Abstract:
A digital analog converter includes a current conversion section and a voltage conversion section. The current conversion section has a first output terminal and a second output terminal. The first output terminal outputs a first current and a second output terminal outputs a second current, the first current varying in value according to inputted digital data, the sum of the first current and the second current becoming a constant current. The voltage conversion section converts the first current to a corresponding first voltage and produces an offset voltage on the basis of the constant current and outputs the sum of the first voltage and the offset voltage as an output voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The contents disclosed in the claims, specification, and drawings of Japanese Patent Application No. 2006-218081 filed on Aug. 10, 2006 are hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a current addition type digital analog converter and in particular to a digital analog converter having an interface circuit with a subsequent circuit. 
     2. Related Art 
       FIG. 8  shows the configuration of a general current addition type digital analog converter (DAC). As shown in  FIG. 8 , a usual current addition type DAC has a current conversion section  101  for converting inputted digital data to a corresponding output current and a voltage conversion section  102  for converting the output current of the current conversion section  101  to voltage. 
     The current conversion section  101  includes a reference power source, a plurality of current source circuits connected to the reference power source, and a switching part controlled by inputted digital data. Current is divided for output by the switching part into a first output terminal (output terminal for conversion)  114  and a second output terminal (output terminal for waste)  115 . 
     As for the voltage conversion section  102 , in the simplest configuration, it suffices to have a resistance element  121  connected between the first output terminal  114  and the ground and to convert current I+ outputted from the first output terminal  114  to voltage. However, in this case, the bottom of the output voltage becomes 0 V, which causes mismatching with an input range of a subsequent circuit connected subsequently to the DAC. For this reason, there is provided an offset voltage producing circuit composed of a resistance element  122  and a constant current source  123  to match the range of the output voltage with the input range of the subsequent circuit. On the other hand, current I− outputted from the second output terminal  115  is wasted to the ground via a resistance element  125 . 
     However, when the offset voltage producing circuit like this is provided, the following problem will be presented. First, since an offset voltage is produced by the use of the constant current source and the resistance element, the consumption current of the DAC will be increased. Further, the footprint of the interface circuit will be also increased. Still further, since the output voltage is varied by variations in the constant current source and the resistance element, it is necessary to use an element of high accuracy and to make a cut-and-try adjustment. 
     Moreover, although there is a case where current (waste current) outputted from the second output terminal is utilized for the purpose of improving the frequency characteristics of the DAC, as disclosed in Japanese Unexamined Patent Publication No. 2003-338759, the current is usually wasted without being utilized. In this manner, a sufficient study has never been made so as to reduce consumption current in the conventional DAC. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to solve the foregoing problems in the related art and to realize a digital analog converter having consumption current and a circuit footprint reduced. 
     To achieve this object, according to the present invention, a digital analog converter employs a configuration of producing an offset voltage on the basis of the sum of a current for output and a current for waste. 
     Specifically, a digital analog converter according to the present invention includes: a current conversion section having a first output terminal for outputting a first current and a second output terminal for outputting a second current, the first current varying in value according to inputted digital data, a sum of the first current and the second current being a constant current; and a voltage conversion section for converting the first current to a corresponding first voltage and for producing an offset voltage on the basis of the constant current and outputs a sum of the first voltage and the offset voltage as an output voltage. 
     The digital analog converter of the present invention produces the offset voltage by the use of the constant current which is the sum of the first current and the second current. For this reason, a current source for producing the offset voltage does not need to be provided separately. Thus, the consumption current of the digital analog converter is hardly increased as compared with a conventional digital analog converter. Moreover, the area of the circuit is hardly increased, either. 
     It suffices for the voltage conversion section to include, for example, a first resistance element connected between the first output terminal and the second output terminal and a second resistance element connected between the second output terminal and the ground. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a digital analog converter according to one embodiment of the present invention. 
         FIG. 2  is a circuit diagram showing an example of a current conversion section of the digital analog converter according to the one embodiment of the present invention. 
         FIG. 3  is a circuit diagram showing an example of a reference voltage source used for the digital analog converter according to the one embodiment of the present invention. 
         FIG. 4  is a circuit diagram showing an example of a circuit connected subsequently to the digital analog converter according to the one embodiment of the present invention. 
         FIG. 5  is a circuit diagram showing a modification of the current conversion section of the digital analog converter according to the one embodiment of the present invention. 
         FIG. 6  is a circuit diagram showing a modification of the digital analog converter according to the one embodiment of the present invention. 
         FIG. 7  is a graph showing an effect of a variable resistance according to the one embodiment of the present invention. 
         FIG. 8  is a circuit diagram showing a digital analog converter according to a related art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the present invention will be specifically described below with reference to the drawings.  FIG. 1  shows a circuit configuration of a digital analog converter (DAC) according to one embodiment of the present invention. As shown in  FIG. 1 , the DAC includes: a current conversion section  11  for converting digital data to a corresponding current value and for outputting the current value; and a voltage conversion section  12  for converting the output current of the current conversion section to voltage. 
     The current conversion section  11  is a digital data-current conversion section of a general current addition type DAC, and includes, for example, a current source circuit  21  and a switch circuit  22  as shown in  FIG. 2 , the current source circuit  21  outputting a reference current I and currents I/ 2 , I/ 4 , and I/ 8  that are obtained by assigning weights to the reference current I. A reference numeral  23  denotes a reference power source. The switch circuit  22  divides the output of the current source circuit  21  into a first output terminal  24  and a second output terminal  25  according to the digital data inputted to the current conversion section  11 . For this reason, the value of a first current (current for conversion) I 1  outputted from the first output terminal  24  is varied according to the inputted digital data. Moreover, the sum of the first current I 1  and a second current (current for waste) I 2  outputted from the second output terminal  25  are constant. 
