Abstract:
A current cell matrix type of digital-to-analog (D/A) converter to prevent deterioration of a.c. characteristics on a current path for digital-to-analog conversion includes a array of current source cells arranged in a matrix configuration. Each current source cell includes a current source transistor to generate the cell current. During the regular operation, the cell current is flowed on output lines via a first transistor connected in cascode to the current source transistor. During the calibration operation, the cell current is flowed into a current comparator via a second transistor connected in cascode to the current source transistor. This prevents parasitic capacitance from being additively caused in switches for the first transistor and in another switch for the second transistor to prevent deterioration of a.c. characteristics on the current path.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a digital-to-analog (D/A) converter, and more particularly to a current cell matrix type of D/A converter for calibrating the current value of a current source cell. 
         [0003]    2. Description of the Background Art 
         [0004]    Conventionally, there is a type of digital-to-analog (D/A) converter having a plurality of current source cells arranged in a matrix to receive digital data to be converted to analog data through a row decoder and a column decoder. This type of D/A converter is able to convert the current value, differential-outputted from each current source cell, to an output voltage by output end resistors to deliver the resultant output voltage as an analog output. 
         [0005]    The current source cells operate as current sources to generate cell currents in proportion to a preset bias voltage and to differential-output the resultant cell currents in response to the input codes derived from digital data. Each current source cell includes a transistor, operating as a current source, and two current source switches, respectively controlling the positive and negative outputs of the cell current. These switches are actuated in accordance with the input codes. 
         [0006]    Thus, the amount of current flowing through the output end resistors of the D/A converter is varied by the current source switch of each current source cell. 
         [0007]    The transistors for each current source cell may involve unevenness caused by variation in manufacturing process to generate the current value involving an error. Such an error of the current should be corrected. For example, Hsin-Hung Chen, et al., “A 14-b 150MS/s CMOS DAC with Digital Background Calibration” 2006 Symposium on VLSI Circuits Digest of Technical Papers, proposes a digital background self-calibration scheme of the CMOS DAC (Complementary Metal-Oxide Semiconductor Digital-to-Analog Converter). In this scheme, a current source for correction, such as a dummy current source cell (CAL_DAC), is provided in a current source cell, such as a digital background calibrating current source cell, to adjust the cell current. 
         [0008]    In this scheme, the D/A converter carries out calibration for determining the correction value of the current source for correction. During the process of the calibration, each current source cell generates current to output the resultant current via a calibration switch provided on the correction path. In the D/A converter, the current value in each current source cell is converted to a corresponding analog voltage value by a resistive current-to-voltage converter. The analog voltage value is converted to corresponding digital data by a ΔΣ modulator and a digital counter. On the basis of the result of the digital-conversion, a calibration decision circuit calculates a digital value to be delivered to the current source for correction, i.e. the correction value, and causes the calculated correction value to be stored in a memory. 
         [0009]    A D/A converter disclosed by Japanese patent laid-open publication No. 289450/1997 operates in accordance with a segment system in which D/A converted outputs of upper bit segments equalized in current value are summed to D/A converted outputs of lower bit segments weighted in current to produce an resultant analog output. To the upper bit segments, a least one segment is added, the voltage value for switching which and the region for outputting the lower bits are controlled to correct an error in that segment. 
         [0010]    Such conventional D/A converters use the calibration technique for ameliorating its accuracy to correct the current value of the current source cells. However, the effect brought by the connections for calibration, i.e. circuit paths for correction, is not taken into account. 
         [0011]    For example, in the constitution for calibration as disclosed by the above Hsin-Hung Chen, et al., the CMOS DAC is provided with two switches for current source outputting and one switch for calibration for a transistor operating as a current source. However, the switches thus connected in the CMOS DAC cause the capacitance additive in a node of the switch for the current source to be increased. For example, when the switch for calibration is turned on in operation for calibration and off in regular operation, the parasitic capacitance by the switch for calibration is additively caused in the course of regular operation. 
