an optical pointing apparatus and an automatic gain control ( agc ) method thereof according to the present invention will be described more fully hereinafter with reference to the accompanying drawings , in which exemplary embodiments of the invention are shown . fig2 is a block diagram of an optical pointing apparatus according to an exemplary embodiment of the present invention . referring to fig2 , the optical pointing apparatus includes an optical unit 100 , an image sensor 200 , a multiplexer 300 , an automatic gain control ( agc ) unit 400 , an analog - to - digital converter ( adc ) 500 , and an image data processor 600 . the image sensor 200 is comprised of a plurality of pixels , and the image data processor 600 includes an image analyzer 610 , a motion value calculator 630 , and a motion detector 650 . functions of the blocks shown in fig2 will now be described . the optical unit 100 is comprised of a light source and a lens . the optical unit 100 radiates light onto a work surface using the light source in response to a light source control signal , and transmits light reflected by the work surface through the lens to the image sensor 200 . the image sensor 200 receives the reflected light through the lens and senses image data . the multiplexer 300 selects one pixel or a small number of pixels out of a pixel array of the image sensor 200 and outputs an electrical signal having image data of the selected pixel . the agc unit 400 receives the electrical signal having the image data of the selected pixel from the multiplexer 300 , controls the gain of the electrical signal in response to gain control signals g_con 1 and g_con 2 which depend on the work surface or the motion speed of the optical pointing apparatus , and outputs an analog gain output signal to the adc 500 . the adc 500 receives the analog gain output signal from the agc unit 400 and converts the analog signal into a digital image signal . up to now , the adc 500 is described to be a simple conversion function only . but , it is natural that the agc unit 400 is embedded into the adc 500 . the image data processor 600 receives the digital image signal from the adc 500 . then , the mage analyzer 610 analyzes brightness of the work surface and outputs a light source control signal lo_con , a shutter control signal st_con , and a first gain control signal g_con 1 . the motion detector 650 detects the motion speed of the optical pointing apparatus and outputs a second gain control signal g_con 2 . thereafter , the motion value calculator 630 calculates a motion value v ( mov ) based on the digital image signal output by the adc 500 , and outputs the motion value v ( mov ). here , a method of generating the light source control signal lo_con , the shutter control signal st_con , and the first gain control signal g_con 1 is known to one of ordinary skill in the art , and thus a detailed description thereof will be omitted . fig3 is an internal block diagram of the agc unit of the optical pointing apparatus shown in fig2 . referring to fig3 , the agc unit includes a first agc unit 420 and a second agc unit 440 . functions of the blocks shown in fig3 will now be described with reference to fig3 . the first agc unit 420 receives an electrical signal of a selected pixel from the multiplexer 300 , controls the gain of the electrical signal in response to the first gain control signal g_con 1 which depends on brightness and darkness from images of the work surface , and outputs a gain output the second agc unit 440 receives an electrical signal of a selected pixel from the multiplexer 300 , controls the gain of the electrical signal in response to the second gain control signal g_con 2 which depends on the motion speed of the optical pointing apparatus , and outputs a gain output signal . although the first and second agc units 420 and 440 are illustrated as being connected in parallel in fig3 , the first and second agc units 420 and 440 may be connected in series . specifically , when a shutter - on time is reduced by a predetermined time due to the bright color of the work surface , the gain of an electrical signal is compared with the maximum gain . and when the gain of the electrical signal is smaller than the maximum gain , the first agc unit 420 maintains the gain of the electrical signal , while when the gain of the electrical signal is larger than the maximum gain , the first agc unit 420 reduces the gain of the electrical signal by a predetermined level . however , when a shutter - on time is increased by a predetermined time due to the dark color of the work surface , the gain of the electrical signal is compared with the minimum gain . and when the gain of the electrical signal is larger than the minimum gain , the first agc unit 420 maintains the gain of the electrical signal , while when the gain of the electrical signal is smaller than the minimum gain , the first agc unit 420 increases the gain of the electrical signal by a predetermined level . meanwhile , the second agc unit 440 maintains the gain of the electrical signal when the average motion speed of the optical pointing apparatus is higher than the minimum speed and lower than the maximum speed , increases the gain of the electrical signal by a predetermined level when the average motion speed is higher than the maximum speed , and decreases the gain of the electrical signal by a predetermined level when the average motion speed is lower than the minimum speed . here , since the light source control signal lo_con , the shutter control signal st_con , and the first gain control signal g_con 1 are output by the image analyzer 610 based on