Patent Document

FIELD OF THE INVENTION 
     The present invention relates to a source driver of a TFT-LCD or TFT-OELD; and, more particularly, to a source driver of a LCD, which is capable of improving an accuracy and resolution. 
     DESCRIPTION OF RELATED ART 
       FIG. 1  is a block diagram of a general TFT-LCD (thin film transistor—liquid crystal display). 
     Referring to  FIG. 1 , the TFT-LCD includes a liquid crystal panel  400 , a timing controller  100 , a plurality of gate drivers  200 , a plurality of source drivers  300 , and a voltage generator  500 . 
     The plurality of gate drivers  200  are enabled by the timing controller  100  and sequentially drives gate lines of the liquid crystal panel  400 . The plurality of source drivers  300  are enabled by the timing controller  100  and drives source lines of the liquid crystal panel  400  to allow the liquid crystal panel  400  to display data. The voltage generator  500  generates various voltages that the system requires. 
     The liquid crystal panel  400  has a plurality of unit pixels, each of which consists of a liquid crystal capacitor C 1  and a switching thin film transistor T 1 . The unit pixels are arranged in matrix. Sources of the thin film transistors T 1  are respectively connected to the source lines that are driven by the source driver  300 , and gates of the thin film transistors T 1  are respectively connected to the gate lines that are driven by the gate driver  200 . 
     In such a TFT-LCD, the gate driver  200  sequentially drives the gate lines under control of the timing controller  100 , and the source driver  300  receives data from the timing controller  100  and applies an analog signal to the source lines. In this manner, the TFT-LCD displays the data. 
       FIG. 2  is a block diagram of the source driver  300  of the TFT-LCD shown in  FIG. 1 . 
     Referring to  FIG. 2 , the source driver  300  includes a digital controller  310 , a register  320  for storing digital data provided from the digital controller  310 , a level shifter  330  for converting a level of a signal provided from the register  320 , a digital-to-analog converter (DAC)  340  for converting a digital signal passing through the level shifter  330  into an analog signal, an analog bias part  350 , and a buffering part for buffering an output of the DAC  340  by a bias provided from the analog bias part  350  and supplying it to the source lines of the liquid crystal panel ( 400  in  FIG. 1 ). 
     The digital controller  310  receives a source driver start pulse (SSP), a data clock and a digital data from the timing controller ( 100  in  FIG. 1 ), transfers the digital data to the register  320 , and controls the register  320 . 
     The register  320  includes a shift register  321 , a sampling register  322  and a holding register  323 . All digital data are stored in the sampling register  322  through the shifter register  321 . The digital data stored in the sampling register  322  are transferred to the DAC  340  through the holding register  323  and the level shifter  330  in response to a control signal LOAD provided from the timing controller ( 100  in  FIG. 1 ). 
     The DAC  340  includes a gradation voltage generator  342  for making an input voltage nonlinearly so as to express brightness linearly, and a decoder  344  for decoding an output of the gradation voltage generator  342  by using the digital signal passing through the level shifter  330  as a select signal. 
     The buffering part  360  is configured with a unity gain amp and supplies a signal having the same voltage level as the analog signal to the source lines of the liquid crystal panel at higher power. 
       FIG. 3  is a circuit diagram of the DAC shown in  FIG. 2 . 
     Referring to  FIG. 3 , the respective outputs of the gradation voltage generator  342  are selected through six switches  344  connected in sequence and are then outputted. In this manner, since the gradation voltage is selected through the six switches controlled by the digital signals D&lt; 6 : 1 &gt;, a separate decoder is not required. 
       FIG. 4  is a circuit diagram of another conventional DAC. The respective outputs of the gradation voltage generator  342  are selected through one switch and are outputted as the analog signal AN_OUT. Accordingly, there is required a 6 64 decoder for generating a control signal to control the respective switches. 
     Also, various DACs can be implemented by combining the DACs shown in  FIGS. 3 and 4 . That is, the DAC having a 6-bit resolution can use one switch to maximum six switches connected in series at the respective outputs, and a 6×64 decoder for generating the control signal can be used. Also, a structure having no decoder can be provided. For example, two switches serially connected to the respective outputs can be used and two 3×8 decoders can be used to select the respective switches. Alternatively, three switches connected in series can be used and three 2×4 decoders can be used. 
