Abstract:
A DA converter includes an IV conversion amplifier with output voltage having good linearity, to thus improve total harmonic distortion (THD) characteristics. In the DA converter, a first current path in which current flows due to differential switches being in the ON state in a differential switch section, and a second current path in which current flows due to differential switches being in the OFF state in the differential switch section are connected to the output side of the IV conversion amplifier. A first current flows in the first current path and a second current flows in the second current path. A current equal to the first current plus the second current that is of fixed current amount is drawn by an amplifier stage of the IV conversion amplifier.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 USC 119 from Japanese Patent Application No. 2008-197419 filed on Jul. 31, 2008, the disclosure of which is incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a digital-analogue converter (referred to as a DA converter) that converts a digital signal into an analogue signal, and in particular relates to a current cell type DA converter. 
     2. Related Art 
     Generally semiconductor integrated circuit current cell type DA converters are controlled by a digital decoder input signal switching differential switches ON or OFF based on a decoded decode signal. The output current from the current cell adjusted by the differential switch is converted into a voltage level that accords with the current by an IV (current-voltage) conversion amplifier. 
     An example of such a DA converter is shown in  FIG. 8 . The conventional DA converter  100  shown in  FIG. 8  is equipped with current a decoder  112 , a current cell  120  including a current source  122 , a current cell array  124 , and a differential switch section  126 , and a current-voltage converting section  128 , including two IV conversion amplifiers A 101  and A 102 . 
     An N-bit decoder input signal D 100  controls PMOS transistors Pw 0  to Pwn, Pz 0  to Pzn (n=2 N −1) of the differential switch section  126  by decoded decode signals S 0 Pw to SnPw and S 0 Pz to SnPz (n=2 N −1), switching ON the P channel MOS (PMOS) transistors Pw 0  to Pwn when the differential switches are in the ON state. A current path w 1 , which collects together the nodes that become the current path when the differential switches are in the ON state, is connected to one of the terminals of the IV converter amplifier A 101 , and is connected to the output side of the IV converter amplifier A 101  via a feedback resistor R 102 . 
     In a similar manner, the PMOS transistors Pz 0  to Pzn are switched ON when the differential switches are in the OFF state. A current path z 1 , which collects together the nodes that become the current path when the differential switches are in the OFF state, is connected to one of the terminals of the IV converter amplifier A 102 , and is connected to the output side of the IV converter amplifier A 102  via a feedback resistor R 103 . 
     Current cell output currents Iw and Iz flow through the current paths w 1  and z 1 , and are drawn in the respective amplifier stages  144  and  145  of the IV converter amplifiers A 101 , A 102 , via the respective feedback resistors R 102  and R 103  of the IV converter amplifiers A 101 , A 102 . 
     In the DA converter  100 , there are large changes in the current amount drawn by the amplifier stages  144  and  145  of the IV converter amplifiers A 101 , A 102  accompanying increases and decreases in the current cell output currents Iw and Iz due to changes in the decode signals S 0 Pw to SnPw and S 0 Pz to SnPz. Such changes are detrimental to the linearity of the output of the IV converter amplifiers A 101 , A 102 , and distortions occur in the output voltage waveform of the IV converter amplifiers A 101 , A 102 , with a worsening in the THD (Total Harmonic Distortion) characteristics. 
     A differential output type DA converter is known that is provided with a folding circuit between a current cell and an IV conversion amplifier in order to obtain good linearity (see for example Japanese Patent Application Laid-Open (JP-A) No. 2002-164788). 
     In the technology described in JP-A No. 2002-164788 the circuit structure is made complicated by provision of the folding circuit, the DA converter scale becomes large, and a problem arises of increased current consumption. 
     SUMMARY 
     The present invention addresses the above problem and provides a DA converter that can give good linearity of output voltage of an IV conversion amplifier without providing a folding circuit, and can improve the THD characteristics. 
     The DA converter of the present invention includes a decoder that decodes a digital decoder input signal into a first decode signal for causing a first current to flow in a first current path and a second decode signal for causing a second current to flow in a second current path; a differential switch that outputs a current that has been output from a current source to one or other of the first current path or the second current path based on the first decode signal and the second decode signal output from the decoder; and a current-voltage converting section that outputs an analogue signal at a voltage level according to the current output from the first current path, wherein the first current path is connected to the output of the current-voltage converting section via a feedback resistor, and the second current path is connected to the output of the current-voltage converting section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a circuit diagram showing an exemplary schematic configuration of a DA converter according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a circuit diagram showing an exemplary schematic configuration of an IV conversion amplifier A 1 ; 
         FIG. 3  is a circuit diagram for explaining another exemplary configuration of a DA converter; 
         FIG. 4  is a circuit diagram for explaining another exemplary configuration of a differential switch section; 
         FIG. 5  is a circuit diagram for explaining another exemplary configuration of a differential switch section; 
         FIG. 6  is a circuit diagram for explaining another exemplary configuration of an IV conversion amplifier; 
         FIG. 7  is a circuit diagram showing an exemplary schematic configuration of a DA converter according to a second exemplary embodiment of the present invention; and 
         FIG. 8  is a circuit diagram showing an exemplary schematic configuration of a conventional DA converter. 
