Abstract:
Digital-to-analog converters are disclosed that include a complementary metal-oxide semiconductor (CMOS) image sensor, which can control the brightness of a stored image delicately. The digital-to-analog converter includes: an array of resistors with weights on their resistance value, to distribute a voltage delicately and develop a plurality of analog signals; a switching circuit to select and output one of the analog signals of the arrayed resistors in response to a digital control code; and an output circuit to buffer and develop the output of the switching circuit.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a digital-to-analog converter used in a complementary metal-oxide semiconductor (CMOS) image sensor circuit; and, more particularly, to a digital-to-analog converter that makes the amplification rate of a pixel signal change linearly according to digital control codes inputted thereto.  
         DESCRIPTION OF RELATED ART  
         [0002]    Generally, an image sensor is a device that captures images by using a photo-reactive characteristic of a semiconductor. With different brightness and wavelength, every aspect of a subject existing in nature shows a different electrical value at each pixel of a sensing device. It is the image sensor that converts the electrical values into signals that can be processed.  
           [0003]    Recently, image sensors have received additional attention as they are applied to a variety of security equipment, video conference cameras, digital still cameras, PC cameras, next-generation PDAs, with a function of transmitting image information and other data.  
           [0004]    Image sensors are classified into two types: One type is a charge-coupled device (CCD) image sensor; and the other type is a complementary metal-oxide semiconductor (CMOS) image sensor. Compared to a CCD image sensor, a CMOS image sensor can be operated more easily and implement a variety of scanning methods. Also, because a CMOS image sensor is capable of integrating a single processing circuit in one chip, the CMOS image sensor may assist in miniaturizing products as well as reducing production costs with its CMOS technology. Additionally, with remarkably low power consumption when compared to the CCD, the CMOs image sensor&#39;s applicable fields continue to expand.  
           [0005]    Conventionally, a CMOS image sensor comprises a comparator for comparing a ramp signal, as a reference signal, which decrease regularly, and an analog data signal from a photodiode; a counter to initiate counting when the ramp signal is outputted; and a latch for storing a counted value in a digital data value according to the value of the comparison result.  
           [0006]    To perform the operation described above, the CMOS image sensor includes a digital control block for outputting various control signals. The digital control block sets up a range of storable digital data values, for example, from 0 to 255, according to the brightness of an image, by inputting a digital control code with a digital-to-analog converter. A digital control code is a control code a user inputs arbitrarily that controls the range of storable digital data values according to the brightness of an external light.  
           [0007]    The CMOS image sensor equips the digital-to-analog converter at the digital control block so as to set up the range of digital data values (e.g. 0-255). According to the output voltage from the digital-to-analog converter, the unit voltage of a ramp signal according to the reference clock is determined.  
           [0008]    For example, when an input digital control code is binary ‘00001’, the range of storable digital data values is binary ‘00000000-11111111’ (0-255). If the input digital control code is binary ‘00010’, the range of digital data values is binary ‘00000000-01111111’ (0-127). Similarly, if the input digital control code is binary ‘00100’, the range of digital data values becomes binary ‘00000000-00111111’ (0-63). When the input digital control code is binary ‘01000’, the range becomes binary ‘00000000-00011111’ (0-31).  
           [0009]    The bigger the range of digital data values is, the brighter the image is stored. When a digital data value is 255, the screen becomes about twice as bright as when the digital data value is 127. Therefore, if a user wants to see a dark part brighter, the range of storable digital data values should be increased by making the value of a digital control code smaller.  
           [0010]    [0010]FIG. 1 is a circuit diagram illustrating a digital-to-analog converter of a conventional CMOS image sensor, which is implemented in three bits.  
           [0011]    Referring to FIG. 1, the digital-to-analog converter comprises a voltage divider  10  for equally distributing a reference voltage; a switching block  20  for switching the distributed resistances to an output block; and an output block  30  for outputting signals through the switching block  20 .  
           [0012]    The voltage divider  10  is composed of eight resistors (R) serially arrayed between the reference voltage (Vref) and the ground voltage level. The switching block  20  is comprised of a plurality of N channel MOS transistors configured so that only one path should be selected by a decoded digital value. The output block  30  is comprised of an operation amplifier that feedbacks the output to a negative (−) input and thus serves as a buffer. When connected as shown in FIG. 1, an operation amplifier operates as a buffer with a gain of 1.  
           [0013]    Referring to FIG. 1, the operation of a digital-to-analog converter will be described below.  
           [0014]    When the reference voltage (Vref) from the circuit shown in FIG. 1 is ‘1V’ and the digital control code ‘001’ is inputted into the switching block  20 , switches receiving the signals /b 1 , /b 2 , /b 3  inputted to the switching block  20  are turned on, and a voltage of  {fraction (1/8)}V is output to the output block 30. Also, when the digital control code is increased from ‘ 0001’ to ‘0100’, the output values of the digital-to-analog converter increase to  {fraction (1/8)}V,    {fraction (2/8)}V,    {fraction (3/8)}V and    {fraction (4/8)}V, respectively. That is, as the resistance values of the digital-to-analog converter is regular, the output voltage differences increase regularly as well according to input digital control codes.    
           [0015]    A conventional digital-to-analog converter of a CMOS image sensor uses an array of 32 resistors and outputs 32 voltage values. The voltage values determine the unit voltage of a ramp signal input to the comparator of the CMOS image sensor. A ramp signal decreases as much as the unit voltage, e.g.,  {fraction (1/256)}, each clock pulse, so the size of the unit voltage is determined by the output value of the digital-to-analog converter before the output value is input to the comparator.    
           [0016]    For instance, when the digital control code is binary ‘00001’, the output voltage of the digital-to-analog converter is  {fraction (1/32)}V. Then, from the output value, the unit voltage of a ramp -signal    {fraction (1/256)}V is output. If the digital control code is binary ‘ 00010’, the voltage output from the digital-to-analog converter is  {fraction (2/32)}V, and from this voltage, the unit voltage of a ramp signal is obtained to be    {fraction (2/256)}V. After that, when the digital control code increases to ‘ 00011’ and ‘00100’, the unit voltages of the ramp signal increases to  {fraction (3/256)}V and    {fraction (4/256)}V, respectively, in the CMOS image sensor.    
           [0017]    The unit voltage of a ramp signal is an amount of decreased voltage per clock cycle. As the unit voltage becomes smaller and smaller, the ramp signal decreases for many clock cycles, and as the unit voltage becomes bigger and bigger, the ramp signal decreases for fewer clock cycles. The number of clock cycles used while the ramp signals are supplied becomes the range of storable digital data values in the image sensor. That is, if the digital control code is 1000011, the ramp signal of the comparator decreases  {fraction (1/256)}V per clock cycle, and thereafter, the ramp signal decreases for a total of  255 clock cycles. This means that the range of storable digital data values is from 0 to 255.  
           [0018]    [0018]FIG. 2A is a schematic diagram showing a ramp signal of a CMOS image sensor wherein the digital control code input to the digital-to-analog converter is binary ‘00001’.  
           [0019]    Referring to FIG. 2A, it is shown that the ramp signal decreases as much as the unit voltage, i.e.,  {fraction (1/256)}V, per clock cycle when a digital control code is ‘ 00001’. Therefore, the range of digital data values that can be stored in the CMOS image sensor is ‘from 0 to 255’.  
           [0020]    [0020]FIG. 2B is a schematic diagram showing a ramp signal of a CMOS image sensor, wherein the digital control code inputted to the digital-to-analog converter is binary ‘00010’.  
           [0021]    Referring to FIG. 2B, when the digital control code is ‘00010’, the ramp signal decreases as much as  {fraction (2/256)}V per clock cycle, i.e., the unit voltage, and the range of digital data values storable in the CMOS image sensor is ‘from  0 to 127’. Also, if the digital control code is ‘00011’, the range of digital data value is ‘from 0 to 63’. As described above, the wider the range of the storable digital values becomes, the brighter the image information is stored.  
           [0022]    [0022]FIG. 3 is a graph showing the range of the storable digital data values to a digital control code, which is input to the digital-to-analog converter in the CMOS image sensor, and it is represented in TABLE 1 below.  
                       TABLE 1                       Digital Control   Analog Output   Range of Digital       Code   Voltage   Data Values                   01000 (8)   8/32   0-16        00110 (6)   6/32   0-32        00100 (4)   4/32   0-63        00010 (2)   2/32   0-127       00001 (1)   1/32   0-255                  
 
           [0023]    Referring to TABLE 1, even if a digital control code is increased gradually, the range of storable digital data values is not gradually decreased. From this, we can see that the brightness of a stored image of a subject does not change linearly, even if a digital control code is changed linearly. In addition, the brightness of an image cannot be controlled delicately, even though the digital control code is controlled to store the image brighter in a dark place.  
           [0024]    Of course, it is possible to control the digital control code and control the brightness of a stored image rather delicately, when the digital control code is relatively big, such as 00110. However, fine controlling here is meaningless and it&#39;s still hard to control the range of storable digital data values finely to change the brightness of an image in a practical area, e.g., when the digital control code ranges from 0001 to 0100.  
           [0025]    Therefore, a CMOS image sensor is needed that can control the brightness of a stored image by making linear relations between the range of digital data values and the digital control codes.  
         SUMMARY OF THE INVENTION  
         [0026]    In accordance with an embodiment of the present invention, there is provided a digital-to-analog converter, comprising: an array of resistors with weights on their resistance value, for distributing a voltage delicately and outputting a plurality of analog signals; a switching block for selecting and outputting one of the outputs of the arrayed resistors according to digital control codes; and an output means for buffering and outputting an output from the switching block.  
           [0027]    In accordance with an embodiment of the present invention, there is provided a CMOS image sensor, comprising: a ramp signal control block for outputting a ramp signal that decreases as much as a unit voltage regularly per clock cycle; and a digital-to-analog converter that outputs a reference voltage to determine the size of the unit voltage. The digital-to-analog converter comprises: a) an array of resistors with weights on their resistance value, for distributing a voltage delicately and outputting a plurality of analog signals; b) a switching block for selecting and outputting one of the outputs of the arrayed resistors according to digital control codes; and c) an output means for buffering and outputting an output from the switching block. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING(S)  
       [0028]    [0028]FIG. 1 is a circuit diagram illustrating a digital-to-analog converter of a conventional CMOS image sensor, which is implemented in three bits;  
         [0029]    [0029]FIG. 2A is a schematic diagram showing a ramp signal when a digital control code input to the digital-to-analog converter is ‘1’;  
         [0030]    [0030]FIG. 2B is a schematic diagram showing a ramp signal when a digital control code input to the digital-to-analog converter is ‘2’;  
         [0031]    [0031]FIG. 3 is a graph representing a range of storable digital data values to a digital control code input to the digital-to-analog converter of a CMOS image sensor;  
         [0032]    [0032]FIG. 4 is a block diagram of a digital-to-analog converter applied to a CMOS image sensor constructed in accordance with the teachings of the invention;  
         [0033]    [0033]FIG. 5 is a circuit diagram illustrating a digital-to-analog converter of a CMOS image sensor, which is implemented in three bits and constructed in accordance with the teachings of the invention; and  
         [0034]    [0034]FIG. 6 is a graph showing a range of storable digital data values to a digital control code input to the digital-to-analog converter of a CMOS image sensor constructed in accordance with the teachings of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    [0035]FIG. 4 is a block diagram of a digital-to-analog converter applied to a complementary metal-oxide semiconductor (CMOS) image sensor. The CMOS image sensor comprises: a comparator  600  for receiving a ramp signal and an analog data signal generated at a unit pixel according to the intensity of external light; a ramp signal control block  500  for outputting a ramp signal to the comparator  600 ; a digital-to-analog converter  400  for determining a unit voltage of a ramp signal; a latch  700  for storing an analog data signal in a digital data signal according to the output of the comparator  600 ; and a counter  800  for counting the number of clock signals and outputting the counted value to the latch  700 .  
         [0036]    As described above, a CMOS image sensor includes a digital-to-analog converter to set up a range of digital data values (e.g., 0-255), and according to the output voltage from the digital-to-analog converter, the unit voltage of a ramp signal decreasing according to the reference clock is determined.  
         [0037]    [0037]FIG. 5 is a circuit diagram illustrating a digital-to-analog converter of a CMOS image sensor implemented in three bits in accordance with the teachings of the invention.  
         [0038]    Referring to FIG. 5, the digital-to-analog converter is composed of a voltage divider  100 , a switching block  200  and an output block  300 .  