     Meanwhile, 64 resistors are required so as to obtain a 6-bit resolution by using the DAC  340 , and the decoder and the switch are required so as to select the gradation voltage. Accordingly, if the DAC is implemented to have an 8-bit or 10-bit resolution, a circuit area increases about 4 times or 16 times. That is, in order to increase a resolution by N-bit, the circuit area increases 2 N  times. 
     Like this, if the area of the DAC  340  increases, the area of the TFT-LCD driver chip increases, so that a manufacturing cost rises. Consequently, price competitiveness is reduced. 
     Accordingly, in order to minimize the increase of the circuit area, the DAC is implemented with two stages, which will be described below with reference to the accompanying drawings. 
       FIG. 5  is a circuit diagram of a conventional two-stage DAC. A first DAC  346  converts the upper 6-bit digital signals D&lt; 8 : 3 &gt; into the analog signals and includes a resistor string  346   a  for dividing an upper voltage VREF_H and a lower voltage VREF_L, and a decoder  346   b  for outputting two consecutive analog voltages V N+1  and V N  in response to the digital signals D&lt; 2 : 1 &gt;. A second DAC  347  converts the lower 2 bits D&lt; 2 : 1 &gt; and includes a capacitor part  347  for dividing voltage levels of the two analog voltages V N+1  and V N  and a switching part  347   a  for controlling the voltage levels divided through the capacitors  347   b.    
     The resistor string  346   a  of the first DAC is shared and is the gradation voltage generator  342  shown in  FIG. 2 . 
     However, in the case of the DAC implemented with the capacitors, the accuracy of the output signal is lowered. This is caused by charge injection and clock feedthrough, which occur in the switches connected to the capacitors. The error of the output voltage due to the charge injection and clock feedthrough is proportional to the driving voltage of the MOS transistors used as the switches. Since the TFT-LCD uses a voltage of 7-16 V as the driving voltage, it is difficult to meet the accuracy aimed at the design. Although the accuracy can be improved by increasing the capacitance, the circuit area is increased and the operating speed is reduced. 
     In order to solve these problems, the two-stage DACs are respectively implemented with the resistor string, as shown in  FIG. 6 . 
     Referring to  FIG. 6 , first and second DACs  348  and  350  include resistor strings  348   a  and 350 a  for dividing the applied voltage and switching parts  348   b  and  350   b  for outputting the analog voltages corresponding to the digital signals D&lt; 8 : 3 &gt; and D&lt; 2 : 1 &gt; among the voltages outputted by the resistor strings  348   a  and  350   a.    
     The first and second DACs  348  and  350  are connected through the unity gain amp  349 , so that the divided voltage level of the front stage cannot be influenced by the resistor string  350   a  of the rear stage. That is, since the resistor strings  348   a  and  350  of the first and second stages are connected in parallel through the switching parts  348   b  and  350   b , it is possible to solve the problem that the outputted analog signals cannot have voltage level difference of a constant ratio and thus the analog signals corresponding to the digital signals cannot be outputted. 
     Meanwhile, since the accuracy of the unity gain amp designed in a general CMOS process is about 20 mV. Therefore, if the DAC is implemented with such a unity gain amp, it is difficult to expect the accuracy of about 20 mV or more in the 6-bit resolution. 
     In addition, since two unity-gain amps are added to the channel, the circuit area is increased. 
     Therefore, due to the offset voltage of the unity gain amp, the DAC implemented with the unity gain amp has a limit in designing the high gradation DAC having the accuracy of more than the offset voltage of the unity gain amp. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a source driver of a liquid crystal display, capable of improving an accuracy and resolution without using a unity gain amp in a DAC. 
     In accordance with an aspect of the present invention, there is provided a TFT-LCD source driver for driving L channels of a liquid crystal panel (where L is a positive integer), the TFT-LCD source driver comprising a plurality of DACs (digital-to-analog converters) for converting (M+N)-bit different digital signals into analog signals (where M and N are positive integers), the DAC including: a coarse gradation voltage generator, configured with resistors connected in series, for generating 2 M  gradation voltages; a first decoder for selecting two consecutive voltages among the 2 M  gradation voltages in response to M-bit digital signals; a fine gradation voltage generator, configured with 2 N  resistors connected in series, for receiving output voltages of the first decoder and outputting 2 N  gradation voltages; and a second decoder for selecting one of the 2 N  gradation voltages in response to the N-bit digital signals and outputting the selected gradation voltage as the analog signal. 