statistical characteristics of an image of the work surface , each of the light source control signal lo_con , the shutter control signal st_con , and the first gain control signal g_con 1 is calculated irrespective of allowed maximum and minimum values . examples of the statistical characteristics include the average , maximum and minimum values , standard deviation , and features of an image value of the work surface . the statistical characteristics are obtained without consideration of anomalies caused by defects of pixels or electrical defects . operation of the optical pointing apparatus according to the present embodiment will now be described with reference to fig2 and 3 . when the optical unit 100 radiates light onto the work surface using the light source , the image sensor 200 receives light reflected by the work surface through the lens , senses brightness and darkness from images of the work surface , and outputs image data . the multiplexer 300 receives the sensed image data from the image sensor 200 , selects a single pixel or a small number of pixels out of the plurality of pixels , and outputs an electrical signal having image data of the selected pixel . the agc unit 400 receives the electrical signal having the image data of the selected pixel from the multiplexer 300 and controls the gain of the electrical signal . when a predetermined shutter - on time elapses , the agc unit 400 controls the gain of the electrical signal in response to the first and second gain control signals g_con 1 and g_con 2 and outputs an analog gain output signal to the adc 500 . the adc 500 receives the analog gain output signal output from the agc unit 400 and converts the analog gain output signal into a digital image . the image analyzer 610 receives the digital image signal from the adc 500 , analyzes brightness and darkness from images of the work surface , outputs the light source control signal lo_con for controlling the illumination of the light source , the shutter control signal st_con for controlling the shutter - on time , and the gain control signal g_con 1 for controlling the gain of the electrical signal , and feeds the light source control signal lo_con , the shutter control signal st_con , and the first gain control signal g_con 1 back to the optical unit 100 , the image sensor 200 , and the agc unit 400 , respectively . that is , when the work surface is bright , the adc 500 outputs a shutter control signal st_con for reducing a shutter - on time , a light source control signal lo_con for reducing a light - on time , and a first gain control signal g_con 1 for reducing the gain of the adc input data . conversely , when the work surface is dark , the adc 500 outputs a shutter control signal st_con for increasing a shutter - on time , a light source control signal lo_con for increasing a light - on time , and a first gain control signal g_con 1 for increasing the gain of the adc input data . also , the motion detector 650 receives the digital image signal from the adc 500 , detects the motion speed of the optical pointing apparatus , outputs the second gain control signal g_con 2 for controlling the gain of the adc input data , and feeds the second gain control signal g_con 2 back to the agc unit 400 . that is , the motion detector 650 outputs a second gain control signal g_con 2 for reducing the gain of the adc input data when the motion speed of the optical pointing apparatus is lower than the minimum speed , and outputs a second gain control signal g_con 2 for increasing the gain of the adc input data when the motion speed of the optical pointing apparatus is higher than the maximum speed . meanwhile , the motion value calculator 630 receives the digital image signal converted by the adc 500 , calculates a correlation value between current image data and previous image data , calculates a coordinate motion value v ( mov ) of the optical pointing apparatus based on the correlation value , and outputs the coordinate motion value v ( mov ). control of the shutter - on time and control of the gain of the electrical signal are performed such that the average of an output signal of the adc 500 is maintained constant . in order to minimize power consumption of the optical pointing apparatus , the agc unit 400 must control the gain of the image signal at the same time to minimize the shutter - on time . each of the first and second agc units 420 and 440 of the agc unit 400 of the optical pointing apparatus shown in fig3 may include a single variable gain amplifier amp as shown in fig4 a , so that a gain output signal may be output in a linear gain range in an analog mode according to voltage levels of the gain control signals g_con 1 and g_con 2 . alternatively , each of the first and second agc units 420 and 440 of the agc unit 400 of the optical pointing apparatus shown in fig3 may include a plurality of variable gain amplifiers amp 1 to ampn as shown in fig4 b , so that an output signal may be selectively output in a discontinuous gain range according to voltage levels of the first and second gain control signals g_con 1 and g_con 2 . fig4 a is a circuit diagram of a first example of the first agc unit of the agc unit shown in fig3 . referring to fig4 a , the first agc unit 420 includes the single variable gain amplifier amp . operation of the agc unit 400 of the optical pointing apparatus will now be described with reference to fig4 a . the variable gain amplifier amp receives an electrical signal of a selected pixel from the multiplexer 300 , controls the gain of the electrical signal by a predetermined level , and outputs a gain output signal . in this case , the