     The voltage conversion section  12  includes: a first resistance element  31  connected between the first output terminal  24  and the second output terminal  25 ; a second resistance element  32  connected between the second output terminal  25  and the ground; and a voltage holding circuit  34  connected to a connection node  33  of the first resistance element  31  and the second resistance element  32 . The voltage holding circuit  34  includes a differential amplifier  34   a  and a reference voltage source  34   b , the differential amplifier  34   a  having an output terminal connected to a minus side input terminal and having the minus side input terminal connected to the connection node  33 , the reference voltage source  34   b  being connected to a plus side input terminal of the differential amplifier  34   a.    
     The first current I 1  flows through the first resistance element  31  and is converted to a first voltage V 1  corresponding to the digital data. The constant current of the sum of the first current I 1  and the second current I 2  flows through the second resistance element  32  and hence a constant offset voltage Vf is produced. With this, an output voltage Vout of the sum of the first voltage V 1  and the constant offset voltage Vf is outputted from an output terminal  13 . 
     Moreover, the voltage holding circuit  34  is connected to the connection node  33 . Thus, this can compensate variations in the offset voltage Vf caused by variations in the resistance element. Here, the reference voltage source  34   b  may be any circuit, if the source can produce a constant reference voltage. It suffices to employ, for example, a band gap reference circuit shown in  FIG. 3  as the reference voltage source  34   b . Moreover, the reference voltage source  34   b  may be built in the circuit or may be integrated externally. It suffices to match the voltage of reference voltage source  34   b  with the offset voltage Vf. 
     In the DAC of this embodiment, the constant offset voltage Vf is added to the output voltage Vout. Thus, when the subsequent circuit receiving the output of the DAC is a sample holding circuit  41  as shown in  FIG. 4 , the range of the output voltage Vout can made to coincide with the input range of a switching circuit  42  or a buffer circuit  43 , which is a constituent element of the sample holding circuit  41 . As a result, it is possible to prevent a distortion of signal from being caused by the mismatching of the input range. Moreover, the offset voltage Vf is produced by the output current of the current conversion section  11 , in other words, the sum of the first current I 1  and the second current I 2 . For this reason, a current source for producing the offset voltage Vf does not need to be provided separately and hence the consumption current of the DAC is not increased. Moreover, the footprint of the DAC is not increased by the current source, either. 
     The voltage holding circuit  34  may be any circuit if the circuit can hold the voltage of the connection node  33  constant. For example, the voltage holding circuit  34  may be simply only a capacitance element connected between the connection node  33  and the ground. Moreover, if the DAC circuit does not have any problem in response speed and accuracy, the DAC does not need to be provided with the voltage holding circuit. 
     The current conversion section  11  may have a latch circuit  26  operating in synchronization with a clock signal, as shown in  FIG. 5 . With this, the inputting of the digital data can be synchronized with the clock signal. With this, when a major code in which all bits are changed (01111→100000) is inputted, it is possible to prevent a glitch caused by the wiring delay of each bit. Moreover, it is possible to operate the DAC in cooperation with the other circuit in terms of system by synchronizing the inputting of the digital data with the clock signal. 
     It suffices to determine the resistance values of the first resistance element  31  and the second resistance element  32  according to the input range of the subsequent circuit receiving the output of the DAC and the magnitude of the output current of the current conversion section  11 . For example, these resistance values can be optimized by setting them in the following manner. 
     First, the resistance value of the second resistance element  32  is set in such a way as to match the offset voltage Vf with the lower operating range (for example, 0.3 V to 1.3 V) of a circuit subsequently connected. The offset voltage Vf becomes voltage (V=R×I) as the product of the second resistance element  32  and the sum of the first current I 1  and the second current I 2 . 
     Next, the resistance value of the first resistance element  31  is set in such a way as to make the maximum value of the output voltage Vout not affect the operation of the current source in the current conversion section  11 . This is because when the resistance value of the first resistance element  31  is made large, the developed voltage can be made large but an output node potential of the current source becomes high and hence the current conversion section  11  does not operate. 
     Moreover, as shown in  FIG. 6 , the first resistance element  31  and the second resistance element  32  may be variable resistance elements. With this, as shown in  FIG. 7 , the ranges of the offset voltage Vf and the first voltage V 1  can be made variable. With this, the range of the output voltage can be finely adjusted. Moreover, this can also respond to a case where the subsequent circuit has a plurality of modes and where the ranges of the required output voltage are different according to the modes. In this case, it is also possible to employ a configuration in which a plurality of resistance elements are switched for use. 
     In this regard, there has been described an example in which the subsequent circuit is the sample holding circuit. However, the subsequent circuit is not limited to the sample holding circuit but may be an amplifier circuit or a comparator circuit. 
     As described above, the current addition type digital analog converter according to the present invention can realize a voltage output having a constant offset voltage added thereto without increasing the consumption current and the circuit footprint. For this reason, the current addition type digital analog converter according to the present invention is very useful, in particular, when a circuit requiring an offset voltage is connected as the subsequent circuit. 
     The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements, and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.