     In accordance with another aspect of the present invention, there is provided a An apparatus for converting a digital signal into an analog signal, including: L DACs (digital-to-analog converters) including: a first decoder for selecting two consecutive voltages among the 2 M  gradation voltages in response to M-bit digital signals; a fine gradation voltage generator, configured with 2 N  resistors connected in series, for receiving output voltages of the first decoder and outputting 2 N  gradation voltages; and a second decoder for selecting one of the 2 N  gradation voltages in response to the N-bit digital signals and outputting the selected gradation voltage as the analog signal; and a coarse gradation voltage generator, configured with 2 M  resistors connected in series, for generating 2 M  gradation voltages, wherein the first decoder and the fine gradation voltage generator are connected together without unity gain amp; and a resistance (R ch ) of the fine gradation voltage generator meets an equation 
                 R   ch     ≥         (       2   M     -   1     )     ·   L   ·   R         2   M     ·     2   N           ,         
where R is a resistance of the coarse gradation voltage generator.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the instant invention will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a general TFT-LCD; 
         FIG. 2  is a block diagram of a source driver of the TFT-LCD shown in  FIG. 1 ; 
         FIG. 3  is a circuit diagram of a conventional DAC shown in  FIG. 2 ; 
         FIG. 4  is a circuit diagram of another conventional DAC; 
         FIG. 5  is a circuit diagram of a further another conventional DAC; 
         FIG. 6  is a circuit diagram of a still further another conventional DAC; 
         FIG. 7  is a circuit diagram of a DAC in accordance with an embodiment of the present invention; 
         FIG. 8  is an equivalent circuit diagram of the DAC shown in  FIG. 7  when an output error of the DAC is largest; 
         FIG. 9  is an equivalent circuit diagram of an actual DAC in accordance with an embodiment of the present invention; 
         FIG. 10  is a graph illustrating an output voltage of the DAC shown in  FIG. 9 ; 
         FIG. 11  is an equivalent circuit diagram of the DAC when a first resistance of resister string is adjusted; and 
         FIG. 12  is a graph illustrating an output voltage of the DAC shown in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 7  is a circuit diagram illustrating a DAC of a source driver in accordance with an embodiment of the present invention. 
     Referring to  FIG. 7 , the DAC includes a coarse gradation voltage generator  820 , a first decoder  840 , a fine gradation voltage generator  920 , and a second decoder  940 . 
     The coarse gradation generator  820  is configured with 2 M  resistors connected in series and generates 2 M  gradation voltages. The first decoder  840  selects two consecutive voltages (for example, VH and VL) among the output voltages of the coarse gradation voltage generator  820  in response to M-bit digital signals D&lt;M+N:N+1&gt;. The fine gradation voltage generator  920  is configured with 2 N  resistors connected in series, and receives the output voltages of the first decoder  840  and outputs 2 N  gradation voltages. The second decoder  940  selects one output voltage among the output voltages of the fine gradation voltage generator  920  and outputs an analog signal AN_OUT in response to N-bit digital signals D&lt;N: 1 &gt;. 
     A first DAC  800  includes the coarse gradation voltage generator  820  and the first decoder  840 , and a second DAC  900  includes the fine gradation voltage generator  920  and the second decoder  940 . (M+N) digital signals D&lt;M+N: 1 &gt; are converted into the analog signals AN_OUT through two stages, that is, the first and second DACs  800  and  900 . 
     Here, the coarse gradation voltage generator  820  is shared by L DACs, which drive L channels of the liquid crystal panel. 
     Meanwhile, unlike the conventional DAC (refer to  FIG. 6 ), the first decoder  840  and the fine gradation voltage generator  920  are connected together without any unity gain amp. Accordingly, the resistor string of the coarse gradation voltage generator  820  is connected in parallel to that of the fine gradation voltage generator  920 . Therefore, in order to minimize the error due to the parallel connection, resistance Rch of the fine gradation voltage generator  920  must meet Equation 1 below. 
     