variable gain amplifier amp continuously outputs the gain output signal in an analog mode in response to voltage levels of the gain control signals g_con 1 and g_con 2 which depend on brightness and darkness from images of the work surface or the motion speed of the optical pointing apparatus . fig4 b is a circuit diagram of a second example of the first agc unit of the agc unit shown in fig3 . referring to fig4 b , the first agc unit 420 includes a plurality of variable gain amplifiers 422 and a variable gain multiplexer 424 . operation of the agc unit of the optical pointing apparatus will now be described with reference to fig4 b . the plurality of variable gain amplifiers amp 1 to ampn are connected in series . thus , a first variable gain amplifier amp 1 receives an electrical signal of a selected pixel from the multiplexer 300 , controls the gain of the electrical signal by a predetermined level , and outputs a first gain output signal . also , a second variable gain amplifier amp 2 receives the first gain output signal from the first variable gain amplifier amp 1 , controls the gain of the first gain output signal by a predetermined level , and outputs a second gain output signal . similarly , each of third through n - th variable gain amplifiers amp 3 to ampn receives a gain output signal from a front - stage variable gain amplifier , controls the gain of the received gain output signal , and outputs the corresponding one of third to n - th gain output signals . the variable gain multiplexer 424 receives the first through n - th gain output signals and selectively outputs one of the first through n - th gain output signals according to voltage levels of the gain control signals g_con 1 and g_con 2 which depend on brightness and darkness from images of the work surface or the motion speed of the optical pointing apparatus . fig5 is an operation flowchart of an optical pointing apparatus according to a first exemplary embodiment of the present invention , in a case where a shutter - on time is controlled prior to gain of an image signal according to the brightness of a work surface . a method of controlling the gain of an adc input signal of the optical pointing apparatus according to the embodiment of the present invention will now be described with reference to fig2 , 3 , and 5 . when the light source radiates light onto the work surface , the image sensor 200 senses brightness and darkness from images of the work surface , and outputs image data . the multiplexer 300 selects a single pixel or a small number of pixels and outputs an electrical signal having image data of the selected pixel . the agc unit 400 receives the electrical signal having the image data of the selected pixel , controls the gain of the electrical signal , and outputs an analog gain output signal to the adc 500 . the adc 500 receives the analog gain output signal from the agc unit 400 and converts the analog signal into a digital image signal . the motion detector 650 detects the motion speed of the optical pointing apparatus , outputs the second gain control signal g_con 2 , and feeds the second gain control signal g_con 2 back to the agc unit 400 . the motion calculator 630 receives the digital image signal from the adc 500 and calculates and outputs a coordinate motion value v ( mov ) of the optical pointing apparatus . since the foregoing operation of the optical pointing apparatus shown in fig2 was described in detail above , a further detailed description thereof will not be described again here . the image analyzer 610 receives the digital image signal from the adc 500 and analyzes brightness and darkness from images of the work surface . referring to fig5 , in step s 100 , the average of the digital image signal output by the adc 500 is calculated , the minimum and maximum values of the average are determined , and a shutter - on time , a light - on time , and the gain of an electrical signal are calculated . in steps s 120 and 140 , the average of the digital image signal is compared with the maximum and minimum values . when the work surface is bright , the average of the digital image signal output by the adc 500 is higher than the maximum value . in this case , the shutter - on time is reduced by a predetermined time in step s 200 . thereafter , the gain of the electrical signal to be input to the adc 500 is compared with the maximum gain in step s 250 , while when the gain of the electrical signal is smaller than the maximum gain , the gain of the electrical signal is maintained in step s 270 . when the gain of the electrical signal is larger than the maximum gain , the gain of the electrical signal is reduced by a predetermined level or the minimum gain is fixed in step s 290 . meanwhile , when the work surface is dark , the average of the digital image signal output by the adc 500 is lower than the maximum value . in this case , the shutter - on time is increased by a predetermined time in step s 300 . thereafter , the gain of the electrical signal to be input to the adc 500 is compared with the minimum gain in step s 350 . when the gain of the electrical signal is larger than the minimum gain , the gain of the electrical signal is maintained in step s 370 , while when the gain of the electrical signal is smaller than the minimum gain , the gain of the electrical signal is increased by a predetermined level or the maximum gain is in step s 390 . after all the steps are finished , the process returns to steps s 120 and s 140 in which the average of the digital image signal is compared with the maximum and minimum values . while it is described above that the gain of the electrical