Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application and is based upon PCT/JP2009/065528, filed on Sep. 4, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The embodiments discussed herein are related to a switched capacitor circuit and a stage circuit for AD converter. 
     BACKGROUND 
     Switched capacitor circuits are widely used in high-resolution, low-power AD (Analog-to-Digital) converters, DA (Digital-to-Analog) converters, filters, etc. 
     More specifically, a switched capacitor circuit includes a capacitor, a switch, and an amplifier, and this type of circuit is applied, for example, to an MDAC (Multiplying DAC) or the like used as a basic building block in a pipelined AD conversion circuit or a cyclic AD conversion circuit. 
     In this patent specification, an AD conversion circuit and a switched capacitor circuit (MDAC) to be used therein will be described as examples, but as stated above, the switched capacitor circuit is also applicable to a DA converter, a filter, etc. 
     Various forms of pipelined AD conversion circuits employing switched capacitor circuits have been proposed. 
     As described above, the switched capacitor circuit is applied, for example, to an MDAC or the like in a pipelined AD conversion circuit. 
     With the rapid growth in digital consumer applications (e.g., DTV and DSC) and wireless communications, it has become increasingly important to provide an AD conversion circuit capable of high-resolution with high-speed operation. 
     In the field of portable apparatus, for example, there has also developed a need to further reduce power consumption while also reducing the die size of the circuit.
     Non-Patent Document 1: Shoji Kawahito, “Low-Power Design of Pipeline A/D converters,” IEEE Custom Integrated Circuits Conference, pp. 505-512, September 2006   Non-Patent Document 2: Kunihiko Gotoh et al., “3 STATES LOGIC CONTROLLED CMOS CYCLIC A/D CONVERTER,” IEEE Custom Integrated Circuits Conference, pp. 366-369, May 1986   Non-Patent Document 3: Chin-Chen Lee, “A NEW SWITCHED-CAPACITOR REALIZATION FOR CYCLIC ANALOG-TO-DIGITAL CONVERTER,” IEEE International Symposium on Circuit and Systems, pp. 1261-1265, May 1983   

