Abstract:
The present invention discloses an on-line calibration method, which utilizes two calibration algorithms running in the background without interrupting the normal operation of the analog signal process. The method includes performing a residue amplifier gain error calibration and performing a DAC non-linearity calibration. The residue amplifier gain error calibration can reduce the gain error of the residue amplifier for a missing code or a missing decision level phenomenon. The DAC non-linearity calibration can relax the matching requirement of passive components in current semiconductor processes. The present invention discloses a two-step ADC (Analog-to-Digital Converter), which includes a first signal processing unit, a second signal processing unit, a programmable gain control unit and a programmable reference voltage generator, performing the on-line calibration method.

Description:
RELATED U.S. APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       REFERENCE TO MICROFICHE APPENDIX  
       [0003]     Not applicable.  
       FIELD OF THE INVENTION  
       [0004]     The present invention relates to an on-line calibration method, and more particularly to an on-line calibration method based on the combination of digital and analog philosophies, which is especially suitable for a multi-step ADC (Analog-to-Digital Converter). The present invention further relates to a multi-step ADC, and more particularly to a multi-step ADC, which utilizes the on-line calibration method as a calibration technique.  
       BACKGROUND OF THE INVENTION  
       [0005]     In the last decade, due to the development of semiconductor processing technology, digital signal processing is getting more and more complex and computing capability is getting more and more powerful. More digital signal processing creates more stringent requirements on the interface to the real analog signal world. Currently, the performance index of analog-to-digital converter and digital-to-analog converter is pushing towards higher resolution, higher conversion rate, and most important of all, lower power dissipation that is suitable for embedded SOC applications.  
         [0006]     Current semiconductor processing technology limits the resolution of most of the ADC to around 10-12 bits. In order to get high resolution ADC circuit design, trimming in passive components or calibration circuit techniques are general solutions. The trimming technique is not a good approach due to its high implementation cost. Therefore, the calibration techniques using circuit design techniques are more popular currently.  
         [0007]     The first self-calibration ADC proposed by U.C. Berkeley researcher Hae-Seung Lee in 1984 is based on a successive approximation ADC. During calibration mode, the capacitor error is measured through a successive-approximation algorithm, and the error is saved to on-chip memory. During normal mode, the conversion output data will recall from on-chip memory, which contains capacitor error information, and these errors will be removed before data is sent out. In this approach, dedicated calibration mode is necessary, which means that analog input data cannot convert during calibration mode. In addition, successive approximation ADC has the disadvantage of low conversion speed, which cannot be used in video applications.  
         [0008]     Since 1990, most researchers have focused on the calibration algorithm of high-speed A/D architecture, especially on pipelined ADC. In 1992, Seung-Hoon Lee at Illinois University proposed a digital calibration algorithm for pipelined ADC. Instead of analog calibration, the digital calibration algorithm measures the pipelined error in the digital domain and corrects the error in the digital domain. Currently, most offline calibrations use a similar technique to Lee&#39;s approach, measuring errors in the digital domain and correcting errors in the digital domain with some extra analog circuits and extra control timing.  
         [0009]     The offline calibration described above needs an extra operation mode for calibration. However, in some data transmission applications, extra timing may not be possible. Since late 1990, researchers have developed a calibration algorithm that runs in the background so that the calibration will not interrupt the normal operations of the entire system. In 1997, Un-Ku Moon at Oregon University proposed a skip-and-fill algorithm to make background calibration possible. This algorithm uses similar calibration algorithms to the aforementioned. However, in order to make background calibration possible, the measurement cycles are spread over a long period of time. The algorithm skips a sample for every few samples, and the ‘stolen’ sample time is used for measuring the error of non-ideality. The skipped-sampled output will be generated by the post digital signal processing.  
         [0010]     In 1998, Joseph Ingino at Stanford University proposed a continuously calibrated pipelined ADC. In this case, an extra-pipelined stage is introduced. When normal operations begin, this extra stage is performing calibration. Once the calibration of the extra stage is done, the first stage of the pipelined ADC and the extra stage will swap. After that, the calibrated extra stage is the same as the first stage of the pipelined ADC and the original first stage of pipelined ADC is performing calibrations. In 2003, Boris Murmann at U.C. Berkeley proposed a pipelined ADC without any feedback loop for calibrations. The entire calibration algorithm is completed based on the digital-post-processing unit, which can run in the background. In this architecture, the pseudo-random signal causes the ADC to be operated in two-mode. Both modes can obtain correct results. In normal cases without any non-linearity error, the residue output difference of the same input between two modes should be the same all the time. However, if non-linearity in the inter-stage circuit occurs, the digital post-processing will correct the error.  
         [0011]     The previous background technique proposed by Boris Murmann needs quite complex digital signal processing like multiplications for all the converted samples.  
