Abstract:
An A/D converter comprises a pipeline stage array in which plural pipeline stages are connected in series, each pipeline stage performing a pipeline operation on an inputted analog voltage to output a digital voltage; a number-of-bits control circuit for outputting a number-of-bits selection signal which indicates whether the operation of each pipeline stage should be carried out or halted, according to a number-of-bits control signal which indicates a resolution; and a correction circuit for compensating a digital value to be output, according to the number-of-bits control signal. Therefore, when resolution of the A/D converter, which is requested by the system, is changed, only the pipeline stages required for realizing the requested resolution are operated while the other pipeline stages are halted, whereby a reduction in power consumption of the A/D converter is realized and, simultaneously, a breakdown of an output from the A/D converter is avoided.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an A/D converter and, more particularly, to an A/D converter performing pipeline processing, which can vary resolution.  
         BACKGROUND OF THE INVENTION  
         [0002]    With digitization and speedup of signal processing in information communication fields as well as downsizing and weight-reduction of information communication devices, speedup and reduction in power consumption are required of A/D converters which become key devices in digital signal processing. In recent years, a pipeline A/D converter has increasingly been employed as a configuration of an A/D converter to meet the requirements. First of all, the construction and operation of a conventional pipeline A/D converter will be described.  
           [0003]    [0003]FIG. 17 is a block diagram illustrating a general pipeline A/D converter of 5-bit output, using three pipeline stages of 1.5-bit output, and a final pipeline stage of 2-bit output (refer to Japanese Published Patent Application No. Hei.6-85672, Japanese Published Patent Application No. Hei.10-173528, and “CMOS DATA CONVERTERS FOR COMMUNICATIONS” by Mikael Gustavsson, J. Jacob Wikner, Nianxiong Nick Tan, KLUWER ACADEMIC PUBLISHERS (ISBN-0-7923-7780-X)).  
           [0004]    The pipeline A/D converter is supplied with an analog input signal  1  as an input, and outputs a digital output signal  2 . The analog input signal  1  indicated by a partial analog voltage value P 0  is connected to a first pipeline stage  3 . An output of the first pipeline stage  3 , which is indicated by a partial analog voltage value P 1 , is connected to a second pipeline stage  4 . An output of the second pipeline stage  4 , which is indicated by a partial analog voltage value P 2 , is connected to a third pipeline stage  5 . An output of the third pipeline stage  5 , which is indicated by a partial analog voltage value P 3 , is connected to a fourth pipeline stage  6 . On the other hand, partial digital values M 1 L 1 ˜M 4 L 4  obtained in the respective pipeline stages are connected to a coding circuit  7 . There is a case where the analog input signal  1  is connected to the first pipeline stage  3  via a sampling/hold circuit (not shown). The foregoing is the construction of the pipeline A/D converter.  
           [0005]    Next, a description will be given of the operation of the pipeline A/D converter for converting the analog voltage values into the digital values. When the analog input signal  1  indicated by the partial analog voltage value P 0  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P1, according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the coding circuit  7  and the second pipeline stage  4 , respectively.  
           [0006]    Likewise, when the partial analog voltage value P 1  is input to the second pipeline stage  4 , the second pipeline stage  4  outputs the partial digital value M2L2 which is a binary code having 1.5 bits of information, and the partial analog voltage value P 2 , according to the partial analog voltage value P 1 . The partial digital value M 2 L 2  and the partial analog voltage value P 2  are input to the coding circuit  7  and the third pipeline stage  5 , respectively. Likewise, when the partial analog voltage value P 2  is input to the third pipeline stage  5 , the third pipeline stage  5  outputs the partial digital value M 3 L 3  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 3 , according to the partial analog voltage value P 2 . The partial digital value M 3 L 3  and the partial analog voltage value P 3  are input to the coding circuit  7  and the third pipeline stage  6 , respectively. Further, when the partial analog voltage value P 3  is input to the fourth pipeline stage  6 , the fourth pipeline stage  6  outputs the partial digital value M 4 L 4  which is a binary code having 2 bits of information and constitutes a least significant bit, according to the partial analog voltage value P 3 . The partial digital value M 4 L 4  is input to the coding circuit  7 .  
           [0007]    The partial digital value L 1  and the partial digital value M 2 , the partial digital value L 2  and the partial digital value M 3 , and the partial digital value L 3  and the partial digital value M 4  have overlap portions (0.5 bit), respectively, to increase reliability in the conversion process, and the digital output signal  2  which is a binary code and has 5-bit resolution is consequently outputted by coding these digital values in the coding circuit  7 . The foregoing is the operation of the pipeline A/D converter.  
           [0008]    Next, a description will be given of the constructions and operations of the general pipeline stages constituting the conventional pipeline A/D converter.  
           [0009]    Hereinafter, the constructions of the i-th (first to fourth) pipeline stages will be described.  
           [0010]    [0010]FIG. 18 is a block diagram illustrating the specific construction of each of the first to third pipeline stages  3 ˜ 5  shown in FIG. 17. Each of the first to third pipeline stages  3 ˜ 5  is supplied with, as an input, a first partial analog voltage value  8  indicated by Pi−1, and outputs a partial digital value  9  indicated by MiLi, and a second partial analog voltage value  10  indicated by Pi. The first partial analog voltage value  8  is connected to a first offset addition unit  11 , an output of the first offset addition unit  11  is connected to a partial A/D converter  12 , an output of the partial A/D converter  12  is connected to a partial D/A converter  13 , an output of the partial D/A converter  13  is connected to a second offset addition unit  14 , an output of the second offset addition unit  14  and the first partial analog voltage value  8  are connected to a subtracter  15 , an output of the subtracter  15  is connected to an arithmetic amplifier  16 , an output of the arithmetic amplifier  16  becomes the second partial analog voltage value  10 , and an output of the partial A/D converter  12  becomes the partial digital value  9 .  
           [0011]    On the other hand, FIG. 19 is a block diagram illustrating the specific construction of the fourth pipeline stage  6  shown in FIG. 17. The fourth pipeline stage  6  is supplied with, as an input, a first partial analog voltage value  8  indicated by Pi−1, and outputs a partial digital value  9  indicated by MiLi. The first partial analog voltage value  8  is connected to a partial A/D converter  12 . An output of the partial A/D converter  12  becomes the partial digital value  9 . The foregoing is the constructions of the pipeline stages.  
           [0012]    Hereinafter, the operations of the i-th (first to fourth) pipeline stages will be described.  
           [0013]    In each of the first to third pipeline stages  3 ˜ 5 , the first offset addition unit  11  adds an offset voltage value equivalent to −0.5 LSB, as a first offset voltage value  17 , to the Pi−1 that is the inputted first partial analog voltage value  8 , and the A/D converter  12  performs A/D conversion on the Pi−1, thereby outputting, as the partial digital value  9 , [00,01,10] which are binary codes each corresponding to 1.5-bit output and having Mi as a higher-order bit and Li as a lower-order bit. Further, the partial D/A converter  13  performs D/A conversion on the partial digital value  9 , and the second offset addition unit  14  adds an offset voltage value equivalent to +0.5 LSB, as a second offset voltage value  18 , to a result of the D/A conversion. Then, the output from the second offset voltage value  18  and the first partial analog voltage value  8  are input to the subtracter  15 , and a difference voltage value between them is obtained, and the difference voltage value is amplified by the arithmetic amplifier  16 , thereby obtaining the Pi as the second partial analog voltage value  10 . The foregoing is the operation of each of the first to third pipeline stages  3 ˜ 5 .  
           [0014]    [0014]FIG. 20 is a diagram illustrating relationships of the first partial analog voltage value Pi 1  which is an input to each of the first to third pipeline stages  3 ˜ 5 , to the partial digital value MiLi and the second partial analog voltage value Pi which are outputs from each of the first to third pipeline stages  3 ˜ 5 . Although, in FIG. 20, a GND level is employed as a reference level of the first and second partial analog voltage values, the reference level may be arbitrarily set. Further, FS indicates a full-scale voltage value of the first to third pipeline stages  3 ˜ 5 .  
           [0015]    On the other hand, in the fourth pipe line stage  6 , the partial A/D converter  12  performs A/D conversion on the Pi−1 that is the first partial analog voltage value  8 , thereby outputting, as the partial digital value  9 , [00,01,10,11] which are binary codes each corresponding to 2-bit output and having Mi as a higher-order bit and Li as a lower-order bit. The foregoing is the operation of the fourth pipeline stage  6 .  
           [0016]    [0016]FIG. 21 is a diagram illustrating a relationship between the first partial analog voltage value Pi−1 that is an input to the fourth pipeline stage  6  and the partial digital value MiLi that is an output from the fourth pipeline stage  6 . Although, in FIG. 21, a GND level is employed as a reference level of the first partial analog voltage value, the reference level may be arbitrarily set, as described for the first to third pipeline stages  3 ˜ 5 . Further, FS indicates a full-scale voltage value of the fourth pipeline stage  6 . The operations of the pipeline stages are as described above.  
           [0017]    Next, the construction and operation of the general coding circuit as a component of the pipeline A/D converter will be described in detail.  
           [0018]    [0018]FIG. 22 is a block diagram illustrating the construction of the coding circuit  7 . The coding circuit  7  receives the partial digital values M 1 L 1 ˜M 3 L 3  which are outputted from the first to third pipeline stages  3 ˜ 5 , each value being constituted by a binary code of 1.5 bits, and the partial digital value M 4 L 4  which is outputted from the fourth pipeline stage  6  and is constituted by a binary code of 2 bits, and the coding circuit  7  outputs the digital output signal  2  (D 4 ˜D 0 ).  
           [0019]    The partial digital value Ml is connected to an Al terminal as an input terminal of a half adder  19 , and the partial digital value L 1  is connected to an A 2  terminal as an input terminal of a full adder  20 . The partial digital value M 2  is connected to a B 2  terminal as an input terminal of the full adder  20 , and the partial digital value L 2  is connected to an A 3  terminal as an input terminal of the full adder  21 . The partial digital value M 3  is connected to a B 3  terminal as an input terminal of the full adder  21 , and the partial digital value L 3  is connected to an A 4  terminal as an input terminal of the half adder  22 . The partial digital value M 4  is connected to a B 4  terminal as an input terminal of the half adder  22 , and the partial digital value L 4  becomes D 0  that is a least significant bit of the digital output signal  2 .  
           [0020]    Further, a result of addition S 4  from the half adder  22  becomes D 1  that is the second bit of the digital output signal  2 . A carry signal Co 4  from the half adder  22  is connected to a carry signal input terminal Ci 3  of the full adder  21 . A result of addition S 3  from the full adder  21  becomes D 2  that is the third bit of the digital output signal  2 . A carry signal Co 3  from the full adder  21  is connected to a carry signal input terminal Ci 2  of the full adder  20 . A result of addition S 2  from the full adder  20  becomes D 3  that is the fourth bit of the digital output signal  2 . A carry signal Co 2  from the full adder  20  is connected to an input terminal B 1  of the half adder  19 . A result of addition S 1  from the half adder  19  becomes D 4  that is a most significant bit of the digital output signal  2 .  
           [0021]    The A 1  to A 4  terminals and the B 1  to B 4  terminals as the input terminals of the half adders and the full adders are interchangeable, and the above-described connections are not necessarily required. Further, although the coding circuit  7  is constituted by the half adders and the full adders, it may be constituted by other logic circuits. The foregoing is the construction of the coding circuit  7 .  
           [0022]    Next, the operation of the coding circuit  7  will be described. The coding circuit  7  outputs the partial digital value L 4  as the least significant bit D 0  of the digital output signal  2 . Further, the half adder  22  adds the partial digital value M 4  and the partial digital value L 3 , and a result of addition is outputted as the second bit D 1  of the digital output signal  2 . At this time, if a carry occurs, a carry signal is sent to the full adder  21 . Further, the full adder  21  adds the partial digital value M 3 , the partial digital value L 2 , and the carry signal of the half adder  22 , and a result of addition is outputted as the third bit D 2  of the digital output signal  2 . When a carry occurs, a carry signal is sent to the full adder  20 . Further, the full adder  20  adds the partial digital value M 2 , the partial digital value L 1 , and the carry signal of the full adder  21 , and a result of addition is outputted as the fourth bit D 3  of the digital output signal  2 . When a carry occurs, a carry signal is sent to the half adder  19 . Further, the half adder  19  adds the partial digital value M 1  and the carry signal of the total adder  20 , and a result of addition is outputted as the most significant bit D 4  of the digital output signal  2 . The foregoing is the operation of the coding circuit.  