     SUMMARY 
     According to one embodiment, there is provided a switched capacitor circuit is configured to be operable in two or more kinds of operation modes including a first operation mode and a second operation mode. 
     The switched capacitor circuit includes an amplifier and two or more internal capacitors with switches for controlling connection/disconnection of the capacitor. 
     In the first operation mode that precedes the second operation mode, the switched capacitor circuit generates the first analog output voltage by using the first internal capacitor connected between an input terminal and output terminal of the amplifier by using its switches, the other internal capacitances connected between an input terminal of the amplifier and each analog input voltage supply by using its switches. 
     In the second operation mode that follows the first operation mode, the switched capacitor circuit generates the second analog output voltage with larger feedback factor of the amplifier than it in the first operation mode, by removing some of the internal capacitors, except the first internal capacitor, from the first operation mode. 
     The object and advantages of the embodiments will be realized and attained by the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the embodiments, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a block diagram illustrating one example of a pipelined AD converter and its pipeline stage circuit; 
         FIG. 1B  is a timing chart for explaining the operation of the AD conversion circuit in  FIG. 1A ; 
         FIG. 1C  is a proceeding for explaining the operation of the AD conversion circuit in  FIG. 1A ; 
         FIG. 2A  is a simplified circuit schematic for explaining one example of a STAGE circuit and its operation; 
         FIG. 2B  is a timing chart (part  1 ) for explaining the STAGE circuit in  FIG. 2A ; 
         FIG. 2C  is a timing chart (part  2 ) for explaining the STAGE circuit in  FIG. 2A ; 
         FIG. 3A  is a simplified circuit schematic of a 1.5-b STAGE for sampling mode and in hold mode operation, respectively; 
         FIG. 3B  is a timing chart for explaining the 1.5-b STAGE circuit in  FIG. 3A ; 
         FIG. 3C  is a transfer function for the 1.5bMDAC of the STAGE circuit in  FIG. 3A ; 
         FIG. 3D  is an operation table for the 1.5b-ADC and the 1.5bMDAC of the STAGE circuit in  FIG. 3A ; 
         FIG. 4A  is a simplified circuit schematic of a 2.5-b STAGE for sampling mode and in hold mode operation, respectively; 
         FIG. 4B  is a transfer function for the 2.5bMDAC of the STAGE circuit in  FIG. 4A ; 
         FIG. 4C  is an operation table for the 2.5b-ADC and the 2.5bMDAC of the STAGE circuit in  FIG. 4A ; 
         FIG. 5  is a simplified circuit model of a switched capacitor circuit (ex. MDAC) for explaining the relationship between its operating speed and the current consumption of an op amp for an MDAC circuit in the hold mode operation; 
         FIG. 6  is a comparison table of the performance for some kind of MDAC circuits with considering some kind of load conditions for the output of each MDAC; 
         FIG. 7  is a simplified circuit model of a switched capacitor circuit (ex. MDAC) in the hold operation mode that is used for both analog computation of MDAC and sampling the output voltage of MDAC to a loading capacitance; 
         FIG. 8A  is a simplified circuit model of a first embodiment switched capacitor circuit (ex. MDAC) in the first hold operation mode that is used for only analog computation of MDAC without loading capacitance; 
         FIG. 8B  is a simplified circuit model of a first embodiment switched capacitor circuit (ex. MDAC) in the second hold operation mode that is used for sampling the output voltage of MDAC in the first operation mode to a loading capacitance by using a sampling switch, the amplifier and the capacitance (C 1   H ); 
         FIG. 9A  is a diagram (part  1 ) for explaining the STAGE circuit of the first embodiment and its operation; 
         FIG. 9B  is a timing chart (part  2 ) for explaining the STAGE circuit of the first embodiment and its operation; 
         FIG. 10A  is a circuit diagram illustrating one example of the STAGE circuit of the first embodiment; 
         FIG. 10B  is a timing chart for explaining the operation of the STAGE circuit in  FIG. 10A ; 
         FIG. 11  is a comparison table of the performance for the 1.5bMDAC of the first embodiment for comparison with the MDAC depicted in  FIG. 3A ; 
         FIG. 12  is a comparison table of the performance for the 2.5bMDAC of the first embodiment for comparison with the MDAC depicted in  FIG. 4A ; 
         FIG. 13A  is a diagram (part  1 ) for explaining a STAGE circuit according to a second embodiment and its operation; 
         FIG. 13B  is a timing chart (part  2 ) for explaining the STAGE circuit of the second embodiment and its operation; 
         FIG. 14A  is a circuit diagram illustrating one example of the STAGE circuit of the second embodiment; 
         FIG. 14B  is a timing chart for explaining the operation of the STAGE circuit of  FIG. 14A ; 
         FIG. 15A  is a diagram (part  1 ) for explaining a STAGE circuit according to a third embodiment and its operation; 
         FIG. 15B  is a timing chart (part  2 ) for explaining the STAGE circuit of the third embodiment and its operation; 
         FIG. 16A  is a circuit diagram illustrating one example of the STAGE circuit of the third embodiment; 
         FIG. 16B  is a timing chart for explaining the operation of the STAGE circuit of  FIG. 16A ; 
         FIG. 17  is a diagram depicting the number of comparators of the sub-ADC needed in the stage of the third embodiment for comparison with the number of comparators needed in each of the MDACs depicted in FIG.  3 A and  FIG. 4A ; 
         FIG. 18A  is a diagram (part  1 ) for explaining another example of the MDAC and its operation; 
         FIG. 18B  is a diagram (part  2 ) for explaining that other example of the MDAC and its operation; 
         FIG. 19A  is a diagram (part  1 ) for explaining the MDAC of the earlier described first embodiment and its operation; 
         FIG. 19B  is a diagram (part  2 ) for explaining the MDAC of the earlier described first embodiment and its operation; 
         FIG. 20A  is a circuit diagram illustrating a first configuration example of the 1.5bMDAC in sampling mode and in hold mode, respectively; 
         FIG. 20B  is a diagram (part  1 ) for explaining the operation of the MDAC of  FIG. 20A ; 
         FIG. 20C  is a diagram (part  2 ) for explaining the operation of the MDAC of  FIG. 20A ; 
         FIG. 20D  is a diagram (part  3 ) for explaining the operation of the MDAC of  FIG. 20A ; 
         FIG. 21A  is a circuit diagram illustrating a second configuration example of the 1.5bMDAC in sampling mode and in hold mode, respectively; 
         FIG. 21B  is a diagram (part  1 ) for explaining the operation of the MDAC of  FIG. 21A ; 
         FIG. 21C  is a diagram (part  2 ) for explaining the operation of the MDAC of  FIG. 21A ; 
         FIG. 21D  is a diagram (part  3 ) for explaining the operation of the MDAC of  FIG. 21A ; 
         FIG. 22A  is a diagram (part  1 ) for explaining the basic operation of the MDAC of the second configuration example; 
         FIG. 22B  is a diagram (part  2 ) for explaining the basic operation of the MDAC of the second configuration example; 
         FIG. 23A  is a diagram (part  1 ) for explaining an MDAC according to a fourth embodiment and its operation; 
         FIG. 23B  is a diagram (part  2 ) for explaining the MDAC of the fourth embodiment and its operation; 
         FIG. 24A  is a circuit diagram illustrating one example of the MDAC of the fourth embodiment; 
         FIG. 24B  is a diagram for explaining the operation of the MDAC of  FIG. 24A ; 
         FIG. 25  is a diagram for explaining the basic operation of the MDAC of the first configuration example as applied in a parallel MDAC system; 
         FIG. 26A  is a diagram (part  1 ) for explaining the basic operation of the MDAC of the second configuration example as applied in the parallel MDAC system; 
         FIG. 26B  is a diagram (part  2 ) for explaining the basic operation of the MDAC of the second configuration example as applied in the parallel MDAC system; 
         FIG. 27A  is a diagram (part  1 ) for explaining an MDAC according to a fifth embodiment and its operation; 
         FIG. 27B  is a diagram (part  2 ) for explaining the MDAC of the fifth embodiment and its operation; 
         FIG. 28A  is a circuit diagram illustrating one example of the MDAC of the fifth embodiment; 
         FIG. 28B  is a diagram for explaining the operation of the MDAC of  FIG. 28A ; 
         FIG. 29  is a diagram illustrating the performance of the MDACs of the fourth and fifth embodiments for comparison with the performance of the MDACs depicted in  FIGS. 22A and 26A ; 
         FIG. 30  is a block diagram schematically illustrating one example of a pipelined AD conversion circuit to which the stage circuit that has the MDAC of each embodiment or the sub-ADC of each embodiment is applied; and 
         FIG. 31  is a block diagram schematically illustrating one example of a cyclic AD conversion circuit to which the stage circuit that has the MDAC of each embodiment or the sub-ADC of each embodiment is applied. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Before describing the embodiments in detail, switched capacitor circuits and AD conversion circuits and their associated problems will be described below. 
       FIG. 1A  is a block diagram illustrating one example of a pipelined AD converter and its pipeline stage circuit,  FIG. 1B  is a timing chart for explaining the operation of the AD conversion circuit in  FIG. 1A , and  FIG. 1C  is a proceeding for explaining the operation of the AD conversion circuit in  FIG. 1A . The AD conversion circuit hereinafter described with reference to  FIG. 1A ,  FIG. 1B , and  FIG. 1C  is a pipelined AD conversion circuit. 
     In the pipelined AD conversion circuit, the circuit that becomes important in achieving higher speed, lower power consumption, and smaller die size is the MDAC (Multiplying DAC: switched capacitor circuit) that is used as the basic building block of the cell array. 
     As illustrated in  FIG. 1A , the pipelined AD conversion circuit  1  includes a sample-and-hold (S/H) circuit  11 , a number, N−1, of stage circuits (STG- 1  to STG-(N−1))  10 - 1  to  10 -(N−1), a flash AD converter (flash ADC)  12  at the last stage, and a digital correction circuit (code conversion circuit)  13 . 
     The sample-and-hold circuit  11  samples an input voltage VIN and holds it, and the flash ADC  12  outputs a signal DON, i.e., the AD-converted result, without further processing to the digital correction circuit  13 . 
     The digital correction circuit  13  receives the output signals DO 1  to DO(N−1) of the respective stage circuits  10 - 1  to  10 -(N−1) as well as the output signal DON of the flash ADC  12 , and outputs a digital signal DO as the result of the AD conversion of the input voltage VIN. 
     Each stage circuit  10  includes a MDAC  100  and a sub-AD converter (ADC)  110 , and the MDAC  100  includes a sub-DA converter (DAC)  101  and an analog computation unit  102 . The sub-DAC  101  outputs a voltage +VR, 0(SG), or −VR to the analog computation unit  102  in accordance with a signal DA(i) supplied from the sub-ADC  110 . 
     As will be described later, the MDAC  100  is constructed from a switched capacitor circuit which includes two or more capacitors (internal capacitors), an amplifier, and a switch (internal switch), and performs an analog computation to add or subtract a constant multiple of the reference voltage VR by using the amplified result of the input signal VIN(i) and the AD-converted result DA(i) of the input signal. 
     The output (VO(i)) of each MDAC (one of the stage circuits  10 - 1  to  10 -(N−1)) is supplied as an input signal to the subsequent stage circuit (one of the stage circuits  10 - 2  to  10 -(N−1) or the flash ADC  12 ). 
     For example, consider the case where the analog input signal VIN is converted into a 4-bit digital signal (N=4) for output, as illustrated in  FIG. 1B ; first, for VIN( 1 ), the signal DO 1 ( 1 ) representing the most significant bit (MSB) is output in period T( 1 ), which is followed by the signal DO 2 ( 1 ) in period T( 2 ). 
     Next, the signal DO 3 ( 1 ) is output in period T( 3 ), and the signal DO 4 ( 1 ) representing the least significant bit (LSB) is output in period T( 4 ). Then, in period T( 5 ), the digital output ADCO( 1 ) binarized by the digital correction circuit  13  is produced. 
     Similarly, for VIN( 2 ), the signal DO 1 ( 2 ) representing the most significant bit is output in period T( 2 ), which is followed by the signal DO 2 ( 2 ) in period T( 3 ). 
     Next, the signal DO 3 ( 2 ) is output in period T( 4 ), and the signal DO 4 ( 2 ) representing the least significant bit is output in period T( 5 ). Then, in period T( 6 ), the digital output ADCO( 2 ) binarized by the digital correction circuit  13  is produced. 
     In the above process, each stage carries out the computation VO(i)=m*[VIN(i)−{DA(i)/m}*VR]. For example, when the signals DO 1  to DO 4  are “1, 0, −1, 1”, respectively, as illustrated in  FIG. 1C , the digital correction circuit  13  outputs the binarized digital output “0111”. Here, m represents the signal amplification factor. 
     By thus performing a plurality of processes concurrently through the cascaded MDACs  100  on a per clock basis, the pipelined AD conversion circuit  1  achieves faster conversion speed, though the delay (latency) from input to output increases. 
     Furthermore, since higher resolution may be achieved by appropriately determining the number of stages according to the resolution needed, the pipelined AD conversion circuit may be designed flexibly according to its performance requirements. 
     Since the pipelined AD conversion circuit covers a wide range of resolution and conversion speed, as described above, this type of conversion circuit is widely used in various electronic apparatus such as digital AV equipment and radio communication circuits. 
       FIG. 2A  is a simplified circuit schematic for explaining one example of a STAGE circuit and its operation,  FIG. 2B  is a timing chart (part  1 ) for explaining the STAGE circuit in  FIG. 2A , and  FIG. 2C  is a timing chart (part  2 ) for explaining the STAGE circuit in  FIG. 2A . 