         [0012]    Thus, in a current source cell, when large parasitic capacitance is caused additively on the node of the switch for the current source, the effect of capacitance mismatch between the cells increases to deteriorate alternating current characteristics of the D/A converter. 
         [0013]    If the number of switches in the current source cell is increased in order to prevent deterioration of the alternating current characteristics of the D/A converter, then the circuit is increased in size. 
       SUMMARY OF THE INVENTION 
       [0014]    It is an object of the present invention to provide a digital-to-analog converter which Is capable of correcting the current generated in current source cells arrayed in matrix configuration without deteriorating its alternating current characteristics. 
         [0015]    In accordance with the present invention, there is provided a digital-to-analog (D/A) converter including a plurality of current source cells, arranged in a matrix configuration. The D/A converter of the present invention comprises an output current oath section or digital-to-analog conversion in the regular operation and a correcting current path section for the calibration operation, as a current path for conducting a cell current generated in the current source cells. Each current source cell includes a first transistor serving as a current source that generates the cell current for a predetermined bias voltage applied thereto. In the current source cell, the first transistor is connected during the regular operation to the output current path section in cascade to flow the cell current through the output current path section and during the calibration operation to the correcting current path section in cascade to flow the cell current through the correcting current path section. The D/A converter also comprises a current corrector operative in response to a correction value obtained on the correcting current path section for generating the correction current that is used for correcting the cell current. 
         [0016]    According to the digital-to-analog (D/A) converter of the present invention, each current source cell comprises two current source transistors functioning as current sources so that, during the regular operation, the two transistors are connected in cascode to provide an output current path section. The transistors then conduct the cell current to flow on the output current path section. When the current source cell operates for calibration, one of the current source transistors and a transistor for calibration are connected in cascade, that is, the transistor for calibration is used in place of the other current source transistor, to provide a correcting current path section. Then, the current source transistor and the calibration transistor conduct the cell current to flow on the correcting current path section. 
         [0017]    Thus, each current source cell in the D/A converter carries out the operation for calibration to determine a correction value. The correction value is used to correct current value of each current source cell. It is therefore possible to correct differences in the current values ascribable to process variations. 
         [0018]    Moreover, with the D/A converter, the current path for calibration is not connected to a node connected to a switch for the current source, but is connected to a junction point between the current source transistors connected in cascode to each other. It is therefore possible to prevent the parasitic capacitance from being additively caused on the node of the switch for the current source. 
         [0019]    Additionally, with the D/A converter, if there is an error caused by the process variations between-the current source transistors used for the regular operation and the calibration transistors used for the operation for calibration, these transistors are arranged in proximity from each other so that the adverse effect caused by the process variations can be reduced extensively. 
         [0020]    Furthermore, the D/A converter may comprise a single current corrector for correcting the cell current generated in the plurality of current source cells, the single current corrector being shared with those current source cells, thereby reducing the circuit size. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The objects and features of the present invention will become more apparent from consideration of the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0022]      FIG. 1  is a schematic block diagram showing a preferred embodiment of a digital-to-analog (D/A) converter according to the present invention; 
           [0023]      FIG. 2  is a schematic block diagram showing an array of current source cells in the D/A converter of the embodiment shown in  FIG. 1 ; and 
           [0024]      FIG. 3  is a schematic block diagram showing part of an alternative embodiment in which the calibration transistor is disposed outside the current source cells of the D/A converter. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    With reference to the accompanying drawings, a preferred embodiment of a digital-to-analog (D/A) converter according to the present invention will be described in detail. As shown in  FIG. 1 , a D/A converter  10  converts digital data to analog data by a plurality of current source cells  12  arranged in a matrix configuration to output the resultant current values to output end resistors  14  and  16 , thereby obtaining the analog data. For that aim, the D/A converter  10  includes a reference current source  18 , transistors  20  and  22 , a current comparator  24  and a logic circuit  26 , which are interconnected as illustrated to calibrate the current values obtained by each current source cell  12 . It is noted that parts or elements not directly relevant to understanding the present invention will neither be described nor shown for avoiding redundancy. 