signal is reduced and increased in steps s 290 and s 390 , alternatively , the gain of the electrical signal may be fixed . in particular , when the gain of the electrical signal reaches the minimum gain and cannot be reduced any more , or the maximum gain and cannot be not increased any more , the gain of the electrical signal may be fixed at the minimum or maximum gain . fig6 is an operation flowchart of an optical pointing apparatus according to a second exemplary embodiment of the present invention , in a case where gain of an image signal is controlled prior to a shutter - on time according to the brightness of a work surface . a method of controlling the gain of an adc input signal of the optical pointing apparatus according to the embodiment of the present invention will now be described with reference to fig2 , 3 , and 6 . again , since operation of the image sensor 200 , the multiplexer 300 , the agc unit 400 , the adc 500 , the motion detector 650 , and the motion value calculator 630 of the optical pointing apparatus shown in fig2 was described in detail above , such description will not be repeated here . the image analyzer 610 receives the digital image signal from the adc 500 and analyzes brightness and darkness from images of the work surface . referring to fig6 , in step s 100 , the average of the digital image signal output by the adc 500 is calculated , the minimum and maximum values of the average are determined , and a shutter - on time , a light - on time , and the gain of an electrical signal are calculated . in steps s 120 and 140 , the average of the digital image signal is compared with the maximum and minimum values . when the work surface is bright , the average of the digital image signal output by the adc 500 is higher than the maximum value . in this case , the gain of the electrical signal to be input to the adc 500 is reduced by a predetermined level in step s 400 . thereafter , the shutter - on time is compared with the maximum time in step s 450 . thus , when the shutter - on time is shorter than the maximum time , the shutter - on time is maintained in step s 470 . when the shutter - on time is longer than the maximum time , the shutter - on time is reduced by a predetermined time or the minimum shutter - on time is fixed in step s 490 . meanwhile , when the work surface is dark , the average of the digital image signal output by the adc 500 is lower than the maximum value . in this case , the gain of the electrical signal to be input to the adc 500 is increased by a predetermined level in step s 500 . thereafter , the shutter - on time is compared with the minimum time in step s 550 thus , when the shutter - on time is longer than the minimum time , the shutter - on time is maintained in step s 570 , and when the shutter - on time is shorter than the minimum time , the shutter - on time is increased by a predetermined time or the maximum shutter - on time is fixed in step s 590 after all the steps are finished , the process returns to steps s 120 and s 140 in which the average of the digital image signal is compared with the maximum and minimum values . while it is described above that the shutter - on time is reduced and increased in steps s 490 and s 590 , alternatively , the gain of the electrical signal may be fixed . in particular , when the shutter - on time reaches the minimum time and cannot be reduced any more , or the maximum time and cannot be increased any more , the shutter - on time may be fixed at the minimum or maximum time . fig7 is an operation flowchart of an optical pointing apparatus according to a third exemplary embodiment of the present invention , in a case where a light - on time is controlled prior to gain of an image signal and a shutter - on time according to the brightness of a work surface . a method of controlling the gain of an adc input signal of the optical pointing apparatus according to the embodiment of the present invention will now be described with reference to fig2 , 3 , and 7 . again , since operation of the image sensor 200 , the multiplexer 300 , the agc unit 400 , the adc 500 , the motion detector 650 , and the motion value calculator 630 of the optical pointing apparatus shown in fig2 was described in detail above , a description thereof will not be repeated here . the image analyzer 610 receives the digital image signal from the adc 500 and analyzes brightness and darkness from images of the work surface . referring to fig7 , in step s 100 , the average of the digital image signal output by the adc 500 is calculated , the minimum and maximum values of the average are determined , and a shutter - on time , a light - on time , and the gain of an electrical signal are calculated . in steps s 120 and 140 , the average of the digital image signal is compared with the maximum and minimum values . when the work surface is bright , the average of the digital image signal output by the adc 500 is higher than the maximum value . in this case , the light - on time is reduced by a predetermined time in step s 600 . thereafter , the shutter - on time is compared with the maximum time in step s 450 . when the shutter - on time is shorter than the maximum time , the shutter - on time is maintained in step s 470 . when the shutter - on time is longer than the maximum time , the shutter - on time is reduced by a predetermined time in step s 490 . it is determined whether the reduced shutter - on time is the minimum time in step s 650 . when the reduced shutter - on time is not the minimum time , the process returns to steps s 120 and s 140 in which the average of the digital image signal is compared with