         [0039]    The voltage divider  100  consists of a plurality of resistors connected in serial, which distribute the reference voltage (Vref) in a predetermined ratio and output the voltages to the switching circuit  200 . The voltage divider  100  illustrated in the drawing is an example of a three-bit digital-to-analog converter with eight resistors serially connected between the reference voltage (Vref) and the ground voltage level. The weights of the eight resistors are  12 R,  12 R,  6 R,  2 R,  2 R,  2 R,  2 R, and  6 R, respectively. Here, R denotes a unit resistor. For example, when the unit resistor (R) is 100Ω,  2 R becomes 200Ω.  
         [0040]    In the switching circuit  200 , a plurality of switches connect the voltage divider  100  and the output circuit  300  so that only one path is selected by a decoded digital control code.  
         [0041]    Still referring to FIG. 5, the switching circuit  200  includes: four NMOS transistors, wherein the transistors&#39; gates receive a first bit signal (b 3 ) of a three-bit decoder signal for electrically connecting first, third, fifth and seventh resistors from a ground voltage level to nodes N 1 , N 2 , N 3 , and N 4 , respectively; four NMOS transistors, wherein the transistors&#39; gates receive an inverted signal (/b 3 ) of the first bit signal of a three-bit decoder signal for electrically connecting second, fourth, sixth, and eighth resistors to nodes N 1 , N 2 , N 3 , and N 4 , respectively; two NMOS transistors, wherein the transistors&#39; gates receive a second bit signal (b 2 ) of a three-bit decoder signal for electrically connecting nodes N 2  and N 4  to nodes N 5  and N 6 , respectively; two NMOS transistors, wherein the transistors&#39; gates receive an inverted signal (/b 2 ) of a second bit signal of the three-bit decoder signal for electrically connecting nodes N 1  and N 3  to nodes N 5  and N 6 ; an NMOS transistor, wherein the transistor&#39;s gate receives a third bit signal (b 1 ) of a three-bit decoder signal supplied for electrically connecting nodes N 6  to N 7 ; and an NMOS transistor, wherein the transistor&#39;s gate receives an inverted signal of the third bit signal (/b 1 ) for electrically connecting nodes N 5  and N 7 .  
         [0042]    The output circuit  300  comprises an operation amplifier which feedback the output to a negative (−) input and serves as a buffer. When the output of the operation amplifier is fed back to the negative (−) input, the operation amplifier operates as a buffer that outputs the voltage of the positive input.  
         [0043]    The operation of the digital-to-analog converter from FIG. 5 will be described hereinafter.  
         [0044]    When the reference voltage (Vref) is ‘1V’ and a digital control code ‘ 110 ’ is input to the switching circuit  200 , switches receiving the signals b 1 , b 2 , and /b 3  that are input to the switching circuit  200  are turned on and a voltage  {fraction (1/2)}V is output to the output block 300. Also, when the digital control code is increased from ‘ 001’ to ‘100’ gradually, the output voltages of the digital-to-analog converter increase to  {fraction (1/8)}V,    {fraction (1/6)}V,    {fraction (5/24)}V and    {fraction (1/4)}V, respectively.    
         [0045]    In short, when the serially connected resistors of the digital-to-analog converter are controlled, the output voltages increase with the weights, not regularly input digital control codes. The output voltages to digital control codes are shown in TABLE 2.  
                                             TABLE 2                                   Digital Control Code   Analog Voltage                                        8       Vref           6   1/2   Vref           4   1/4   Vref           2   1/6   Vref           1   1/8   Vref                      
 
         [0046]    [0046]FIG. 6 is a graph showing the range of digital data values that the CMOS image sensor can output, to a digital control code input to the digital-to-analog converter of FIG. 5, in accordance with the teachings of the invention.  
         [0047]    Referring to FIG. 6, the range of the storable digital data values to the input digital control codes has a relatively linear shape. The brightness of an image can thus be controlled delicately by controlling the digital control codes. Also, when a dark part needs to be shown brighter, the brightness of an image can be controlled delicately by handling a digital control code.  
         [0048]    The range of storable digital data values provide a linear shape when compared to a digital control code. This occurs by controlling the resistor array of the digital-to-analog converter in the digital control block. The line shown in FIG. 6 is formed by controlling the resistor array. Therefore, the 32 resistors described above are divided in several parts and the divided resistors are assigned different weights. When a more delicate line is needed to be shaped, the weights of the all resistors are controlled.  
         [0049]    The digital-to-analog converter constructed in accordance with the teachings of the invention controls the brightness of an image more delicately, particularly, the brightness of a dark part of an image.  
         [0050]    While the teachings of the invention have been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.