         [0026]    Referring to  FIG. 2 , the D/A converter  10  may be formed by a matrix array of current source cells  12 , to which digital data for D/A conversion are fed as input by a row decoder  70  and a column decoder  72 . Although a multiplicity of current source cells may actually be arranged in the D/A converter  10 , only a smaller number of current source cells  12  are shown in  FIGS. 1 and 2  for simplicity. 
         [0027]    The D/A converter  10  is adapted to receive the currents differentially outputted from the current source cells  12  on output lines  28  and  30 , which are in turn connected to output end resistors  14  and  16 , respectively, and thence to a reference voltage, e.g. power supply voltage VDD. On these output lines  28  and  30 , output terminals  32  and  34  are provided between the current source cells  12  and the output end resistors  14  and  16 , respectively. 
         [0028]    In accordance with the currents flowing from the current source cells  12 , the voltages applied to the output end resistors  14  and  16  are varied, that is, the voltages outputted from the output terminals or contact pads  32  and  34  are varied. The digital input is converted in this manner to a corresponding analog output. 
         [0029]    The current source cell  12  includes current source transistors  36  and  38  functioning as current sources, one  36  of which is connected in cascode to the other  38 . The one transistor  36  has its source electrode connected to another reference voltage, e.g. grounded, while the other transistor  38  has its drain electrode connected via switches  40  and  42  to the output lines  28  and  30 , respectively. The circuitry consisted of the transistors  36  and  38  and the switches  40  and  42  is referred to hereinbelow as a current source block  51 . 
         [0030]    The current source block  51  runs, for instance, to supply the bias voltage to the current source transistors  36  and  38 . The current source block  51  controls the switches  40  and  42  in accordance with the digital data for D/A conversion, i.e. input codes obtained from the row decoder  70  and the column decoder  72 , so as to switch the ON and OFF conduction states between the drain electrode of the transistor  38  and the output lines  28  and  30 , respectively. 
         [0031]    The current source cell  12  also includes a current corrector  50  adapted to calibrate the differential current values delivered from the current source block  51 , to which the current corrector  50  is electrically connected. The current corrector  50  includes a plurality of correction transistors  52  and  54  serving as current sources for calibration correction, and a plurality of switching transistors  56  and  58  serving as switches for switching the ON and OFF conduction states between the correction transistors  52  and  54 , respectively, and the current source block  51 . 
         [0032]    In the current source cell  12  of the illustrative embodiment, particularly, a calibration transistor  60  is connected in cascode to the current source transistor  36 , which is connected via the transistor  60  to the current comparator  24 . The current source cell  12  also includes a switch  62  for switching its ON and OFF states of the connection to the current comparator  24 , and the calibration transistor  60  connected to the current comparator  24  via the switch  62 . 
         [0033]    The current source cell  12  may preferably have a path for use in calibration operation of the calibration transistor  60  connected to a junction point between the current source transistors  36  and  38 . That causes in calibration operation the current source transistor  36  and the calibration transistor  60  to function as a couple of transistors operating as a current source. That is, the transistor  60  is used in place of the current source transistor  38 . In the illustrative embodiment, the same bias voltage is supplied to the transistors  38  and  60 . 
         [0034]    In the current source cell  12 , if the switch  40  or  42  is in its ON state the transistor  38  allows a cell current  102  to flow therethrough, while if the switch  62  is in its ON state the transistor  60  allows the cell current to flow therethrough. 
         [0035]    The reference current source  18  generates a reference current  104 . With the illustrative embodiment, this current  104  is set beforehand. 