     
    
    
     DETAILED DESCRIPTION 
     First Exemplary Embodiment 
     A detailed explanation will now be given of an exemplary embodiment of the present invention, with reference to the drawings.  FIG. 1  is a circuit diagram showing an exemplary schematic configuration of a DA converter  10  of the present exemplary embodiment. The DA converter  10  of the present exemplary embodiment is configured to include: a decoder  12 ; a current cell  20 ; a current-voltage converting section  28 ; a buffer circuit  30 ; and an output terminal  32  for externally outputting a DA converted output voltage (analogue signal). 
     The decoder  12  outputs decode signals S 0 Px to SnPx (referred to collectively as decode signals SPx below) and decode signals S 0 Py to SnPy (referred to collectively as decode signals SPy below) (n=2 N −1) decoded by an N-bit decoder input signal D 0 . The decode signals SPx and decode signals SPy are signals for switching differential switches ON or OFF, namely, they are signals for adjusting the proportions ON or OFF of differential switches included in a differential switch section  26 . 
     The current cell  20  is configured to include a current source  22 , a current cell array  24 , and the differential switch section  26 . 
     The current source  22  is configured to include an operational amplifier OP 1 , an N-channel MOS (NMOS) transistor N 1 , and a resistor R 1 . The current source  22  converts an external reference voltage into current and supplies current to the current cell array  24  via a PMOS transistor P 1 . 
     The current cell array  24  is configured to include plural (n individual transistors in the present exemplary embodiment, where n=a natural number other than 0) PMOS transistors Pa 0  to Pan (referred to collectively as PMOS transistors Pa below). 
     The differential switch section  26  is configured to include n individual differential switches. Each of the differential switches is configured by a pair of one of PMOS transistors Px 0  to Pxn (referred to collectively as PMOS transistors Px below) and one of PMOS transistors Py 0  to Pyn (referred to collectively as PMOS transistors Py below). The sources of the PMOS transistors Px and the PMOS transistors Py are connected to the drains of the PMOS transistors Pa. The decode signals SPx are input to the gates of the PMOS transistors Px, and the decode signals SPy are input to the gates of the PMOS transistors Py. When the differential switches are in the ON state due to the decode signals SPx and the decode signals SPy, the gates of the PMOS transistors Px are switched ON, and the gates of the PMOS transistors Py are switched OFF. The drains of the PMOS transistors Px are connected to a current path x 1 . When the differential switches are in the OFF state, the gates of the PMOS transistors Px are switched OFF, and the gates of the PMOS transistors Py are switched ON. The drains of the PMOS transistors Py are connected to a current path y 1 . 
     The IV converter section  28  is configured to include an IV conversion amplifier A 1  that is an OP amplifier and a feedback resistor R 2 . The positive terminal of the IV conversion amplifier A 1  is connected to a signal line, serving as a reference voltage, and the negative terminal of the IV conversion amplifier A 1  is connected to the current path x 1 . The feedback resistor R 2  is connected between the negative terminal of the IV conversion amplifier A 1  and the output side thereof. The output of the IV conversion amplifier A 1  is connected to the positive terminal of a buffer circuit  30 , the negative terminal of the buffer circuit  30  is connected to the output side of the buffer circuit  30 , and the output voltage (analogue signal) output from the buffer circuit  30  is externally output from the output terminal  32 . 
     An exemplary specific configuration of the IV conversion amplifier A 1  will now be explained in detail, with reference to  FIG. 2 .  FIG. 2  is a circuit diagram showing an exemplary schematic configuration of the IV conversion amplifier A 1 . The IV conversion amplifier A 1  of the present exemplary embodiment is configured to include a buffer  40 , a differential amplifier  42 , an amplifier stage  44 , a resistor R 3 , and a phase compensation capacitor C 3 . 
     The buffer  40  is configured to include a PMOS transistor P 10 , and an NMOS transistor N 10  that has a gate connected to an Ibias and is connected in series to the PMOS transistor P 10 . 