         [0012]     Although calibration techniques have been proposed to correct the mismatch errors (e.g., see U.S. Pat. Nos. 5,499,027, 6,529,149, 6,563,445, 6,720,895), they need other overheads to achieve and suffer from a variety of disadvantages. They are generally time-consuming, difficult to implement, and require additional structures.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     The objective of the present invention is to provide a multi-step (for example, two-step) analog-to-digital converter and an on-line calibration method thereof. The on-line calibration method uses two background calibration algorithms which can run in the background without interrupting the normal operations of analog signal processing and thus mitigates both the phenomena of missing code or missing decision level, which are caused by the gain error of the residue amplifier, and the nonlinearity error of the sub-DAC (Digital-to-Analog Converter). Therefore, the matching requirement of passive components by the current semiconductor process and the precision requirement of the residue amplifier are reduced. Consequently, the power dissipation decreases.  
         [0014]     The multi-step ADC of the present invention is based on a two-step ADC with conventional architecture, which does not change any normal analog signal processing. The on-line calibration method of the present invention is performed through the multi-step ADC by combining a first signal processing unit, a second signal processing unit, a programmable gain control unit and a programmable reference voltage generator.  
         [0015]     In order to achieve the objective, the present invention discloses an on-line calibration method comprising the steps of: (1) performing a residue amplifier gain error calibration and (2) performing a DAC non-linearity calibration. The step of performing the residue amplifier gain error calibration gathers the residue plot information between coarse digital output and fine digital output, and obtains the real gain error value via statistical analysis and then fixes the real gain error by one of three modes. The step of performing the DAC non-linearity calibration, with the same approach, gathers the residue plot information between coarse digital output and fine digital output, and obtains the difference (i.e., the error) between the ideal value and the real output value via statistical analysis. The error is then reduced with only simple digital subtraction or accumulation, which requires very few digital circuits.  
         [0016]     In order to perform the on-line calibration method, the present invention discloses a multi-step ADC, which is formed by adding four new blocks into a conventional multi-step ADC. These four new functional blocks are a first signal processing unit, a second signal processing unit, a programmable gain control unit and a programmable reference voltage generator. The first two functional blocks, which operate in the digital domain, are used to perform the residue amplifier gain calibration and to perform the DAC non-linearity calibration, respectively. The last two functional blocks, which operate in the analog domain, are used to determine the mode of adjustable gain control, and to generate an adjustable reference voltage, respectively. All the four added functional blocks are not involved in the process paths of analog signals, and therefore, the conversion rate will not be affected. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0017]     The invention will be described according to the appended drawings.  
         [0018]      FIG. 1  is a schematic view of a block diagram of an embodiment of a multi-step ADC of the present invention.  
         [0019]      FIG. 2  is a schematic view of an illustration of a residue plot of the residue signal (M=5).  
         [0020]      FIG. 3  is a flow chart of the residue amplifier gain error calibration of the present invention.  
         [0021]      FIG. 4  is another schematic view of an illustration showing the relative maximums and the relative minimums of the second digital codes, and the first code jump differences in the residue plot of the residue signal.  
         [0022]      FIG. 5 ( a ) and  FIG. 5 ( b ) show schematic views of illustrations of the residue plots of the residue signals with the gain error above one and below one, respectively.  
         [0023]      FIG. 6  is a flow chart of the DAC non-linearity calibration.  
         [0024]      FIG. 7  shows another schematic view of an illustration of the relative maximums and the relative minimums of the gain calibration codes, and the second code jump differences in the residue plot of the residue signal.  
         [0025]      FIG. 8  is a schematic view of an illustration of a residue plot of the residue signal with DAC non-linearity (M=5).  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     In order to explain the on-line calibration method of the present invention more smoothly, the apparatus to perform the method is described first.  FIG. 1  is a block diagram of an embodiment of a multi-step ADC  1  of the present invention. A two-step hardware architecture is used, hereinafter, as an example to explain the present invention. The two-step ADC  1  converts a first analog signal to a digital output code. The detailed description of  FIG. 1  is as follows. Then, the (25−2) nonlinear calibration codes DO 2 _DAC_CAL are fed to the third signal processing unit  20 .  
         [0027]     An M-bit sub-ADC  10  receives and converts a first analog signal to an M-bit first digital code DO 1 . In general, the first digital code DO 1  is called a coarse part. Then the first digital code DO 1  is fed to an M-bit DAC  12  and converted to a second analog signal. An analog subtractor  13  is used to subtract the second analog signal from the first analog signal to generate an error signal. The error signal is then amplified through a residue amplifier  14  to generate a residue signal VOUT 1 . After that, an N-bit ADC  16  is used to convert VOUT 1  to a second digital code DO 2 . In general, the second digital code DO 2  is called a fine part. The least significant bit (LSB) of the first digital code DO 1  and the most significant bit (MSB) of the corresponding second digital code DO 2  overlap.  