         [0036]    The D/A converter  10  further includes transistors  20  and  22  respectively positioned on the side of the reference current source  18  and the side of the current source cell  12  to form a current mirror circuit  23  together. In the illustrative embodiment, the transistor  20  is connected simply to the reference current source  18 , while the other transistor  22  is connected to, in particular, the calibration transistor  60  of the current source cell  12  and also to the current comparator  24 . 
         [0037]    The current mirror circuit  23  including the transistors  20  and  22  generates the current in proportion to the size ratio between the side of the reference current source  18  and the side of the current source cell  12 . In the illustrative embodiment, the current substantially equal to the reference current  104  is generated on the side of the current source cell  12 . 
         [0038]    In the illustrative embodiment, the switch  62  on the side of the current source cell  12  is connected to the calibration transistor  60  and the current comparator  24 . Thus, according to the current mirror circuit  23 , if an error caused by the mirror circuit  23  is not taken into account, the sum of a cell current  102  flowing through the calibration transistor  60  and the current  106  supplied to the comparator  24  is substantially equalized to the reference current  104 . 
         [0039]    More specifically, by the current mirror circuit  23 , the input current  106  substantially equal to the difference between the reference current  104  and the cell current  102  is supplied to the current comparator  24 . When the reference current  104  is larger than the cell current  102 , the current  106  becomes of a positive value to flow into the comparator  24 . When the reference current  104  is smaller than the cell current  102 , the current  106  becomes of a negative value to flow out from the comparator  24 . 
         [0040]    The current comparator  24  is adapted to compare the input current  106  to, for example, a predetermined threshold value. Specifically, the comparator  24  determines which is larger of the reference current  104  and the cell current  102  and outputs a decision result  108  to the logic circuit  26 . 
         [0041]    The logic circuit  26  is adapted for deciding a code directing the quantity of the current  112 , i.e. a correction value  110 , conducted through the current corrector  50 . In the illustrative embodiment, the correction value  110  for the current quantity  112  to be used during the regular operation is determined in accordance with the decision result  108  by the current comparator  24  in the course of calibration operation. Preferably, the logic circuit  26  may have, e.g. a memory, not shown, adapted for storing the correction value  110 . 
         [0042]    The D/A converter  10  of the illustrative embodiment carries out the respective calibration operation on the plurality of current source cells  12 . That is, the current comparator  24  finds a decision result  108  from each current source cell  12  and the logic circuit  26  determines the code  110  from that current source cell  12 . 
         [0043]    Preferably, when the D/A converter  10  carries out the calibration operation, the current corrector  50  of the current source cell  12  controls the correction current value  112  to be varied little by little. In addition, the current corrector  50  preferably causes the correction current value  112  at a transition point of the sign of the input current  106  of the current comparator  24  to be stored in, e.g. a memory. 
         [0044]    Now, the operation of the D/A converter  10  of the illustrative embodiment will be described directed to an example of regular operation for D/A conversion. 
         [0045]    In the D/A converter  10 , the bias voltage and the input code corresponding to digital data for conversion are supplied to each current source cell  12 . In the current source cell  12 , a cell current  103  is generated between the node  44  of the current source block  51  and the predetermined reference voltage, i.e. ground (GND) in the embodiment. 
         [0046]    During the regular operation, the switch  62  is in its OFF state initially. However, when the input codes corresponding to the digital data are supplied to the switches  40  and  42  of the current source cell  12 , the switches  40  and  42  are changed over in response to the digital data. 
         [0047]    If the switch  40  and/or the switch  42  are in the ON state thereof, then the cell current  103  flows through the current source transistor  38  in-to the output lines  28  and/or  30 . On these output lines  28  and  30 , the cell current  103  is converted by the output end resistors  14  and  16  to a corresponding output voltage, which is in turn delivered as an output via the output terminals  32  and  34 . 