     The differential amplifier  42  is configured to include PMOS transistors P 12 , P 14  and P 16 , and NMOS transistors N 14  and N 16 . The NMOS transistors N 14  and N 16  configure a current mirror. The current path x 1  is connected to the gate of the PMOS transistor P 14 , and a signal line is connected to the gate of the PMOS transistor P 16 . One electrode of the PMOS transistor  12  is connected to one electrode of the PMOS transistors P 14  and P 16 . The other electrode of the PMOS transistor P 14  is connected to one electrode of the NMOS transistor N 14  and the gates of the NMOS transistor N 14  and N  16 . The other electrode of the PMOS transistor P 16  is connected to one electrode of the NMOS transistor N 16 . The output voltage outputs to the amplifier stage  44  via the resistor R 3  and the phase compensation capacitor C 3 . 
     The amplifier stage  44  is configured to include a PMOS transistor P 18  and an NMOS transistor N 1  whose source is connected to the drain of the PMOS transistor P 18 . The output from the differential amplifier  42  is connected to between the serially connected PMOS transistor P 13  and NMOS transistor N 18 . The gate electrode of the NMOS transistor N 18  is connected to one of the electrodes of the NMOS transistor N 16 . The gate electrodes of the PMOS transistors P 10 , P 12  and P 18  are connected to between the PMOS transistor P 10  and the NMOS transistor N 10 . The output voltage output from the differential amplifier  42  is amplified by the amplifier stage  44  and externally output from the IV conversion amplifier A 1 . 
     It should be noted that the IV conversion amplifier A 1  is not limited to the configuration shown in  FIG. 2 , and there are no particular limitations to the specific configuration thereof with other configurations of OP amplifiers also applicable. 
     Detailed explanation will now be given of the operation of the DA converter  10  of the present exemplary embodiment. 
     Current is supplied to the source of the PMOS transistors Pa of the current cell array  24  by current output from the current source  22 . The gates are switched ON, and currents Ia 0  to Ian (referred to collectively as currents Ia below) are output from the drains of the PMOS transistors Pa to the differential switch section  26 . 
     When input with the decoder input signal D 0 , which is an N-bit digital signal, the decoder  12  decodes and outputs the decode signals SPx and the decode signals SPy corresponding to the input code to the differential switch section  26 . 
     The decode signals SPx and the decode signals SPy are input to the gates of the PMOS transistors Px and Py of the differential switch section  26 , and one or other of the gates is switched ON by the input signal. When the differential switch is in the ON state, the gate of the Px is switched on, and when the differential switch is in the OFF state, the gate of the Py is switched on. Namely, one or other of the differential switches, switches ON the gate of a PMOS transistor (PMOS transistor Px or Py), and a current Ia is output from the drain thereof. 
     The current Ia that is output when the differential switch is in the ON state, is output to the current path x 1  that collects together the nodes (drains of the PMOS transistors Px) that become the current path when the differential switch is in the ON state. Namely, a current Ix that is the sum of currents Ia for the number of differential switches that are in the ON state is output through the current path x 1  to the current-voltage converting section  28 . In the same manner, the current Ia that is output when the differential switch is in the OFF state, is output to the current path y 1  that collects together the nodes (drains of the PMOS transistors Py) that become the current path when the differential switch is in the OFF state. Namely, a current Iy that is the sum of currents Ia for the number of differential switches that are in the OFF state is output through the current path y 1  to the current-voltage converting section  28 . 
     The current Ix flows to the output side of the IV conversion amplifier A 1  of the current-voltage converter section  28  via the feedback resistor R 2 . The current Iy flows to the output side of the IV conversion amplifier A 1  of the current-voltage converter section  28 . Consequently, the current Ix+Iy is drawn in the amplifier stage  44  of the IV conversion amplifier A 1 . The current Ix+Iy is the sum of output current from the differential switches, which is independent of the proportion of differential switches that are in the ON state or OFF state in the differential switch section  26 , and is therefore a constant value of current Ia×n. Consequently, the current Ix+Iy (current amount) drawn in the amplifier stage  44  is fixed. 
     In this manner, in the IV conversion amplifier A 1 , the current amount drawn in the amplifier stage  44  of the IV conversion amplifier A 1  is fixed and is not related to the decode signals SPx and SPy. 
     A voltage proportional to the current Ix and the resistance value of the feedback resistor R 2  is output as an output voltage from the IV converter section  28 . 
     The output voltage that has been output from the IV converter section  28  is stabilized by the buffer circuit  30 , which is voltage follower, and output externally to the DA converter  10  from the output terminal  32 . 