         [0028]     In order to perform the residue amplifier gain error calibration, it is necessary to deliver a plurality of first digital codes DO 1  and the corresponding second digital codes DO 2  to a first signal processing unit  18  that mainly performs the algorithm of the residue amplifier gain error calibration. Then the first signal processing unit  18  sends a reference control signal RC to a programmable reference voltage generator  17  or sends a gain control signal GC to a programmable gain control unit  15 . The first signal processing unit  18  also provides a plurality of gain calibration codes DO 2 _GAIN_CAL to a second signal processing unit  19  for performing the DAC non-linearity calibration. The programmable gain control unit  15 , which is an analog circuit, receives the gain control signal GC and sends an adjustable gain signal AV_CAL to the residue amplifier  14  and adjusts the gain thereof. The programmable reference voltage generator  17 , which is also an analog circuit, receives the reference control signal RC and provides a first calibration reference voltage VRP 2 _CAL and a second calibration reference voltage VRN 2 _CAL to an N-bit sub-ADC  16  and then adjusts the voltage range of the second digital codes DO 2 . After receiving the first digital codes DO 1  and the gain calibration codes DO 2 _GAIN_CAL, the second signal processing unit  19  performs the DAC non-linearity calibration. The result of performing the DAC non-linearity calibration is that a plurality of nonlinear calibration codes DO 2 _DAC_CAL, and the first digital codes DO 1  are fed to a third signal processing unit  20 . After that, the third signal processing unit  20  generates a plurality of digital output codes, which are with an (M+N−2) effective bit. A reference voltage generator  11  is used to provide a first fixed reference voltage VRP 1  and a second fixed reference voltage VRN 1  to determine the voltage range of the first digital codes DO 1 .  
         [0029]      FIG. 2  is a residue plot of the residue signal VOUT 1  with M=5, where the horizontal axis indicates the voltage range of the first digital codes DO 1 , which is defined by the first fixed reference voltage VRP 1  and the second fixed reference voltage VRN 1 . The horizontal range is divided to (2 5 −1) domains. Each domain is arranged and marked in order of the increment of the first digital codes DO 1 , i.e., from “0” to “2 5 −2”, in decimal format. The vertical axis indicates the voltage arrangement of the second digital codes DO 2 , which is defined by a first reference voltage VRP 2 , the current upper bound of the voltage range, and a second reference voltage VRN 2 , the current lower bound of the voltage range. The first reference voltage VRP 2  and the second reference voltage VRN 2  are generated by the programmable reference voltage generator  17 . The nodes  200 ,  201  to  229  are representative of the comparator thresholds of the M-bit sub-ADC  10 . In order to obtain a correct residue plot of the residue signal VOUT 1 , the second analog signal, which is the output level of the M-bit sub-DAC  12 , should have a half-LSB shift. After the half-LSB shift, the positions of the second analog signal are shown as node  100 ,  101 ,  102  to  130 .  
         [0030]     The following describes a flow chart of the residue amplifier gain error calibration as an embodiment with M=5 and N=7.  FIG. 3  is a flow chart of the residue amplifier gain error calibration of the present invention. First, the step of S 302  provides K third digital codes, where K is a positive integer. The K third digital codes are obtained by sampling an analog signal and converting K analog samples. Each third digital code contains a 5-bit first digital code DO 1  and a 7-bit second digital code DO 2 , which are generated by a 5-bit sub-ADC  10  and a 7-bit sub-ADC  16 , respectively. Then, the step of S 303  provides (2 5 −1) relative maximums and (2 5 −1) relative minimums of the second digital codes DO 2 . At this step, a statistical approach is utilized to find a relative maximum and a relative minimum of the second digital codes DO 2  that are corresponding to a certain first digital code DO 1 . The detailed algorithm (M=5) is explained as follows.  
         [0031]     When DO 1 (I)=“0”, where I=0˜(K−1), a relative maximum of the second digital codes DO 2  is determined by choosing an absolute maximum of the second digital codes DO 2  and is marked as U 00 ; when DO 1 (I)=“30”, where I=0˜(K−1), a relative minimum of the second digital codes DO 2  is determined by choosing an absolute minimum of the second digital codes DO 2  and marked as V 30 ; when DO 1 (I)=“1”˜“29”, where I=0˜(K−1), a relative maximum and a relative minimum of the second digital codes DO 2  are determined by choosing an absolute maximum and an absolute minimum of the second digital codes DO 2 , respectively, for each domain of DO 1 (I)=“1”˜“ 29 ”. For example, for a first digital code DO 1  belonging to the domain of DO 1 (I)=“1”(i.e., the decimal value of the first digital code DO 1 (I) equals 1), a relative maximum and a relative minimum of the second digital codes DO 2  with the corresponding first digital code (i.e., DO 1 (I)=1) are determined and marked as U 01  and V 01 , respectively. Similarly, for a first digital code DO 1  belonging to the domain of DO 1 (I)=“2”(i.e., the decimal value of the first digital code DO 1 (I) equals 2), a relative maximum and a relative minimum of the second digital codes DO 2  with the corresponding first digital code (i.e., DO 1 (I)=2) are determined and marked as U 02  and V 02 , respectively. The algorithm continues until the domain of DO 1 =“29”. After the step of S 303 , thirty relative maximums (U 00 ˜U 29 ) and thirty relative minimums (V 01 ˜V 30 ) are obtained.  
         [0032]     Then, the step of S 304  calculates (2 5 −2) first code jump differences of the relative maximums and the relative minimums of the second digital codes DO 2 . These thirty first code jump differences (W 00 ˜W 29 ) are defined by formula set (1).
 