the maximum and minimum values . when the reduced shutter - on time is equal to or smaller than the minimum time , the gain of the electrical signal to be input to the adc 500 is compared with the maximum gain in step s 250 . and when the gain of the electrical signal is smaller than the maximum gain , the gain of the electrical signal is maintained in step s 270 , while when the gain of the electrical signal is larger than the maximum gain , the gain of the electrical signal is reduced by a predetermined level in step s 290 . meanwhile , when the work surface is dark , the average of the digital image signal output by the adc 500 is lower than the maximum value . in this case , the light - on time is increased by a predetermined time in step s 700 . thereafter , the shutter - on time is compared with the minimum time in step s 550 . when the shutter - on time is longer than the minimum time , the shutter - on time is maintained in step s 570 , and when the shutter - on time is shorter than the minimum time , the shutter - on time is increased by a predetermined time in step s 590 . it is determined if the increased shutter - on time is the maximum time in step s 750 . when the reduced shutter - on time is not the maximum time , the process returns to steps s 120 and s 140 . when the increased shutter - on time is the maximum time , the gain of the electrical signal to be input to the adc 500 is compared with the minimum gain in step s 350 . thus , when the gain of the electrical signal is larger than the minimum gain , the gain of the electrical signal is maintained in step s 370 . when the gain of the electrical signal the gain of the electrical signal is smaller than the minimum gain , the gain of the electrical signal is increased by a predetermined level in step s 390 . after all the steps are finished , the process returns to steps s 120 and s 140 . like in fig5 , it is described above that the gain of the electrical signal is reduced and increased in steps s 290 and s 390 , respectively , for brevity , but the gain of the electrical signal may be fixed . in particular , when the gain of the electrical signal reaches the minimum gain and cannot be reduced any more , or the maximum gain and cannot be increased any more , the gain of the electrical signal may be fixed at the minimum or maximum gain . fig8 is an operation flowchart of an optical pointing apparatus according to a fourth another exemplary embodiment of the present invention , in a case where gain of an image signal is controlled according to motion speed . a method of controlling the gain of an adc input signal of the optical pointing apparatus according to the embodiment of the present invention will now be described with reference to fig2 , 3 , and 8 . again , since operation of the image sensor 200 , the multiplexer 300 , the agc unit 400 , the adc 500 , the motion detector 650 , and the motion value calculator 630 of the optical pointing apparatus shown in fig2 was described in detail above , a description thereof will not be repeated here . the motion detector 650 receives a digital image signal from the adc 500 and detects the motion speed of the optical pointing apparatus . referring to fig8 , in step s 800 , the average motion speed of the optical pointing apparatus is calculated and the maximum and minimum values of the average are determined . in steps s 820 and 840 , the average motion speed of the optical pointing apparatus is compared with the maximum and minimum speeds . when the optical pointing apparatus moves at high speed , the average motion speed of the optical pointing apparatus may be higher than the maximum speed . in this case , the gain of the electrical signal to be input to the adc 500 is increased by a predetermined level in step s 830 . when the optical pointing apparatus moves at low speed , the average motion speed of the optical pointing apparatus may be lower than the minimum speed . in this case , the gain of the electrical signal to be input to the adc 500 is reduced by a predetermined level in step s 850 . when the optical pointing apparatus moves at an appropriate speed , the average motion speed of the optical pointing apparatus may be lower than the maximum speed and higher than the minimum speed . in this case , the gain of the electrical signal to be input to the adc 500 is maintained in step s 860 . after all the steps are finished , the process returns to steps s 820 and s 840 . as described above , when the work surface is dark and the optical pointing apparatus moves fast , the gain of the electrical signal to be input to the adc 500 is increased . otherwise , that is , when the work surface is bright or the optical pointing apparatus moves slowly , the gain of the electrical signal to be input to the adc 500 is adaptively controlled according to brightness and darkness from images of the work surface or the motion speed of the optical pointing apparatus to be in a normal state . according to the present invention , power consumption of the optical pointing apparatus can be minimized . also , even if the optical pointing apparatus is placed on a dark work surface or moves at high speed , a required light intensity can be fluidly ensured , and a precise image can be obtained in response to an input signal having at least a predetermined light intensity . exemplary embodiments of the present invention have been disclosed herein and , although specific terms are employed , they are used and are to be interpreted in a generic and descriptive sense only and not for purposes of limitation . accordingly , it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims .