Abstract:
A digital-to-analog converter includes a first input end for reversely receiving a digital signal, a second input end for receiving multiple reference voltages arranged in a best order, wherein the best order is determined by reordering an initial order several times, multiple first switches coupled to each other stage by stage like a tree for selecting one of the reference voltages to be a first selection result, multiple second switches coupled in series for selecting one of the reference voltages to be a second selection result.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/212,589 filed on Aug. 31, 2015, the contents of which are incorporated herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a digital-to-analog converter and source driving circuit, and more particularly to a digital-to-analog converter and source driving circuit requiring fewer switches by reordering reference voltages. 
         [0004]    2. Description of the Prior Art 
         [0005]    A liquid crystal display (LCD) monitor has characteristics of light weight, low power consumption, zero radiation, etc. and is widely used in many information technology (IT) products, such as televisions, mobile phones, and laptop computers. The operating principle of the LCD monitor is based on the fact that different twist states of liquid crystals result in different polarization and refraction effects on light passing through the liquid crystals. Thus, the liquid crystals can be used to control amount of light emitted from the LCD monitor by arranging the liquid crystals in different twist states, so as to produce light outputs at various brightnesses. 
         [0006]    Please refer to  FIG. 1 , which is a schematic diagram of a thin film transistor (TFT) LCD monitor  10  of the prior art. The LCD monitor  10  includes an LCD panel  100 , a source driver  102 , a gate driver  104  and a voltage generator  106 . The LCD panel  100  is composed of two substrates, and space between the substrates is filled with liquid crystal materials. One of the substrates is installed with a plurality of data lines  108 , a plurality of scan lines (or gate lines)  110  and a plurality of TFTs  112 , and another substrate is installed with a common electrode for providing a common signal Vcom outputted by the voltage generator  106 . The TFTs  112  are arranged as a matrix on the LCD panel  100 . Accordingly, each data line  108  corresponds to a column of the LCD panel  100 , each scan line  110  corresponds to a row of the LCD panel  100 , and each TFT  112  corresponds to a pixel. Note that the LCD panel  100  composed of the two substrates can be regarded as an equivalent capacitor  114 . 
         [0007]    The source driver  102  and the gate driver  104  input signals to the corresponding data lines  108  and scan lines  110  based upon a desired image data, to control whether or not to enable the TFT  112  and a voltage difference between two ends of the equivalent capacitor  114 , so as to change alignment of the liquid crystals as well as the penetration amount of light. As a result, the desired image data can be correctly displayed on the LCD panel  100 . In order to display various gray levels, the source driver  102  has to provide hundreds of voltage levels to the LCD panel  100 . For example, to display 256 gray levels, the source driver  102  has to select one reference voltage from 256 reference voltages according to an 8-bit digital signal to be a source driving signal. However, implementation for the 256-level selection requires a large number of transistor switches, which occupy a large circuit layout area and cause parasitic resistors, which is a heavy burden on display driving efficiency. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore a primary objective of the claimed invention to provide a digital-to-analog converter and source driving circuit, which require smaller circuit layout area and cause less parasitic resistance. 
         [0009]    The present invention discloses a digital-to-analog converter for converting a digital signal into a first analog signal and a second analog signal, the digital signal comprising N bits, N being a positive integer, the digital-to-analog converter comprising a plurality of first input ends, each for receiving one of the N bits; a plurality second input ends, for receiving a plurality of reference voltages arranged in a best order, wherein the plurality of reference voltages ordered by magnitude determines an original order, wherein the original order is performed with (N−1) reorder processes to determine the best order; a plurality of first switches, electrically coupled to each other in a tree structure and electrically coupled to the plurality first input ends and the plurality second input ends, comprising N stages, each for selecting one of the plurality reference voltages according to one of the N bits to generate a first selection result; at least N second switches, coupled in series, each for selecting a second selection result of a previous second switch of the second switch or the first selection result of one of the plurality of first switches according to one of the N bits, to generate the second selection result; wherein the first selection result generated by the first switch of the Nth stage is the first analog signal, and the second selection result generated by the last one of the at least N second switches is the second analog signal; wherein the Nth second switch is utilized for receiving the first selection result generated by the second first switch of the (N−1)th stage. 