       
         
           
             
               
                 
                   
                     R 
                     ch 
                   
                   ≥ 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             M 
                           
                           - 
                           1 
                         
                         ) 
                       
                       · 
                       L 
                       · 
                       R 
                     
                     
                       
                         2 
                         M 
                       
                       · 
                       
                         2 
                         N 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     In Equation 1, R denotes the resistance of the coarse gradation voltage generator  820 . If resistances are different, R denotes the largest resistance among them. 
     That is, the DAC of the source driver adjusts the resistance of the resistor string contained in the fine gradation voltage generator  920 , which is connected in parallel without using the unity gain amp. Thus, the DAC of the source driver can minimize the influence of the parallel connection. Consequently, since there is no limit due to the offset voltage of the unity gain amp, the accuracy can be improved and the bits of the digital signal can be increased. In addition, the area occupied by the unity gain amp can be reduced. 
     Therefore, the high-gradation DAC having the high accuracy can be implemented. 
     Meanwhile, the resistance Rch of the fine gradation voltage generator  920  is a resistance given when a voltage level difference between an ideal voltage level V 1LSB  and an actual voltage level V 1LSB , in a 1-bit digital signal meets Equation 2 below. 
     
       
         
           
             
               
                 
                   
                     
                       V 
                       
                         1 
                         ⁢ 
                         LSB 
                       
                     
                     - 
                     
                       V 
                       
                         1 
                         ⁢ 
                         
                           LSB 
                           ′ 
                         
                       
                     
                   
                   ≤ 
                   
                     
                       1 
                       2 
                     
                     ⁢ 
                     
                       V 
                       
                         1 
                         ⁢ 
                         LSB 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     That is, the ideal voltage level V 1LSB  is a voltage level in case where the resistor string ratio of the front stage is not influenced by the resistor string of the rear stage, and the actual voltage level V 1LSB , is a voltage level in case where the resistor string ratio of the front stage is influenced by the resistor string of the rear stage. 
     A degree of the output error is about ⅓V 1LSB . However, The degree of the output error can be reduced below ⅓V 1LSB  by changing the coefficient of Equation 2. 
     In addition, in case where the L channels output the same analog signals, the largest error occurs due to the influence of the parallel connection. In such a case, the resistor string of the L fine gradation voltage generators  920  is connected in parallel to one resistor of the coarse gradation voltage generator  820 , as shown in  FIG. 8 . 
       FIG. 8  is an equivalent circuit diagram of the DAC shown in  FIG. 7 , in which the resistor string of the L fine gradation voltage generator  920  are connected in parallel to the resistor string of the coarse gradation voltage generator  820  when the 1 channels generate the same output. 
     Referring to  FIG. 8 , it can be seen that the actual voltage level V 1LSB  corresponding to 1-bit digital signal has a relationship of (VH′−VL′)/2 N  Accordingly, substituting in Equation 2, the result is given by 
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         
                           V 
                           H 
                         
                         - 
                         
                           V 
                           L 
                         
                       
                       ) 
                     
                     - 
                     
                       ( 
                       
                         
                           V 
                           H 
                           ′ 
                         
                         - 
                         
                           V 
                           L 
                           ′ 
                         
                       
                       ) 
                     
                   
                   ≤ 
                   
                     
                       1 
                       2 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           V 
                           H 
                         
                         - 
                         
                           V 
                           L 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     Referring to  FIG. 8 , (V H ′−V L ′) is a voltage applied on both terminals of the resistor R′ of the coarse gradation voltage generator connected in parallel to the resistor string of the fine gradation voltage generator  920  and is R′×(VREF_H−VREF_L)/R total ′. In the ideal case, a voltage applied on both terminals of the resistor R of the coarse gradation voltage generator  820  is R×(VREF_H—VREF_L)/R total . Accordingly, substituting in Equation 3, the result is given as Equation 4 below. 
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         R 
                         
                           R 
                           total 
                         
                       
                       ) 
                     
                     - 
                     
                       ( 
                       
                         
                           R 
                           ′ 
                         
                         
                           R 
                           total 
                           ′ 
                         
                       
                       ) 
                     
                   
                   ≤ 
                   
                     
                       1 
                       2 
                     
                     ⁢ 
                     
                       ( 
                       
                         R 
                         
                           R 
                           total 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
     R total ′ denotes a total resistance of the coarse voltage generator  820  when the resistor string of the L fine gradation voltage generator  920  is connected in parallel to the resistor string of the coarse voltage generator  820 . R total  denotes a total resistance of 2 M  serially-connected resistor strings connected of the coarse voltage generator  820 . 
     Referring to  FIG. 8 , the total resistance R total ′ of the coarse voltage generator  820  is R×(2 M −1)+R′. Also, in the ideal case, the total resistance R total  of the coarse voltage generator  820  is R×2 M . Substituting in Equation 4, the result is obtained as follows. 
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         R 
                         
                           R 
                           × 
                           
                             2 
                             M 
                           
                         
                       
                       ) 
                     
                     - 
                     
                       ( 
                       
                         
                           R 
                           ′ 
                         
                         
                           