       FIG. 2B  illustrates the processing performed by MDAC 1  and MDAC 2 , while  FIG. 2C  illustrates only the processing performed by MDAC 1 . Further, in  FIG. 2A , reference characters OP 1  and OP 2  designate operational amplifiers (op amps). 
     In  FIG. 2A ,  FIG. 28 , and  FIG. 2C , the conversion time (T) is divided into four periods ( 1 ) to ( 4 ) to correspond with the description of each embodiment to be given later, but actually, the operation may be described by dividing it into two periods made up of the period ( 1 )+( 2 ) and the period ( 3 )+( 4 ). 
     That is, the time during which a series of operations is repeated (the conversion time T) is depicted as being divided into the four periods ( 1 ) to ( 4 ). Accordingly, the length of each period is defined by ( 1 )+( 2 )=( 3 )+( 4 )=T/2. 
     In this patent specification and the accompanying drawings, the description is given by dealing with the case where the signal to be processed is a single-ended signal, but the configuration is basically the same for the case of a differential signal. 
     Further, while two MDACs, MDAC 1  and MDAC 2 , are depicted in  FIG. 2A , the basic operation is described for the first-stage MDAC 1 , and the second-stage MDAC 2  is depicted for the purpose of facilitating an understanding of the load condition of the first-stage MDAC 1 . 
     As illustrated in  FIG. 2A , the MDAC as a circuit for processing an analog signal is constructed using a switched capacitor (SC) circuit that includes a capacitor (C), a switch (SW), and an operational amplifier (OP: OP AMP). 
     To describe the basic operation of the MDAC, first in the period ( 1 )+( 2 ), the MDAC 1  samples the analog input signal (VIN) by using the sampling capacitor C 1   s  (=C 1   n1 +C 1   n2 ). Further, in the same period ( 1 )+( 2 ), the digital output result DO (DO 1 ) and the add/subtract coefficient DA (DA 1 ) of the reference voltage VR are determined by using the sub-AD converter ADC 1  ( 110 ) which includes a comparator. 
     Next, in the period ( 3 )+( 4 ), the analog computation result VO 1  is output by applying a DAC output voltage using the op amp OP 1 , the capacitors C 1   n1  and C 1   n2 , and the comparison result from the ADC 1 . 
     The output result is supplied as the input signal VIN 2  to the second-stage MDAC (MDAC 2 ) and sampled on the sampling capacitor C 2   s  (=C 2   n1 +C 2   n2 ); the output result is also supplied as an input signal to the second-stage sub-AD converter ADC 2  ( 110 ). 
     In  FIG. 2C , during the period ( 3 )+( 4 ) when the MDAC 1  performs computation, the capacitor C 1   n1  acts as a computation capacitor (C 1   MDAC ) and the capacitor C 1   n2  as a hold capacitor (C 1   H ), and the sampling capacitor C 2   s  (=C 2   n1 +C 2   n2 ) in the second-stage MDAC 2  acts as the load. 
     More specifically, in the period ( 3 )+( 4 ), the hold capacitor C 1   H  (C 1   n2 ) is connected between the output terminal and negative input terminal of the op amp OP 1 , and the computation capacitor C 1   MDAC  (C 1   n1 ) is connected between the output terminal of the sub-DA converter ( 101 ) and the negative input terminal of the op amp OP 1 . Then, the sampling capacitor C 2 , (=C 2   n1 +C 2   n2 ) in the second-stage MDAC 2  is coupled to the output terminal of the op amp OP 1 . 
       FIG. 3A  is a simplified circuit schematic of a 1.5-b STAGE for sampling mode and in hold mode operation, respectively, and  FIG. 3B  is a timing chart for explaining the 1.5-b STAGE circuit in  FIG. 3A . Further,  FIG. 3C  is a transfer function for the 1.5bMDAC of the STAGE circuit in  FIG. 3A , and  FIG. 3D  is an operation table for the 1.5b-ADC and the 1.5bMDAC of the STAGE circuit in  FIG. 3A . 
     In  FIG. 3A , reference character SWC 1  designates a switch control unit which receives signals MCLK and SHSEL and outputs switch control signals; further,  101  is a sub-DAC, CMP 1  and CMP 2  are comparators, DFF 1  and DFF 2  are flip-flops, and LO 1  is a logic unit. 
     First, as illustrated in the left half of  FIG. 3A  and in the periods ( 1 ) and ( 2 ) (( 1 )+( 2 )) of  FIG. 3B , in the sampling (S) mode of the MDAC 1  the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 1 B/ 2 A/ 2 B and SWADCIN to a high level “H”, causing these switches to turn on. 
     When the switch SWADCIN is turned on, the comparators CMP 1  and CMP 2  compare the input voltage V, applied as the compare voltage V CMP , with the reference voltages (¼)*VR and −(¼)*VR, respectively, and supply the comparison results to the input terminals of the flip-flops DFF 1  and DFF 2 , respectively. 
     Further, in the sampling mode of the MDAC 1  in the period ( 1 )+( 2 ), the switch control unit SWC 1  sets the control signals for the switches SWH 1 A/ 1 B/ 2 B and CLKADC to a low level “L”. In this means, the switches SWH 1 A/ 1 B/ 2 B to turn off, and the flip-flops DFF 1  and DFF 2  are disabled. 
     As earlier described, in the sampling capacitor C 1   s  on which the MDAC 1  samples the input signal VIN, the capacitors C 1   n1  and C 1   n2  are connected in parallel with each other with the switches SWS 1 A, SWS 1 B, and SWS 2 B turning on; as a result, the sampling capacitor C 1   s  is C 1   s =C 1   n1 +C 1   n2 . Here, if C 0 =C 1   s  and C 1   n1 =C 1   n2 , then C 1   n1 =C 1   n2 =C 0 /2. 
     Next, as illustrated in the right half of  FIG. 3A  and in the periods ( 3 ) and ( 4 ) (( 3 )+( 4 )) of  FIG. 3B , in the hold (H: computation) mode of the MDAC 1  the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 1 B/ 2 A/ 2 B and SWADCIN to “L”, causing these switches to turn off. 
     Further, in the hold mode in the period ( 3 )+( 4 ), the switch control unit SWC 1  sets the control signals for the switches SWH 1 A/ 1 B/ 2 B and CLKADC to “H”. This causes the switches SWH 1 A/ 1 B/ 2 B to turn on, and the flip-flops DFF 1  and DFF 2  are enabled to latch and hold the input data. 
     Here, the output signals from the flip-flops DFF 1  and DFF 2  are supplied to the logic unit LO 1 , and the logic unit LO 1  outputs the digital output DO and the add/subtract coefficient DA 1 . The add/subtract coefficient DA 1  is supplied to the sub-DAC  101 . 
     Further, the hold capacitor C 1   H  and the computation capacitor C 1   MDAC  are C 1   H =C 0 /2 and C 1   MDAC =C 0 /2, respectively, the feedback ratio β is β=C 1   H (C 1   H +C 1   MDAC )=½, and the signal amplification factor, m, is m=C 1   s /C 1   H =2. 
     That is, in the period ( 3 )+( 4 ), the hold capacitor C 1   H  (C 1   n2 ) is connected between the output terminal and negative input terminal of the op amp OP 1 , and the computation capacitor C 1   MDAC  (C 1   n1 ) is connected between the output terminal of the sub-DA converter ( 101 ) and the negative input terminal of the op amp OP 1 . 
     When the signal amplification is m=2, the relation depicted in  FIG. 3C  holds between VIN/VR and VO/VR. Further, the input voltage VIN (the compare voltage V CMP ), the digital output DO, the add/subtract coefficient DA 1 , the output voltage VDA 1  of the sub-DAC  101 , and the output voltage VO of the op amp OP 1  are as depicted in  FIG. 3D . 
     Here, since the output voltage VO is VO=m*{VIN−(DA/m)*VR}, and m=2, it follows that VO=2*VIN−DA*VR. 
     That is, when the input voltage VIN is in the range defined by +VR≧VIN≧+(¼)*VR, DO is +01, DA is +1, VDA 1  is +VR, and VO is 2*VIN−VR; on the other hand, when the input voltage VIN is in the range defined by +(¼)*VR≧VIN≧−(¼)*VR, DO is 00, DA is 0, VDA 1  is 0, and VO is 2*VIN. 
     Further, when the input voltage VIN is in the range defined by −(¼)*VR≧VIN≧−VR, DO is −01, DA is −1, VDA 1  is −VR, and VO is 2*VIN+VR. 
       FIG. 4A  is a simplified circuit schematic of a 2.5-b STAGE for sampling mode and in hold mode operation, respectively,  FIG. 4B  is a transfer function for the 2.5bMDAC of the STAGE circuit in  FIG. 4A , and  FIG. 4C  is an operation table for the 2.5b-ADC and the 2.5bMDAC of the STAGE circuit in  FIG. 4A . 
     As is apparent from a comparison between  FIG. 4A  and the previously described  FIG. 3A , the capacitor C 1   n1  in the 2.5bMDAC is divided into two capacitors C 1   n11  and C 1   n12  each of which is provided with a sub-DAC  101   a  or  101   b  and switches SWS 11 B and SWH 11 B or SWS 12 B and SWH 12 B. 
     Further, the two comparators CMP 1  and CMP 2  in  FIG. 3A  are replaced by six comparators CMP 11  to CMP 16 , and six split voltages ⅝*VR, ⅜*VR, ⅛*VR, −⅛*VR, −⅜*VR, and −⅝*VR are applied to the respective comparators and compared with the input voltage VIN (V CMP ). 
     The output signals from the respective comparators CMP 11  to CMP 16  are supplied to the logic unit LO 1  via the respective flip-flops DFF 11  to DFF 16 , and the logic unit LO 1  outputs the digital output DO along with two add/subtract coefficients DA 1  and DA 2  that are supplied to the sub-DACs  101   a  and  101   b.    
     Then, as illustrated in the left half of  FIG. 4A , in the sampling mode of the MDAC 1  the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 2 B/ 11 B/ 12 B and SWADCIN to “H”, causing these switches to turn on. 
     Further, in the sampling mode of the MDAC 1 , the switch control unit SWC 1  sets the control signals for the switches SWH 1 A/ 2 B/ 11 B/ 12 B and CLKADC to “L”, causing the switches SWH 1 A/ 2 B/ 11 B/ 12 B to turn off and disabling the flip-flops DFF 1  to DFF 16 . 
     At this time, since the capacitors C 1   n11 , C 1   n12 , and C 1   n2  are connected in parallel with each other with the switches SWS 11 B, SWS 12 B, and SWS 2 B turning on, the sampling capacitor C 1   s  is C 1   s =C 1   n11 +C 1   n12 +C 1   n2 . Here, if C 0 =C 1   s , C 1   n2 =C 1   s /4 and C 1   n11 =2*C 1   n2 , then C 1   n2 =C 0 /4, C 1   n12 =C 0 /4, C 1   n11 =C 0 /2. 
     Next, as illustrated in the right half of  FIG. 4A , in the hold mode of the MDAC 1  the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 2 B/ 11 B/ 12 B and SWADCIN to “L”, causing these switches to turn off. 
     Further, in the hold mode of the MDAC 1 , the switch control unit SWC 1  sets the control signals for the switches SWH 1 A/ 2 B/ 11 B/ 12 B and CLKADC to “H”. This causes the switches SWH 1 A/ 2 B/ 11 B/ 12 B to turn on, and the flip-flops DFF 1  to DFF 6  are enabled to latch and hold the input data. 
     That is, the flip-flops DFF 1  to DFF 6  latch and hold the output signals of the corresponding comparators CMP 11  to CMP 16 . Here, the output signals from the flip-flops DFF 1  to DFF 6  are supplied to the logic unit LO 1 , and the logic unit LO 1  outputs the digital output DO and the add/subtract coefficients DA 1  and DA 2 . The add/subtract coefficients DA 1  and DA 2  are supplied to the sub-DACs  101   b  and  101   a , respectively. 
     Further, the hold capacitor C 1   H  and the computation capacitor C 1   MDAC  are C 1   H =C 0 /4 and C 1   MDAC =(¾)*C o , respectively, the feedback ratio β is β C 1   H /(C 1   H +C 1   MDAC )=¼, and the signal amplification, m, is m=C 1   s /C 1   H =4. 
     When the signal amplification is m=4, the relation depicted in  FIG. 4B  holds between VIN/VR and VO/VR. 
     Further, the input voltage VIN (the compare voltage V CMP ), the digital output DO, the add/subtract coefficients DA 1  and DA 2 , the output voltages VDA 1  and VDA 2  of the sub-DACs  101   b  and  101   a , and the output voltage VO of the op amp OP 1  are as depicted in  FIG. 4C . 
     Here, since the output voltage VO is VO=m*{VIN−(DA/m)*VR}, and m=4, it follows that VO=4*VIN−DA*VR. 
     That is, when the input voltage VIN is in the range defined by +VR≧VIN≧+(⅝)*VR, DO is +011, DA is +3, VDA 2  is +VR, VDA 1  is +VR, and VO is 4*VIN−3*VR; on the other hand, when the input voltage VIN is in the range defined by +(⅝)*VR≧VIN≧+(⅜)*VR, DO is +010, DA is +2, VDA 2  is +VR, VDA 1  is 0, and VO is 4*VIN−2*VR. 
     Further, when the input voltage VIN is in the range defined by +(⅜)*VR≧VIN≧+(⅛)*VR, DO is +001, DA is +1, VDA 2  is 0, VDA 1  is +VR, and VO is 4*VIN−VR; on the other hand, when the input voltage VIN is in the range defined by +(⅛)*VR≧VIN≧−(⅛)*VR, DO is 000, DA is 0, VDA 2  is 0, VDA 1  is 0, and VO is 4*VIN. 
     Further, when the input voltage VIN is in the range defined by −(⅛)*VR≧VIN≧−(⅜)*VR, DO is −001, DA is −1, VDA 2  is 0, VDA 1  is −VR, and VO is 4*VIN+VR; on the other hand, when the input voltage VIN is in the range defined by −(⅜)*VR≧VIN≧−(⅝)*VR, DO is −010, DA is −2, VDA 2  is −VR, VDA 1  is 0, and VO is 4*VIN+2*VR. 
     Finally, when the input voltage VIN is in the range defined by −(⅝)*VR≧VIN≧−VR, DO is −011, DA is −3, VDA 2  is −VR, VDA 1  is −VR, and VO is 4*VIN+3*VR. 
     It will be recognized that each of the embodiments to be described later is also applicable to MDACs of other configurations such as 3.5-b and 4.5-b, though such applications will not be described herein. 
       FIG. 5  is a simplified circuit model of a switched capacitor circuit (ex. MDAC) for explaining the relationship between its operating speed and the current consumption of an op amp for an MDAC circuit in the hold mode operation, and more specifically, the relationship between the speed of computation in the operation mode of the MDAC 1  and the current consumption of the amplifier. 
     Here, denoting the load as CL T , the feedback ratio as β 1 , and the current of the op amp OP 1  as I AMP , the time T 1  taken to accomplish the conversion is defined by the following relation (see equation (5)). 
                     [     MATHEMATICAL   ⁢           ⁢   1     ]     ⁢                                     CL   T     =       CL   1     +     C   ⁢           ⁢     2   S                 (   1   )                 CL   1     =       C   ⁢           ⁢       1   H     ·   C     ⁢           ⁢     1   MDAC         (       C   ⁢           ⁢     1   H       +     C   ⁢           ⁢     1   MDAC         )               (   2   )                 β   1     =       C   ⁢           ⁢     1   H         (       C   ⁢           ⁢     1   H       +     C   ⁢           ⁢     1   MDAC         )               (   3   )                 1     β   1       =         (       C   ⁢           ⁢     1   H       +     C   ⁢           ⁢     1   MDAC         )       C   ⁢           ⁢     1   H         =     1   +       C   ⁢           ⁢     1   MDAC         C   ⁢           ⁢     1   H                     (   4   )                 T   1     =       k     I   AMP       ·       CL   T       β   1                 (   5   )               
where k is a proportionality constant independent of β 1  and I AMP . That is,
 