         [0048]    In addition, if the switch  56  and/or the switch  58  of the current corrector  50  are in the ON state thereof during the regular operation, the cell current  103  may be expressed as the sum of a current  114  flowing through the current source transistor  36  and a current  112  flowing through the transistors  52  and/or  54  of the correction current source of the current corrector  50 . The cell current  112  thus serves as a current source for correcting an error caused by current mismatch of the current  114  of the current source transistor  36 . Although the illustrative embodiment configures the current corrector  50  by four transistors  52 ,  54 ,  56  and  58 , the block  50  may consist of more transistors than this embodiment. 
         [0049]    Next, the operation of the D/A converter  10  of the illustrative embodiment will be described in terms of an example of operation for calibration. 
         [0050]    In the D/A converter  10 , as with the regular operation, the bias voltage and the input codes corresponding to digital data for conversion are supplied to the respective current source cells  12 . 
         [0051]    During the operation for calibration, the switch  62  is in its ON state, while the switches  40  and  42  are not in the ON state thereof. The cell current  102  therefore flows through the calibration transistor  60  and the switch  62 . 
         [0052]    In addition, during the operation for calibration, the current mirror circuit  23  including the transistors  20  and  22  equalizes the current flowing through the current source cell  12  to the reference current  104 . The sum of the cell current  102  and the current  106  flowing into the current comparator  24  is therefore equal to the reference current  104 . That renders the current substantially equivalent to the difference between the reference current  104  and the cell current  102  flow into the current comparator  24 . 
         [0053]    The current comparator  24  compares the input current  106  to the predetermined threshold value. The comparator  24  determines which is larger of the reference current  104  and the cell current  102  and outputs the decision result  108  to the logic circuit  26 . 
         [0054]    The logic circuit  26  determines, in response to the decision result  108  of the current comparator  24 , the correction value  110  which directs the current quantity  112  for the current corrector  50  during the regular operation. 
         [0055]    The D/A converter  10  of the illustrative embodiment carries out the above-described operation for calibration for each of the plurality of current source cells  12 . Thus, if the currents developed by the current source cells  12  are different between the cells due to a variation in manufacturing process, the D/A converter  10  may efficiently correct the currents in terms of the difference. 
         [0056]    In an alternative embodiment, the D/A converter  10  may include, as shown in  FIG. 3 , a sole calibration transistor  60  on the outside of the current source cells  12  so as to be shared with the plurality of current source cells  12 , each of which includes a switch  62  which is arranged to be controlled to change over its conductive state to the common calibration transistor  60 . 
         [0057]    Moreover, with the D/A converter  10  of the illustrative embodiment, it is possible to provide with a sole current corrector  50  on the outer side of the current source cell  12  to correct the cell current  102  in the current source block  51  of the current source cell  12 . In this case, the current corrector  50  may be used to be shared with the plurality of current source cells  12 . 
         [0058]    According to the illustrative embodiment, the D/A converter  10  can correct the cell current  103  ( 102 ) flowing in the current source block  51  of the current source cell  12  without using the current comparator  24 . For example, the D/A converter  10  may be configured so as to calibrate by converting the cell current to a corresponding voltage, further converting the voltage to a digital data by A/D conversion and determining the digital-converted result in comparison. 
         [0059]    In addition, the D/A converter  10  of the embodiments is able to execute the calibration no matter whether the respective cell current quantities of the plurality of current source cells  12  are equal to or different from each other. For example, there is a type of current source cells which can execute weighting correction, to which the calibration transistor  60  of the present invention may be applied without influencing the weighting correction. 
         [0060]    In the D/A converter  10  of the embodiments, the current source in the current source block  51  of the current source cells  12  may be constructed by applying a gain boost cascode connection. Alternatively, the current source may be constructed to include two cascode connections. The transistors in the current source of the current source cell  12  may be an NMOS or a PMOS transistor. 
         [0061]    The entire disclosure of Japanese patent application No. 2007-181972 filed on Jul. 11, 2007, including the specification, claims, accompanying drawings and abstract of the disclosure is incorporated herein by reference in its entirety. 
         [0062]    While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.