Patent Publication Number: US-2005134493-A1

Title: Analog-to-digital converting module capable of converting data at an increased resolution

Description:
BACKGROUND OF INVENTION  
      1. Field of the Invention  
      The present invention relates to an analog-to-digital converter, and more specifically, to an analog-to-digital converting module with an adjustable saturation point.  
      2. Description of the Prior Art  
      Analog-to-digital converters are widely used in industry for converting analog input voltages into digital output voltages. Analog-to-digital converters are utilized in applications input devices such as pointing devices. Pointing devices are commonly used in conjunction with computers to control the movement of graphical cursers or pointers on display screens and to select objects and operate controls displayed on the screen. Recently, small laptop and notebook computers have become very popular. Laptop and notebook computers may be used in conjunction with a docking station so that a standard keyboard, mouse, and CRT display may be used for the user interface. However, laptop and notebook computers are designed to be used while traveling away from the office or home. In such remote locations, the user does not always have available a flat surface upon which to use a mouse. Accordingly, laptop and notebook computers typically have a built-in pointing device, such as a touchpad.  
      A touchpad comprises a rectangular surface that is mapped to correspond to a display screen. By touching the location on the touchpad, the user causes the computer to move the pointer to the corresponding location on the screen. Since a typical touchpad is substantially smaller than the screen, accurate positioning of the pointer can be difficult. In order to be useable, a touchpad must be large enough to permit the user to position the pointer accurately. The large size of touchpads makes them relatively expensive.  
      Please refer to  FIG. 1 .  FIG. 1  is a diagram of a touchpad input system  10  according to the prior art. The touchpad input system  10  contains a touchpad  20  capable of sensing position of a point on the surface of the touchpad  20 . The touchpad  20  includes resistors  22  and  24  for generating voltage values according to position of the point on the surface of the touchpad  20 . A voltage Vp is applied to each of the resistors  22  and  24 , with the other end of the resistors  22  and  24  being connected to ground. The position of the point with respect to each of the resistors  22  and  24  generates voltages Vx and Vy, respectively. The values of the voltages Vx and Vy are directly related to a position of the point on the touchpad  20 . In  FIG. 1 , the point contacts the touchpad  20  at point (x 1 , y 1 ). The touchpad input system  10  includes a switch  30  for alternatively selecting either an x-axis or a y-axis for measuring a position of the point. A controller  60  outputs a control signal SW_CNT for controlling the switch  30  to sequentially read the analog voltages Vx and Vy generated by the resistors  22  and  24 , respectively. A buffer  40  is electrically connected to the switch  30  for impedance matching and for reducing a loading effect in the touchpad input system  10 . The voltage value read from the touchpad  20  is stored in the buffer  40 . An analog-to-digital converter  50  reads the analog voltage value from the buffer  40 , and converts the analog voltage into a digital voltage value. The controller  60  then reads the digital voltage value from the analog-to-digital converter  50 , and position of the point on the touchpad  20  is determined.  
      The touchpad input system  10  shown in  FIG. 1  is a common structure used extensively as an input device for electronics. The resolution of the touchpad  20  is limited by the resolution of the analog-to-digital converter  50 . For example, suppose that the analog-to-digital converter  50  has an 8-bit resolution. The touchpad  20  can then have a resolution of 256×256. Increasing the resolution of the touchpad  20  is usually accomplished through the use of a high-resolution analog-to-digital converter. Unfortunately, high costs of high-resolution analog-to-digital converters limit the extent to which the resolution of the touchpad  20  can be increased.  
     SUMMARY OF INVENTION  
      It is therefore an objective of the claimed invention to provide a converting module with an adjustable saturation point. The converting module can be used in applications such as a touchpad for increasing the resolution of the touchpad.  
      According to the claimed invention, a converting module for converting a first analog voltage into a digital output value includes a comparator for comparing the first analog voltage with a threshold voltage and generating a comparison result, the threshold voltage corresponding to a first group of digits containing at least one digit. A subtracting circuit generates a second analog voltage by subtracting the threshold voltage from the first analog voltage if the comparison result indicates that the first analog voltage is greater than or equal to the threshold voltage. An analog-to-digital converter converts the second analog voltage into a second group of digits and concatenates the first group of digits and the second group of digits to form the digital output value.  