                             R 
                             × 
                             
                               ( 
                               
                                 
                                   2 
                                   M 
                                 
                                 - 
                                 1 
                               
                               ) 
                             
                           
                           + 
                           
                             R 
                             ′ 
                           
                         
                       
                       ) 
                     
                   
                   ≤ 
                   
                     ( 
                     
                       R 
                       
                         R 
                         × 
                         
                           2 
                           
                             M 
                             + 
                             1 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
     When the resistor strings of the L fine gradation voltage generators  920  are connected in parallel to the resistors of the fine gradation voltage generator  920 , the resistance R′ is given as 
     
       
         
           
             
               R 
               ′ 
             
             = 
             
               
                 R 
                 × 
                 
                   ( 
                   
                     
                       R 
                       ch_total 
                     
                     L 
                   
                   ) 
                 
               
               
                 R 
                 + 
                 
                   ( 
                   
                     
                       R 
                       ch_total 
                     
                     L 
                   
                   ) 
                 
               
             
           
         
       
     
     R ch     —     total  denotes a total resistance of 2 N  serially-connected resistor strings of the fine gradation voltage generator  920 . Substituting in Equation 5, the result is obtained as follows. 
     
       
         
           
             
               
                 
                   
                     R 
                     ch_total 
                   
                   ≥ 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             M 
                           
                           - 
                           1 
                         
                         ) 
                       
                       · 
                       L 
                       · 
                       R 
                     
                     
                       2 
                       M 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ) 
                 
               
             
           
         
       
     
     The total resistance R ch     —     total  of the fine gradation voltage generator  920  is R ch ×2 N . Substituting in Equation 6, the result of Equation 1 can be obtained. 
     Meanwhile, when the resistor R 1  and the resistor R 2  are connected in parallel, a voltage level applied to the resistor R 1 ∥R 2  becomes ½ of a voltage level applied to the resistor R 1  when the resistor R 2  has the same resistance as the resistor R 1 . That is, in view of the resistance of the fine gradation voltage generator  930 , R ch     —     total /L=R, that is, R ch     —     total =R·L. 
     If 2 M −1≅2 M  because M is sufficiently large in Equation 6, it can be intuitively seen that the resistance of the fine gradation voltage generator is identical to Equation 6. 
     As described above, if the DAC is implemented with two stages, the rear stage adjusts the resistance and thus the gap between the stages can be connected without any unity gain amp. Accordingly, since the limit in the accuracy of the DAC due to the offset voltage of the conventional unity gain amp can be removed, the DAC having high accuracy can be implemented. In addition, the unity gain amp required at channels can be removed, thereby reducing the area. 
     The first decoder  840  of the DAC is implemented with one MOS switch to M MOS switch arrays connected in series. It is presumed that a total resistance of an ideal first decoder  840  is 0Ω. However, the first decoder  840  of an actual DAC has a resistance that cannot be ignored compared with the resistance of the fine gradation voltage generator  920 . A description will be made about a problem due to the resistance of the first decoder  840  actually implemented. 
       FIG. 9  is an equivalent circuit diagram of a DAC in accordance with the present invention. The output voltages V H1 /V L1  and V H2 /V L2  from the adjacent resistors R N  and R N−1  of the coarse gradation voltage generator  820  is decoded by the fine gradation voltage generator  920 . 
     As shown in  FIG. 9 , the resistors R SW11 /R SW12  and R SW21 /R SW22  respectively connected to both ends of the resistor strings of the fine gradation voltage generators  920  and  920 ′ are turn-on resistors within the first decoders  840  and  840 ′. 
       FIG. 10  is a graph illustrating an output voltage of the DAC shown in  FIG. 9 . X axis represents the analog signal AN_OUT of the DAC corresponding to the applied digital signal, and Y axis represents the voltage level of the analog signal AN_OUT. Also, a reference symbol “★” represents the analog output of the ideal DAC, and a reference symbol “∘” represents the analog output of the actually-implemented DAC. 
     Referring to  FIGS. 9 and 10 , the fine gradation voltage generator  920  receives the voltages V H1  and V L1  applied on both terminals of the resistor RN of the coarse gradation voltage generator  820  and divides the voltages. At this point, due to the turn-on resistance of the switch in the first decoder  840 , the voltage level V N  of the first output signal AN_OUT N  rises higher than the level V ORG     —     N  of the expected first output signal, and the voltage level of the last output signal AN_OUT N+3  drops lower than the voltage level V ORG     —     N+3  of the expected last output signal. Also, the voltage level V N  of the first output signal AN_OUT N  rises and the voltage level V N  of the last output signal AN_OUT N+3  drops. Therefore, the voltage levels of the signals AN_OUT N+1  and AN_OUT N+2 , which are divided through the resistors R ch12  and R ch13  arranged in series between the output node of the first voltage V N  and the output node of the last voltage V N+3 , are also higher or lower than the expected voltage levels. 
     The voltage level difference (V N −V N−1 ) between the voltage V N−1  of the last analog signal AN_OUT N−1  and the voltage V N  of the first analog signal AN_OUT N  are greater than the voltage level difference corresponding to 1-bit digital signal. 
     That is, it can be seen that voltage level gaps of the analog signals are not equal due to the turn-on resistance of the switches within the first decoder  840 . 
     Meanwhile, the problem due to the turn-on resistance of the MOS switch can be solved by extending the width of the MOS switch making the size of the resistor string of the fine gradation voltage generator larger. However, this may cause the increase of the circuit area and serves as a limit factor in the conversion speed of the DAC. 
     Accordingly, in the resistor string of the fine gradation voltage generator  920 , one resistance of the two resistors connected to the first decoder  840  is added to the turn-on resistance of the entire switch within the first decoder  840  in order to equalize the voltage level gaps of the analog signal. In this manner, it is adjusted to meet the resistance R ch  proposed in Equation 1. That is, the resistance can be expressed as
 