W00=U00−V01
 
W01=U01−V02
 
W02=U02−V03
 
W29=U29−V30  (1)
 
         [0033]     Each first code jump difference between two adjacent domains is the difference between the relative maximum of the second digital codes DO 2  in one domain and the relative minimum of the second digital codes DO 2  in the following domain. After that, the step of S 305  calculates an average W_AVG of the thirty first code jump differences, where W_AVG is the average of W 00 , W 01 , W 02  to W 29 . Then, the step of S 306  provides a gain error GAIN_ERR, where GAIN_ERR is defined by formula (2)
 
GAIN_ERR=(W_AVG+1−2 N-1 )/2 N-1   ( 2 )
 
         [0034]     In this embodiment of N− 7 , thus from formula (2) GAIN_ERR=(W_AVG+1−2 7-1 )/2 7-1 =(W_AVG−63)/64. If GAIN_ERR is zero, it means no gain error is generated and no calibration is required. If GAIN_ERR is not equal to zero, a calibration procedure is performed (refer to S 307 ). The aforementioned relative maximums, relative minimums of the second digital codes DO 2  and the first code jump differences are shown in  FIG. 4 , where the coordinates of the horizontal axis are presented in decimal form of the first digital code DO 1  and those of the vertical axis are the voltage levels of the second digital code DO 2 .  
         [0035]     There are three calibration procedures in the present invention: a digital gain calibration, an analog programmable gain calibration and an analog programmable reference voltage calibration. Only one of the three calibration procedures is required to achieve the purpose of the calibration.  
         [0036]     In the digital gain calibration, K gain calibration codes are obtained by dividing each of K second digital codes DO 2  from the 7-bit sub-ADC  16  by (1+GAIN_ERR). In the analog programmable gain calibration, the gain AV of the residue amplifier  14  and the gain error GAIN_ERR are used to generate an adjustable gain signal AV_CAL according to formula (3) (shown below). Then, the adjustable gain signal AV_CAL updates the gain AV of the residue amplifier  14 .
 