         [0010]    The present invention further discloses a digital-to-analog converter for converting a digital signal into a first analog signal and a second analog signal, the digital signal comprising N bits, N being a positive integer, the digital-to-analog converter comprising a plurality of first input ends, each for receiving one of the N bits; a plurality second input ends, for receiving a plurality of reference voltages arranged in a second best order, wherein the plurality of reference voltages ordered by magnitude determines an original order, wherein the original order is performed with (N−2) reorder processes to determine the second best order; a plurality of first switches, electrically coupled to each other in a tree structure and electrically coupled to the plurality first input ends and the plurality second input ends, comprising N stages, each for selecting one of the plurality reference voltages according to one of the N bits to generate a first selection result; at least (N+1) second switches, coupled in series, each for selecting a second selection result of a previous second switch of the second switch or the first selection result of one of the plurality of first switches according to one of the N bits, to generate the second selection result; wherein the first selection result generated by the first switch of the Nth stage is the first analog signal, and the second selection result generated by the last one of the at least (N+1) second switches is the second analog signal; wherein the Nth second switch is utilized for receiving the first selection result generated by the second first switch of the (N−1)th stage. 
         [0011]    The present invention further discloses a source driving circuit, comprising a Gamma circuit, for providing a plurality of reference voltages between a high voltage and a low voltage; a digital-to-analog converter, electrically coupled to the Gamma circuit, for selecting two from the plurality of reference voltages according to a digital signal comprising N bits to be a first analog signal and a second analog signal; and an interpolation circuit, electrically coupled to the digital-to-analog converter, for providing an interpolation voltage of the first analog signal and second analog signal according to a second digital signal to be a source driving signal; wherein the digital-to-analog converter comprises a plurality of first input ends, each for receiving one of the N bits; a plurality second input ends, for receiving the plurality of reference voltages arranged in a best order, wherein the plurality of reference voltages ordered by magnitude determines an original order, wherein the original order is performed with (N−1) reorder processes to determine the best order; a plurality of first switches, electrically coupled to each other in a tree structure and electrically coupled to the plurality first input ends and the plurality second input ends, comprising N stages, each for selecting one of the plurality reference voltages according to one of the N bits to generate a first selection result; at least N second switches, coupled in series, each for selecting a second selection result of a previous second switch of the second switch or the first selection result of one of the plurality of first switches according to one of the N bits, to generate the second selection result; wherein the first selection result generated by the first switch of the Nth stage is the first analog signal, and the second selection result generated by the last one of the at least N second switches is the second analog signal; wherein the Nth second switch is utilized for receiving the first selection result generated by the second first switch of the (N−1)th stage. 
         [0012]    The present invention further discloses a source driving circuit, comprising a Gamma circuit, for providing a plurality of reference voltages between a high voltage and a low voltage; a digital-to-analog converter, electrically coupled to the Gamma circuit, for selecting two from the plurality of reference voltages according to a digital signal comprising N bits to be a first analog signal and a second analog signal; and an interpolation circuit, electrically coupled to the digital-to-analog converter, for providing an interpolation voltage of the first analog signal and second analog signal according to a second digital signal to be a source driving signal; wherein the digital-to-analog converter comprises a plurality of first input ends, each for receiving one of the N bits; a plurality second input ends, for receiving the plurality of reference voltages arranged in a second best order, wherein the plurality of reference voltages ordered by magnitude determines an original order, wherein the original order is performed with (N−2) reorder processes to determine the second best order; a plurality of first switches, electrically coupled to each other in a tree structure and electrically coupled to the plurality first input ends and the plurality second input ends, comprising N stages, each for selecting one of the plurality reference voltages according to one of the N bits to generate a first selection result; at least (N+1) second switches, coupled in series, each for selecting a second selection result of a previous second switch of the second switch or the first selection result of one of the plurality of first switches according to one of the N bits, to generate the second selection result; wherein the first selection result generated by the first switch of the Nth stage is the first analog signal, and the second selection result generated by the last one of the at least (N+1) second switches is the second analog signal; wherein the Nth second switch is utilized for receiving the first selection result generated by the second first switch of the (N−1)th stage. 
         [0013]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic diagram of a thin film transistor LCD monitor of the prior art. 
           [0015]      FIG. 2A  is a schematic diagram of a source driving circuit. 
           [0016]      FIG. 2B  is a schematic diagram of reference voltages of the source driving circuit of  FIG. 2A . 
           [0017]      FIG. 2C  is a schematic diagram of an alternative embodiment of the source driving circuit of  FIG. 2A . 
           [0018]      FIG. 2D  is a schematic diagram of reference voltages of the source driving circuit of  FIG. 2C . 
