094]    In the analog front-end circuits  10 A and  10 B shown in  FIGS. 1 and 2 , the data calculator  14  performs averaging processing by adding together two successive output data of the switch circuit  13 . The switch circuit  13  can be omitted if the data calculator  14  is configured so as to calculate a difference between two successive input data.