           [0023]    The conventional A/D converter is constructed as described above. Since a mobile communication device such as a handy phone is driven by a battery or the like, reduction in power consumption is required of an A/D converter which is employed in a circuit for improving communication quality in digital communication, for downsizing and weight-reduction of the mobile communication device and long-hours drive of the battery or the like. On the other hand, when the mobile communication device receives information, the intensity of radio wave to be received varies due to obstructions such as buildings or a change in communication distance from a base station to the mobile communication device, and therefore, resolution required of the A/D converter varies according to variations in the radio wave intensity that depends on the place where the device is used.  
           [0024]    In the conventional pipeline A/D converter described above, however, the number of bits in the digital output does not change even when the resolution requested by the mobile communication system changes, and all of the pipeline stages are operated, resulting in an impediment to reduction in power consumption of the A/D converter.  
           [0025]    On the other hand, in order to solve the above-mentioned problem, Japanese Published Patent Application No. Hei.6-85672 proposes a pipeline A/D converter which can vary resolution of a digital value to be output, by halting operations of A/D converters for determining lower-order bits, according to a given request signal.  
           [0026]    However, as described above, in the construction of the pipeline A/D converter which is commonly used at present, the digital output of each pipeline stage has an overlap portion with the digital output of the pipeline stage constituting the higher-order bit, and therefore, the higher-order bit cannot be determined until the lower-order bit is determined. For example, when [M 1 L 1 ,M 2 L 2 ,M 3 L 3 ] are [01,01,01] as shown in FIG. 26( a ), [0111] is outputted as an arithmetic result. However, when [M 1 L 1 ,M 2 L 2 ,M 3 L 3 ] are [01,01,10] as shown in FIG. 26( b ), an arithmetic result, which should be [1000] actually, becomes [011] because [M 3 L 3 ] is unknown by halting the pipeline stage  5 . In this way, since the output of the most significant bit might be determined by the output of the least significant bit, if the operation of the A/D converter for determining the lower-order bit is simply halted, the requested resolution cannot be realized and, worse yet, a breakdown might occur in the output from the A/D converter.  
         SUMMARY OF THE INVENTION  
         [0027]    The present invention is made to solve the above-described problems and has for its object to provide a pipeline A/D converter which can operate with reduced power consumption, by driving only pipeline stages that are required for realizing requested resolution while halting pipeline stages that are not required for realizing the resolution, when resolution of the A/D converter requested by the system is changed, and further, an A/D converter which can prevent a breakdown in an output from the A/D converter, which breakdown might occur when the operations of the unnecessary pipeline stages are halted.  
           [0028]    Other objects and advantages of the invention will become apparent from the detailed description that follows. The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will be apparent to those of skill in the art from the detailed description.  
           [0029]    According to a first aspect of the present invention, an A/D converter comprises a pipeline stage array in which plural pipeline stages are connected in series, each pipeline stage performing a pipeline operation on an inputted analog voltage to output a digital voltage; a number-of-bits control circuit for outputting a number-of-bits selection signal which indicates whether the operation of each pipeline stage should be carried out or halted, according to a number-of-bits control signal which indicates a resolution; and a correction circuit for compensating a digital value to be output, according to the number-of-bits control signal. Therefore, when resolution of the A/D converter which is requested by the system is changed, only the pipeline stages required for realizing the requested resolution are operated while the other pipeline stages are halted, whereby a reduction in power consumption of the A/D converter is realized and, simultaneously, resolution of a digital value to be output can be compensated by the correction circuit, resulting in an A/D converter that never causes a breakdown of an output from the A/D converter.  
           [0030]    According to a second aspect of the present invention, in the A/D converter according to the first aspect, the correction circuit comprises a correction A/D converter for receiving inputs of all of the pipeline stages, comparing an input that is selected from the inputs according to the number-of-bits selection signal, with a reference voltage value, and outputting a result of comparison as a partial digital value for correction which comprises at least one binary code; and a correction code conversion circuit for receiving the partial digital values which are outputted from all of the pipeline stages in the pipeline stage array, and the partial digital value for correction which is outputted from the correction A/D converter, and outputting at least one binary code according to the number-of-bits selection signal. Therefore, when resolution of a digital output signal which is requested by the system changes over a range of 1˜N bits, a series of pipeline operations performed at the pipeline stages whose operations are not needed can be halted, without causing a breakdown of data output.  
           [0031]    According to a third aspect of the present invention, in the A/D converter according to the first aspect, the correction circuit comprises a selection means for selecting an analog voltage value to be processed in a final pipeline stage, from among analog voltage values which are outputted from the plural pipeline stages except the final pipeline stage, according to the number-of-bits selection signal; and a correction code conversion circuit for receiving the partial digital value outputted from the final pipeline stage, and the partial digital values outputted from the respective pipeline stages in the pipeline stage array other than the final pipeline stage, and outputting corrected binary codes according to the number-of-bits selection signal. Therefore, when resolution of a digital output signal which is requested by the system changes over a range of 2˜N bits, a series of pipeline operations performed at the pipeline stages whose operations are not needed can be halted, without causing a breakdown of data output.  
           [0032]    According to a fourth aspect of the present invention, in the A/D converter according to the first aspect, the correction circuit comprises an offset control means for determining as to whether offset addition should be performed or not in the plural pipeline stages except the final pipeline stage, according to the number-of-bits control signal, and controlling the plural pipeline stages so as to output corrected binary codes. Therefore, when resolution of a digital output signal which is requested by the system changes over a range of 1˜N−2 bits, or to N bits, a series of pipeline operations performed at the pipeline stages whose operations are not needed can be halted, without causing a breakdown of data output. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    [0033]FIG. 1 is a block diagram illustrating the construction of a pipeline A/D converter according to a first embodiment of the invention.  
         [0034]    [0034]FIG. 2 is a block diagram illustrating a number-of-bits control circuit as a component of the pipeline A/D converter according to the first embodiment.  
         [0035]    [0035]FIG. 3 is a block diagram illustrating each of first to third pipeline stages as components of the pipeline A/D converter according to the first embodiment.  
         [0036]    [0036]FIG. 4 is a block diagram illustrating a fourth pipeline stage as a component of the pipeline A/D converter according to the first embodiment.  
         [0037]    [0037]FIG. 5 is a block diagram illustrating a correction A/D converter as a component of the pipeline A/D converter according to the first embodiment.  
         [0038]    [0038]FIG. 6 is a block diagram illustrating a correction code conversion circuit as a component of the pipeline A/D converter according to the first embodiment.  
         [0039]    [0039]FIG. 7 is a block diagram illustrating the construction of a pipeline A/D converter according to a second embodiment of the invention.  
         [0040]    [0040]FIG. 8 is a block diagram illustrating a number-of-bits control circuit as a component of the pipeline A/D converter according to the second embodiment.  
         [0041]    [0041]FIG. 9 is a block diagram illustrating each of first to third pipeline stages as components of the pipeline A/D converter according to the second embodiment.  
         [0042]    [0042]FIG. 10 is a block diagram illustrating a fourth pipeline stage as a component of the pipeline A/D converter according to the second embodiment.  
         [0043]    [0043]FIG. 11 is a block diagram illustrating a correction code conversion circuit as a component of the pipeline A/D converter according to the second embodiment.  
         [0044]    [0044]FIG. 12 is a block diagram illustrating the construction of a pipeline A/D converter according to a third embodiment of the invention.  
         [0045]    [0045]FIG. 13 is a block diagram illustrating a number-of-bits control circuit as a component of the pipeline A/D converter according to the third embodiment.  
         [0046]    [0046]FIG. 14 is a block diagram illustrating each of first to third pipeline stages as components of the pipeline A/D converter according to the third embodiment.  
         [0047]    [0047]FIG. 15 is a block diagram illustrating a fourth pipeline stage as a component of the pipeline A/D converter according to the third embodiment.  
         [0048]    [0048]FIG. 16 is a diagram illustrating an input-output relationship in each of the first to third pipeline stages included in the pipeline A/D converter according to the third embodiment.  
         [0049]    [0049]FIG. 17 is a block diagram illustrating the construction of a conventional pipeline A/D converter.  
         [0050]    [0050]FIG. 18 is a block diagram illustrating each of first to third pipeline stages as components of the conventional pipeline A/D converter.  
         [0051]    [0051]FIG. 19 is a block diagram illustrating a fourth pipeline stage as a component of the conventional pipeline A/D converter.  
         [0052]    [0052]FIG. 20 is a diagram illustrating an input-output relationship in each of the first to third pipeline stages of the conventional pipeline A/D converter.  
         [0053]    [0053]FIG. 21 is a diagram illustrating an input-output relationship in the fourth pipeline stage as a component of the conventional pipeline A/D converter.  
         [0054]    [0054]FIG. 22 is a block diagram illustrating a coding circuit as a component of the conventional pipeline A/D converter.  
         [0055]    [0055]FIG. 23 is a diagram illustrating a relationship between a pipeline stage output and a digital value for correction, for explaining the operation of the correction A/D converter as a component of the A/D converter according to the first embodiment.  
         [0056]    [0056]FIG. 24 is a diagram illustrating a relationship between a pipeline stage output and a digital value for correction, for explaining the operation of the correction A/D converter as a component of the A/D converter according to the first embodiment.  
         [0057]    [0057]FIG. 25 is a diagram illustrating an input-output relationship in each of the first to third pipeline stages of the A/D converter according to the third embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0058]    Hereinafter, specific embodiments of A/D converters according to the present invention will be described with reference to the drawings.  
       Embodiment 1  
       [0059]    [0059]FIG. 1 is a block diagram illustrating the construction of a pipeline A/D converter according to a first embodiment of the invention, which can vary resolution of a digital output signal over a range of 1˜5 bits. The pipeline A/D converter according to the first embodiment is supplied with an analog input signal  1 , and outputs a digital output signal  2 . More specifically, the analog input signal  1  indicated by a partial analog voltage P 0  is input to a first pipeline stage  3 . An output of the first pipeline stage  3 , which is indicated by a partial analog voltage value P 1 , is input to a second pipeline stage  4 . An output of the second pipeline stage  4 , which is indicated by a partial analog voltage value P 2 , is input to a third pipeline stage  5 . An output of the third pipeline stage  5 , which is indicated by a partial analog voltage value P 3 , is input to a fourth pipeline stage  6 .  
         [0060]    Partial digital values M 1 L 1 ˜M 4 L 4  obtained in the respective pipeline stages are connected to a correction code conversion circuit  23 , and an output of the correction code conversion circuit  23  is connected to a coding circuit  7 . On the other hand, a number-of-bits control signal  24  is connected to a number-of-bits control circuit  25 , and a number-of-bits selection signal  26  as an output of the number-of-bits control circuit  25  is connected to the first to fourth pipeline stages  3 ˜ 6 , a correction A/D converter  27 , and the correction code conversion circuit  23 . Further, the partial analog voltage values P 0 ˜P 3  are connected to the correction A/D converter  27 , and a partial digital value  28  for correction (hereinafter referred to as “correction partial digital value”), which is an output of the correction A/D converter  27  and is indicated by Cout, is connected to the correction code conversion circuit  23 . The foregoing is the construction of the pipeline A/D converter according to the first embodiment.  
         [0061]    Next, a description will be given of the operation of the pipeline A/D converter according to the first embodiment when it outputs a digital output signal having 5-bit resolution.  
         [0062]    When the number-of-bits control signal  24  for outputting a digital output signal having 5-bit resolution is applied to the number-of-bits control circuit  25 , the number-of-bits control circuit  25  outputs the number-of-bits selection signal  26  so as to operate the first to fourth pipeline stages  3 ˜ 6 , and halt the correction A/D converter  27 .  
         [0063]    It is assumed that the halted correction A/D converter  27  outputs either [0] or [1] as the correction partial digital value Cout. At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P 1 , according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the correction code conversion circuit  23  and the second pipeline stage  4 , respectively.  
         [0064]    Likewise, when the partial analog voltage value P 1  is input to the second pipeline stage  4 , the second pipeline stage  4  outputs the partial digital value M 2 L 2  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 2 , according to the partial analog voltage value P 1 . The partial digital value M 2 L 2  and the partial analog voltage value P 2  are input to the correction code conversion circuit  23  and the third pipeline stage  5 , respectively. Likewise, when the partial analog voltage value P 2  is input to the third pipeline stage  5 , the third pipeline stage  5  outputs the partial digital value M 3 L 3  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 3 , according to the partial analog voltage value P 2 . The partial digital value M 3 L 3  and the partial analog voltage value P 3  are input to the correction code conversion circuit  23  and the fourth pipeline stage  6 , respectively. When the partial analog voltage value P 3  is input to the fourth pipeline stage  6 , the fourth pipeline stage  6  outputs the partial digital value M 4 L 4  which is a binary code having 2 bits of information and constitutes a least significant bit of the digital output signal  2 , according to the partial analog voltage value P 3 , and the partial digital value M 4 L 4  is input to the correction code conversion circuit  23 .  