           [0019]      FIG. 3A  and  FIG. 3B  are schematic diagrams of a digital-to-analog converter. 
           [0020]      FIG. 4A  and  FIG. 4B  are schematic diagrams of a digital-to-analog converter according to an embodiment of the present invention. 
           [0021]      FIG. 5  is a schematic diagram of a reorder process according to an embodiment of the present invention. 
           [0022]      FIG. 6A  is a schematic diagram of a digital-to-analog converter. 
           [0023]      FIG. 6B  is a schematic diagram of a digital-to-analog converter according to an embodiment of the present invention. 
           [0024]      FIG. 6C  is a schematic diagram of a digital-to-analog converter according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Please refer to  FIG. 2A , which is a schematic diagram of a source driving circuit  20 . The source driving circuit  20  includes a Gamma circuit  200 , a digital-to-analog converter  210  and an interpolation circuit  220 . The Gamma circuit  200  is utilized for providing M+1 reference voltages Vr 0 , Vr 1 , . . . , VrM between a high voltage VDD and a low voltage VSS, e.g. 5V=VDD&gt;=Vr 0 &gt;Vr 1 &gt; . . . &gt;VrM&gt;=VSS=0V, which are candidates for a source driving voltage. Based upon chromatic needs, the reference voltages Vr 0 , Vr 1 , . . . , VrM are arranged to depict a non-linear curve, as illustrated in  FIG. 2B . According to an alternative embodiment, the reference voltages Vr 0 , Vr 1  . . . VrM could be negative, i.e. 0V=VSS&gt;=VrM&gt; . . . &gt;Vr 1 &gt;Vr 0 &gt;=VCL=−5V, where VCL denotes another low voltage, as illustrated in  FIG. 2C . In such an arrangement, the reference voltages Vr 0 , Vr 1 , . . . , VrM depict another non-linear curve shown in  FIG. 2D . The Gamma circuit  200  could be implemented by resistors connected in series. The digital-to-analog converter  210  is utilized for selecting two reference voltages from the reference voltages Vr 0 , Vr 1 , . . . , VrM according to an N-bit digital signal DIG 1  to be a first analog signal VH and a second analog signal VL. The interpolation circuit  220  is utilized for providing an interpolation voltage of the first analog signal VH and the second analog signal VL according to a 2-bit second digital signal DIG 2  to be a source driving signal SR. 
         [0026]    Note that, the first analog signal VH and the second analog signal VL are two adjacent reference voltages. For example, if the source driving circuit  20  can provide 2 8  candidate voltages to be the source driving signal SR, the digital-to analog converter  210  would be a 6-bit digital-to-analog converter  310  as illustrated in  FIG. 3A  and  FIG. 3B . In  FIG. 3A  and  FIG. 3B , there are  65  reference voltages (M=64), and the digital signal DIG 1  includes six bits D 0 , D 1 , D 2 , D 3 , D 4 , D 5 . The digital-to-analog converter  310  selects two adjacent reference voltages from the reference voltage Vr 0 , Vr 1 , . . . , Vr 64  according to the bits D 0 , D 1 , D 2 , D 3 , D 4 , D 5  to be the first analog signal VH and the second analog signal VL. Further, the interpolation circuit  220  inserts four voltages between the first analog signal VH and the second analog signal VL and selects one from the four voltages according to the 2-bit second digital signal DIG 2  to be the source driving signal SR. As a result, the source driving circuit  20  can generates 2 6 *2 2 =2 8  possible voltage levels for the source driving signal SR. Note that, the digital-to-analog converter  310  requires 126 switches. If each of the 126 switches is implemented by two transistors,  252  transistors are totally required. Therefore, the present invention further provides another digital-to-analog converter, which requires fewer transistors than the digital-to-analog converter  310 . Please refer to  FIG. 4A  and  FIG. 4B , which is a schematic diagram of an N-bit (N=6) digital-to-analog converter  410  according to an embodiment of the present invention. The digital-to-analog converter  410  is utilized for converting a digital signal DIG 1  into a first analog signal VH and a second analog signal VL. The digital signal DIG 1  includes N bits D 0 , D 1  . . . DN−1, where N denotes a positive integer. The digital-to-analog converter  410  includes multiple first input ends  412 , multiple second input ends  414 , multiple first switches  416  and at least N second switches  418 . The first input ends  412  are respectively utilized for receiving the bits DN- 1 , DN- 2  . . . D 1 , D 0  (contrary to the receiving order of the digital-to-analog converter  310 ). The second input ends  414  are respectively utilized for