      It is an advantage of the claimed invention that the subtracting circuit subtracts the threshold voltage from the first analog voltage if the first analog voltage is greater than or equal to the threshold voltage for allowing the converting module to attain higher resolution without increasing the resolution of the analog-to-digital converter.  
      These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.  
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a diagram of a touchpad input system according to the prior art.  
       FIG. 2  is a diagram of a touchpad input system utilizing a converting module according to the present invention.  
       FIG. 3  is a detailed diagram of a subtracting circuit for choosing an output voltage that is sent to an analog-to-digital converter. 
    
    
     DETAILED DESCRIPTION  
      Please refer to  FIG. 2 .  FIG. 2  is a diagram of a touchpad input system  100  utilizing a converting module  125  according to the present invention. The touchpad input system  100  contains a touchpad  110  capable of sensing position of a point on the surface of the touchpad  110 . The touchpad  110  includes resistors  116  and  118  for generating voltage values according to position of the point on the surface of the touchpad  110 . A voltage 2Vp is applied to each of the resistors  116  and  118 , with the other end of the resistors  116  and  118  being connected to ground. The position of the point with respect to each of the resistors  116  and  118  generates voltages Vx and Vy, respectively. The values of the voltages Vx and Vy are directly related to a position of the point on the touchpad  110 .  
      The touchpad  110  is divided into a plurality of regions  111 - 114 . Although only four regions  111 - 114  are shown in  FIG. 2 , the touchpad  110  can also be divided into other numbers of regions. The number of regions is preferably a perfect square, such as 4, 9, 16, or so on. As will be explained below, each of the regions  111 - 114  in the present invention touchpad  110  has the same resolution as the entire touchpad  20  of the prior art. In order for this to be possible, the voltage 2Vp provided to resistors  116  and  118  in the present invention is higher than the voltage Vp provided to resistors  22  and  24  in the prior art. Take resistor  116  as an example. The voltage value 2Vp is applied to end Rx 2  of resistor  116 . Since the other end of resistor  116  is grounded, a midpoint Rx 1  of the resistor  116  would have a voltage value of Vp. Therefore, the voltage measured across the resistor  116  between points Rx 2  and Rx 1  is equal to Vp, which is the same voltage measured across the resistor  116  between point Rx 1  and ground. As shown in  FIG. 2 , the section of resistor  116  between points Rx 2  and Rx 1  corresponds to regions  112  and  114  of the touchpad  110 , while the section of resistor  116  between point Rx 1  and ground corresponds to regions  111  and  113  of the touchpad  110 . Resistor  118  has a structure identical to that of resistor  116 . That is, a section of resistor  118  between points Ry 2  and Ry 1  corresponds to regions  111  and  112  of the touchpad  110 , while a section of resistor  118  between point Ry 1  and ground corresponds to regions  113  and  114  of the touchpad  110 .  
      In  FIG. 2 , the point contacts the touchpad  110  at point (x 2 , y 2 ). The touchpad input system  100  includes a first switch  120  for alternatively selecting either an x-axis or a y-axis for measuring a position of the point. A controller  126  of the converting module  125  is used to issue a control signal SW 1 _CNT to control the first switch  120  to sequentially read the analog voltages Vx and Vy generated by the resistors  116  and  118 , respectively. A buffer  122  is electrically connected to the first switch  120  for impedance matching and for reducing a loading effect in the touchpad input system  100 . Depending on the region of the touchpad  110  contacted by the point, the voltage value Vin can have a value between 0 and 2Vp. Since an analog-to-digital converter  140  of the converting module  125 , which is used to convert the analog voltage Vin into a digital voltage value, can only handle voltages in the range of 0 to Vp, the voltage value Vin must first be altered to match input requirements of the analog-to-digital converter  140 .  
      When the voltage value Vin is between Vp and 2Vp, a subtracting circuit  128  of the converting module  125  is used to subtract a voltage value of Vp from the voltage Vin. Output from the subtracting circuit  128  is labeled as voltage Vo 1 , and is equal to the difference Vin−Vp.  
