Abstract:
A scheme to reduce output variations in a column driver for a flat-panel display by sharing the characteristics of analog circuit is disclosed. An input multiplexer is provided between two neighboring digital inputs, and an output multiplexer is provided between two neighboring analog outputs so that the characteristics of neighboring analog circuits can be shared by multiplexing. The averaging effect by sharing reduces variations in the output. The multiplexing may be done either in time division or on a frame-by-frame basis.

Description:
RELATED APPLICATION 
     This application claims the benefit of co-pending U.S. Provisional Application Ser. No. 60/325,258, filed Sep. 26, 2001, entitled “Method and Apparatus for Reducing Output Variation by Sharing Analog Circuit Characteristics.” 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention in general relates to semiconductor circuits. More specifically, this invention relates to circuits for sharing analog circuit characteristics in flat-panel displays to compensate for variations in the outputs. 
     2. Description of the Related Art 
       FIG. 1  shows a conventional driver circuit for a flat panel display in general. Each digital input is converted to an analog value by a digital-to-analog (D/A) converter and buffered before an output is generated. For example, Data  1  of n-bits is converted by D/A 1  to an analog value, which is then buffered to produce Out 1 . 
     Ideally, one digital input should produce the same analog output in different columns. In practice, however, for the same digital input, there are column-to-column deviations in the output because there are variations in the analog characteristics of the D/A converters and buffers due to many reasons such as processing variations. 
     Therefore, there is a need for a scheme to compensate for the output deviations due to variations in the analog circuit characteristics. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to compensate for any output deviations due to variations in the analog circuit characteristics. 
     The foregoing and other objects are accomplished by sharing the characteristics of multiple neighboring analog circuits. Provided for each column are an input multiplexer for multiplexing neighboring digital inputs into one and an output multiplexer for multiplexing neighboring analog outputs into one. Sharing the characteristics of the neighboring analog circuits through multiplexing may be done in time division. Alternatively, sharing the characteristics of the neighboring analog circuits may be done on a frame basis. For example, at every n frames, different analog circuits may be selected for driving the outputs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a prior art output driver. 
         FIG. 2  is a schematic of an output driver of the present invention using multiplexing. 
         FIGS. 3A and 3B  are illustrations of an averaging effect by sharing the characteristics of neighboring analog circuits. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows a scheme of the present invention for reducing output variation. The driver circuit shown in  FIG. 2  includes multiple columns, where each column corresponds to one of the digital inputs (Data 1 , Data 2 , . . . , DataX) and one of the analog outputs (Out 1 , Out 2 , . . . , Outx), respectively. The driver circuit includes a plurality of input multiplexers (In-MUX 1 , In-MUX 2 , . . . , In-MUXx), and each input multiplexer selects an input from a plurality of the digital inputs (Data 1 , Data 2 , . . . , DataX). The driver circuit also includes a plurality of digital-to-analog converters (D/A 1 , D/A 2 , . . . , D/Ax), and each digital-to-analog converter connects to one of the input multiplexers to receive input from the corresponding input multiplexer and generate analog output data corresponding to the received digital input data. The driver circuit also includes a plurality of buffers (Buffer 1 , Buffer 2 , . . . , Bufferx), and each buffer is connected to one of the digital-to-analog converters to receive and buffer the corresponding analog output data. The driver circuit also includes a plurality of output multiplexers for outputting the analog outputs (Out 1 , Out 2 , . . . , Outx), and each output multiplexer selects an input from a plurality of buffers to output the analog output data. In other words, each column is provided with an input multiplexer (In-MUX) for selecting among inputs from multiple neighboring digital inputs and an output multiplexer (Out-MUX) selecting one among outputs from multiple neighboring analog outputs. For example, In-MUX 2  is provided for the column corresponding to Data 2  and Out 2  to select one among three inputs, Data 1 , Data 2 , and Data 3 . Similarly, Out-MUX 2  is provided for the column corresponding to Data 2  and Out 2  to select one among three outputs Buffer 1 , Buffer 2 , and Buffer 3 . The input multiplexers and the output multiplexers are controlled to select different digital-to-analog converters in different time slots for driving the analog outputs, whereby the analog outputs from the driver share neighboring analog characteristics of the digital-to-analog converters used. 
       FIGS. 3A and 3B  illustrate an averaging effect obtained by sharing the characteristics of the analog circuits. The example shows the case where the effective output time is divided into three time slots, and a different analog circuit drives the output during each time slot. The averaging effect reduces the output variations to any variation in the analog device characteristics. For example, referring to  FIG. 2  in conjunction with  FIGS. 3A and 3B , input multiplexers In-MUX 3 , In-MUX 2 , and In-MUX 4  correspond to digital inputs Data 3 , Data 2 , and Data 4 , respectively. In-MUX 2  selectively outputs Data 3  during a first time slot (“ts” or period), In-MUX 3  selectively outputs Data 3  during a second time slot, and In-MUX 4  selectively outputs the Data 3  during a third time slot. D/A converters, D/A 3 , D/A 2 , and D/A 4 , are coupled to In-MUX 3 , In-MUX 2 , and In-MUX 4 , respectively. D/A 2  converts the output of In-MUX 2  to analog output data Out 3  during the first time slot, D/A 3  converts the output of In-MUX 3  to analog output data Out 3  during the second time slot, and D/A 4  converts the output of In-MUX 4  to analog output data Out 3  during the third time slot. Buffer 2  buffers the analog output data Out 3  received from the D/A 2  during the first time slot, Buffer 3  buffers the analog output data Out 3  received from the D/A 3  during the second time slot, and Buffer 4  buffers the analog output data Out 3  received from the D/A 4  during the third time slot. Output multiplexer Out-MUX 3  selectively outputs the analog output data Out 3  received from D/A 2  and Buffer 2  during the first time slot, selectively outputs the analog output data Out 3  received from D/A 3  and Buffer 3  during the second time slot, and the analog output data Out 3  received from D/A 4  and Buffer 4  during the third time slot. Therefore, during the first time slot, the digital input Data 3  is selected by In-MUX 2 , converted to analog output data Out 3  by D/A 2 , which is buffered by Buffer 2 , and selectively output by Out-MUX 3 . During the second time slot, the digital input Data 3  is selected by In-MUX 3 , converted to analog output data Out 3  by D/A 3 , which is buffered by Buffer 3 , and selectively output by Out-MUX 3 . During the third time slot, the digital input Data 3  is selected by In-MUX 4 , converted to analog output data Out 3  by D/A 4 , which is buffered by Buffer 4 , and selectively output by Out-MUX 3 . As a result, the analog characteristics of D/A 2  and Buffer 2 , D/A 3  and Buffer 3 , and D/A 4  and Buffer 4  are averaged during the effective output time including the first time slot, the second time slot, and the third time slot, when generating the output data Out 3 , as shown in  FIGS. 3A and 3B . 
     Sharing the characteristics of the analog circuits may be done on a frame-by-frame basis. For example, in every n frames, the multiplexers may switch the analog circuits driving the outputs. 
     While the invention has been described with reference to preferred embodiments, it is not intended to be limited to those embodiments. It will be appreciated by those of ordinary skilled in the art that many modifications can be made to the structure and form of the described embodiments without departing from the spirit and scope of this invention.