receiving multiple reference voltages Vr 0 −Vr 2   N  arranged in a best order shown in  FIG. 4A  and  FIG. 4B . In order to narrate the present invention more easily, the reference voltages Vr 0 −Vr 2   N  ordered by magnitude (Vr 0 &gt;Vr 1 &gt; . . . &gt;Vr 2   N ) determines an original order (Vr 0 →Vr 1 → . . . →Vr 2   N ), and the original order is performed with (N−1) reorder processes to determine the best order (Vr 0 →Vr 32 →Vr 16 → . . . →Vr 2   N ). The first switches  416  are electrically coupled to each other in a tree structure, and include N stages, each for selecting one of the reference voltages Vr 0 −Vr 2   N  according to one of the N bits to generate a first selection result. Each of the second switches  418 _ 1 - 418 _ 6  is utilized for selecting a second selection result of a previous second switch ( 418 _ 1 - 418 _ 5 ) of the second switch or the first selection result of the corresponding first switch according to one of the N bits DN−1, D 1 , D 0 , to generate the second selection result. For N&gt;=2, the Nth second switch  418 _N is utilized for receiving the first selection result generated by the second first switch of the (N−1)th stage. Finally, through the switching operations of N stages, the first selection result generated by the first switch  416  of the Nth stage is the first analog signal VH, and the second selection result generated by the last second switch  418 _ 6  is the second analog signal VL. Note that, the N stages of the first switches  416  correspond to the N bits D 0 , D 1 , . . . , DN−1 according to a reverse order, which means that the first switches  416  of the first stage are controlled by the bit DN−1, the first switches  416  of the second stage are controlled by the bit and the first switch  416  of the Nth stage is controlled by the bit D 0 . Similarly, Similarly, the second switches  418  also correspond to the N bits D 0 , D 1 , . . . , DN−1 according to the reverse order, which means that the first second switch  418 _ 1  is controlled by the bit DN−1, the second second switch  418 _ 2  is controlled by the bit DN- 2 , . . . , and the Nth second switch  418 _N is controlled by the bit D 0 . In comparison with the digital-to-analog converter  310  requiring  252  transistors, the digital-to-analog converter  410  of  FIG. 4A  and  FIG. 4B  merely requires  138  transistors, but can generate the same first analog signal VH and the second analog signal VL. Therefore, the digital-to-analog converter  410  requires less circuit layout area and causes less parasitic capacitance than the digital-to-analog converter  310 . In other words, the digital-to-analog converter  410  arranges the input order of the reference voltages Vr 0 −Vr 2   N , such that the second switches  418  can use a part of the first selection result of the first switches  416 , so as to reduce the number of the second switches  418 . 
         [0027]    How the reference voltages Vr 0 −Vr 2   N  are reordered is illustrated in  FIG. 5 , in which the reference voltages Vr 0 −Vr 15  are reordered for example. In the embodiment of  FIG. 5 , N=4, and the reference voltages Vr 0 −Vr 15  are performed with N−1=3 reorder processes. During the first reorder process, odd components {Vr 0 , Vr 2 , Vr 4  . . . Vr 14 } of an original sequence SEQ 0 ={Vr 0 −Vr 15 } are acquired to form a first half part of a new first sequence, and even components {Vr 1 , Vr 3 , Vr 5  . . . Vr 15 } of the original sequence SEQ 0  are acquired to form a second half part of the first sequence, i.e. SEQ 1 ={Vr 0 , Vr 2 , Vr 4 , Vr 6 , Vr 8 , Vr 10 , Vr 12 , Vr 14 , Vr 1 , Vr 3 , Vr 5 , Vr 7 , Vr 9 , Vr 11 , Vr 13 , Vr 15 }. During the second reorder process:
   (1) the first sequence is divided into two sub-sequences seq 11 ={Vr 0 , Vr 2 , Vr 4 , Vr 6 , Vr 8 , Vr 10 , Vr 12 , Vr 14 } and seq 12 ={Vr 1 , Vr 3 , Vr 5 , Vr 7 , Vr 9 , Vr 11 , Vr 13 , Vr 15 };   (2) odd components {Vr 0 , Vr 4 , Vr 8 , Vr 12 } of the first sub-sequence seq 11  of the sub-sequences seq 11 , seq 12  are acquired to form a 1/2 2  part of a second sequence SEQ 2 ;   (3) even components {Vr 2 , Vr 6 , Vr 10 , Vr 14 } of the first sub-sequence seq 11  of the sub-sequences seq 11 , seq 12  are acquired to form a 2/2 2  part of the second sequence SEQ 2 ;   (4) odd components {Vr 1 , Vr 5 , Vr 9 , Vr 13 } of the second sub-sequence seq 12  of the sub-sequences seq 11 , seq 12  are acquired to form a 3/2 2  part of the second sequence SEQ 2 ; and   (5) even components {Vr 3 , Vr 7 , Vr 11 , Vr 15 } of the second sub-sequence seq 12  of the sub-sequences seq 11 , seq 12  are acquired to form a 4/2 2  part of the second sequence SEQ 2 .   
 
         [0033]    Therefore, the whole second sequence SEQ 2  is {Vr 0 , Vr 4 , Vr 8 , Vr 12 , Vr 2 , Vr 6 , Vr 10 , Vr 14 , Vr 1 , Vr 5 , Vr 9 , Vr 13 , Vr 3 , Vr 7 , Vr 11 , Vr 15 }. Similarly, during the third reorder process:
   (1) the second sequence is divided into 2 2  sub-sequences seq 21 ={Vr 0 , Vr 4 , Vr 8 , Vr 12 }, seq 22 ={Vr 2 , Vr 6 , Vr 10 , Vr 14 }, seq 23 ={Vr 1 , Vr 5 , Vr 9 , Vr 13 } and seq 24 ={Vr 3 , Vr 7 , Vr 11 , Vr 15 };   (2) odd components of the sub-sequences seq 21 , seq 22 , seq 23 , seq 24  are respectively acquired to form 1/2 3 , 3/2 3 , 5/2 3 , 7/2 3  parts of a third sequence SEQ 3 ; and   (3) even components of the sub-sequences seq 21 , seq 22 , seq 23 , seq 24  are respectively acquired to form 2/2 3 , 4/2 3 , 6/2 3 , 8/2 3  parts of the third sequence SEQ 3 .   
 
         [0037]    As a result, the whole third sequence SEQ 3  is {Vr 0 , Vr 8 , Vr 4 , Vr 12 , Vr 2 , Vr 10 , Vr 6 , Vr 14 , Vr 1 , Vr 9 , Vr 5 , Vr 13 , Vr 3 , Vr 11 , Vr 7 , Vr 15 }. 
         [0038]    The above first to third reorder processes can be summarized into a general rule, so as to derive the (N−1)th reorder process, which includes the following steps:
   (1) dividing an (N−2)th sequence into 2 N−2  sub-sequences seq(N−2)1−seq(N−2)2 N−2 ;   (2) acquiring odd components of K (K=1−2 N−2 ) sub-sequences seq(N−2)1−seq(N−2)2 N−2  to form a (2K−1)/2 N−1  part of an (N−1)th sequence; and   (3) acquiring even components of K (K=1−2 N−2 ) sub-sequences seq(N−2)1−seq(N−2)2 N−2  to form a 2K/2 N−1  part of the (N−1)th sequence.   
 
         [0042]    For example, the order of the reference voltages of  FIG. 4A  and  FIG. 4B  is the result of N- 1 = 5  reorder processes. Details of the five reorder processes can be acquired by setting K=N−1=1, 2, 3, 4 or 5 in the above steps. 
         [0043]    Note that, the digital signal DIG 1  has to be received according to the reverse order in the digital-to-analog converter  410 , such that the first analog signal VH and the second analog signal VL of the digital-to-analog converter  410  can be the same as the first analog signal VH and the second analog signal VL of the digital-to-analog converter  310 . Particularly, the reverse order is defined herein to be a reversed bit order of the digital signal DIG 1 . For example, according to the embodiment of  FIG. 4A  and  FIG. 4B , the original bit order is {D 0 , D 1 , D 2 , D 3 , D 4 , D 5 }, and the reverse order is {D 5 , D 4 , D 3 , D 2 , D 1 , D 0 }. 
         [0044]    In addition, a special case is that there is no previous second switch for the first second switch  418 _ 1  according to  FIG. 4A  and  FIG. 4B . In such a case, the first second switch  418 _ 1  selects the (2 (N−1) +1=33)th reference voltage Vr 32  numbered by the original order or the (2 N +1=65)th reference voltage Vr 64  numbered by the original order according to the (N=6)th bit D 5 . 
         [0045]    According to an embodiment, the first switches  416  and the second switches  418  could be implemented by transistor switches. 
         [0046]    Notably, although N−1=5 reorder processes are performed according to the embodiment of  FIG. 4A  and  FIG. 4B , a second best order can be formed for the reference voltages Vr 0 , Vr 1  . . . VrM if only N−2 reorder processes are performed. For example, if N=3, and the reference voltages Vr 0 −Vr 8  are received according to the original order, a 3-bit digital-to-analog converter  610  requires 28 transistors, as illustrated in  FIG. 6A . If the reference voltages Vr 0 −Vr 8  are performed with N−2=1 reorder process, and are received according to the second best order, a 3-bit digital-to-analog converter  620  requires 22 transistors, as illustrated in  FIG. 6B . If the reference voltages Vr 0 −Vr 8  are performed with N−1=2 reorder processes, and are received according to the best order, a 3-bit digital-to-analog converter  630  requires 20 transistors, as illustrated in  FIG. 6C . Therefore, receiving the reference voltages according to the second best order also can reduce the required transistors. 
         [0047]    According to  FIG. 6B  and  FIG. 6C , the circuits of the best order and the second best order differ in receiving orders for the bits D 0 -D 2  and the reference voltages Vr 0 −Vr 8 , and differ in connections of the second switches  628 ,  638 . Specifically, first input ends  622  receive the bits D 0 -D 2  according to a semi-reverse order (D 1 →D 2 →D 0 ), second input ends  624  receive the reference voltages Vr 0 −Vr 7  according to the second best order (Vr 0 →Vr 2 →Vr 4 →Vr 6 →Vr 1 →Vr 3 →Vr 5 →Vr 7 ), and the second switches  628 _ 1 - 628 _ 3  of the first and second stages are specially connected. Other than the different parts, the remaining part of the second best order circuit is identical to the best order circuit, which can be referred to the digital-to analog converter  410  in the above and is not further narrated herein. 
         [0048]    As mentioned before, the second best order is the result of performing N−2 reorder processes on the original order, and the N−2 reorder processes are identical to those for acquiring the best order. For example, the (N−2)th reorder process includes the following steps:
   (1) dividing the (N−3)th sequence into 2 N−3  sub-sequences seq(N−3)1−seq(N−3)2 N−3 ;   (2) acquiring odd components of K (K=1−2 N−3 ) sub-sequences seq(N−3)1−seq(N−3)2 N−3  to form a (2K−1)/2 N−2  part of a (N−2)th sequence; and   (3) acquiring even components of K (K=1−2 N−3 ) sub-sequences seq(N−3)1−seq(N−3)2 N−3  to form a 2K/2 N−2  part of the (N−2)th sequence.   
 
         [0052]    Furthermore, the semi-reverse order is formed by the following steps:
   (1) reversing a bit order of the digital signal DIG 1  to be the semi-reverse order, i.e. D 0 →D 1 →D 2 ;   (2) determining a second highest bit D 1  of the bit order to be a lowest bit of the semi-reverse order; and   (3) determining a highest bit D 2  of the bit order to be a second lowest bit of the semi-reverse order.   
 
         [0056]    For example, the semi-reverse order is D 1 →D 2 →D 0  in the embodiment of  FIG. 6B . 
         [0057]    Finally, the first second switch  628 _ 1  selects the (2 (N−1) −1=3)th reference voltage Vr 2  numbered by the original order or the (2 (N−1) +1=5)th reference voltage Vr 4  numbered by the original order according to the (N=3)th bit D 2 . The second second switch  628 _ 2  selects the (2 N −1=7)th reference voltage Vr 6  numbered by the original order or the (2 N +1=9)th reference voltage Vr 8  numbered by the original order according to the (N=3)th bit D 2 . The third second switch  628 _ 3  selects the second selection result of the first second switch  628 _ 1  or the second selection result of the second second switch  628 _ 2  according to the (N−1=2)th bit D 1 . 
         [0058]    Therefore, regardless of the digital-to analog converters  410 ,  630  applying the best order or the digital-to analog converter  620  applying the second best order, node voltages within the generation process of the first analog signal VH are further utilized for generating the second analog signal VL since the first analog signal VH and the second analog signal VL are two adjacent reference voltages, so as to reduce employed transistors. 
         [0059]    To sum up, the present invention shares switches based on the relation between the output analog signals to employ fewer transistors for the digital-to-analog converter, so as to minimize circuit layout area and parasitic resistance. 
         [0060]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.