AV_CAL=AV/(1+GAIN_ERR)  (3)
 
         [0037]     In the analog programmable reference voltage calibration, the gain error GAIN_ERR, the first reference voltage VRP 2  and the second reference voltage VRN 2  are used to generate, according to formula (4) and (5), the first calibration reference voltage VRP 2 _CAL and the second calibration reference voltage VRN 2 _CAL, which update a voltage range of the second digital codes DO 2 .
 
VRP2_CAL=VRP2×(1+GAIN_ERR)  (4)
 
VRN2_CAL=VRN2×(1+GAIN_ERR)  (5)
 
         [0038]     The first calibration reference voltage VRP 2 _CAL, the second calibration reference voltage VRN 2 _CAL and the average W_AVG of the first code jump differences satisfy formula (6).
 
W_AVG=(VRP2_CAL−VRN2_CAL)/2  (6)
 
         [0039]     The advantage of the digital gain calibration is that all the sampled data is still available after running this calibration. If the analog programmable gain calibration or the analog programmable reference voltage calibration is used, the first K digital output codes will not be calibrated.  
         [0040]      FIG. 5 ( a ) and  FIG. 5 ( b ) show the residue plots of the residue signals with the gain error GAIN_ERR above one and below one, respectively. In  FIG. 5 ( a ), the solid line  502  indicates the real transfer curve of the residue plot. “AV” on the solid line  502  means the solid line  502  is obtained according to the gain AV of the residue amplifier  14 . The vertical coordinate of the initial point of the solid line  502  is VRN 2 _CAL, which means the solid line  502  is obtained according to the analog programmable reference voltage calibration. The dashed line  501  indicates the ideal transfer curve (called the calibrated transfer curve). “AV_CAL” on the dashed line  501  means the dashed line  501  is obtained according to the analog programmable gain calibration. Similarly, the solid line  551  (real transfer curve) in  FIG. 5 ( b ) is obtained according to the analog programmable reference voltage calibration and the dashed line  552  (ideal transfer curve) is obtained according to the analog programmable gain calibration.  
         [0041]      FIG. 6  is a flow chart of the DAC non-linearity calibration. First, the step of S 602  provides K first digital codes DO 1  and K gain calibration codes DO 2 _GAIN_CAL by sampling an analog signal. Then, the step of S 603  provides (2 5 −2) relative maximums and (2 5 −2) relative minimums of the gain calibration codes DO 2 _GAIN_CAL. At this step, a statistical approach is utilized to find a relative maximum and a relative minimum of the gain calibration codes DO 2 _GAIN_CAL that are corresponding to a certain first digital code DO 1 . The detailed algorithm (M=5) is explained as follows.  
         [0042]     When DO 1 (I)=“0”, where I=0˜(K−1), a relative maximum of the gain calibration codes DO 2 _GAIN_CAL is determined by choosing an absolute maximum of the gain calibration codes DO 2 _GAIN_CAL and is marked as R 00 ; when DO 1 (I)=“30”, where I=0˜(K−1), a relative minimum of the gain calibration codes DO 2 _GAIN_CAL is determined by choosing an absolute minimum of the gain calibration codes DO 2 _GAIN_CAL and marked as S 30 ; when DO 1 (I)=“1”˜“29”, where I=0˜(K−1), a relative maximum and a relative minimum of the gain calibration codes DO 2 _GAIN_CAL are determined by choosing an absolute maximum and an absolute minimum of the gain calibration codes DO 2 _GAIN_CAL, respectively, for each domain of DO 1 (I)=“1”˜“29”. For example, for a first digital code DO 1  belonging to the domain of DO 1 (I)=“1”(i.e., the decimal value of the first digital code DO 1 (I) equals 1), a relative maximum and a relative minimum of the gain calibration codes DO 2 _GAIN_CAL with the corresponding first digital code (i.e., DO 1 (I)=1) are determined and marked as R 01  and S 01 , respectively. Similarly, for a first digital code DO 1  belonging to the domain of DO 1 (I)=“2” (i.e., the decimal value of the first digital code DO 1 (I) equals 2), a relative maximum and a relative minimum of the gain calibration codes DO 2 _GAIN_CAL with the corresponding first digital code (i.e., DO 1 (I)=2) are determined and marked as R 02  and S 02 , respectively. The algorithm continues until the domain of DO 1 =“29”. After the step of S 603 , thirty relative maximums (R 00 ˜R 29 ) and thirty relative minimums (S 01 ˜S 30 ) are obtained.  
         [0043]     After that, the step of S 604  calculates (2 5 −2) second code jump differences (T 00 ˜T 29 ) of the relative maximums and the relative minimums of the gin calibration codes DO 2 _GAIN_CAL. These thirty second code jump differences (T 00 ˜T 29 ) are defined by formula set (7).
 