     
       
         
           
             
               
                 
                   
                     
                       T 
                       1 
                     
                     · 
                     
                       I 
                       AMP 
                     
                   
                   = 
                   
                     
                       k 
                       · 
                       
                         
                           
                             CL 
                             1 
                           
                           + 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               2 
                               S 
                             
                           
                         
                         
                           β 
                           1 
                         
                       
                     
                     = 
                     
                       
                         
                           TL 
                           1 
                         
                         · 
                         
                           I 
                           AMP 
                         
                       
                       + 
                       
                         T 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
                             2 
                             S 
                           
                           · 
                           
                             I 
                             AMP 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Here, TL 1 *I AMP  and T 2   S *I AMP  are related as illustrated below (see equations (7) and (8)). While  FIG. 3A ,  FIG. 3B ,  FIG. 3C ,  FIG. 3D ,  FIG. 4A ,  FIG. 4B , and  FIG. 4C  have been described by assuming that C 1   MDAC +C 1   H =C 1   s  for ease of explanation, it makes no difference if this assumption is eliminated. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             TL 
                             1 
                           
                           · 
                           
                             I 
                             AMP 
                           
                         
                         = 
                         
                           k 
                           · 
                           
                             
                               CL 
                               1 
                             
                             
                               β 
                               1 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           k 
                           · 
                           
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 
                                   1 
                                   H 
                                 
                                 · 
                                 C 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 1 
                                 MDAC 
                               
                             
                             
                               ( 
                               
                                 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     1 
                                     H 
                                   
                                 
                                 + 
                                 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     1 
                                     MDAC 
                                   
                                 
                               
                               ) 
                             
                           
                           · 
                           
                             
                               ( 
                               
                                 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     1 
                                     H 
                                   
                                 
                                 + 
                                 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     1 
                                     MDAC 
                                   
                                 
                               
                               ) 
                             
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 1 
                                 H 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             k 
                             · 
                             C 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             MDAC 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
             
               
                 
                   
                     T 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         2 
                         S 
                       
                       · 
                       
                         I 
                         AMP 
                       
                     
                   
                   = 
                   
                     
                       k 
                       · 
                       
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             2 
                             S 
                           
                         
                         
                           β 
                           1 
                         
                       
                     
                     = 
                     
                       
                         
                           k 
                           · 
                           C 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
                             2 
                             S 
                           
                           · 
                           
                             
                               
                                 C 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   1 
                                   H 
                                 
                               
                               + 
                               
                                 C 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   1 
                                   MDAC 
                                 
                               
                             
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 1 
                                 H 
                               
                             
                           
                         
                       
                       = 
                       
                         
                           k 
                           · 
                           C 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
                             2 
                             S 
                           
                           · 
                           
                             ( 
                             
                               
                                 C 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   1 
                                   S 
                                 
                               
                               
                                 C 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   1 
                                   H 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     Since the sampling capacitor C 1   s  in the MDAC (MDAC 1 ) is determined by thermal noise (kT/C), a constant value C 0  is used as the reference value in  FIG. 3A ,  FIG. 3B ,  FIG. 3C ,  FIG. 3D ,  FIG. 4A ,  FIG. 4B , and  FIG. 4C . The signal amplification factor (m) is expressed by C 1   s /C 1   H . 
     More specifically, in the case of  FIG. 3A ,  FIG. 3B ,  FIG. 3C , and  FIG. 3D  (1.5bMDAC), m=2, and in the case of  FIG. 4A ,  FIG. 4B , and  FIG. 4C  (2.5bMDAC), m=4. Accordingly, the MDAC in  FIG. 3A ,  FIG. 3B ,  FIG. 3C , and  FIG. 3D  and the MDAC in  FIG. 4A ,  FIG. 4B , and  FIG. 4C  are defined as illustrated below if C 1   H  and C 1   MDAC  are expressed using the signal amplification factor, m (see equations (10) and (11)). 
     
       
         
           
             
               
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       1 
                       S 
                     
                   
                   = 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     0 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
             
               
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       1 
                       H 
                     
                   
                   = 
                   
                     
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           1 
                           S 
                         
                       
                       m 
                     
                     = 
                     
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         0 
                       
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
             
               
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       1 
                       MDAC 
                     
                   
                   = 
                   
                     
                       
                         
                           ( 
                           
                             
                               m 
                               - 
                               1 
                             
                             m 
                           
                           ) 
                         
                         · 
                         C 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         1 
                         S 
                       
                     
                     = 
                     
                       
                         
                           ( 
                           
                             
                               m 
                               - 
                               1 
                             
                             m 
                           
                           ) 
                         
                         · 
                         C 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       0 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     The sampling capacitor C 2 , in the next-stage MDAC (MDAC 2 ) may generally be multiplied by (1/m) relative to the signal amplification factor (m), but the limitation by the minimum capacitance value needs to be considered. That is, the following two points (A) and (B) need to be considered. 
     (A) Primarily in the first half stage: The sampling capacitor C 2   s  in the MDAC 2  is scaled relative to C 1   s  by a factor of (1/m). 
                     C   ⁢           ⁢     2   S       =         C   ⁢           ⁢     1   S       m     =       C   ⁢           ⁢   0     m               (   12   )               
(B) Primarily in the second half stage: NO scaling is applied to C 2   s  relative to C 1   s  (the size is the same) because the former is limited by the minimum capacitance value.
 
                     C   ⁢           ⁢     2   S       =         C   ⁢           ⁢     1   S       m     =       C   ⁢           ⁢   0     m               (   13   )               
Arranging equations (7) and (8) by using equations (9) to (13)
 
     
       
         
           
             
               
                 
                   
                     
                       TL 
                       1 
                     
                     · 
                     
                       I 
                       AMP 
                     
                   
                   = 
                   
                     
                       
                         k 
                         · 
                         C 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         1 
                         MDAC 
                       
                     
                     = 
                     
                       
                         k 
                         · 
                         
                           ( 
                           
                             
                               m 
                               - 
                               1 
                             
                             m 
                           
                           ) 
                         
                         · 
                         C 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       0 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
             
               
                 
                   
                     T 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         2 
                         S 
                       
                       · 
                       
                         I 
                         AMP 
                       
                     
                   
                   = 
                   
                     
                       
                         k 
                         · 
                         C 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           2 
                           S 
                         
                         · 
                         
                           ( 
                           
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 1 
                                 S 
                               
                             
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 1 
                                 H 
                               
                             
                           
                           ) 
                         
                       
                     
                     = 
                     
                       { 
                       
                         
                           
                             
                               
                                 k 
                                 · 
                                 C 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               0 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     2 
                                     s 
                                   
                                 
                                 , 
                                 
                                   with 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   scaling 
                                 
                               
                               ) 
                             
                           
                         
                         
                           
                             
                               
                                 k 
                                 · 
                                 m 
                                 · 
                                 C 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               0 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     2 
                                     s 
                                   