         [0065]    In order to output a digital output signal having 5-bit resolution, it is not necessary to correct the partial digital values M 1 L 1 ˜M 4 L 4  outputted from the first to fourth pipeline stages  3 ˜ 6 . So, the partial digital values M 1 L 1 ˜M 4 L 4  are input to the coding circuit  7  as they are, and the coding circuit  7  codes these values M 1 L 1 ˜M 4 L 4  to output, as a result, the digital output signal  2  which is a binary code and has 5-bit resolution. In this way, the pipeline A/D converter outputs the digital output signal having 5-bit resolution.  
         [0066]    Next, a description will be given of the operation of the pipeline A/D converter of the first embodiment when it outputs a digital output signal having  4 -bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 4-bit resolution is input to the number-of-bits control circuit  25 , the number-of-bits control circuit  25  outputs the number-of-bits selection signal  26  so as to operate the first to third pipeline stages  3 ˜ 5 , halt the fourth pipeline stage  6 , and operate the correction A/D converter  27 .  
         [0067]    It is assumed that the halted fourth pipeline stage  6  outputs [00] as the partial digital value M 4 M 4 . At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P 1 , according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the correction code conversion circuit  23  and the second pipeline stage  4 , respectively. Likewise, when the partial analog voltage value P 1  is input to the second pipeline stage  4 , the second pipeline stage  4  outputs the partial digital value M 2 L 2  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 2 , according to the partial analog voltage value P 1 . The partial digital value M 2 L 2  and the partial analog voltage value P 2  are input to the correction code conversion circuit  23  and the third pipeline stage  5 , respectively. Likewise, when the partial analog voltage value P 2  is input to the third pipeline stage  5 , the third pipeline stage  5  outputs the partial digital value M 3 L 3  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 3 , according to the partial analog voltage value P 2 . The partial digital value M 3 L 3  and the partial analog voltage value P 3  are input to the correction code conversion circuit  23  and the correction A/D converter  27 , respectively.  
         [0068]    The correction A/D converter  27  selects the partial analog voltage value P 3  from among the partial analog voltage values P 0 ˜P 3  inputted to the correction A/D converter  27 , according to the number-of-bits control signal  26 , and outputs the correction partial digital value  28  which is a binary code having 1 bit of information and is indicated by Cout, according to the partial analog voltage value P 3 , by a method described later. The correction partial digital value  28  is input to the correction code conversion circuit  23 .  
         [0069]    In order to output a digital output signal having 4-bit resolution, the partial digital value M 4  outputted from the fourth pipeline stage  6  is corrected using the correction partial digital value  28  and, thereafter, the partial digital values M 1 L 1 ˜M 4 L 4  are input to the coding circuit  7 , and coded in the coding circuit  7 , thereby outputting the digital output signal  2  which is a binary code and has 4-bit resolution. In this way, the pipeline A/D converter outputs the digital output signal having 4-bit resolution.  
         [0070]    Next, a description will be given of the operation of the pipeline A/D converter of the first embodiment when it outputs a digital output signal having 3-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 3-bit resolution is input to the number-of-bits control circuit  25 , the number-of-bits control circuit  25  outputs the number-of-bits selection signal  26  so as to operate the first and second pipeline stages  3  and  4 , halt the third and fourth pipeline stages  5  and  6 , and operate the correction A/D converter  27 .  
         [0071]    It is assumed that the halted third and fourth pipeline stages  5  and  6  output [00] as the partial digital values M 3 M 3  and M 4 M 4 . At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P 1 , according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the correction code conversion circuit  23  and the second pipeline stage  4 , respectively. Likewise, when the partial analog voltage value P 1  is input to the second pipeline stage  4 , the second pipeline stage  4  outputs the partial digital value M 2 L 2  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 2 , according to the partial analog voltage value P 1 . The partial digital value M 2 L 2  and -the partial analog voltage value P 2  are input to the correction code conversion circuit  23  and the correction A/D converter  27 , respectively.  
         [0072]    The correction A/D converter  27  selects the partial analog voltage value P 2  from among the partial analog voltage values P 0 ˜P 3  inputted to the correction A/D converter  27 , according to the number-of-bits control signal  26 , and outputs the correction partial digital value  28  which is a binary code having 1 bit of information and is indicated by Cout, according to the partial analog voltage value P 2 , by a method described later. The correction partial digital value  28  is input to the correction code conversion circuit  23 . In order to output a digital output signal having 3-bit resolution, the partial digital value M 3  outputted from the third pipeline stage  5  is corrected using the correction partial digital value  28  and, thereafter, the partial digital values M 1 L 1 ˜M 4 L 4  are input to the coding circuit  7  and coded in the coding circuit  7 , thereby outputting the digital output signal  2  which is a binary code and has 3-bit resolution.  
         [0073]    In this way, the pipeline A/D converter outputs the digital output signal having 3-bit resolution.  
         [0074]    Next, a description will be given of the operation of the pipeline A/D converter of the first embodiment when it outputs a digital output signal having 2-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 2-bit resolution is input to the number-of-bits control circuit  25 , the number-of-bits control circuit  25  outputs the number-of-bits selection signal  26  so as to operate the first pipeline stage  3 , halt the second to fourth pipeline stages  4 ˜ 6 , and operate the correction A/D converter  27 .  
         [0075]    It is assumed that the halted second to fourth pipeline stages  4 ˜ 6  output [00] as the partial digital values M 2 M 2  and M 4 M 4 . At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P 1 , according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the correction code conversion circuit  23  and the correction A/D converter  27 .  
         [0076]    The correction A/D converter  27  selects the partial analog voltage value P 1  from among the partial analog voltage values P 0 ˜P 3  inputted to the correction A/D converter  27 , according to the number-of-bits control signal  26 , and outputs the correction partial digital value  28  which is a binary code having 1 bit of information and is indicated by Cout, according to the partial analog voltage value P 1 , by a method described later. The correction partial digital value  28  is input to the correction code conversion circuit  23 . In order to output a digital output signal having 2-bit resolution, the partial digital value M 2  outputted from the second pipeline stage  4  is corrected using the correction partial digital value  28  and, thereafter, the partial digital values M 1 L 1 ˜M 4 L 4  are input to the coding circuit  7  and coded in the coding circuit  7 , thereby outputting the digital output signal  2  which is a binary code and has 2-bit resolution. In this way, the pipeline A/D converter outputs the digital output signal having 2-bit resolution.  
         [0077]    Next, a description will be given of the operation of the pipeline A/D converter of the first embodiment when it outputs a digital output signal having 1-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 1-bit resolution is input to the number-of-bits control circuit  25 , the number-of-bits control circuit  25  outputs the number-of-bits selection signal  26  so as to halt the first to fourth pipeline stages  3 ˜ 6 , and operate the correction A/D converter  27 .  
         [0078]    It is assumed that the halted first to fourth pipeline stages  3 ˜ 6  output [00] as the partial digital values M 1 M 1 ˜M 4 M 4 . The correction A/D converter  27  selects the partial analog voltage value P 0  from among the partial analog voltage values P 0 ˜P 3  inputted to the correction A/D converter  27 , according to the number-of-bits control signal  26 , and outputs the correction partial digital value  28  which is a binary code having 1 bit of information and is indicated by Cout, according to the partial analog voltage value P 0 , by a method described later. The correction partial digital value  28  is input to the correction code conversion circuit  23 . In order to output a digital output signal having 1-bit resolution, the partial digital value Ml outputted from the first pipeline stage  3  is corrected using the correction partial digital value  28  and, thereafter, the partial digital values M 1 L 1 ˜M 4 L 4  are input to the coding circuit  7  and coded in the coding circuit  7 , thereby outputting the digital output signal  2  which is a binary code and has 1-bit resolution. In this way, the pipeline A/D converter outputs the digital output signal having 1-bit resolution.  
         [0079]    [0079]FIG. 2 is a block diagram illustrating the specific construction of the number-of-bits control circuit employed in the pipeline A/D converter according to the first embodiment. Among the number-of-bits control signals  24 , a number-of-bits control signal A 0  is connected to the first NOT gate and to the first input terminals of the first, third, and fifth NOR gates, a number-of-bits control signal A 1  is connected to the second NOT gate and to the second input terminals of the first, second, and fifth NOR gates, and a number-of-bits control signal A 2  is connected to the third NOT gate and to the third input terminals of the first, second, third, and fourth NOR gates. Further, an output of the first NOT gate is connected to the first input terminals of the second and fourth NOR gate, an output of the second NOT gate is connected to the second input terminals of the third and fourth NOR gates, and an output of the third NOT gate is connected to the third input terminal of the fifth NOR gate. Furthermore, an output of the first NOR gate is outputted as a bit selection signal  26  indicated by B 1 , an output of the second NOR gate is outputted as a bit selection signal  26  indicated by B 2 , an output of the third NOR gate is outputted as a bit selection signal  26  indicated by B 3 , an output of the fourth NOR gate is outputted as a bit selection signal  26  indicated by B 4 , and an output of the fifth NOR gate is outputted as a bit selection signal  26  indicated by B 5 .  
         [0080]    Next, the operation of the number-of-bits control circuit  25  will be described. When [000] is input to the number-of-bits control signals A 2 ˜A 0 , only the output of the first NOR gate becomes 1 level while the outputs of the second to fifth NOR gates become 0 level, whereby the number-of-bits selection signals B 5 ˜B 1  [00001] are outputted. When [001] is input to the number-of-bits control signals A 2 ˜A 0 , only the output of the second NOR gate becomes 1 level while the outputs of the first, third, fourth, and fifth NOR gates become 0 level, whereby the number-of-bits selection signals B 5 ˜B 1  [00010] are outputted. When [ 010 ] is input to the number-of-bits control signals A 2 ˜A 0 , only the output of the third NOR gate becomes 1 level while the outputs of the first, second, fourth, and fifth NOR gates become 0 level, whereby the number-of-bits selection signals B 5 ˜B 1  [00100] are outputted. When [011] is input to the number-of-bits control signals A 2 ˜A 0 , only the output of the fourth NOR gate becomes 1 level while the outputs of the first, second, third, and fifth NOR gates become 0 level, whereby the number-of-bits selection signals B 5 ˜B 1  [01000] are outputted. When [100] is input to the number-of-bits control signals A 2 ˜A 0 , only the output of the fifth NOR gate becomes 1 level while the outputs of the first to fourth NOR gates become 0 level, whereby the number-of-bits selection signals B 5 ˜B 1  [10000] are outputted.  
         [0081]    When the number-of-bits selection signal B 5  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter according to the first embodiment operates as an A/D converter having 5-bit resolution. Likewise, when the number-of-bits selection signal B 4  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter operates as an A/D converter having 4-bit resolution. When the number-of-bits selection signal B 3  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter operates as an A/D converter having 3-bit resolution. When the number-of-bits selection signal B 2  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter operates as an A/D converter having 2-bit resolution. When the number-of-bits selection signal B 1  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter operates as an A/D converter having 1-bit resolution. The foregoing is the operation of the number-of-bits control circuit  25  used in the pipeline A/D converter according to the first embodiment.  
         [0082]    While the number-of-bits control circuit  25  shown in FIG. 2 is constituted by using the NOT gates and the NOR gates, it can be constituted by other logic circuits with the same effects as mentioned above.  
         [0083]    Further, in the construction shown in FIG. 2, when the logical states of the number-of-bits control signals A 2 ˜A 0  are [000], [001], [010], [011], and [100], the number-of-bits selection signals B 5 ˜B 1  are outputted as [00001], [00010], [00100], [01000], and [10000], respectively. However, the logical states of the number-of-bits control signals A 2 ˜A 0  may be arbitrarily selected so long as the number-of-bits selection signals B 5 ˜B 1  are uniquely determined with respect to the number-of-bits control signals A 2 ˜A 0 , and the circuits receiving the number-of-bits selection signals correspond to the logical states, so that the same effects as mentioned above can be obtained.  
         [0084]    Furthermore, in the construction shown in FIG. 2, only one selection signal among the number-of-bits selection signals B 5 ˜B 1  is output as 1 level while the remaining four selection signals are output as 0 level. However, the same effects as mentioned above can be achieved even when only one selection signal among the number-of-bits selection signals B 5 ˜B 1  is output as 0 level while the remaining four selection signals are output as 1 level or when arbitrary logical states are outputted as the number-of-bits selection signals B 5 ˜B 1 , so long as the number-of-bits selection signals B 5 ˜B 1  are uniquely determined with respect to the number-of-bits control signals A 2 ˜A 0 , and the circuits receiving the number-of-bits selection signals correspond to the logical states.  