T00=R00−S01
 
T01=R01−S02
 
T02=R02−S03
 
T29=R29−S30  (7)
 
         [0044]     Each second code jump difference between two adjacent domains is the difference between the relative maximum of the gain calibration codes DO 2 _GAIN_CAL in one domain and the relative minimum of the gain calibration codes DO 2 _GAIN_CAL in the following domain.  
         [0045]     The aforementioned relative maximums (R 00 ˜R 29 ), relative minimums (S 01 ˜S 30 ) of the gain calibration codes DO 2 _GAIN_CAL and the second code jump differences (T 00 ˜T 29 ) are shown in  FIG. 7 , where the coordinates of the horizontal axis are presented in decimal form of the first digital code DO 1  and those of the vertical axis are the voltage levels that are bounded by the first calibration reference voltage VRP 2 _CAL and the second calibration reference voltage VRN 2 _CAL.  
         [0046]     The step of S 605  calculates an average T_AVG of the (2 5 −2) second code jump differences (T 00 ˜T 29 ), and calculates (2 5 −2) offset values. The thirty offset values (DEL_T 00 ˜DEL_T 29 ) are obtained by formula set (8).
 
DEL_T00=T00−T_AVG
 
DEL_T01=T01−T_AVG
 
DEL_T02=T02−T_AVG
 
DEL_T29=T29−T_AVG  (8)
 
         [0047]     Then, the step of S 606  removes the DAC non-linearity, which utilizes the (2 5 −2) offset values and the K gain calibration codes DO 2 _GAIN_CAL to generate (2 5 −2) nonlinear calibration codes DO 2 _DAC_CAL. The (2 5 −2) nonlinear calibration codes DO 2 _DAC_CAL are defined by formula set (9).
 
If DO1=“0”, then DO2_DAC_CAL=DO2_GAIN_CAL;
 
If DO1=“1”, then DO2_DAC_CAL=DO2_GAIN_CAL+DEL_T00;
 
If DO1=“2”, DO2_DAC_CAL=DO2_GAIN_CAL+DEL_T00+DEL_T01;
 
If DO1=“3”, DO2_DAC_CAL=DO2 —GAIN _CAL+DEL_T00+DEL_T01+DEL_T02;
 
If DO1=“29”, DO2_DAC_CAL=DO2_GAIN_CAL+DEL_T00+DEL_T01+DEL_T02+ . . . +DEL_T28;
 
If DO1=“30”, DO2_DAC —CAL=DO 2_GAIN_CAL+DEL_T00+DEL_T01+DEL_T02+ . . . +DEL_T28+DEL_T29.  ( 9 )
 
         [0048]     Then, the (2 5 −2) nonlinear calibration codes DO 2 _DAC_CAL are fed to the third signal processing unit  20 .  
         [0049]      FIG. 8  is a residue plot of the residue signal VOUT 1  with DAC non-linearity (M=5). In  FIG. 8 , the real transfer curve is indicated by the solid lines  801 ,  802  to  830 . The ideal transfer curve is indicated by the dashed lines  851 ,  852  to  880 .  FIG. 8  shows that the issue of the DAC non-linearity is solved by shifting the residue voltage transfer curve from the positions of the solid lines to those of the dashed lines. That is, when DO 1 =“1”, the residue voltage transfer curve is shifted from the solid line  801  to the dashed line  851 ; when DO 1 =“2”, the residue voltage transfer curve is shifted from the solid line  802  to the dashed line  852 ; when DO 1 =“3”, the residue voltage transfer curve is shifted from the solid line  803  to the dashed line  853 , and so on.  
         [0050]     The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by persons skilled in the art without departing from the scope of the following claims.