                                 
                                 , 
                                 
                                   without 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   scaling 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
       FIG. 6  is a comparison table of the performance for some kind of MDAC circuits with considering some kind of load conditions for the output of each MDAC; that is, the performance under no load conditions and the performance under load conditions with and without scaling are compared for each of the signal amplification factors m=2 and m=4. 
     Here, the term “with scaling” indicates that the capacitance of the MDAC is reduced, for example, in increments of ½ for each subsequent stage relative to the preceding one in the case of m=2 (1.5bMDAC), and in increments of ¼ for each subsequent stage relative to the preceding one in the case of m=4 (2.5bMDAC). On the other hand, the term “without scaling” indicates that the capacitance of the MDAC remains the same at each stage. 
     As illustrated in  FIG. 6 , under load conditions, the conversion time (see T*I AMP ) increases by a factor of two or more compared with no load conditions; in particular, it is seen that when no scaling is applied to the capacitor C 2   s  in  FIG. 5 , the decrease in conversion speed becomes more pronounced. 
     Further, in the multi-bit case (m=4), it is seen that when no scaling is applied to the capacitor C 2 , the conversion speed (operation speed) decreases by a factor of six or more compared with no load conditions. This indicates that when the supply current to the amplifier (op amp) is held constant, the time taken to accomplish the conversion increases by a factor of two or more compared with no load conditions (the conversion speed decreases by a factor of two or more). 
     In this way, in the MDAC (switched capacitor circuit), when there is a load associated with the sampling capacitor at the subsequent stage, for example, the time taken to accomplish the conversion (computation) increases by a factor of two or more compared with the time taken when there is no such load. Furthermore, if no scaling is applied, the decrease in the conversion speed of the MDAC becomes more pronounced; further, as the number of bits increases, the decrease in speed becomes greater. 
       FIG. 7  is a simplified circuit model of a switched capacitor circuit (ex. MDAC) in the hold operation mode that is used for both analog computation of MDAC and sampling the output voltage of MDAC to a loading capacitance; more specifically, the operation in the earlier described analog computation mode (hold mode) is illustrated in simplified form. 
     As illustrated in  FIG. 7 , in the SC circuit  11  (MDAC 1 ) that performs analog computation, the capacitors C 1   MDAC  and C 1   H  are connected (used). Further, the sampling capacitor C 2   s  in the SC circuit  12  (MDAC 2 ) at the subsequent stage is coupled to the output of the operational amplifier (op amp) OP 1  in order to sample its output voltage VO( 0 ). 
     More specifically, in the SC circuit  11 , the hold capacitor C 1   H  is connected between the output terminal and negative input terminal of the op amp OP 1 , and the computation capacitor C 1   MDAC  is connected between the output terminal of the sub-DA converter ( 101 ) and the negative input terminal of the op amp OP 1 . Then, the sampling capacitor C 2   s  in the SC circuit  12  at the subsequent stage is coupled to the output terminal of the op amp OP 1 . 
     Accordingly, when the SC circuit  11  is performing analog computation, since the sampling capacitor C 2   s  in the SC circuit  12  at the subsequent stage is connected as the load for the op amp OP 1 , the supply current to the amplifier needs to be increased. 
     This not only increases power consumption but also increases the size of the amplifier, thus increasing the die size it occupies and hence leading to an increase in cost. 
     Next, embodiments of a switched capacitor circuit and an AD conversion circuit will be described in detail with reference to the accompanying drawings. 
       FIG. 8A  is a simplified circuit model of a first embodiment switched capacitor circuit (ex. MDAC) in the first hold operation mode that is used for only analog computation of MDAC without loading capacitance, and  FIG. 8B  is a simplified circuit model of a first embodiment switched capacitor circuit (ex. MDAC) in the second hold operation mode that is used for sampling the output voltage of MDAC in the first operation mode to a loading capacitance by using a sampling switch, the amplifier and the capacitance C 1   H . 
     As is apparent by comparing the above-described  FIG. 7  with  FIG. 8A  and  FIG. 8B , in the first embodiment the analog computation (hold operation) is performed by dividing it into two modes, the first-half operation mode and the second-half operation mode. 
     More specifically, in the first-half operation mode depicted in  FIG. 8A , the analog computation is performed in the SC circuit  11  (MDAC 1 ) by disconnecting the sampling capacitor C 2   s  in the subsequent-stage SC circuit  12  (MDAC 2 ). 
     On the other hand, in the second-half operation mode depicted in  FIG. 8B , the feedback coefficient β is set to “1”, i.e., full feedback, by disconnecting the computation capacitor C 1   MDAC  in the SC circuit  11 , and the output voltage is stored on the sampling capacitor C 2 , in the subsequent-stage SC circuit  12 . 
     By thus performing the analog computation in two separate modes, it becomes possible to enhance the speed of computation, reduce the power consumption of the amplifier, or reduce the footprint of the circuit, and so on. 
     While the present specification deals primarily with examples in which the SC circuit is employed as the MDAC, the embodiments described herein are basically intended to enhance the speed of computation of the SC circuit itself and are therefore extensively applicable not only to SC circuits but also to various circuits employing SC circuits. 
       FIG. 9A  and  FIG. 9B  are diagrams for explaining the STAGE circuit of the first embodiment and its operation with the conversion time (T) divided into four periods ( 1 ) to ( 4 ). 
     As is apparent by comparing  FIG. 9A  and  FIG. 9B  with the previously described  FIG. 2A  and  FIG. 2C , the operation of the MDAC according to the first embodiment is characterized in that, in the period ( 1 ), the MDAC 1  and the ADC 1  (sub-AD converter) are not used, and the MDAC 2  performs computation (hold: H). 
     Here, just like the op amp OP 1  in the MDAC 1  in the period ( 3 ) to be described later, the output of the op amp OP 2  in the MDAC 2  is decoupled from the load (C 3   s ) in the subsequent-stage MDAC (MDAC 3 ) and the op amp OP 2  is thus at no load. 
     Next, in the period ( 2 ), the ADC 1  is used and the MDAC 1  performs sampling (S), while the MDAC 2  performs computation (full feedback operation). 
     In the period ( 3 ), the MDAC 2  and the ADC 2  (sub-AD converter) are not used, and the MDAC 1  performs computation. Here, the output of the op amp OP 1  in the MDAC 1  is decoupled from the load (C 2   s  (=C 2   n1 +C 2   n2 )) in the subsequent-stage MDAC 2  and the op amp OP 1  is thus at no load. 
     The operation of the MDAC 1  in the period ( 3 ) corresponds to the operation of the SC 11  (MDAC 1 ) described with reference to  FIG. 8A . Here, the capacitor C 1   n1  acts as the computation capacitor C 1   MDAC  and the capacitor C 1   n2  as the hold capacitor C 1   H . 
     Then, in the period ( 4 ), the MDAC 1  performs computation (full feedback operation), while on the other hand, the ADC 2  is used and the MDAC 2  performs sampling. The operation of the MDAC 1  in the period ( 4 ) corresponds to the operation of the SC 11  (MDAC 1 ) described with reference to  FIG. 83 . 
     In this way, according to the MDAC of the first embodiment, it becomes possible to enhance the speed of computation, reduce the power consumption of the amplifier, or reduce the footprint of the circuit, and so on. 
     Here, the ratio between the periods ( 1 ) and ( 2 ) (or the periods ( 3 ) and ( 4 )) may be varied as needed according to such factors as the operating speed of the circuit and the size of the capacitors used. Further, between the periods ( 1 ) and ( 2 ) (or the periods ( 3 ) and ( 4 )), the supply current to the op amp OP 1  may be set to different values. 
     The control of the ratio between the periods ( 1 ) and ( 2 ) (or the periods ( 3 ) and ( 4 )) and the control of the supply current to the op amp in the periods ( 1 ) and ( 2 ) (or the periods ( 3 ) and ( 4 )) may be performed not only in the first embodiment but also in the second to fifth embodiments to be described later. 
       FIG. 10A  is a circuit diagram illustrating one example of the STAGE circuit of the first embodiment, and  FIG. 10B  is a timing chart for explaining the operation of the STAGE circuit in  FIG. 10A . The MDAC illustrated in  FIG. 10A  and  FIG. 103  is a 1.5bMDAC (MDAC 1 ). 
     The circuit of the MDAC 1  in the periods ( 1 ) to ( 4 ) in  FIG. 102  corresponds to that of the MDAC 1  in ( 1 ) to ( 4 ) depicted in  FIG. 9A . 
     In  FIG. 10A , reference character SWC 1  designates a switch control unit which receives signals MCLK and SHSEL and outputs switch control signals; further,  101  is a sub-DAC, CMP 1  and CMP 2  are comparators, DFF 1  and DFF 2  are flip-flops, and LO 1  is a logic unit. 
     As illustrated in  FIG. 10A , the MDAC 1  (switched capacitor circuit) includes capacitors C 1   n1  and C 1   n2  (two or more internal capacitors), an op amp OP 1  (one or more amplifiers), and switches SWS 1 A/ 1 B/ 2 A/ 2 B, SWH 1 A/ 1 B, SWH 2 A/ 2 B, and SWADCIN (two or more internal switches). 
     As is apparent by comparing  FIG. 10A  and  FIG. 10B  with the previously described  FIG. 3A  and  FIG. 3B , the MDAC 1  is similar between the two, but differs in the way the switch control unit SWC 1  controls the respective switches. 
     First, in the period ( 1 ) of  FIG. 10B , the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 1 B/ 2 B, SWH 1 A/ 1 B, SWH 2 A/ 2 B, and SWADCIN to a low level “L”, causing these switches to turn off. The signal CLKADC is at “L”, so that the flip-flops DFF 1  and DFF 2  are disabled. 
     Next, in the period ( 2 ) of  FIG. 103 , the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 1 B/ 2 A/ 2 B and SWADCIN to a high level “H”, causing these switches to turn on. Here, the control signals for the switches SWH 1 A/ 1 B and SWH 2 A/ 2 B and the signal CLKADC remain at “L”. 
     As a result, in the period ( 2 ), the switches SWS 1 A/ 1 B/ 2 A/ 2 B and SWADCIN turn on, and the MDAC 1  performs sampling (S). That is, the input voltage VIN (the compare voltage V CMP ) is coupled to the comparators CMP 1  and CMP 2  where it is compared with the reference voltages (¼)*VR and −(¼)*VR, respectively, and the comparison results are supplied to the input terminals of the respective flip-flops DFF 1  and DFF 2 . 
     Further, in the period ( 2 ), the input voltage VIN is sampled by the sampling capacitor C 1   s  (C 1   n1 +C 1   n2 ). 
     Next, in the period ( 3 ) of  FIG. 10B , the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 1 B/ 2 A/ 2 B and SWADCIN to “L”, causing these switches to turn off, and sets the control signals for the switches SWH 1 A/ 1 B and SWH 2 A/ 2 B to “H”. The signal CLKADC is also set to “H”. 
     As a result, in the period ( 3 ), the comparison results from the comparators CMP 1  and CMP 2  are latched into the flip-flops DFF 1  and DFF 2  and held therein. Since similar control is performed on the subsequent-stage MDAC 2 , the op amp OP 1  is disconnected from the load (C 2   s ) in the subsequent-stage MDAC 2  and is thus held at no load. 
     More specifically, the hold capacitor C 1   H  (C 1   n2 ) is connected between the output terminal and negative input terminal of the op amp OP 1 , and the computation capacitor C 1   MDAC  (C 1   n1 ) is connected between the output terminal of the sub-DA converter  101  and the negative input terminal of the op amp OP 1 . Then, the op amp OP 1  operates at no load with its output terminal decoupled from the sampling capacitor C 2   s  (C 2   n1 +C 2   n2 ) in the subsequent-stage MDAC 2 . 
     The operation of the MDAC 1  in the period ( 3 ) is the same as that described with reference to  FIG. 8A ,  FIG. 9A , and  FIG. 9B . 
     Next, in the period ( 4 ) of  FIG. 10B , the switch control unit SWC 1  sets the control signal for the switches SWH 1 A/ 1 B from “H” to “L” to turn off the switches SWH 1 A/ 1 B thereby disconnecting the capacitor C 1   n1 . The other switches SWS 1 A/ 1 B/ 2 A/ 2 B, SWADCIN, and SWH 2 A/ 2 B are each held in the same state as in the period ( 3 ). 
     Thus, in the period ( 4 ), the MDAC 1  performs full feedback operation. The operation of the MDAC 1  in the period ( 4 ) is the same as that described with reference to  FIG. 8A ,  FIG. 9A , and  FIG. 9B . 
       FIG. 11  is a comparison table of the performance for the 1.5bMDAC of the first embodiment for comparison with the MDAC depicted in  FIG. 3A , that is, the amount of performance improvement achieved by the 1.5bMDAC, i.e., the MDAC with m=2. 
     As seen from the “T*I AMP ” section in  FIG. 11  that relates to the operating speed or power consumption of the MDAC, the MDAC of the first embodiment is capable of improving the speed or the power consumption by about 33% when scaling is applied and by about 40% when no scaling is applied, as compared with the MDAC of  FIG. 3A . 
     More specifically, when the MDAC operating speed (T) is the same, the power consumption (the op amp current I AMP ) may be reduced, while on the other hand, when the op amp current (I AMP : power consumption) is the same, the MDAC operating speed (T) may be enhanced. 
     Regarding the operating speed and power consumption described above, their magnitude may be designed appropriately as needed by giving priority to one or the other of the two factors. In this case, an alteration may be made, for example, by adjusting the duration of each of the periods ( 1 ) to ( 4 ) (duty ratio). 
     Further, when applying the MDAC to a cyclic AD conversion circuit, the circuit may be designed by adjusting the switch timing (clock period) so as to increase the processing time in the starting or first-half period and reduce the processing time in the last or second-half period. 
       FIG. 12  is a comparison table of the performance for the 2.5bMDAC of the first embodiment for comparison with the MDAC depicted in  FIG. 4A , that is, the amount of performance improvement achieved by the 2.5bMDAC, i.e., the MDAC with m=4. 
     As seen from the “T*I AMP ” section in  FIG. 12 , the 2.5bMDAC as a modified example of the first embodiment is capable of improving the speed or the power consumption by about 43% when scaling is applied and by about 63% when no scaling is applied, as compared with the MDAC of  FIG. 4A . 
     Regarding “T*I AMP ”, by giving priority to the operating speed (T) or the power consumption (I AMP ), whichever is desired, their magnitude may be designed appropriately as needed, as just described with reference to  FIG. 11 ; further, the duration of each of the periods ( 1 ) to ( 4 ) may also be adjusted as needed. 
     Here, the duration of the first-half operation mode (period ( 3 )) during analog computation of the MDAC (MDAC 1 ) and the duration of the second-half operation mode (period ( 4 )) are denoted by TL 1  and TL 2 , respectively, and the feedback ratio β in the first-half operation mode and that in the second-half operation mode are denoted by β 1  and β 2 , respectively. 
     The first-half operation mode (period ( 3 )) during analog computation of the MDAC of the first embodiment is the same as the operation under no load conditions for the case of m=2 described in the previously given  FIG. 6 . 
     On the other hand, in the second-half operation mode (period ( 4 )) during analog computation of the MDAC of the first embodiment, since the capacitor C 1   MDAC  is disconnected, the feedback ratio β becomes equal to unity, and the capacitor CL 1  may be regarded as almost zero. 
     That is, the following equations (16) to (18) hold. 
     That is, 
     
       
         
           
             
               
                 
                   
                     
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     When the value of T 2   s *I AMP  is compared between the equation (15) for the MDAC of  FIG. 3A  and the equation (18) for the MDAC of the first embodiment, it is seen that the MDAC of the first embodiment is faster by a factor of m. 
     The above description has dealt with the case where only the switch timing of each switch is controlled by the switch control unit SWC 1 , but if, for example, the duty ratio between the periods ( 1 ) and ( 2 ) (or the periods ( 3 ) and ( 4 )) or the supply current to the op amp is also controlled, performance close to the ideal may be achieved. 
       FIG. 13A  and  FIG. 13B  are diagrams for explaining a STAGE circuit according to a second embodiment and its operation with the conversion time (T) divided into four periods ( 1 ) to ( 4 ). 
     As is apparent by comparing  FIG. 13A  and  FIG. 13B  with the previously described  FIG. 9A  and  FIG. 9B , the operation of the MDAC according to the second embodiment is characterized in that, in the period ( 1 ), the MDAC 1  is not used but the ADC 1  is used. On the other hand, the MDAC 2  performs computation (hold: H). 
     That is, in the case of the MDAC of the second embodiment, the ADC 1  is used in the period ( 1 ) so that in the period ( 1 ) the add/subtract coefficient DA 1  is supplied to the sub-DAC not depicted (refer, for example, to the sub-DAC  101  in  FIG. 3 ) in the MDAC 1 . On the other hand, in the case of the MDAC of the first embodiment, the ADC 1  supplies the add/subtract coefficient DA 1  to the sub-DAC in the period ( 2 ). 
     Next, in the period ( 2 ), the MDAC 1  performs sampling (S), while the MDAC 2  performs computation (full feedback operation). The ADC 1  continues to perform the same operation as that in the period ( 1 ). 
     In the period ( 3 ), the MDAC 2  is not used but the ADC 2  is used, and the MDAC 1  performs computation. Here, the output of the op amp OP 1  in the MDAC 1  is decoupled from the load (C 2   s  (=C 2   n1 +C 2   n2 )) in the subsequent-stage MDAC 2  and the op amp OP 1  is thus at no load, as in the first embodiment. 
     That is, in the second embodiment, the ADC 2  is used in the period ( 3 ) so that in the period ( 3 ) the add/subtract coefficient DA 2  is supplied to the sub-DAC (not depicted) in the MDAC 2 . 
     Then, in the period ( 4 ), the MDAC 1  performs computation (full feedback operation), while on the other hand, the MDAC 2  performs sampling. The ADC 2  continues to perform the same operation as that in the period ( 3 ). 
     In this way, the MDAC of the second embodiment aims to relax the constraints on the conversion speed of the comparators in the ADC 1  (for example, the comparators CMP 1  and CMP 2  in  FIG. 10A ) by utilizing, for example, the fact that the analog computation result of the MDAC 1  is output in the two periods ( 1 ) and ( 2 ). 
     That is, according to the second embodiment, the comparators CMP 1  and CMP 2  in the ADC 1  need only perform the comparisons over the entire duration of the period ( 2 ) by using the final data obtained from the period ( 1 ); this serves to alleviate the need for higher operating speeds demanded of the comparators CMP 1  and CMP 2 . 
       FIG. 14A  is a circuit diagram illustrating one example of the STAGE circuit of the second embodiment, and  FIG. 14B  is a timing chart for explaining the operation of the STAGE circuit of  FIG. 14A . The STAGE circuit (MDAC) illustrated in  FIG. 14A  and  FIG. 14B  is a 1.5bMDAC (MDAC 1 ). 
     The circuit of the MDAC 1  in the periods ( 1 ) to ( 4 ) in  FIG. 14B  corresponds to that of the MDAC 1  in ( 1 ) to ( 4 ) in the above-described  FIG. 13A . 
     As is apparent from a comparison between  FIG. 10A  and the previously described  FIG. 10A , the MDAC (MDAC 1 ) of the second embodiment differs from the MDAC 1  of the first embodiment by the inclusion of a capacitor C s (CMP) which is provided between the switch SWASCIN and the ADC 1  and which acts as a sampling capacitor C s  in the period ( 1 ). 
     As illustrated in  FIG. 14A , the MDAC 1  (switched capacitor circuit) includes capacitors C n1 , C n2 , and C s (CMP) (two or more internal capacitors), the op amp OP 1  (one or more amplifiers), and switches SWS 1 A/ 1 B/ 2 A/ 2 B, SWH 1 A/ 1 B, SWH 2 A/ 2 B, and SWADCIN (two or more internal switches). 
     In the period ( 1 ) of  FIG. 14B , the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 1 B/ 2 A/ 2 B, SWH 1 A/ 1 B, and SWH 2 A/ 2 B to a low level “L”, causing these switches to turn off, and sets the control signal for the switch SWADCIN to a high level “H”, causing the switch SWADCIN to turn on. The signal CLKADC is at “L”, so that the flip-flops DFF 1  and DFF 2  are disabled. 
     That is, the difference from the embodiment earlier described with reference to  FIG. 10A  and  FIG. 105  is that, in the period ( 1 ), the switch SWADCIN is turned on so that the compare voltage V CMP  (the input voltage VIN) is sampled onto the sampling capacitor C s (CMP). 
     Next, in the period ( 2 ) of  FIG. 14B , the switch control unit SWC 1  sets the control signal for the switches SWS 1 A/ 1 B/ 2 A/ 2 B to “H”, causing these switches to turn on, and sets the control signal for the switch SWADCIN to “L”, causing the switch SWADCIN to turn off. Here, the control signals for the switches SWH 1 A/ 1 B and SWH 2 A/ 2 B and the signal CLKADC remain at “L”. 
     That is, upon entering the period ( 2 ), the compare voltage V CMP  sampled by the sampling capacitor C s (CMP) in the period ( 1 ) is coupled to the comparators CMP 1  and CMP 2  in the ADC 1  where it is compared with the reference voltages (¼)*VR and −(¼)*VR, respectively, and the comparison results are supplied to the input terminals of the respective flip-flops DFF 1  and DFF 2 . 
     Next, in the period ( 3 ) of  FIG. 14B , the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 1 B/ 2 A/ 2 B and SWADCIN to “L”, causing these switches to turn off, and sets the control signals for the switches SWH 1 A/ 1 B and SWH 2 A/ 2 B to “H”, causing these switches to turn on. The signal CLKADC is also set to “H”. 
     As a result, in the period ( 3 ), the comparison results from the comparators CMP 1  and CMP 2  are latched into the flip-flops DFF 1  and DFF 2  and held therein. Here, the op amp OP 1  is disconnected from the load (C 2 ) in the subsequent-stage MDAC 2 , but its output voltage VO 1  is sampled onto the sampling capacitor C S (CMP) in the ADC 2  at the subsequent stage. 
     Next, in the period ( 4 ) of  FIG. 14B , the switch control unit SWC 1  sets the control signal for the switches SWH 1 A/ 1 B from “H” to “L” to turn off the switches SWH 1 A/ 1 B thereby disconnecting the capacitor C 1   n1 . 
     The other switches SWS 1 A/ 1 B/ 2 A/ 2 B, SWADCIN, and SWH 2 A/ 2 B are each held in the same state as in the period ( 3 ). Thus, in the period ( 4 ), the MDAC 1  performs full feedback operation. 
       FIG. 15A  and  FIG. 152  are diagrams for explaining a STAGE circuit according to a third embodiment and its operation with the conversion time (T) divided into four periods ( 1 ) to ( 4 ). Further,  FIG. 16A  is a circuit diagram illustrating one example of the STAGE circuit of the third embodiment, and  FIG. 162  is a timing chart for explaining the operation of the STAGE circuit of  FIG. 16A . 
     As is apparent by comparing  FIG. 15A ,  FIG. 15B ,  FIG. 16A , and  FIG. 162  with the previously described  FIG. 9A ,  FIG. 92 ,  FIG. 10A , and  FIG. 10B , the third embodiment differs from the first embodiment in that the two comparators CMP 1  and CMP 2  in the first embodiment are replaced by one common comparator CMP 0 . 
     That is, in the third embodiment, as is apparent from a comparison between  FIG. 16A  and the previously described  FIG. 10A , the ADC 1  is provided with two switches SELADC 1  and SELADC 2 , and the one comparator CMP 0  is made to perform the same functions as the comparators CMP 1  and CMP 2  of the first embodiment in the periods ( 1 ) and ( 2 ), respectively. 
     Further, the common signal CLKADC supplied to the clock terminals of the flip-flops DFF 1  and DFF 2  in the first embodiment is replaced by two separate signals CLKADC 1  and CLKADC 2  so that the activation of each of the flip-flops DFF 1  and DFF 2  is controlled independently of each other. 
     As illustrated in  FIG. 15A  and  FIG. 15B , the MDAC according to the third embodiment is characterized in that, in the period ( 1 ), the MDAC 1  is not used but the ADC 1  is used, while on the other hand, the MDAC 2  performs computation (H). The purpose of using the ADC 1  in the period ( 1 ) is, for example, to compare the input voltage VIN (the compare voltage C CMP ) with the reference voltage (¼)*VR and output the result to the flip-flop DFF 1 . 
     Next, in the period ( 2 ), the ADC 1  is used and the MDAC 1  performs sampling (S), while the MDAC 2  performs computation. The purpose of using the ADC 1  in the period ( 2 ) is, for example, to compare the compare voltage C CMP  with the reference voltage −(¼)*VR and output the result to the flip-flop DFF 2 . 
     That is, in the third embodiment, in the period ( 1 ) the ADC 1  is used to compare the compare voltage C CMP  with the reference voltage (¼)*VR, and in the period ( 2 ) the ADC 1  is used to compare the compare voltage C CMP  with the reference voltage −(¼)*VR. In this way, in the periods ( 1 ) and ( 2 ), the same comparator is used to compare the compare voltage with the respective reference voltages. 
     In the period ( 3 ), the MDAC 2  is not used but the ADC 2  is used, while on the other hand, the MDAC 1  performs computation. Then, in the period ( 4 ), the ADC 2  is used, the MDAC 1  performs computation, and the MDAC 2  performs sampling. 
     The purpose of using the ADC 2  in the periods ( 3 ) and ( 4 ) is to compare the compare voltage at the subsequent stage with difference reference voltages, and the same comparator is used for this purpose. 
     As illustrated in  FIG. 16A , in the third embodiment, the compare voltage C CMP  is applied to one input of the comparator CMP 0  in the ADC 1 , and the reference voltage (¼)*VR or −(¼)*VR, whichever is selected, is applied to the other input via the switch SELADC 1  or SELADC 2 , respectively. Here, the switches SELADC 1  and SELADC 2  are controlled by signals from the switch control unit SWC 1 . 
     As illustrated in  FIG. 16A , the MDAC 1  (switched capacitor circuit) includes capacitors C 1   n1  and C 1   n2  (two or more internal capacitors), an op amp OP 1  (one or more amplifiers), and switches SWS 1 A/ 1 B/ 2 A/ 2 B, SWH 1 A/ 1 B, SWH 2 A/ 2 B, and SWADCIN (two or more internal switches). 
     In the period ( 1 ) of  FIG. 16B , the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 1 B/ 2 A/ 2 B, SWH 1 A/ 1 B, SWH 2 A/ 2 B, and SELADC 2  to “L”, causing these switches to turn off, and sets the control signals for the switches SWADCIN and SELADC 1  to “H”, causing these switches to turn on. 
     As a result, the compare voltage C CMP  (the input voltage VIN) and the reference voltage (¼)*VR selected via the switch SELADC 1  are applied to the comparator CMP 0  which then compare these voltages and outputs the result of the comparison. Here, the signals CLKADC 1  and CLKADC 2  are both “L”, so that the flip-flops DFF 1  and DFF 2  are disabled. 
     Next, in the period ( 2 ) of  FIG. 16B , the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 1 B/ 2 A/ 2 B and SELADC 2  to “H”, causing these switches to turn on, and sets the control signal for the switch SELADC 1  to “L”, causing the switch SELADC 1  to turn off. The other switches are each held in the same state as in the period ( 1 ). 
     In the period ( 2 ), the signal CLKADC 1  changes from “L” to “H”, so that the flip-flop DFF 1  is enabled to latch and hold the result of the comparison made between the compare voltage C CMP  and the reference voltage (¼)*VR by the comparator CMP 0 . 
     Further, in the period ( 2 ), since the switch SELADC 1  turns off and the switch SELADC 2  turns on, as described above, the comparator CMP 0  compares the compare voltage C CMP  with the reference voltage −(¼)*VR applied via the switch SELADC 2 . 
     Next, in the period ( 3 ) of  FIG. 16B , the switch control unit SWC 1  sets the control signals for the switches SWS 1 A/ 1 B/ 2 A/ 2 B, SWADCIN, and SELADC 2  to “L”, causing these switches to turn off, and sets the control signals for the switches SWH 1 A/ 1 B and SWH 2 A/ 2 B to “H”, causing these switches to turn on. 
     In the period ( 3 ), the signal CLKADC 2  changes from “L” to “H”, so that the flip-flop DFF 2  is enabled to latch and hold the result of the comparison made between the compare voltage C CMP  and the reference voltage −(¼)*VR by the comparator CMP 0 . 
     As a result, the result of the comparison between the compare voltage C CMP  and the reference voltage (¼)*VR, held in the flip-flop DFF 1 , and the result of the comparison between the compare voltage C CMP  and the reference voltage −(¼)*VR, held in the flip-flop DFF 2 , are supplied to the logic unit LO 1  which performs a prescribed logic operation between them. 
     Then, in the period ( 4 ) of  FIG. 16B , the switch control unit SWC 1  sets the control signal for the switches SWH 1 A/ 1 B from “H” to “L” to turn off the switches SWH 1 A/ 1 B. In other respects, the operation in each of the periods ( 1 ) to ( 4 ) is apparent from the description given in the first and second embodiments, and therefore will not be further described herein. 
     Thus, according to the third embodiment, it becomes possible to reduce the amount of hardware by making provisions to share the same comparator for the operation of the ADC 1 . 
       FIG. 17  is a diagram depicting the number of comparators of the sub-ADC needed in the stage of the third embodiment for comparison with the number of comparators needed in each of the MDACs depicted in  FIG. 3A  and  FIG. 4A . 
     As illustrated in  FIG. 17 , in the case of the signal amplification factor m=2 (1.5bMDAC), the comparators CMP 1  and CMP 2  in the ADC (ADC 1 ) may be replaced by one common comparator CMP 0 , thus halving the number of comparators. 
     Further, in the case of the signal amplification factor m=4 (2.5bMDAC), the number of comparators CMP 11  to CMP 16  (six comparators) in  FIG. 14 , for example, may be reduced by one half, i.e., to three, when the third embodiment is applied. 
     The examples given in  FIG. 17  are only illustrative and not restrictive, and it is to be understood that the effect of reducing the number of comparators becomes greater as the signal amplification factor, m, (the number of bits) becomes larger. 
       FIG. 18A  and  FIG. 18B  are diagrams for explaining another example of the MDAC and its operation; in the illustrated MDAC configuration, provisions are made to share one common op amp in order to reduce the footprint and power consumption of a pipelined AD conversion circuit. 
     In the drawings hereinafter described, the earlier described sub-AD converts ADC 1  and ADC 2  are not depicted since they are not directly relevant to the fourth and fifth embodiments. 
     As is apparent by comparing  FIG. 18A  and  FIG. 18B  with  FIG. 2A  and  FIG. 2B , the MDAC configuration depicted in  FIG. 18A  and  FIG. 18B  is characterized in that the op amps OP 1  and OP 2  in the MDACs depicted in  FIG. 2A  and  FIG. 2B  are substituted by one common op amp (OP 1 ). 
     That is, in the MDAC 1 , the op amp is not needed in the sampling mode (the period ( 1 )+( 2 ): S) that samples the input voltage VIN, but is needed only in the hold mode (the period ( 3 )+( 4 ): H) that performs computation. 
     On the other hand, the MDAC 2  which operates 180 degrees out of phase with the MDAC 1 , the op amp is not needed in the sampling mode (the period ( 3 )+( 4 ): S) that samples the input voltage (the output voltage VO 1 =VIN 2  of the MDAC 1  at the preceding stage). The op amp is used only in the hold mode (the period ( 1 )+( 2 ): H) that performs computation. 
     In view of the fact that the period in which the op amp is needed differs between the MDAC 1  and the MDAC 2 , the op amp (OP 1 ) is used as the op amp  2  for the MDAC 2  during the period ( 1 )+( 2 ), and the op amp OP 1  is used as the op amp  1  for the MDAC 1  during the period ( 3 )+( 4 ). In like manner, the comparators in the sub-AD converts ADC 1  and ADC 2  not depicted may also be substituted by one common comparator. 
     However, the capacitors in the respective MDACs (the computation capacitor C 1   n1 , C 1   n2  in the MDAC 1  and the computation capacitor C 2   n1 , C 2   n2  in the MDAC 2 ) need to be provided separately, and it is therefore not possible to share the same capacitor between them. 
       FIG. 19A  and  FIG. 19B  are diagrams for explaining the MDAC of the earlier described first embodiment and its operation. 
     As illustrated in  FIG. 19A  and  FIG. 19B , it is possible to share one op amp (OP 1 ) between adjacent MDACs (for example, between the MDAC 1  and the MDAC 2 ) even in the case of the MDAC of the first embodiment such as depicted, for example, in  FIG. 9A . 
     Further, as earlier described with reference to  FIG. 15A ,  FIG. 15B ,  FIG. 16A ,  FIG. 16B , and  FIG. 17 , in the MDAC of the third embodiment, it has been possible to reduce the number of comparators by taking advantage of the fact that the computation result of the MDAC is output twice (H( 1 ) and H( 2 )). 
     Here, as illustrated in  FIG. 19A  and  FIG. 193 , C 1   n1  is not used in the periods ( 1 ) and ( 4 ), C 1   n2  is not used in the period ( 1 ), C 2   n1  is not used in the periods ( 2 ) and ( 3 ), and C 2   n2  is not used in the period ( 3 ). 
     In the MDACs of the fourth and fifth embodiments to be described later, the capacitors are also shared by utilizing the periods in which the respective capacitors in the MDACs (C 1   n1  and C 1   n2  in the MDAC 1  and C 2   n1  and C 2   n2  in the MDAC 2 ) are not used. 
     For the MDAC circuit configuration, more specifically, in the case of the 1.5bMDAC, for example, two types of circuit configuration are possible, that is, the first configuration example (type I) and the second configuration example (type II). 
       FIG. 20A  is a circuit diagram illustrating the first configuration example (type I) of the 1.5bMDAC in sampling mode and in hold mode, respectively, and  FIG. 20B ,  FIG. 200 , and  FIG. 20D  are diagrams for explaining the operation of the MDAC of  FIG. 20A . 
     On the other hand,  FIG. 21A  is a circuit diagram illustrating the second configuration example (type II) of the 1.5bMDAC in sampling mode and in hold mode, respectively, and  FIG. 21B ,  FIG. 21C , and  FIG. 21D  are diagrams for explaining the operation of the MDAC of  FIG. 21A . 
     Here,  FIG. 20A ,  FIG. 203 ,  FIG. 20C , and  FIG. 20D  correspond to the previously described  FIG. 3A ,  FIG. 3B ,  FIG. 3C , and  FIG. 3D , except that the signals associated with the ADC 1  (sub-AD converter) in the MDAC 1  in  FIG. 3A ,  FIG. 3B ,  FIG. 3C , and  FIG. 3D  are omitted. 
     The MDACs of the first to third embodiments illustrated in  FIG. 8A ,  FIG. 8B ,  FIG. 9A ,  FIG. 9B ,  FIG. 10A ,  FIG. 10B ,  FIG. 12 ,  FIG. 13A ,  FIG. 13B ,  FIG. 14A ,  FIG. 143 ,  FIG. 15A ,  FIG. 153 ,  FIG. 16A ,  FIG. 16B , and  FIG. 17  are each based on the type I circuit, but it is also possible to apply the MDACs of the first to third embodiments to the type II circuit. 
     First, as illustrated in  FIG. 20A ,  FIG. 203 ,  FIG. 20C , and  FIG. 20D , that is, as previously described with reference to  FIG. 3A ,  FIG. 3B ,  FIG. 3C , and  FIG. 3D , the relations C 1   s =C 1   n1 +C 1   n2 =C 0 , C 1   H =C 0 /2, and C 1   MDAC =C 0 /2 hold in the 1.5bMDAC 1  of the type I illustrated in  FIG. 20A . Here, the feedback ratio β is β=C 1   H /(C 1   H +C 1   MDAC )=½, and the signal amplification factor, m, is m=C 1   s /C 1   H =2. 
     On the other hand, the relations C 1   s =C 1   s11 +C 1   s12 =C 0 , C 1   H =C 0 /2, and C 1   MDAC =C 1   s =C 0  hold in the 1.5bMDAC 1  of the type II illustrated in  FIG. 21A . Here, the feedback ratio β is β=C 1   H /C 1   H +C 1   MDAC )=⅓, and the signal amplification factor, m, is m=C 1   s /C 1   H =2. 
     More specifically, as illustrated in the left half of  FIG. 21A  and in the periods ( 1 ) and ( 2 ) (( 1 )+( 2 )) of  FIG. 21B , in the sampling (S) mode of the MDAC 1  the switch control unit SWC 1  sets the control signal for the switches SWS 1 A/ 11 B/ 2 A/ 12 B to a high level “H”, causing these switches to turn on. 
     Further, in the sampling mode of the MDAC 1  in the period ( 1 )+( 2 ), the switch control unit SWC 1  sets the control signal for the switches SWH 1 A/ 11 B/ 12 B/ 2 B to a low level “L”. This causes the switches SWH 1 A/ 11 B/ 12 B/ 2 B to turn off. 
     Here, in the sampling capacitor C 1   s  on which the MDAC 1  samples the input signal VIN, the capacitors C 1   s11  and C 1   s12  are connected in parallel with each other with the switches SWH 1 A/ 11 B/ 12 B turning on; as a result, the sampling capacitor C 1   s  is C 1   s =C 1   s11 +C 1   s12 . Here, if C 1   s11 =C 1   s12 =C 0 /2, then C 1   s =C 1   s11 +C 1   s12 =C 0 , as earlier described. 
     Next, as illustrated in the right half of  FIG. 21A  and in the periods ( 3 ) and ( 4 ) (( 3 )+( 4 )) of  FIG. 21B , in the hold (H: computation) mode of the MDAC 1  the switch control unit SWC 1  sets the control signal for the switches SWS 1 A/ 11 B/ 12 B/ 2 B to “L”, causing these switches to turn off. 
     Further, in the hold mode in the period ( 3 )+( 4 ), the switch control unit SWC 1  sets the control signal for the switches SWH 1 A/ 11 B/ 12 B/ 2 B to “H”. This causes the switches SWH 1 A/113/123/23 to turn on. 
     As a result, as earlier described, the hold capacitor C 1   H  and the computation capacitor C 1   MDAC  are C 1   H =C 0 /2 and C 1   MDAC =C 1   s =C 0 , respectively, the feedback ratio β is β=C 1   H /(C 1   H +C 1   MDAC )=⅓, and the signal amplification factor, m, is m=C 1   S /C 1   H =2. 
     When the signal amplification is m=2, the relation depicted in  FIG. 21C  holds between VIN/VR and VO/VR; on the other hand, the input voltage VIN (the compare voltage V CMP ), the digital output DO, the add/subtract coefficient DA 1 , the output voltage VDA 1  of the sub-DAC  101 , and the output voltage VO of the op amp OP 1  are as depicted in  FIG. 21D . Here,  FIG. 21C  and  FIG. 21D  are the same as the earlier given  FIG. 20C  and  FIG. 20D , respectively. 
       FIG. 22A  and  FIG. 22B  are diagrams for explaining the basic operation of the MDAC of the second configuration example (type II); the above-described  FIG. 21A  and  FIG. 21B  are redrawn here in an easier to understand manner. In the MDAC configuration depicted in  FIG. 22A , one common op amp (OP 1 ) is shared between two MDACs (MDAC 1  and MDAC 2 ). 
     To simplify the explanation, C 1   s =C 1   MDAC =C 0 , C 1   H =C 0 /m, C 2   s =C 2   MDAC =C 0 /m, and C 2   H =C 0 /m. Here, m represents the signal amplification factor. 
     First, as illustrated in the top half of  FIG. 22A  and in the period ( 1 )+( 2 ) of  FIG. 22B , when the MDAC 1  is in the sampling (S) mode and the MDAC 2  in the computation (H) mode, the op amp (OP 1 ) in the MDAC 1  is not used, but the op amp (OP 2 ) in the MDAC 2  is used (operating). 
     Further, in the period ( 1 )+( 2 ), the capacitor C 1   s  in the MDAC 1  and the capacitors C 2   MDAC  and C 2   H  in the MDAC 2  are used, while the capacitor C 1   H  in the MDAC 1  is reset. 
     On the other hand, as illustrated in the bottom half of  FIG. 22A  and in the period ( 3 )+( 4 ) of  FIG. 22B , when the MDAC 1  is in the computation mode and the MDAC 2  in the sampling mode, the op amp (OP 1 ) in the MDAC 1  is used, but the op amp (OP 2 ) in the MDAC 2  is not used. 
     Further, in the period ( 3 )+( 4 ), the capacitors C 1   MDAC  and C 1   H  in the MDAC 1  and the capacitor C 2 , in the MDAC 2  are used, while the capacitor C 2   H  in the MDAC 2  is reset. 
     In view of the above, the op amp (OP 1 : common op amp) is shared between the two MDACs (MDAC 1  and MDAC 2 ). However, in the MDAC configuration of  FIG. 22A  and  FIG. 22B , it is not possible to share the capacitors between the MDAC 1  and the MDAC 2 . 
     In the fourth and fifth embodiments hereinafter described, the same capacitor is shared between the MDAC 1  and the MDAC 2  in order to further reduce the footprint of the switched capacitor circuit or AD conversion circuit. 
       FIG. 23A  and  FIG. 238  are diagrams for explaining the MDAC of the fourth embodiment and its operation. The MDAC of the fourth embodiment is a MDAC of the second configuration example (type II), and is controlled by dividing the conversion time T into four periods ( 1 ) to ( 4 ), as in the case of the MDACs of the earlier described first to third embodiments. 
     First, as is apparent from a comparison between  FIG. 23A  and the above-described  FIG. 22A , in the MDAC configuration according to the fourth embodiment, not only the op amp (OP 1 : common op amp) but also the capacitor (CSC: common capacitor) is shared between the two MDACs (MDAC 1  and MDAC 2 ). 
     That is, as illustrated in  FIG. 23A  and  FIG. 23B , the capacitor CSC is used as C 2   MDAC  in the period ( 1 ), as C 1   s  in the period ( 2 ), as C 1   MDAC  in the period ( 3 ), and as C 2   s  in the period ( 4 ). 
     As is apparent from  FIG. 23A  and  FIG. 238 , the hold capacitors C 1   H  and C 2   H  need to be provided exclusively for the MDAC 1  and the MDAC 2 , respectively, but the sampling capacitors C 1   s  and C 2   s  and computation capacitors C 1   MDAC  and C 2   MDAC  in the MDAC 1  and MDAC 2  may be replaced by one common capacitor. 
     In the MDAC of the fourth embodiment, the op amp (common op amp OP 1 ) is shared between the MDAC 1  and the MDAC 2  in the same manner as earlier described with reference to  FIG. 22A  and  FIG. 223 . 
     In the case of a pipelined AD conversion circuit constructed by cascading a plurality of MDACs, the value of the sampling capacitor in the MDAC at the subsequent stage (for example, the capacitor C 2   s  in the MDAC 2 ) may be made smaller than the value of the sampling capacitor in the MDAC at its preceding stage (for example, the capacitor C 1   s  in the MDAC 1 ). That is, the sampling capacitor (C 2   s ) in the MDAC at the subsequent stage may be formed using a portion of the sampling capacitor (C 1   s ) in the MDAC at its preceding stage. 
       FIG. 24A  is a circuit diagram illustrating one example of the MDAC of the fourth embodiment, and  FIG. 24B  is a diagram for explaining the operation of the MDAC of  FIG. 24A . 
     In  FIG. 24A , reference characters CSC 11  and CSC 12  designate the common capacitor (CSC) shared for use, while CH 1  designates the hold capacitor (C 1   H ) when used for the first MDAC (MDAC 1 ) and CH 2  designates the hold capacitor (C 2   H ) when used for the second MDAC (MDAC 2 ). 
     In the description of the MDAC of the fourth embodiment, CH 1 =CH 2 =C 0 /2 and CSC 1 =CSC 11 +CSC 12 =C 0 , assuming the case of no scaling. 
     As illustrated in  FIG. 24A , the MDAC 0  (switched capacitor circuit) includes the capacitors CH 1 , CH 2 , CSC  11 , and CSC 12  (two or more internal capacitors) and the op amp OP 0  (one or more amplifiers). The MDAC 0  further includes switches SWVIN 1 , SWVIN 2 , SWSC 1 A/ 11 B/ 12 B, SWHC 1 A/ 11 B/ 12 B, SWSH 1 A/ 1 B, SWHH 1 A/ 1 B, SWSH 2 A/ 2 B, and SWHH 2 A/ 2 B (two or more internal switches). 
     First, in the period ( 1 ) of  FIG. 24B , the switch control unit SWC 1  sets the control signals for the switches SWVIN 1 , SWHC 1 A/ 11 B/ 12 B, and SWHH 2 A/ 2 B to a high level “H”, causing these switches to turn on. 
     Further, in the period ( 1 ), the switch control unit SWC 1  sets the control signals for the switches SWVIN 2 , SWSC 1 A/ 11 B/ 12 B, SWSH 1 A/ 1 B, SWHH 1 A/ 1 B, and SWSH 2 A/ 2 B to a low level “L”, causing these switches to turn off. 
     As a result, in the MDAC 0 , the capacitor CSC 11  acts as the computation capacitor C 2   MDAC  for the second MDAC (MDAC 2 ), as illustrated in the period ( 1 ) of  FIG. 23A . On the other hand, the capacitor CH 2  is connected so as to act as the hold capacitor C 2   H  for the second MDAC (MDAC 2 ). 
     Next, in the period ( 2 ) of  FIG. 24B , the switch control unit SWC 1  sets the control signals for the switches SWSC 1 A/ 11 B/ 12 B and SWSH 1 A/ 1 B to “H”, causing these switches to turn on, and sets the control signal for the switches SWHC 1 A/ 11 B/ 12 B to “L”, causing these switches to turn off. 
     In the period ( 2 ), the other switches SWVIN 1 , SWVIN 2 , SWHH 1 A/ 1 B, SWSH 2 A/ 2 B, and SWHH 2 A/ 2 B are each held at the same level as in the period ( 1 ). 
     As a result, the capacitors CSC 11  and CSC 12  act as the sampling capacitor C 1   s  for the first MDAC (MDAC 1 ), as illustrated in the period ( 2 ) of  FIG. 23A . The capacitor CH 2  remains connected so as to act as the hold capacitor C 2   H  for the second MDAC (MDAC 2 ), while the capacitor CH 1  is reset. 
     Next, in the period ( 3 ) of  FIG. 24B , the switch control unit SWC 1  sets the control signals for the switches SWVIN 2 , SWHC 1 A/ 11 B/ 12 B, and SWHH 1 A/ 1 B to “H”, causing these switches to turn on. Further, the switch control unit SWC 1  sets the control signals for the switches SWVIN 1 , SWSC 1 A/ 11 B/ 12 B, SWSH 1 A/ 1 B, and SWHH 2 A/ 2 B to “L”, causing these switches to turn off. In the period ( 3 ), the control signal for the SWSH 2 A/ 2 B is held at the same level as in the period ( 2 ). 
     As a result, in the MDAC 0 , the capacitor CSC 11  acts as the computation capacitor C 1   MDAC  for the first MDAC (MDAC 1 ), as illustrated in the period ( 3 ) of  FIG. 23A . On the other hand, the capacitor CH 1  is connected so as to act as the hold capacitor C 1   H  for the first MDAC (MDAC 1 ). 
     Then, in the period ( 4 ) of  FIG. 24B , the switch control unit SWC 1  sets the control signals for the switches SWSC 1 A/ 11 B/ 12 B and SWSH 2 A/ 2 B to “H”, causing these switches to turn on, and sets the control signal for the switches SWHC 1 A/ 11 B/ 12 B to “L”, causing these switches to turn off. 
     In the period ( 4 ), the other switches SWVIN 1 , SWVIN 2 , SWSH 1 A/ 1 B, SWHH 1 A/ 1 B, and SWHH 2 A/ 2 B are each held at the same level as in the period ( 3 ). 
     As a result, in the MDAC 0 , the capacitors CSC 11  and CSC 12  act as the sampling capacitor C 2 , for the second MDAC (MDAC 2 ), as illustrated in the period ( 4 ) of  FIG. 23A . The capacitor CH 1  remains connected so as to act as the hold capacitor C 1   H  for the first MDAC (MDAC 1 ), while the capacitor CH 2  is reset. 
     In the MDAC of the fourth embodiment, the op amp OP 1  (common op amp) may be shared between the MDAC 1  and the MDAC 2  in the same manner as earlier described with reference to  FIG. 22A  and  FIG. 22B . 
     In this way, according to the MDAC configuration of the fourth embodiment, not only the op amp but the capacitor (CSC: CSC 11  and CSC 12 ) may also be shared between the two MDACs, thus making is possible to further reduce the footprint of the switched capacitor circuit or AD conversion circuit. 
     In the above-described fourth embodiment, scaling may be applied, and the sampling capacitor C 2   s  in the MDAC 2  at the subsequent stage, for example, may be made smaller in size than the sampling capacitor C 1   s  in the MDAC 1  at its preceding stage. 
     More specifically, when the scaling factor is denoted by γ, generally γ=½ in the case of a 1.5bMDAC and γ=¼ in the case of a 2.5bMDAC; accordingly, the sampling capacitor C 2   s  in the MDAC 2  at the subsequent stage may be formed using a portion of the sampling capacitor C 1   s  in the MDAC 1  at its preceding stage. 
     Next, before describing the MDAC configuration according to the fifth embodiment, two configuration examples of a parallel MDAC (double-sampling MDAC) system will be described below with reference to  FIG. 25 ,  FIG. 26A , and  FIG. 263 . 
     The double-sampling AD conversion circuit includes two paralleled MDACs and operates them in interleaved fashion thereby aiming to double the conversion speed of the AD conversion circuit without increasing the power consumption. 
       FIG. 25  is a diagram for explaining the basic operation of the MDAC of the first configuration example (type I) as applied in the parallel MDAC system. The double-sampling MDAC (parallel MDAC) system depicted here corresponds, for example, to one that performs processing by paralleling two MDACs (MDAC 1  and MDAC 2 ) which perform processing in time sequential fashion as described above. 
     Further, since channel  1  and channel  2  operate 180 degrees out of phase of each other, the double-sampling MDAC system depicted in  FIG. 25  may be implemented, for example, by reconfiguring the MDAC 1  and MDAC 2  in the earlier described  FIG. 18A  to operate as MDAC 1 (E: Even mode) and MDAC 2 (O: Odd mode), respectively. 
     More specifically, as illustrated in  FIG. 25 , in the period ( 1 )+( 2 ), the MDAC 1 (E) samples the input voltage VIN 1 (E) by using the sampling capacitor C 1   n1 (E)+C 1   n2 (E), while on the other hand, the MDAC 1 (O) performs computation. 
     Here, the computation capacitor in the MDAC 1 (O) is C 1   n1 (O), and the hold capacitor is C 1   n2 (O). In the period ( 1 )+( 2 ), only the op amp OP 1 (O) in the MDAC 1 (O) that performs computation is used, and the op amp OP 1 (E) in the MDAC 1 (E) that performs the sampling is not used. 
     Next, in the period ( 3 )+( 4 ), the MDAC 1 (E) performs computation, and the MDAC 1 (O) samples the input voltage VIN 1 (O) by using the sampling capacitor C 1   n1 (O)+C 1   n2 (O). 
     Here, the computation capacitor in the MDAC 1 (E) is C 1   n1 (E), and the hold capacitor is C 1   n2 (E). In the period ( 3 )+( 4 ), only the op amp OP 1 (E) in the MDAC 1 (E) that performs computation is used, and the op amp OP 1 (O) in the MDAC 1 (O) that performs the sampling is not used. 
     In view of the above, the op amp (OP 1 (E)) is configured to act at the op amp OP 1 (O) in the period ( 1 )+( 2 ) and as the op amp OP 1 (E) in the period ( 3 )+( 4 ). However, while the op amp may thus be shared between the MDAC 1 (E) and the MDAC 1 (O), it is not possible to share the capacitors in the parallel MDAC configuration of type I depicted in  FIG. 25 . 
       FIG. 26A  and  FIG. 26B  are diagrams for explaining the basic operation of the MDAC of the second configuration example (type II) as applied in the parallel MDAC system. Here, the double-sampling MDAC (parallel MDAC) system depicted in  FIG. 26A  and  FIG. 26B  may be implemented, for example, by reconfiguring the MDAC 1  and MDAC 2  in the earlier described  FIG. 22A  and  FIG. 22B  to operate as MDAC 1 (E) and MDAC 2 (O), respectively. 
     That is, as illustrated in the left half of  FIG. 26A  and in the period ( 1 )+( 2 ) of  FIG. 26B , when the MDAC 1 (E) is in the sampling (S) mode and the MDAC 1 (O) in the computation (H) mode, the op amp (OP 1 (E)) in the MDAC 1 (E) is not used, but the op amp (OP 1 ( 0 )) in the MDAC 1 (O) is used (operating). 
     Further, in the period ( 1 )+( 2 ), the capacitor C 1   s (E) in the MDAC 1 (E) and the capacitors C 1   MDAC (O) and C 1   H (O) in the MDAC 1 (O) are used, while the capacitor C 1   H (E) in the MDAC 1 (E) is reset. 
     On the other hand, as illustrated in the right half of  FIG. 26A  and in the period ( 3 )+( 4 ) of  FIG. 26B , when the MDAC 1 (E) is in the computation mode and the MDAC 1 (O) in the sampling mode, the op amp (OP 1 (E)) in the MDAC 1 (E) is used, but the op amp (OP 1 ( 0 )) in the MDAC 1 (O) is not used. 
     Further, in the period ( 3 )+( 4 ), the capacitors C 1   MDAC (E) and C 1   H (E) in the MDAC 1 (E) and the capacitor C 1   s (O) in the MDAC 1 (O) are used, while the capacitor C 1   H (O) in the MDAC 1 (O) is reset. 
     In view of the above, the op amp (OP 1 : common op amp) is shared between the two MDACs (MDAC 1 (E) and MDAC 1 (O)). However, in the MDAC configuration of  FIG. 26A  and  FIG. 26B , it is not possible to share the capacitors between the MDAC 1 (E) and the MDAC 1 (O). 
       FIG. 27A  and  FIG. 27B  are diagrams for explaining the MDAC of the fifth embodiment and its operation; the MDAC illustrated here is a MDAC of the second configuration example (type II). 
     The MDAC of the fifth embodiment is implemented by applying the MDAC of the fourth embodiment to a double-sampling MDAC (parallel MDAC) system. More specifically, in  FIG. 27A ,  FIG. 27B ,  FIG. 28A , and  FIG. 28B , the MDAC 1  and MDAC 2  in the earlier described  FIG. 23A ,  FIG. 23B ,  FIG. 24A , and  FIG. 24B  are reconfigured to operate as MDAC 1 (E) and MDAC 1 (O), respectively. 
     That is, as illustrated in  FIG. 27A  and  FIG. 27B , the capacitor CSC is used as C 1   MDAC (O) in the period ( 1 ), as C 1   s (E) in the period ( 2 ), as C 1   MDAC (E) in the period ( 3 ), and as C 1   s (O) in the period ( 4 ). 
     As is apparent from  FIG. 27A  and  FIG. 27B , the hold capacitors C 1   H (E) and C 1   H (O) need to be provided exclusively for the MDAC 1 (E) and the MDAC 1 (O), respectively. However, the sampling capacitors C 1   s (E) and C 1   s (O) and computation capacitors C 1   MDAC (E) and C 1   MDAC (O) in the MDAC 1 (E) and MDAC 1 (O) may be replaced by one common capacitor. 
     In the MDAC of the fifth embodiment, the op amp (common op amp OP 1 (E)) is shared between the MDAC 1 (E) and the MDAC 1 (O) in the same manner as earlier described. 
       FIG. 28A  is a circuit diagram illustrating one example of the MDAC of the fourth embodiment, and  FIG. 28B  is a diagram for explaining the operation of the MDAC of  FIG. 28A . 
     In  FIG. 28A , reference characters CSC 11  and CSC 12  designate the common capacitor (CSC) shared for use, while CH 1 E designates the hold capacitor (C 1   H (E)) when used for the first MDAC (MDAC 1 (E)) and CH 1 O designates the hold capacitor (C 1   H (O)) when used for the second MDAC (MDAC 1 (O)). 
     In the description of the MDAC of the fifth embodiment, CH 1 E=CH 1 O=C 0 /2 and CSC 1 =CSC 11 +CSC 12 =C 0 , assuming the case of the signal amplification factor m=2. 
     As illustrated in  FIG. 28A , the MDAC 0  (switched capacitor circuit) includes the capacitors CH 1 E, CH 1 O, CSC  11 , and CSC 12  (two or more internal capacitors) and the op amp OP 0  (one or more amplifiers). The MDAC 0  further includes switches SWVIN 1 E, SWVIN 1 O, SWSC 1 A/ 11 B/ 12 B, SWHC 1 A/ 11 B/ 12 B, SWSH 1 A/ 1 B, SWHH 1 A/ 1 B, SWSH 2 A/ 2 B, and SWHH 2 A/ 2 B (two or more internal switches). 
     First, in the period ( 1 ) of  FIG. 28B , the switch control unit SWC 1  sets the control signals for the switches SWVIN 1 E, SWHC 1 A/ 11 B/ 12 B, and SWHH 2 A/ 2 B to a high level “H”, causing these switches to turn on. 
     Further, in the period ( 1 ), the switch control unit SWC 1  sets the control signals for the switches SWVIN 1 O, SWSC 1 A/ 11 B/ 12 B, SWSH 1 A/ 1 B, SWHH 1 A/ 1 B, and SWSH 2 A/ 2 B to a low level “L”, causing these switches to turn off. 
     As a result, in the MDAC 0 , the capacitor CSC 11  acts as the computation capacitor C 1   MDAC (O) for the second MDAC (MDAC 1 (O)), as illustrated in the period ( 1 ) of  FIG. 27A . On the other hand, the capacitor CH 1 O is connected so as to act as the hold capacitor C 1   H (O) for the second MDAC (MDAC 1 (O)). 
     Next, in the period ( 2 ) of  FIG. 28B , the switch control unit SWC 1  sets the control signals for the switches SWSC 1 A/ 11 B/ 12 B and SWSH 1 A/ 1 B to “H”, causing these switches to turn on, and sets the control signal for the switches SWHC 1 A/ 11 B/ 12 B to “L”, causing these switches to turn off. 
     In the period ( 2 ), the other switches SWVIN 1 E, SWVIN 1 O, SWHH 1 A/ 1 B, SWSH 2 A/ 2 B, and SWHH 2 A/ 2 B are each held at the same level as in the period ( 1 ). 
     As a result, the capacitors CSC 11  and CSC 12  act as the sampling capacitor C 1   s (E) for the first MDAC (MDAC 1 (E)), as illustrated in the period ( 2 ) of  FIG. 27A . The capacitor CH 1 O remains connected so as to act as the hold capacitor C 1   H (O) for the second MDAC (MDAC 1 (O)), while the capacitor CH 1 E is reset. 
     Next, in the period ( 3 ) of  FIG. 28B , the switch control unit SWC 1  sets the control signals for the switches SWVIN 1 O, SWHC 1 A/ 11 B/ 12 B, and SWHH 1 A/ 1 B to “H”, causing these switches to turn on. Further, the switch control unit SWC 1  sets the control signals for the switches SWVIN 1 E, SWSC 1 A/ 11 B/ 12 B, SWSH 1 A/ 1 B, and SWHH 2 A/ 2 B to “L”, causing these switches to turn off. In the period ( 3 ), the control signal for the SWSH 2 A/ 2 B is held at the same level as in the period ( 2 ). 
     As a result, in the MDAC 0 , the capacitor CSC 11  acts as the computation capacitor C 1   MDAC (E) for the first MDAC (MDAC 1 (E)), as illustrated in the period ( 3 ) of  FIG. 27A . On the other hand, the capacitor CH 1 E is connected so as to act as the hold capacitor C 1   H (E) for the first MDAC (MDAC 1 (E)). 
     Then, in the period ( 4 ) of  FIG. 28B , the switch control unit SWC 1  sets the control signals for the switches SWSC 1 A/ 11 B/ 12 B and SWSH 2 A/ 2 B to “H”, causing these switches to turn on, and sets the control signal for the switches SWHC 1 A/ 11 B/ 12 B to “L”, causing these switches to turn off. 
     In the period ( 4 ), the other switches SWVIN 1 E, SWVIN 1 O, SWSH 1 A/ 1 B, SWHH 1 A/ 1 B, and SWHH 2 A/ 2 B are each held at the same level as in the period ( 3 ). 
     As a result, in the MDAC 0 , the capacitors CSC 11  and CSC 12  act as the sampling capacitor C 1   s (O) for the second MDAC (MDAC 1 (O)), as illustrated in the period ( 4 ) of  FIG. 27A . The capacitor CH 1 E remains connected so as to act as the hold capacitor C 1   H (E) for the first MDAC (MDAC 1 (E)), while the capacitor CH 1 O is reset. 
     In the MDAC of the fifth embodiment, the op amp OP 1 (E) may be shared between the MDAC 1 (E) and the MDAC 1 (O) in the same manner as earlier described. 
     In this way, according to the MDAC configuration of the fifth embodiment, not only the op amp but the capacitor (CSC: CSC 11  and CSC 12 ) may also be shared between the two MDACs, thus making is possible to further reduce the footprint of the switched capacitor circuit or AD conversion circuit. 
       FIG. 29  is a diagram illustrating the performance of the MDACs of the fourth and fifth embodiments for comparison with the performance of the MDACs depicted in  FIGS. 22A and 26A .  FIG. 29  provides data not only for the case of the signal amplification factor m=2 but also for the case of m=4. 
     While the MDACs depicted in  FIGS. 22A and 26A  and the MDACs of the fourth and fifth embodiments have each been described by dealing with a 1.5bMDAC with m=2,  FIG. 29  also deals with 2.5bMDACs with m=4 such as described with reference to  FIG. 4A ,  FIG. 4B , and  FIG. 4C . 
     That is,  FIG. 29  also provides data for the m=4 version of the MDACs depicted in  FIGS. 22A and 26A  and the m=4 version of the MDACs of the fourth and fifth embodiments. 
     In  FIG. 29 , for the fourth embodiment, data is provided for the case of no scaling applied as well as the case of scaling applied, and the coefficient  2  is given by considering the capacitance for two MDACs. Further, for the input voltage (signal amplitude), data has been obtained by calculating (C 1   MDAC +C 1   H )×coefficient  2 , without regard to the magnitude of the input voltage. 
     As is apparent from  FIG. 29 , the MDAC of the fourth embodiment achieves, by virtue of the capacitor sharing, a reduction in capacitance, i.e., the footprint of the capacitors (the circuit), by about 33% in the case of no scaling applied and about 22% in the case of scaling applied. 
     It is also seen that the m=4 version of the MDAC of the fourth embodiment achieves a reduction in capacitance, i.e., the footprint of the circuit, by about 40% in the case of no scaling applied and about 16% in the case of scaling applied. 
     It is further seen that the MDAC of the fifth embodiment achieves a reduction in capacitance, i.e., the footprint of the circuit, by about 33% and also that the m=4 version of the MDAC of the fifth embodiment achieves a reduction in capacitance, i.e., the footprint of the circuit, by about 40%. 
     Here, the signal amplification factors m=2 and m=4 are only examples, and it will be appreciated that larger bit-width versions of the MDACs of the fourth and fifth embodiment also achieve the effect of reducing the footprint of the circuit. 
       FIG. 30  is a block diagram schematically illustrating one example of a pipelined AD conversion circuit to which the stage circuit that has the MDAC of each embodiment or the sub-ADC of each embodiment is applied, and  FIG. 31  is a block diagram schematically illustrating one example of a cyclic AD conversion circuit to which the stage circuit that has the MDAC of each embodiment or the sub-ADC of each embodiment is applied. 
     The MDAC of any one of the first to fifth embodiments described above may be applied, for example, as an MDAC in a cascade of MDAC circuits  202 - 1  to  202 -( n− 1) such as used in the pipelined AD conversion circuit  200  of  FIG. 30 . 
     As illustrated in  FIG. 30 , the pipelined AD conversion circuit  200  includes a sample-and-hold (S/H) circuit  201 , (n−1) stages of MDAC circuits  202 - 1  to  202 -( n− 1), a flash ADC  203  at the last stage, and a logic operation circuit (digital correction circuit)  204 . 
     The sample-and-hold circuit  201  samples the input voltage VIN and holds it, and supplies its output signal to the MDAC circuits  202 - 1  to  202 -( n− 1). 
     The logic operation circuit  204  receives the output signals DB( 1 ) to DB(n−1) of the MDAC circuits  202 - 1  to  202 -( n− 1) as well as the output signal DB(n) of the flash ADC  203  at the last stage, and produces an output code (ADC output) by analog-to-digital converting the input voltage VIN with a resolution corresponding to the number of stages of the MDAC circuits. 
     Further, the MDAC of any one of the first to fifth embodiments described above may be applied, for example, as an MDAC circuit  303  such as used in the cyclic AD conversion circuit  300  of  FIG. 31 . 
     That is, as illustrated in  FIG. 31 , the cyclic AD conversion circuit  300  includes, in addition to the MDAC circuit  303 , a switch  301 , a sample-and-hold (S/H) circuit  302 , and a logic operation circuit  304 . The sample-and-hold circuit  302  may be omitted. 
     The sample-and-hold circuit  302  samples the input voltage VIN or the output voltage VO( i )=VI(i+1) of the MDAC circuit  303 , whichever is selected by the switch  301 , and holds the sampled voltage, and the switch  301  causes the output voltage VO( i ) of the MDAC circuit  303  to cycle a plurality of times. 
     The signal DB(i) output from the MDAC circuit  303  in each cycle is supplied to the logic operation circuit  304 , and the logic operation circuit  304  produces an output code (ADC output) by analog-to-digital converting the voltage with a resolution corresponding to the number of cycles through the MDAC circuit. 
     While the MDACs (switched capacitor circuits) of the first to fifth embodiments have been described in detail above, it will be appreciated that various modifications may be made, for example, to the number of switches or capacitors in each MDAC or their connections or to the switch timing of each switch controlled by the switch control unit. 
     It will also be appreciated that the MDACs of the first to fifth embodiments may be applied not only to pipelined AD conversion circuits and cyclic AD conversion circuits but also extensively to various other circuits such as DA converts and filters. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.

Technology Category: 5