         [0085]    Hereinafter, the constructions of the i-th (first to fourth) pipeline stages will be described in more detail. FIG. 3 is a block diagram illustrating the construction of each of the first to third pipeline stages  3 ˜ 5  employed in the pipeline A/D converter according to the first embodiment. Each of the first to third pipeline stages  3 ˜ 5  is supplied with, as an input, a first partial analog voltage value  8  indicated by Pi−1, and outputs a partial digital value  9  indicated by MiLi, and a second partial analog voltage value  10  indicated by Pi. Further, each of the first to third pipeline stages is supplied with, as inputs, the number-of-bits selection signals B 1 ˜Bi which are outputted from the number-of-bits control circuit  25  shown in FIG. 2, and it is determined whether each pipeline stage should carry out a series of pipeline operations or halt it, according to the number-of-bits selection signals B 1 ˜Bi.  
         [0086]    The first partial analog voltage value  8  is input to a first offset addition unit  11 . An output of the first offset addition unit  11  is input to a partial A/D converter  12 . An output of the partial A/D converter  12  is input to a partial D/A converter  13 . An output of the partial D/A converter  13  is input to a second offset addition unit  14 . An output of the second offset addition unit  14  and the first partial analog voltage value  8  are input to a subtracter  15 . An output of the subtracter  15  is input to an arithmetic amplifier  16 . An output of the arithmetic amplifier  16  becomes the second partial analog voltage value  10 .  
         [0087]    Only the number-of-bits selection signal B 1  is connected to an OR circuit  26  in the first pipeline stage  3 , and the number-of-bits selection signals B 1 ˜B 3  (i.e., the number-of-bits selection signals B 1 ˜Bi described above) are connected to an OR circuit  26  in the third pipeline stage  5 . An output Ii of the OR circuit  26  is connected to the first offset addition unit  11 , the partial A/D converter  12 , the partial D/A converter  13 , the second offset addition unit  14 , the subtracter  15 , and the arithmetic amplifier  16 .  
         [0088]    On the other hand, FIG. 4 is a block diagram illustrating the fourth pipeline stage  6  which is the final stage. The fourth pipeline stage is supplied with, as an input, a first partial analog voltage value  8  indicated by Pi−1, and outputs a partial digital value  9  indicated by MiLi. The first partial analog voltage value  8  is input to a partial A/D converter  12 . The number-of-bits selection signals B 1 ˜B 4  are input to an OR circuit  26 , and an output Ii of the OR circuit  26  is input to the partial A/D converter  12 . The foregoing is the constructions of the respective pipeline stages used in the pipeline A/D converter according to the first embodiment.  
         [0089]    Hereinafter, the operation of the i-th (first to fourth) pipeline stages will be described.  
         [0090]    In each of the first to third pipeline stages  3 ˜ 5 , when all of the inputted number-of-bits selection signals B 1 ˜Bi are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution equal to or higher than (i+1)-bit precision, the output Ii from the OR circuit  26  becomes  0  level. At this time, a series of pipeline operations in which all of the pipeline stages are operated is carried out as usual. The first offset addition unit  11  adds an offset voltage value equivalent to −0.5 LSB, as a first offset voltage value  17 , to the Pi−1 which is the inputted partial analog voltage value  8 , and the partial A/D converter  12  performs A/D conversion on the Pi−1 to output, as the partial digital value  9 , [00,01,10] which are binary codes each corresponding to 1.5-bit output and having Mi as a higher-order bit and Li as a lower-order bit.  
         [0091]    Furthermore, the partial D/A converter  13  performs D/A conversion on the partial digital value  9 , and the second offset addition unit  14  adds, to a result of D/A conversion, an offset voltage value equivalent to +0.5 LSB as a second offset voltage value  18 . Then, an output of the second offset addition unit  14  and the first partial analog voltage value  8  are input to the subtracter  15 , and a difference voltage value between them is obtained. Thereafter, the arithmetic amplifier  16  amplifies the difference voltage value to output the Pi as the second partial analog voltage value  10 .  
         [0092]    On the other hand, when at least one of the inputted number-of-bits selection signals B 1 ˜Bi is 1 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution equal to or lower than i-bit precision, the output Ii from the OR circuit  26  becomes 1 level. At this time, the first offset addition unit  11 , the partial A/D converter  12 , the partial D/A converter  13 , the second offset addition unit  14 , the subtracter, and the arithmetic amplifier  16  halt their operations. The partial A/D converter  12  outputs [00] as the partial digital value  9  indicated by MiLi.  
         [0093]    Further, in the fourth pipeline stage  6 , when all of the inputted number-of-bits selection signals B 1 ˜B 4  are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 5-bit precision, the output Ii from the OR circuit  26  becomes 0 level. At this time, the fourth pipeline stage  6  performs a series of pipeline operations as usual. That is, the Pi−1, which is the inputted first partial analog voltage value  8 , is A/D converted by the partial A/D converter  12 , thereby outputting, as the partial digital value  9 , [00,01,10,11] which are binary codes each corresponding to 2-bit output and having Mi as a higher-order bit and Li as a lower-order bit.  
         [0094]    On the other hand, when at least one of the inputted number-of-bits selection signals B 1 ˜B 4  is 1 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution equal to or lower than 4-bit precision, the output Ii from the OR circuit  26  becomes 1 level. At this time, the partial A/D converter  12  halts its operation. It is assumed that the partial A/D converter  12  outputs [00] as the partial digital value  9  indicated by MiLi. The foregoing is the operations of the pipeline stages used in the pipeline A/D converter according to the first embodiment.  
         [0095]    [0095]FIG. 5 is a block diagram illustrating the specific construction of the correction A/D converter  27  employed in the pipeline A/D converter according to the first embodiment. The correction A/D converter  27  is supplies with, as inputs, the partial analog voltage values P 0 ˜P 3  of the respective pipeline stages, selects one of the partial analog voltage values P 0 ˜P 3  according to the number-of-bits selection signals B 5 ˜B 1 , performs voltage comparison, and outputs the correction partial digital value  28 .  
         [0096]    To be specific, the partial analog voltage values P 0 ˜P 3  of the respective pipeline stages, which are supplied as inputs, are connected through switches SW 1 ˜SW 4  to a Vin terminal as an input voltage terminal of a voltage comparator  29 , respectively. A Vref terminal as a reference voltage terminal of the voltage comparator  29  is connected to GND level. Further, a C terminal output which indicates a result of voltage comparison performed by the voltage comparator  29  is outputted as the correction partial digital value  28  that is indicated by Cout.  
         [0097]    Among the number-of-bits selection signals  26 , B 1 ˜B 4  are connected to the switches SW 1 ˜SW 4 , respectively, to determine the ON/OFF states of the switches SW 1 ˜SW 4 , while B 5  is connected to a PD terminal that is a power-off terminal of the voltage comparator  29 . The foregoing is the construction of the correction A/D converter  27  employed in the pipeline A/D converter of the first embodiment.  
         [0098]    Hereinafter, the operation of the correction A/D converter  27  will be described in detail with reference to FIG. 23. Among the number-of-bits selection signals B 1 ˜B 5  inputted to the correction A/D converter  27 , one is 1 level while the others are 0 level. When the number-of-bits selection signal B 5  is 1 level while the other number-of-bits selection signals B 1 ˜B 4  are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 5-bit precision (refer to FIG. 23( a )), the voltage comparator  29  is halted. At this time, the voltage comparator  29  outputs either 0 level or 1 level as the correction partial digital value Cout.  
         [0099]    When the number-of-bits selection signal B 4  is 1 level while the other number-of-bits selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 4-bit precision (refer to FIG. 23( b )), the switch SW 4  is ON and the other switches are OFF. Then, the voltage comparator  29  compared the partial analog voltage value P 3  that is applied to the Vin terminal through the switch SW 4 , with the GND level connected to the Vref terminal, and outputs a binary code corresponding to 1-bit output as the correction partial digital value Cout. Likewise, when the number-of-bits selection signal B 3  is 1 level while the other number-of-bits selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 3-bit precision (refer to FIG. 23( c )), the switch SW 3  is ON and the other switches are OFF. Then, the voltage comparator  29  compares the partial analog voltage value P 2  that is applied to the Vin terminal through the switch SW 3 , with the GND level connected to the Vref terminal, and outputs a binary code corresponding to 1-bit output as the correction partial digital value Cout. When the number-of-bits selection signal B 2  is 1 level while the other number-of-bits selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 2-bit precision (refer to FIG. 23( d )), the switch SW 2  is ON and the other switches are OFF. Then, the voltage comparator  29  compares the partial analog voltage value P 1  that is applied to the Vin terminal through the switch SW 2 , with the GND level connected to the Vref terminal, and outputs a binary code corresponding to 1-bit output as the correction partial digital value Cout. When the number-of-bits selection signal B 1  is 1 level and the other number-of-bits selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 1-bit precision (refer to FIG. 23( e )), the switch SW 1  is ON and the other switches are OFF. Then, the voltage comparator  29  compares the partial analog voltage value P 0  that is applied to the Vin terminal through the switch SW 1 , with the GND level connected to the Vref terminal, and outputs a binary code corresponding to 1-bit output as the correction partial digital value Cout.  
         [0100]    The foregoing is the operation of the correction A/D converter  27  employed in the pipeline A/D converter according to the first embodiment.  
         [0101]    While in this first embodiment the partial analog voltage values P 0 ˜P 3  are selected using the switches as shown in FIG. 5, other circuits may be employed so long as the circuits can select voltage values.  
         [0102]    Further, while in this first embodiment the voltage comparator  29  shown in FIG. 5 compares the partial analog voltage values P 0 ˜P 3  with the GND level, since a reference level of the partial analog voltage values P 0 ˜P 3  can be arbitrarily set, these values P 0 ˜P 3  may be compared with the reference level.  
         [0103]    [0103]FIG. 6 is a block diagram illustrating the specific constructions of the correction code conversion circuit  23  and the coding circuit  7  which are employed in the pipeline A/D converter according to the first embodiment. The correction code conversion circuit  23  is supplied with the partial digital values M 1 L 1 ˜M 4 L 4  outputted from the first to fourth pipeline stages  3 ˜ 6 , the correction partial digital value Cout outputted from the correction A/D converter  27 , and the number-of-bits selection signals B 1 ˜B 4  (26), and outputs partial digital values m 1 ˜m 4  which are obtained by correcting the higher-order bits M 1 ˜M 4  of the partial digital values, using the correction partial digital value Cout, according to the number-of-bits selection signals B 1 ˜B 4 . In FIG. 6, an end of each of switches SW 11 ˜SW 41  is connected to an end of each of switches SW 12 ˜SW 42 , and the connected ends of the switches are outputted as the partial digital values m 1 ˜m 4  to be connected to the coding circuit  7 .  
         [0104]    The partial digital values M 1 ˜M 4  are connected to ends of the switches SW 11 ˜SW 41 , which are not connected to the switches SW 12 ˜SW 42 . On the other hand, the partial digital values L 1 ˜L 4  bypass the correction code conversion circuit  23 , and are directly connected to the coding circuit  4 . The correction partial digital value Cout is connected to the ends of the switches SW 12 ˜SW 42 , which are not connected to the switches SW 11 ˜SW 41 . The number-of-bits selection signals B 1 ˜B 4  (26) are connected to the switches SW 11 ˜SW 41  and to the switches SW 12 ˜SW 42 , respectively, to determine the ON/OFF states of the respective switches.  
         [0105]    Next, the operation of the correction code conversion circuit  23  will be described. As for the number-of-bits selection signals B 1 ˜B 4  (26) to be input to the correction code conversion circuit  23 , all of these signals are 0 level, or one of these signals is 1 level while the others are 0 level. When all of the number-of-bits selection signals B 1 ˜B 4  are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 5-bit precision, the switches SW 11 ˜SW 41  are ON and the switches SW 12 ˜SW 42  are OFF, and the partial digital values M 1 ˜M 4  are output through the switches SW 11 ˜SW 41 , respectively, to be input to the coding circuit  7  without being corrected.  
         [0106]    When the number-of-bit selection signal B 4  is 1 level while the other number-of-bit selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 4-bit precision, the switches SW 11 ˜SW 31  and SW 42  are ON and the switches SW 12 ˜SW 32  and SW 41  are OFF. Then, the partial digital values M 1 ˜M 3  and the correction partial digital value Cout are output through the switches SW 11 ˜SW 31  and SW 42 , respectively, and the partial digital value M 4  is corrected, and these partial digital values are input to the coding circuit  7 .  