      In order to decide if the output voltage Vo 1  from the subtracting circuit  128  or the voltage Vin is to be selected for conversion with the analog-to-digital converter  140 , the converting module  125  contains a comparator  124  for comparing the voltage value Vin to the voltage Vp. A result CMP of this comparison is then sent to the controller  126 , which uses control signal SW 2 _CNT to operate a second switch  132  accordingly. When the comparison result CMP indicates that Vin is less than Vp, the voltage Vin is selected as an output value Vout of the second switch  132 . On the other hand, when the comparison result CMP indicates that Vin is greater than or equal to Vp, the voltage Vo 1  is selected as the output value Vout of the second switch  132 . Once the voltage value Vout has been selected with the second switch  132 , the analog-to-digital converter  140  converts the analog voltage value Vout into a digital voltage value. The controller  126  controls operation of the analog-to-digital converter  140  with a control bus CNT_BUS, and the digital result outputted by the analog-to-digital converter  140  is sent to the controller  126  through a data bus DATA_BUS.  
      Please refer to  FIG. 3 .  FIG. 3  is a detailed diagram of the subtracting circuit  128  used in conjunction with the second switch  132  for choosing the output voltage Vout that is sent to the analog-to-digital converter  140 . The subtracting circuit  128  shown in  FIG. 3  is provided as an example only, and by no means is used to limit the scope of the present invention. The subtracting circuit  128  contains an operational amplifier  130  serving as a summing amplifier. The operational amplifier  130  receives inputs Vin and Vp, and outputs the difference Vin−Vp as the voltage value Vo 1 . Then the second switch  132  selects either the voltage Vin or the voltage Vo 1  according to the control signal SW 2 _CNT output from the controller  126 .  
      Based on the comparison result CMP given by the comparator  124 , the controller  126  is able to determine if the voltage Vin was originally above Vp or not. This allows the controller  126  to determine which region  111 - 114  of the touchpad  110  that the point is in located in. The present invention touchpad input system  100  uses the full resolution of the analog-to-digital converter  140  to convert analog voltage values into digital voltage values indicating position of the point in each region  111 - 114  of the touchpad  110 . Therefore, the more regions that the touchpad  110  is segmented into, the higher the overall resolution of the touchpad  110 .  
      If the voltage Vin was above Vp, then the controller  126  will set a first group of binary digits to correspond to the value of Vp. On the other hand, if the voltage Vin was below Vp, then the first group of binary digits will be set equal to zero. The first group of binary digits contains at least one binary digit. The analog-to-digital converter  140  will then convert the voltage value Vout into a second group of binary digits. To form the final digital output result, the analog-to-digital converter  140  will concatenate the first group of binary digits with the second group of binary digits.  
      As an example, suppose that the analog-to-digital converter  140  has a resolution of 8 bits. That means the analog-to-digital converter  140  can generate digital output values from 0 to 255. A digital representation of Vp would have a corresponding digital value of 256. Therefore, if the value of Vin was greater than the value of Vp, Vp would be subtracted from Vin to form Vout. The voltage Vout would then be converted into an 8-bit digital value, and the digital value would be concatenated with a leading digit of “1” to form the digital output value. In this example, the present invention effectively adds one bit of resolution to the touchpad  110 . In general, if the analog-to-digital converter  140  has a resolution of n bits, the touchpad  110  will have a resolution of n+1 bits, assuming the touchpad  110  is divided into four regions  111 - 114 . If the touchpad  110  is divided into a greater number of regions, then the resolution of the touchpad  110  can be increased further using the spirit of the present invention.  
      The prior art touchpad  20  had a resolution of 256×256 when used with the analog-to-digital converter  50  having a resolution of 8 bits. If the analog-to-digital converter  140  of the present invention also has a resolution of 8 bits, each of the regions  111 - 114  will also have a resolution of 256×256. Thus, the total resolution of the touchpad  110  will be 512×512. The increase in resolution is accomplished without increasing the resolution of the analog-to-digital converter  140 , thereby eliminating the need for an expensive high-resolution analog-to-digital converter.  
      Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.