 R   ch   ′=R   ch   −R   SW     —     TOTAL   (Eq. 7)
 
     In Equation 7, R ch ′ denotes the resistance adjusted by one of the resistors connected to the first decoder, and R ch  denotes the resistance of the fine gradation voltage generator, which is calculated by Equation 7. Also, R SW     —     TOTAL  denotes the turn-on resistance of all the switches in the first decoder. 
       FIG. 11  is an equivalent circuit diagram of the DAC when the first resistance of resister string is adjusted. 
     As shown in  FIG. 11 , the resistance of the resistor string in the fine gradation voltage generator  920  is calculated based on Equation 1 and one resistance R ch  of the resistor string is 300 KΩ. Also, the total resistance of the switches in the first decoder  840  is 200 KΩ. Therefore, the first resistance R ch ′ of the resistor string in the fine gradation voltage generator  920  is 100 KΩ. 
       FIG. 12  is a graph illustrating the output voltage of the DAC shown in  FIG. 11 . 
     Referring to  FIG. 12 , the voltage level V RL  of the analog signal of the DAC implemented considering the resistance of the first decoder  840  is slightly higher than the analog signal of the ideal DAC as a whole. However, the rising level is the same as the resistance of the switches disposed at one side of the first decoder  840 . Consequently, the analog signal of the DAC in accordance with the present invention has the equal voltage level difference. 
     That is, the differential non-linearity (DNL) is equal. Here, the DNL is the voltage level difference of the analog signal outputted from the DAC. 
     If adjusting the upper voltage V REF     —     H  and the lower voltage V REF     —     L  supplied to the coarse gradation voltage generator  820 , the analog signal of the DAC can have the same voltage level as the analog signal of the ideal DAC. 
     Meanwhile, since the resistance of the rear stage is adjusted when the DAC is implemented in two-stage parallel structure, each stage can be connected without any unity gain amp. Accordingly, since it is possible to remove the limit of the accuracy of the DAC due to the offset voltage of the conventional unity gain amp, the DAC having the high accuracy can be implemented. In addition, the unity gain amp required in the respective channels can be removed, thus reducing the area. 
     Further, the constant gradation gaps can be made by adjusting the resistance of the resistors connected to the first decoder in the fine gradation voltage generator, considering the resistance of the switches between the respective stages. 
     Although the TFT-LCD has been described as one example, the present invention can also be applied to a TFT-OELD. 
     In accordance with the inventive source driver, the DAC having the two-stage parallel structure can be implemented by adjusting the resistance of the resistor string of the rear stage without any unity gain amp. Therefore, the accuracy and the resolution can be improved and the chip area can be reduced. Further, the analog signals having the equal gradation gaps can be outputted by adjusting one resistance of the resistor string contained in the DAC of the rear stage. 
     The present application contains subject matter related to Korean patent application No. 2004-60389, filed in the Korean Patent Office on Jul. 30, 2004, the entire contents of which being incorporated herein by reference. 
     While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.

Technology Category: 3