         [0107]    Likewise, when the number-of-bit selection signal B 3  is 1 level while the other number-of-bit selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 3-bit precision, the switches SW 11 , SW 21 , SW 32 , and SW 41  are ON while the switches SW 12 , SW 22 , SW 31 , and SW 42  are OFF. Then, the partial digital values M 1  and M 2 , the correction partial digital value Cout, and the partial digital value M 4  are output through the switches SW 11 , SW 21 , SW 32 , and SW 41 , respectively, and the partial digital value M 3  is corrected, and these partial digital values are input to the coding circuit  7 .  
         [0108]    Likewise, when the number-of-bit selection signal B 2  is 1 level while the other number-of-bit selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 2-bit precision, the switches SW 11 , SW 22 , SW 31 , and SW 41  are ON while the switches SW 12 , SW 21 , SW 32 , and SW 42  are OFF. Then, the partial digital value M 1 , the correction partial digital value Cout, and the partial digital values M 3  and M 4  are output through the switches SW 11 , SW 22 , SW 31 , and SW 41 , respectively, and the partial digital value M 2  is corrected, and these partial digital values are input to the coding circuit  7 .  
         [0109]    Likewise, when the number-of-bit selection signal B 1  is 1 level while the other number-of-bit selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 1-bit precision, the switches SW 12  and SW 21 ˜SW 41  are ON while the switches SW 11  and SW 22 ˜SW 42  are OFF. Then, the correction partial digital value Cout and the partial digital values M 2 ˜M 4  are output through the switches SW 12  and SW 21 ˜SW 41 , respectively, and the partial digital value M 1  is corrected, and these partial digital values are input to the coding circuit  7 .  
         [0110]    On the other hand, the partial digital values L 1 ˜L 4  are input to the coding circuit  7  as they are, regardless of the number-of-bits selection signals B 1 ˜B 4  (26).  
         [0111]    While in this first embodiment the correction code conversion circuit  23  selects the partial digital values M 1 ˜M 4  using the switches as shown in FIG. 6, it may be constituted by other circuits than the switches so long as the circuits can select the digital values.  
         [0112]    As described above, according to the pipeline A/D converter of the first embodiment, when resolution of a digital output signal requested by the system varies over a range of  1 ˜ 5  bits, a series of pipeline operations performed at the pipeline stages whose operations are not needed can be halted, whereby power consumption of the pipeline A/D converter is reduced. Simultaneously, even when the pipeline operations of the pipeline stages which output the partial digital values that are not required for constituting the digital value to be output, are halted, a compensation bit Cout for determining a least significant bit is generated by the correction A/D converter  27 , and this compensation bit Cout is added to desired bits by the correction code conversion circuit  23 , whereby resolution of the digital value to be output is compensated.  
         [0113]    While in this first embodiment a pipeline A/D converter which can vary resolution of a digital output signal over a range of  1 ˜ 5  bits is described, the number of bits of the digital output signal can be arbitrarily set.  
         [0114]    Further, while in this first embodiment a A/D converter in which a partial digital value to be output from each pipeline stage is 1.5 bits or 2 bits is described, the number of bits of the partial digital value to be output from each pipeline stage may be arbitrarily set.  
         [0115]    Furthermore, while in this first embodiment a A/D converter in which resolution of a correction A/D converter is 1 bit is described, the number of bits of a partial digital value for correction to be output from the correction A/D converter can be arbitrarily set.  
       Embodiment 2  
       [0116]    [0116]FIG. 7 is a block diagram illustrating the construction of a pipeline A/D converter according to a second embodiment of the present invention, wherein the same reference numerals as those shown in FIG. 1 denote the same or corresponding parts. More specifically, FIG. 7 shows the construction of a pipeline A/D converter capable of varying resolution of a digital output signal over a range of  2 ˜ 5  bits.  
         [0117]    The pipeline A/D converter according to the second embodiment is supplied with an analog input signal  1 , and outputs a digital output signal  2 . To be specific, the analog input signal  1  indicated by a partial analog voltage P 0  is input to a first pipeline stage  3 . An output of the first pipeline stage  3 , which is indicated by a partial analog voltage value P 1 , is input to a second pipeline stage  4 . An output of the second pipeline stage  4 , which is indicated by a partial analog voltage value P 2 , is input to a third pipeline stage  5 . An output of the third pipeline stage  5 , which is indicated by a partial analog voltage value P 3 , is input to a fourth pipeline stage  6 .  
         [0118]    Partial digital values M 1 L 1 ˜M 4 L 4  obtained in the respective pipeline stages are connected to a correction code conversion circuit  23   a , and an output of the correction code conversion circuit  23   a  is connected to a coding circuit  7 . On the other hand, a number-of-bits control signal  24  is connected to a number-of-bits control circuit  25   a , and a number-of-bits selection signal  26  as an output of the number-of-bits control circuit  25   a  is connected to the first to fourth pipeline stages  3 ˜ 6  and to the correction code conversion circuit  23   a . Further, the partial analog voltage values P 0 ˜P 2  are connected to the fourth pipeline stage  6 .  
         [0119]    Next, the operation of the pipeline A/D converter will be described with reference to FIG. 24. Initially, a description will be given of a case where the pipeline A/D converter outputs a digital output signal having 5-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 5-bit resolution is applied to the number-of-bits control circuit  25   a , the number-of-bits control circuit  25   a  outputs the number-of-bits selection signal  26  so as to operate the first to fourth pipeline stages  3 ˜ 6 . At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P 1 , according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the correction code conversion circuit  23   a  and the second pipeline stage  4 , respectively.  
         [0120]    Likewise, when the partial analog voltage value P 1  is input to the second pipeline stage  4 , the second pipeline stage  4  outputs the partial digital value M 2 L 2  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 2 , according to the partial analog voltage value P 1 . The partial digital value M 2 L 2  and the partial analog voltage value P 2  are input to the correction code conversion circuit  23   a  and the third pipeline stage  5 , respectively. Likewise, when the partial analog voltage value P 2  is input to the third pipeline stage  5 , the third pipeline stage  5  outputs the partial digital value M 3 L 3  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 3 , according to the partial analog voltage value P 2 . The partial digital value M 3 L 3  and the partial analog voltage value P 3  are input to the correction code conversion circuit  23   a  and the fourth pipeline stage  6 , respectively. When the partial analog voltage value P 3  is input to the fourth pipeline stage  6 , the fourth pipeline stage  6  outputs the partial digital value M 4 L 4  which is a binary code having 2 bits of information and constitutes a least significant bit of the digital output signal  2 , according to the partial analog voltage value P 3 , and the partial digital value M 4 L 4  is input to the correction code conversion circuit  23   a.    
         [0121]    In order to output a digital output signal having 5-bit resolution, it is not necessary to correct the partial digital values M 1 L 1 ˜M 4 L 4  outputted from the first to fourth pipeline stages  3 ˜ 6 . So, the partial digital values M 1 L 1 ˜M 4 L 4  are input to the coding circuit  7  as they are, and coded in the coding circuit  7 , thereby outputting the digital output signal  2  which is a binary code and has 5-bit resolution (refer to FIG. 24( a )). In this way, the pipeline A/D converter outputs the digital output signal having 5-bit resolution.  
         [0122]    Next, a description will be given of the operation of the pipeline A/D converter when it outputs a digital output signal having 4-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 4-bit resolution is input to the number-of-bits control circuit  25   a , the number-of-bits control circuit  25   a  outputs the number-of-bits selection signal  26  so as to operate the first, second, and fourth pipeline stages  3 ,  4 , and  6 , and halt the third pipeline stage  5 . It is assumed that the halted third pipeline stage  5  outputs [00] as the partial digital value M 3 L 3 . At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P 1 , according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the correction code conversion circuit  23   a  and the second pipeline stage  4 , respectively.  
         [0123]    Likewise, when the partial analog voltage value P 1  is input to the second pipeline stage  4 , the second pipeline stage  4  outputs the partial digital value M 2 L 2  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 2 , according to the partial analog voltage value P 1 . The partial digital value M 2 L 2  and the partial analog voltage value P 2  are input to the correction code conversion circuit  23   a  and the fourth pipeline stage  6 , respectively. When the partial analog voltage value P 2  is input to the fourth pipeline stage  6 , the fourth pipeline stage  6  outputs the partial digital value M 4 L 4  which is a binary code having 2 bits of information, according to the partial analog voltage value P 2 , and the partial digital value M 4 L 4  is input to the correction code conversion circuit  23   a.    
         [0124]    In order to output a digital output signal having 4-bit resolution, the partial digital value M 3 L 3  outputted from the third pipeline stage  5  is corrected using the partial digital value M 4 L 4 , and the partial digital value M 4 L 4  is corrected to [00], and thereafter, the partial digital values M 1 L 1 ˜M 4 L 4  are input to the coding circuit  7 , and coded in the coding circuit  7 , thereby outputting the digital output signal  2  which is a binary code and has 4-bit resolution (refer to FIG. 24( b )). In this way, the pipeline A/D converter outputs the digital output signal having 4-bit resolution.  
         [0125]    Next, a description will be given of the operation of the pipeline A/D converter when it outputs a digital output signal having 3-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 3-bit resolution is input to the number-of-bits control circuit  25   a , the number-of-bits control circuit  25   a  outputs the number-of-bits selection signal  26  so as to operate the first and fourth pipeline stages  3  and  6 , and halt the second and third pipeline stages  4  and  5 . It is assumed that the halted second and third pipeline stages  4  and  5  output [00] as the partial digital values M 2 L 2  and M 3 L 3 , respectively. At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P 1 , according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the correction code conversion circuit  23   a  and the fourth pipeline stage  6 , respectively. When the partial analog voltage value P 1  is input to the fourth pipeline stage  6 , the fourth pipeline stage  6  outputs the partial digital value M 4 L 4  which is a binary code having 2 bits of information, according to the partial analog voltage value P 1 , and the partial digital value M 4 L 4  is input to the correction code conversion circuit  23   a . In order to output a digital output signal having 3-bit resolution, the partial digital value M 2 L 2  outputted from the second pipeline stage  4  is corrected using the partial digital value M 4 L 4 , and the partial digital value M 4 L 4  is corrected to [00], and thereafter, the partial digital values M 1 L 1 ˜M 4 L 4  are input to the coding circuit  7 , and coded in the coding circuit  7 , thereby outputting the digital output signal  2  which is a binary code and has 3-bit resolution (refer to FIG. 24( c )). In this way, the pipeline A/D converter outputs the digital output signal having 3-bit resolution.  
         [0126]    Next, a description will be given of the operation of the pipeline A/D converter when it outputs a digital output signal having 2-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 2-bit resolution is input to the number-of-bits control circuit  25   a , the number-of-bits control circuit  25   a  outputs the number-of-bits selection signal  26  so as to operate the fourth pipeline stage  6 , and halt the first to third pipeline stages  3 ˜ 5 . It is assumed that the halted first to third pipeline stages  3 ˜ 5  output [00] as the partial digital values M 1 L 1 ˜M 3 L 3 , respectively. At this time, the analog input signal  1  indicated by the partial analog voltage value P 0  is input to the fourth pipeline stage  6 . When the partial analog voltage value P 0  is input to the fourth pipeline stage  6 , the fourth pipeline stage  6  outputs the partial digital value M 4 L 4  which is a binary code having 2 bits of information, according to the analog input signal  1 , and the partial digital value M 4 L 4  is input to the correction code conversion circuit  23   a . In order to output a digital output signal having 2-bit resolution, the partial digital value M 1 L 1  outputted from the first pipeline stage  3  is corrected using the partial digital value M 4 L 4 , and the partial digital value M 4 L 4  is corrected to [00], and thereafter, the partial digital values M 1 L 1 ˜M 4 L 4  are input to the coding circuit  7 , and coded in the coding circuit  7  codes, thereby outputting the digital output signal  2  which is a binary code and has 2-bit resolution (refer to FIG. 24( d )). In this way, the pipeline A/D converter outputs the digital output signal having 2-bit resolution.  
         [0127]    [0127]FIG. 8 is a block diagram illustrating the specific construction of the number-of-bits control circuit  25   a  employed in the pipeline A/D converter according to the second embodiment. the number-of-bits control signal A 0  is connected to the first NOT gate and to the first input terminals of the first and third NOR gates, and the number-of-bits control signal A 1  is connected to the second NOT gate and to the second input terminals of the first and second NOR gates. An output of the first NOT gate is connected to the first input terminals of the second and fourth NOR gates, and an output of the second NOT gate is connected to the second input terminals of the third and fourth NOR gates. An output of the first NOR gate is outputted as a bit selection signal  26  indicated by B 3 , an output of the third NOR gate is outputted as a bit selection signal  26  indicated by B 3 , an output of the third NOR gate is outputted as a bit selection signal  26  indicated by B 4 , and an output of the fourth NOR gate is outputted as a bit selection signal  26  indicated by B 5 . The foregoing is the construction of the number-of-bits control circuit used in the pipeline A/D converter of the second embodiment.  
         [0128]    Next, the operation of the number-of-bits control circuit  25   a  will be described in detail. When [00] is input to the number-of-bits control signals A 1 ˜A 0 , only the output of the first NOR gate becomes 1 level while the outputs of the second to fourth NOR gates become 0 level, whereby the number-of-bits selection signals B 5 ˜B 2  [0001] are outputted. When [01] is input to the number-of-bits control signals A 1 ˜A 0 , only the output of the second NOR gate becomes 1 level while the outputs of the first, third, and fourth NOR gates become 0 level, whereby the number-of-bits selection signals B 5 ˜B 2  [0010] are outputted. When [10] is input to the number-of-bits control signals A 1 ˜A 0 , only the output of the third NOR gate becomes 1 level while the outputs of the first, second, and fourth NOR gates become 0 level, whereby the number-of-bits selection signals B 5 ˜B 2  [0100] are outputted. When [11] is input to the number-of-bits control signals A 1 ˜A 0 , only the output of the fourth NOR gate becomes 1 level while the outputs of the first to third NOR gates become 0 level, whereby the number-of-bits selection signals B 5 ˜B 2  [1000] are outputted.  
         [0129]    When the number-of-bits selection signal B 5  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter according to the second embodiment operates as an A/D converter having 5-bit resolution. Likewise, when the number-of-bits selection signal B 4  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter operates as an A/D converter having 4-bit resolution. When the number-of-bits selection signal B 3  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter operates as an A/D converter having 3-bit resolution. When the number-of-bits selection signal B 2  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter operates as an A/D converter having 2-bit resolution.  
         [0130]    While the number-of-bits control circuit  25   a  shown in FIG. 8 is constituted using the NOT gates and the NOR gates, it may be constituted by other logic circuits with the same effects as mentioned above.  
         [0131]    Further, in FIG. 8, when the logical states of the number-of-bits control signals A 1 ˜A 0  are [00], [01], [10], and [11], the number-of-bits selection signals B 5 ˜B 2  are outputted as [0001], [0010], [0100], and [1000], respectively. However, the logical states of the number-of-bits control signals A 1 ˜A 0  may be arbitrarily selected so long as the number-of-bits selection signals B 5 ˜B 2  are uniquely determined according to the number-of-bits control signals A 1 ˜A 0 , and the circuits receiving the number-of-bits selection signals B 5 ˜B 2  correspond to the logical states, so that the same effects as mentioned above are obtained.  
         [0132]    Furthermore, in FIG. 8, only one selection signal among the number-of-bits selection signals B 5 ˜B 2  is outputted as 1 level while the remaining three selection signals are outputted as 0 level. However, the same effects as mentioned above can be obtained even when only one selection signal among the number-of-bits selection signals B 5 ˜B 2  is outputted as 0 level while the remaining three selection signals are outputted as 1 level, or when arbitrary logical states are outputted as the number-of-bits selection signals B 5 ˜B 2 , so long as the number-of-bits selection signals B 5 ˜B 2  are uniquely determined according to the number-of-bits control signals A 1 ˜A 0 , and the circuits receiving the number-of-bits selection signals correspond to the logical states.  
         [0133]    Hereinafter, the constructions of the i-th (first to fourth) pipeline stages will be described in more detail. FIG. 9 is a block diagram illustrating the construction of each of the first to third pipeline stages  3 ˜ 5  employed in the pipeline A/D converter according to the second embodiment. Each of the first to third pipeline stages  3 ˜ 5  is supplied with, as an input, a first partial analog voltage value  8  indicated by Pi−1, and outputs a partial digital value  9  indicated by MiLi, and a second partial analog voltage value  10  indicated by Pi. Further, each of the first to third pipeline stages is supplied with, as inputs, the number-of-bits selection signals B 2 ˜Bi+ 1 , and it is determined whether each pipeline stage should perform a series of pipeline operations or halt it, according to the number-of-bits selection signals.  
         [0134]    The first partial analog voltage value  8  is input to a first offset addition unit  11 . An output of the first offset addition unit  11  is input to a partial A/D converter  12 . An output of the partial A/D converter  12  is input to a partial D/A converter  13 . An output of the partial D/A converter  13  is input to a second offset addition unit  14 . An output of the second offset addition unit  14  and the first partial analog voltage value  8  are input to a subtracter  15 . An output of the subtracter  15  is input to an arithmetic amplifier  16 . An output of the arithmetic amplifier  16  becomes the second partial analog voltage value  10 .  
         [0135]    Only the number-of-bits selection signal B 2  is connected to an OR circuit in the first pipeline stage  3 , and the number-of-bits selection signals B 2 ˜B 4  (i.e., the number-of-bits selection signals B 2 ˜Bi+1) are connected to an OR circuit in the third pipeline stage  5 , and an output Ii of the OR circuit is connected to the first offset addition unit  11 , the partial A/D converter  12 , the partial D/A converter  13 , the second offset addition unit  14 , the subtracter  15 , and the arithmetic amplifier  16 .  
         [0136]    [0136]FIG. 10 is a block diagram illustrating the fourth pipeline stage  6  which is used in the pipeline A/D converter. The fourth pipeline stage is supplied with, as inputs, the partial analog voltage values P 0 ˜P 3  of the respective pipeline stages, selects one of the partial analog voltage values P 0 ˜P 3  according to the number-of-bits selection signals B 2 ˜B 5 , and outputs the partial digital value  9  indicated as MiLi. The inputted partial analog voltage values P 0 ˜P 3  of the respective pipeline stages are connected through switches SW 1 ˜SW 4  to the partial A/D converter  12 , and the partial digital value  9  indicated as MiLi is outputted. The number-of-bits selection signals B 2 ˜B 5  are connected to the switches SW 1 ˜SW 4 , respectively, and determine the ON/OFF states of the respective switches. In this way, a selection means S, which selects an analog voltage value to be processed in the fourth pipeline stage  6  according to the number-of-bits selection signals, is constituted.  
         [0137]    Hereinafter, the operations of the i-th (first to fourth) pipeline stages will be described in detail. In each of the first to third pipeline stages  3 ˜ 5 , when all of the inputted number-of-bits selection signals B 2 ˜Bi+1 are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution equal to or higher than (i+2)-bit precision, the output Ii from the OR circuit ( 26 ) becomes 0 level. At this time, a series of pipeline operations is carried out as usual. The first offset addition unit  11  adds an offset voltage value equivalent to −0.5 LSB, as a first offset voltage value  17 , to the Pi−1 which is the inputted partial analog voltage value  8 , and the partial A/D converter  12  performs A/D conversion on the Pi−1, thereby outputting, as the partial digital value  9 , [00,01,10] which are binary codes each corresponding to 1.5-bit output and having Mi as a higher-order bit and Li as lower-order bit. Furthermore, the partial D/A converter  13  performs D/A conversion on the partial digital value  9 , and the second offset addition unit  14  adds an offset voltage value equivalent to +0.5 LSB, as a second offset voltage value  18 , to a result of D/A conversion. Then, an output of the second offset addition unit  14  and the first partial analog voltage value  8  are input to the subtracter  15 , and a difference voltage value between them is obtained. Thereafter, the arithmetic amplifier amplifies the difference voltage value to obtain the Pi as the second partial analog voltage value  10 .  
         [0138]    When at least one of the inputted number-of-bits selection signals B 2 ˜Bi+1 is 1 level, i.e., when the pipeline A/D converter serves as an A/D converter having resolution equal to or lower than (i+1)-bit precision, the output Ii from the OR circuit ( 26 ) becomes 1 level. At this time, the first offset addition unit  11 , the partial A/D converter  12 , the partial D/A converter  13 , the second offset addition unit  14 , the subtracter  15 , and the arithmetic amplifier  16  halt their operations. The partial A/D converter  12  outputs [00] as the partial digital value  9  indicated by MiLi.  
         [0139]    Further, in the fourth pipeline stage  6 , when the number-of-bit selection signal B 5  is 1 level and the other number-of-bits selection signals are 0 level, i.e., when the pipeline A/D converter serves as an A/D converter having resolution of 5-bit precision, the switch SW 4  is ON and the other switches are OFF, and the partial A/D converter  12  performs A/D conversion on the partial analog voltage value P 3  that is connected through the switch SW 4 , thereby outputting, as the partial digital value  9 , [00,01,10,11] which are binary codes each corresponding to 2-bit output and having Mi as a higher-order bit and Li as a lower-order bit.  
         [0140]    Further, when the number-of-bits selection signal B 4  is 1 level while the other number-of-bits selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 4-bit precision, the switch SW 3  is ON and the other switches are OFF, and the partial A/D converter  12  performs A/D conversion on the partial analog voltage value P 2  connected through the switch SW 3 , thereby outputting, as the partial digital value  9 , [00,01,10,11] which are binary codes each corresponding to 2-bit output and having Mi as a higher-order bit and Li as a lower-order bit.  
         [0141]    Further, when the number-of-bits selection signal B 3  is 1 level while the other number-of-bits selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 3-bit precision, the switch SW 2  is ON and the other switches are OFF, and the partial A/D converter  12  performs A/D conversion on the partial analog voltage value P 1  is connected through the switch SW 2 , thereby outputting, as the partial digital value  9 , [00,01,10,11] which are binary codes each corresponding to 2-bit output and having Mi as a higher-order bit and Li as a lower-order bit.  
         [0142]    Furthermore, when the number-of-bits selection signal B 2  is 1 level while the other number-of-bits selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 2-bit precision, the switch SW 1  is ON and the other switches are OFF, and the partial A/D converter  12  performs A/D conversion on the partial analog voltage value P 0  that is connected through the switch SW 1 , thereby outputting, as the partial digital value  9 , [00,01,10,11] which are binary codes each corresponding to 2-bit output and having Mi as higher-order bits and Li as lower-order bits.  
         [0143]    [0143]FIG. 11 is a block diagram illustrating the specific constructions of the correction code conversion circuit  23   a  and the coding circuit  7  which are employed in the pipeline A/D converter according to the second embodiment. The correction code conversion circuit  23   a  is supplied with the partial digital values M 1 L 1 ˜M 4 L 4  outputted from the first to fourth pipeline stages  3 ˜ 6 , and the number-of-bits selection signals B 2 ˜B 5 , and outputs partial digital values m 1 l 1 ˜m 4 l 4  which are obtained by correcting the partial digital values M 1 L 1 ˜M 4 L 4  according to the number-of-bits selection signals B 2 ˜B 5 . An end of each of switches SW 111 ˜SW 411  is connected to an end of each of switches SW 112 ˜SW 412 , and the connected ends of these switches are outputted as partial digital values m 1 ˜m 4 . An end of each of switches SW 121 ˜SW 421  is connected to an end of each of switches SW 122 ˜SW 422 , and the connected ends of these switches are outputted as partial digital values l 1 ˜l 4  to be connected to the coding circuit  7 .  
         [0144]    The partial digital values M 1 ˜M 3  are connected to ends of the switches SW 111 ˜SW 311 , which are not connected to the switches SW 112 ˜SW 312 . Further, the partial digital value M 4  is connected to an end of the switch SW 411 , which is not connected to the switch SW 412 , and to ends of the switches SW 112 ˜SW 312 , which are not connected to the switches SW 111 ˜SW 311 . On the other hand, the partial digital values L 1 ˜L 3  are connected to ends of the switches SW 121 ˜SW 321 , which are not connected to the switches SW 122 ˜SW 322 . Further, the partial digital value L 4  is connected to an end of the switch SW 421 , which is not connected to the switch SW 422 , and to ends of the switches SW 122 ˜SW 322 , which are not connected to the switches SW 121 ˜SW 321 .  
         [0145]    Further, ends of the switches SW 412  and SW 422 , which are not connected to the switches SW 411  and SW 421 , are connected to the GND level. The number-of-bits selection signals B 2 ˜B 5  are connected to the switches SW 111 ˜SW 411 , the switches SW 112 ˜SW 412 , the switches SW 121 ˜SW 421 , and the switches SW 122 ˜SW 422 , to determine the ON/OFF states of the respective switches.  
         [0146]    Next, the operation of the correction code conversion circuit  23   a  employed in the pipeline A/D converter will be described. Among the number-of-bits selection signals B 2 ˜B 5  inputted to the correction code conversion circuit  23   a , one of these signals is 1 level while the others are 0 level. When the number-of-bit selection signal B 5  is 1 level while the other number-of-bit selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 5-bit precision, the switches SW 111 ˜SW 411  and SW 121 ˜SW 421  are ON and the switches SW 112 ˜SW 412  and SW 122 ˜SW 422  are OFF, whereby the partial digital values M 1 ˜M 4  are outputted through the switches SW 111 ˜SW 411  to be the partial digital values m 1 ˜m 4  without being corrected, respectively, and the partial digital values L 1 ˜L 4  are outputted through the switches SW 121 ˜SW 421  to be the partial digital values l 1 ˜l 4  without being corrected, respectively. The outputted partial digital values m 1 ˜m 4  and l 1 ˜l 4  are input to the coding circuit  7 .  
         [0147]    Likewise, when the number-of-bit selection signal B 4  is 1 level while the other number-of-bit selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 4-bit precision, the switches SW 111 ˜SW 211 , SW 312 ˜SW 412 , SW 121 ˜SW 221 , and SW 322 ˜SW 422  are ON and the switches SW 112 ˜SW 212 , SW 311 ˜SW 411 , SW 122 ˜SW 222 , and SW 321 ˜SW 421  are OFF. Thereby, the partial digital values M 1 , M 2 , and M 4  are outputted through the switches SW 111 , SW 211 , and SW 312 , respectively, the 0 level is output through the switch SW 412 , the partial digital values M 1  and M 2  are not corrected, the partial digital value M 3  is corrected by the partial digital value M 4 , and the partial digital value M 4  is corrected to the 0 level, resulting in the partial digital values m 1 ˜m 4 . The partial digital values m 1 ˜m 4  are input to the coding circuit  7 . The partial digital values L 1 , L 2 , and L 4  are outputted through the switches SW 121 , SW 221 , and SW 322 , respectively, the 0 level is output through the switch SW 422 , the partial digital values L 1  and L 2  are not corrected, the partial digital value L 3  is corrected by the partial digital value L 4 , and the partial digital value L 4  is corrected to the 0 level, resulting in the partial digital values l 1 ˜l 4 . The partial digital values l 1 ˜l 4  are input to the coding circuit  7 .  
         [0148]    Further, when the number-of-bit selection signal B 3  is 1 level while the other number-of-bit selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 3-bit precision, the switches SW 111 , SW 212 , SW 311 , SW 412 , SW 121 , SW 222 , SW 321 , and SW 422  are ON and the switches SW 112 , SW 211 , SW 312 , SW 411 , SW 122 , SW 221 , SW 322 , and SW 421  are OFF. Thereby, the partial digital values M 1 , M 4 , and M 3  are outputted through the switches SW 111 , SW 212 , and SW 311 , respectively, the 0 level is output through the switch SW 412 , the partial digital values MI and M 3  are not corrected, the partial digital value M 2  is corrected by the partial digital value M 4 , and the partial digital value M 4  is corrected to the 0 level, resulting in the partial digital values m 1 ˜m 4 . The partial digital values m 1 ˜m 4  are input to the coding circuit  7 . The partial digital values L 1 , L 4 , and L 3  are output through the switches SW 121 , SW 222 , and SW 321 , respectively, the 0 level is output through the switch SW 422 , the partial digital values LI and L 3  are not corrected, the partial digital value L 2  is corrected by the partial digital value L 4 , and the partial digital value L 4  is corrected to the 0 level, resulting in the partial digital values l 1 ˜l 4 . The partial digital values l 1 ˜l 4  are input to the coding circuit  7 .  
         [0149]    Furthermore, when the number-of-bit selection signal B 2  is 1 level while the other number-of-bit selection signals are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of 2-bit precision, the switches SW 112 , SW 211 ˜SW 311 , SW 412 , SW 122 , SW 221 ˜SW 321 , and SW 422  are ON and the switches SW 111 , SW 212 ˜SW 312 , SW 411 , SW 121 , SW 222 ˜SW 322 , and SW 421  are OFF. Thereby, the partial digital values M 4 , M 2 , and M 3  are output through the switches SW 112 , SW 211 , and SW 311 , respectively, the 0 level is output through the switch SW 412 , the partial digital values M 2  and M 3  are not corrected, the partial digital value M 1  is corrected by the partial digital value M 4 , and the partial digital value M 4  is corrected to the 0 level, resulting in the partial digital values m 1 ˜m 4 . The partial digital values m 1 ˜m 4  are input to the coding circuit  7 . The partial digital values L 4 , L 2 , and L 3  are output through the switches SW 122 , SW 221 , and SW 321 , respectively, the 0 level is output through the switch SW 422 , the partial digital values L 2  and L 3  are not corrected, the partial digital value L 1  is corrected by the partial digital value L 4 , and the partial digital value L 4  is corrected to the 0 level, resulting in the partial digital values l 1 ˜l 4 . The partial digital values l 1 ˜l 4  are input to the coding circuit  7 .  
         [0150]    While in FIG. 11 the correction code conversion circuit  23   a  selects the partial digital values M 1 L 1 ˜M 4 L 4  using the switches, it may be constituted by other circuits than the switches so long as the circuits can select the digital values.  
         [0151]    As described above, according to the pipeline A/D converter of the second embodiment, when resolution of a digital output signal requested by the system varies over a range of 2˜5 bits, a series of pipeline operations performed at the pipeline stages whose operations are not needed can be halted, whereby power consumption of the pipeline A/D-converter is reduced. Simultaneously, even when the pipeline operations of the pipeline stages which output the partial digital values that are not required for constituting the digital value to be output, are halted, resolution of the digital value to be output can be compensated.  
         [0152]    While in this second embodiment a pipeline A/D converter which can vary resolution of a digital output signal over a range of 2˜5 bits is described, the number of bits of the digital output signal may be set to an arbitrary value equal to or larger than 2.  
         [0153]    Further, while in this second embodiment an A/D converter in which a partial digital value to be outputted from each pipeline stage is 1.5 bits or 2 bits is described, the number of bits of the partial digital value to be output from each pipeline stage may be arbitrarily set.  
       Embodiment 3  
       [0154]    [0154]FIG. 12 is a block diagram illustrating the construction of a pipeline A/D converter according to a third embodiment of the present invention, which can change resolution of a digital output signal over a range of 1˜3 bits, or to 5 bits.  
         [0155]    The pipeline A/D converter according to the third embodiment is supplied with an analog input signal  1 , and outputs a digital output signal  2 . To be specific, the analog input signal  1  indicated by a partial analog voltage P 0  is input to a first pipeline stage  3 .- An output of the first pipeline stage  3 , which is indicated by a partial analog voltage value P 1 , is input to a second pipeline stage  4 . An output of the second pipeline stage  4 , which is indicated by a partial analog voltage value P 2 , is input to a third pipeline stage  5 . An output of the third pipeline stage  5 , which is indicated by a partial analog voltage value P 3 , is input to a fourth pipeline stage  6 . Partial digital values M 1 L 1 ˜M 4 L 4  obtained in the respective pipeline stages are connected to a coding circuit  7 . On the other hand, a number-of-bits control signal  24  is connected to a number-of-bits control circuit  25   b , and a number-of-bits selection signal  26  as an output of the number-of-bits control circuit  25   b  is connected to the first to fourth pipeline stages  3 ˜ 6 .  
         [0156]    Next, the operation of the pipeline A/D converter according to the third embodiment will be described. Initially, a description will be given of a case where the pipeline A/D converter outputs a digital output signal having 5-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 5-bit resolution is applied to the number-of-bits control circuit  25   b , the number-of-bits control circuit  25   b  outputs the number-of-bits selection signal  26  so as to operate the first to fourth pipeline stages  3 ˜ 6 . At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P 1 , according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the coding circuit  7  and the second pipeline stage  4 , respectively. Likewise, when the partial analog voltage value P 1  is input to the second pipeline stage  4 , the second pipeline stage  4  outputs the partial digital value M 2 L 2  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 2 , according to the partial analog voltage value P 1 . The partial digital value M 2 L 2  and the partial analog voltage value P 2  are input to the coding circuit  7  and the third pipeline stage  5 , respectively. Likewise, when the partial analog voltage value P 2  is input to the third pipeline stage  5 , the third pipeline stage  5  outputs the partial digital value M 3 L 3  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 3 , according to the partial analog voltage value P 2 . The partial digital value M 3 L 3  and the partial analog voltage value P 3  are input to the coding circuit  7  and the fourth pipeline stage  6 , respectively. When the partial analog voltage value P 3  is input to the fourth pipeline stage  6 , the fourth pipeline stage  6  outputs the partial digital value M 4 L 4  which is a binary code having 2 bits of information and constitutes a least significant bit of the digital output signal  2 , according to the partial analog voltage value P 3 , and the partial digital-value M 4 L 4  is input to the coding circuit  7 . The coding circuit  7  codes the inputted partial digital values to output the digital output signal  2  which is a binary code and has 5-bit resolution.  
         [0157]    Next, a description will be given of the operation of the pipeline A/D converter when it outputs a digital output signal having 3-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 3-bit resolution is input to the number-of-bits control circuit  25   b , the number-of-bits control circuit  25   b  outputs the number-of-bits selection signal  26  so as to operate the first to third pipeline stages  3 ˜ 5 , and halt the fourth pipeline stage  6 . It is assumed that the halted fourth pipeline stage  6  outputs [00] as the partial digital value M 4 L 4 . At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P 1 , according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the coding circuit  7  and the second pipeline stage  4 , respectively.  
         [0158]    When the partial analog voltage value P 1  is input to the second pipeline stage  4 , the second pipeline stage  4  outputs the partial digital value M 2 L 2  which is a binary code having 1.5 bits of information, and the partial analog voltage value P 2 , according to the partial analog voltage value P 1 . The partial digital value M 2 L 2  and the partial analog voltage value P 2  are input to the coding circuit  7  and the third pipeline stage  5 , respectively. When the partial analog voltage value P 2  is input to the third pipeline stage  5 , the third pipeline stage  5  outputs the partial digital value M 3  which is a binary code having 1 bit of information, and a 0 level as the partial digital value L 3 , according to the partial analog voltage value P 2 . The partial digital value M 3  and the 0 level are input to the coding circuit  7 . The fourth pipeline stage  6  outputs [00] as the partial digital value M 4 L 4  to the coding circuit  7 . The coding circuit  7  codes the M 1 L 1 ˜M 4 L 4 , actually, M 1 L 1 ˜M 3 L 3 , thereby outputting the digital output signal  2  which is a binary code and has 3-bit resolution.  
         [0159]    Next, a description will be given of the operation of the pipeline A/D converter when it outputs a digital output signal having 2-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 2-bit resolution is input to the number-of-bits control circuit  25   b , the number-of-bits control circuit  25   b  outputs the number-of-bits selection signal  26  so as to operate the pipeline stages  3  and  4 , and halt the pipeline stages  5  and  6 . It is assumed that the halted pipeline stages  5  and  6  output [00] as the partial digital values M 3 L 3  and M 4 L 4 , respectively. At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1 L 1  which is a binary code having 1.5 bits of information and constitutes a most significant bit of the digital output signal  2 , and the partial analog voltage value P 1 , according to the analog input signal  1 . The partial digital value M 1 L 1  and the partial analog voltage value P 1  are input to the coding circuit  7  and the second pipeline stage  4 , respectively.  
         [0160]    When the partial analog voltage value P 1  is input to the second pipeline stage  4 , the second pipeline stage  4  outputs the partial digital value M 2  which is a binary code having 1 bit of information, and a 0 level as the partial digital value L 2 , according to the partial analog voltage value P 1 . The partial digital value M 2  and the 0 level are input to the coding circuit  7 . The pipeline stages  5  and  6  output [00] as the partial digital values M 3 L 3  and M 4 L 4  to the coding circuit  7 . The coding circuit  7  codes the M 1 L 1 ˜M 4 L 4 , actually, M 1 L 1 ˜M 2 L 2 , thereby outputting the digital output signal  2  which is a binary code and has 2-bit resolution.  
         [0161]    Next, a description will be given of the operation of the pipeline A/D converter when it outputs a digital output signal having 1-bit resolution. When the number-of-bits control signal  24  for outputting a digital output signal having 1-bit resolution is input to the number-of-bits control circuit  25   b , the number-of-bits control circuit  25   b  outputs the number-of-bits selection signal  26  so as to operate the pipeline stage  3 , and halt the pipeline stages  4 ˜ 6 . It is assumed that the halted pipeline stages  4 ˜ 6  output [00] as the partial digital values M 2 L 2 ˜M 4 L 4 , respectively. At this time, when the analog input signal  1  is input to the first pipeline stage  3 , the first pipeline stage  3  outputs the partial digital value M 1  which is a binary code having 1 bit of information, and a 0 level as the partial digital value L 1 , according to the analog input signal  1 . The partial digital value M 1  and the 0 level are input to the coding circuit  7 . The partial digital value M 2  and the 0 level are input to the coding circuit  7 . The pipeline stages  4 ˜ 6  outputs [00] as the partial digital values M 2 L 2 ˜M 4 L 4  to the coding circuit  7 . The coding circuit  7  codes the M 1 L 1 ˜M 4 L 4 , actually, M 1 L 1 , thereby outputting the digital output signal  2  which is a binary code and has 1-bit resolution.  
         [0162]    [0162]FIG. 13 is a block diagram illustrating the specific construction of the number-of-bits control circuit  25   b  employed in the pipeline A/D converter according to the third embodiment. the number-of-bits control signal A 0  is connected to the first NOT gate and to the first input terminals of the first and third NOR gates, and the number-of-bits control signal A 1  is connected to the second NOT gate and to the second input terminals of the first and second NOR gates. An output of the first NOT gate is connected to the first input terminal of the second NOR gate, and an output of the second NOT gate is connected to the second input terminal of the third NOR gate. An output of the first NOR gate is outputted as a bit selection signal  26  indicated by B 1 , an output of the second NOR gate is outputted as a bit selection signal  26  indicated by B 2 , and an output of the third NOR gate is outputted as a bit selection signal  26  indicated by B 3 .  
         [0163]    Next, the operation of the number-of-bits control circuit  25   b  employed in the pipeline A/D converter will be described in detail. When [00] is input to the number-of-bits control signals A 1 ˜A 0 , only the output of the first NOR gate becomes 1 level while the outputs of the second and third NOR gates become 0 level, whereby the number-of-bits selection signals B 3 ˜B 1  [001] are outputted. When [01] is input to the number-of-bits control signals A 1 ˜A 0 , only the output of the second NOR gate becomes 1 level while the outputs of the first and third NOR gates become 0 level, whereby the number-of-bits selection signals B 3 ˜B 1  [010] are outputted. When [10] is input to the number-of-bits control signals A 1 ˜A 0 , only the output of the third NOR gate becomes 1 level while the outputs of the first and second NOR gates become 0 level, whereby the number-of-bits selection signals B 3 ˜B 1  [100] are outputted. When [11] is input to the number-of-bits control signals A 1 ˜A 0 , the outputs of the first to third NOR gates become 0 level, whereby the number-of-bits selection signals B 3 ˜B 1  [000] are outputted.  
         [0164]    When all of the number-of-bits selection signal B 3 ˜B 1  indicate 0 level, the pipeline A/D converter operates as an A/D converter having resolution of 5-bit precision. Likewise, when the number-of-bits selection signal B 3  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter operates as an A/D converter having resolution of 3-bit precision. Likewise, when the number-of-bits selection signal B 2  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter operates as an A/D converter having resolution of 2-bit precision. Likewise, when the number-of-bits selection signal B 1  indicates 1 level while the other number-of-bits selection signals indicate 0 level, the pipeline A/D converter operates as an A/D converter having resolution of 1-bit precision.  
         [0165]    While in FIG. 13 the number-of-bits control circuit  25   b  is constituted using the NOT gates and the NOR gates, it may be constituted by other logic circuits with the same effects as mentioned above.  
         [0166]    Further, in FIG. 13, when the logical states of the number-of-bits control signals A 1 ˜A 0  are [00], [01], [10], and [11], the number-of-bits selection signals B 3 ˜B 1  are outputted as [001], [010], [100], and [000], respectively. However, the logical states of the number-of-bits control signals A 1 ˜A 0  may be arbitrarily selected so long as the number-of-bits selection signals B 3 ˜B 1  are uniquely determined with respect to the number-of-bits control signals A 1 ˜A 0 , and the circuits receiving the number-of-bits selection signals B 3 ˜B 1  correspond to the logical states, so that the same effects as mentioned above are obtained.  
         [0167]    Furthermore, in FIG. 13, only one selection signal among the number-of-bits selection signals B 3 ˜B 1  is outputted as 1 level while the remaining two selection signals are outputted as 0 level. However, the same effects as mentioned above can be achieved even when only one selection signal among the number-of-bits selection signals B 3 ˜B 1  is output as 0 level while the remaining two selection signals are output as 1 level, or when arbitrary logical states are output as the number-of-bits selection signals B 3 ˜B 1 , so long as the number-of-bits selection signals B 3 ˜B 1  are uniquely determined with respect to the number-of-bits control signals A 1 ˜A 0 , and the circuits receiving the number-of-bits selection signals B 3 ˜B 1  correspond to the logical states.  
         [0168]    Hereinafter, the constructions of the i-th (first to fourth) pipeline stages will be described. FIG. 14 is a block diagram illustrating the construction of each of the first to third pipeline stages  3 ˜ 5  employed in the pipeline A/D converter according to the third embodiment. Each of the first to third pipeline stages  3 ˜ 5  is supplied with, as an input, a first partial analog voltage value  8  indicated by Pi−1, and outputs a partial digital value  9  indicated by MiLi, and a second partial analog voltage value  10  indicated by Pi. Further, the pipeline stage is supplied with, as inputs, the number-of-bits selection signals B 1 ˜Bi (26), and determines whether a series of pipeline operations should be carried out or halted, and whether a first offset voltage value should be added or not, according to the number-of-bits selection signals B 1 ˜Bi. The first partial analog voltage value  8  is input to a first offset addition unit  11 . An output of the first offset addition unit  11  is input to a partial A/D converter  12 . An output of the partial A/D converter  12  is input to a partial D/A converter  13 . An output of the partial D/A converter  13  is input to a second offset addition unit  14 . An output of the second offset addition unit  14  and the first partial analog voltage value  8  are input to a subtracter  15 . An output of the subtracter  15  is input to an arithmetic amplifier  16 . An output of the arithmetic amplifier  16  becomes the second partial analog voltage value  10 . Further, the first offset addition unit  11  is supplied to a bit selection signal Bi, thereby constituting an offset control means OFC which controls as to whether offset addition should be performed or not, according to the bit selection signal Bi.  
         [0169]    Only the number-of-bits selection signal B 1  is connected to an OR circuit ( 26 ) in the second pipeline stage, and the number-of-bits selection signals B 1 ˜B 2  (i.e., the number-of-bits selection signals B 1 ˜Bi−1) are connected to an OR circuit ( 26 ) in the third pipeline stage  5 . An output Ii of the OR circuit ( 26 ) is connected to the first offset addition unit  11 , the partial A/D converter  12 , the partial D/A converter  13 , the second offset addition unit  14 , the subtracter  15 , and the arithmetic amplifier  16 , and it is determined whether each circuit should be operated or not, according to the output Ii of the OR circuit ( 26 ).  
         [0170]    Since the first pipeline stage is always operated, it needs no OR circuit. The number-of-bit selection signal Bi is connected to the first offset addition unit  11 .  
         [0171]    [0171]FIG. 15 is a block diagram illustrating the fourth pipeline stage  6  which is employed in the pipeline A/D converter according to the third embodiment. The fourth pipeline stage is supplied with, as inputs, the first partial analog voltage value  8  indicated by Pi−1, and the number-of-bits selection signals B 1 ˜B 3  ( 26 ), and outputs the partial digital value  9  indicated by MiLi. The first partial analog voltage value  8  is input to the A/D converter  12 .  
         [0172]    Hereinafter, the operations of the i-th (first to fourth) pipeline stages employed in the pipeline A/D converter will be described with reference to FIG. 25. In the first pipeline stage  3 , when the number-of-bits selection signal B 1  is 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution equal to or higher than 2-bit precision, a series of pipeline operations is carried out as usual. In the second and third pipeline stages  4  and  5 , when all of the inputted number-of-bits selection signals B 1 ˜Bi are 0 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution equal to or higher than (i+1)-bit precision, the output Ii from the OR circuit ( 26 ) becomes 0 level. At this time, a series of pipeline operations is carried out as usual. The first offset addition unit  11  adds an offset voltage value equivalent to −0.5 LSB, as a first offset voltage value  17 , to the Pi−1 that is the inputted partial analog voltage value  8 , and then the partial A/D converter  12  performs A/D conversion on the Pi−1, thereby outputting, as the partial digital value  9 , [00,01,10] which are binary codes each corresponding to 1.5-bit output and having Mi as a higher-order bit and Li as a lower-order bit.  
         [0173]    Furthermore, the partial D/A converter  13  performs D/A conversion on the partial digital value  9 , and the second offset addition unit  14  adds an offset voltage value equivalent to +0.5 LSB to the result of the D/A conversion, as a second offset voltage value  18 . Then, an output of the second offset addition unit  14  and the first partial analog voltage value  8  are input to the subtracter  15 , and a difference voltage value between them is obtained. Thereafter, the difference voltage value is amplified by the arithmetic amplifier  16  to obtain the Pi as the second partial analog voltage value  10 .  
         [0174]    On the other hand, when at least one of the inputted number-of-bits selection signals B 1 ˜Bi−1 is 1 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution equal to or lower than (i−1)-bit precision, the output Ii from the OR circuit becomes 1 level. At this time, the first offset addition unit  11 , the partial A/D converter  12 , the partial D/A converter  13 , the second offset addition unit  14 , the subtracter  15 , and the arithmetic amplifier  16  halt their operations. The partial A/D converter  12  outputs [00] as the partial digital value  9  indicated by MiLi. On the other hand, when the inputted number-of-bit selection signal Bi is 1 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution of i-bit precision, the number-of-bit selection signal Bi is input to the first offset addition unit  11  to halt the offset addition operation.  
         [0175]    Thereby, the relationship of the first partial analog voltage value Pi−1 as an input, to the partial digital value MiLi and the second partial analog voltage value Pi as outputs, changes from the input-output relationship shown in FIG. 20 to the input-output relationship shown in FIG. 16. That is, the 0 level is outputted as the partial digital value Mi when the first partial analog voltage value Pi−1 is lower than the GND level, and the 1 level is outputted as the partial digital value Mi when the first partial analog voltage value Pi−1 is higher than the GND level. At this time, assuming that the 0 level is output as the partial digital value Li, the partial digital value Mi as the output of the i-th pipeline stage has no overlap with the partial digital values Mi+1Li+1˜M 4 L 4  which are outputted from the (i+1)th pipeline stage and subsequent pipeline stages, whereby an output of i-bit precision can be determined by only˜the i-th pipeline stage.  
         [0176]    On the other hand, in the fourth pipeline stage  6 , when all of the inputted number-of-bits selection signals B 1 ˜B 3  are 0 level, i.e., when the pipeline A/D converter severs as an A/D converter having resolution of 5-bit precision, the output Ii from the OR circuit ( 26 ) becomes 0 level. At this time, a series of pipeline operations is carried out as usual. The partial analog voltage value Pi−1 is subjected to A/D conversion, thereby outputting, as the partial digital value  9 , [00,01,10,11] which are binary codes each corresponding to 2-bit output and having Mi as a higher-order bit and Li as a lower-order bit. Further, when at least one of the number-of-bits selection signals B 1 ˜B 3  is 1 level, i.e., when the pipeline A/D converter operates as an A/D converter having resolution equal to or lower than 3-bit precision, the output Ii from the OR circuit ( 26 ) becomes 1 level. At this time, the A/D converter  12  halts its operation. The partial A/D converter  12  outputs [00] as the partial digital value  9  indicated by MiLi.  
         [0177]    As described above, according to the pipeline A/D converter of the third embodiment, when resolution of a digital output signal requested by the system changes over a range of 1˜3 bits, or to 5 bits, a series of pipeline operations performed at the pipeline stages whose operations are not needed can be halted, whereby power consumption of the pipeline A/D converter is reduced. Simultaneously, even when the pipeline operations of the pipeline stages which output the partial digital values that are not necessary in constructing the digital value to be output, are halted, precision of the digital value to be output can be compensated.  
         [0178]    While in this third embodiment a pipeline A/D converter which can change resolution of a digital output signal over a range of 1˜3 bits or to 5 bits is described, the number of bits of a digital output signal can be arbitrarily set, like 1˜n−2 bits or n bits, by changing the number of the pipeline stages.  
         [0179]    Furthermore, while in this third embodiment an A/D converter in which a partial digital value outputted from each pipeline stage is 1.5 bits or 2 bits is described, the number of bits of the partial digital value outputted from each pipeline stage can be arbitrarily set.