     In the DA converter  10  of the present exemplary embodiment the current paths x 1  and y 1  are connected to the output side of the IV conversion amplifier A 1 , and the voltage value of the output voltage is proportional to the current Ix flowing through the current path x 1  connected via the feedback resistor R 2 , and is not proportional to the current Iy flowing through the current path y 1 . There is therefore no problem of influence from changes to the current Iy. 
     As explained above, in the DA converter  10  of the present exemplary embodiment, the current path x 1  in which current flows due to the differential switches of the differential switch section  26  that are in the ON state, and the current path y 1  in which current flows due to the differential switches that are in the OFF state, are connected to the output side of the IV conversion amplifier A 1 , hence the current amount (current Ix+Iy) drawn in the amplifier stage of the IV conversion amplifier A 1  is fixed and is not related to the decode signals SPx and SPy. There is therefore no detriment to the linearity of the output voltage of the IV conversion amplifier A 1 , no matter whether or not a folding circuit is provided, and distortion of the output voltage waveform can be suppressed. Consequently the THD characteristics can be improved. As a specific example, when the current Ix+Iy is from 0 to 300 μA (when the current Ix and the current Iy change between 0 and 300 μA), the THD characteristics of the output voltage of the IV conversion amplifier A 101  in the DA converter  100  shown in  FIG. 8  is −61.74 dB, however the THD characteristics of the output voltage of the IV conversion amplifier A 1  of the DA converter  10  of the present exemplary embodiment is −108.6 dB, i.e. the THD characteristics can be improved. 
     In order to obtain a differential output with the DA converter  10  of the present exemplary embodiment, configuration may be made, for example, with one output voltage being the output voltage of the IV conversion amplifier A 1 , and the other output voltage being a turnover voltage that is the IV conversion amplifier A 1  output voltage that has been inverted by an inverting circuit configured by an operational amplifier or the like, or other such configuration. 
     It should be noted that there is no limitation to the above configuration of the DA converter  10 , and other configurations of DA converters are suitable as long as configuration is made such that the current paths x 1  and y 1  are connected to the output side of the IV conversion amplifier A 1  and the current amount drawn in the amplifier stage of the IV conversion amplifier A 1  is fixed (current Ix+Iy). 
     For example, as in the DA converter  10 A shown in  FIG. 3 , configuration may be made in which the current path y 1  that collects together the nodes of the current paths when the differential switches of the differential switch section  26  are in the OFF state is connected to one terminal of the IV conversion amplifier A 1  and to the output side of the IV conversion amplifier A 1  via the feedback resistor R 2 , and the current path x 1  that collects together the nodes of the current paths when the differential switches of the differential switch section  26  are in the ON state is connected the output side of the IV conversion amplifier A 1 . In such a case the output voltage is a voltage value proportional to the current Iy. 
     The differential switch section  26  may also be configured, like the differential switch section  26 B shown in  FIG. 4 , with NMOS transistors (NMOS transistors Nx, Ny), or with a differential switch section (not shown in the drawings) configured by PMOS transistors and NMOS transistors. In addition, the differential switch section  26  may be configured from bipolar transistors.  FIG. 5  shows a case in which a differential switch section  26 C is configured by PNP junction bipolar transistors Qx and Qy. The decode signals SQx and SQy are input to the respective bases of bipolar transistors Qx and Qy, a current cell array  24  is connected to the respective emitters thereof, the respective collectors of the bipolar transistors Qx are connected to a current path x 1 , and the respective collectors of the bipolar transistors Qy are connected to the current path y 1 . It should be noted that a differential switch section (not shown in the drawings) may also be configured with NPN junction bipolar transistors. 
     It should be noted that when the differential switch sections are configured like those shown in  FIG. 3  to  FIG. 5 , the decode mode of each of the decoders may be defined as being when the differential switch is switched ON (or OFF) according to the decoder input signal. 
     There is also no limitation of the configuration of the IV conversion amplifier A 1  to the above configurations. For example, a signal ground is connected to the positive terminal that is the reference voltage of the IV conversion amplifier A 1  of the present exemplary embodiment, however, there are no particular limitations and, as shown in  FIG. 6 , the bias ground applying a bias voltage may be connected. The current source  22  is also not limited to the above configuration. For example, the current source  22  may be configured as a current mirror using a transistor, and a configuration where an external reference current is output to the current cell array  24  may also be made, or other configurations with no particular limitation thereto. The current cell array  24  is also not limited to the configuration above. For example, the current cell array  24  may be configured from NMOS transistors and bipolar transistors, or other configurations with no particular limitation thereto. 
     Second Exemplary Embodiment 
     Explanation will now be given of a second exemplary embodiment of the present invention with reference to the drawings.  FIG. 7  is a circuit diagram showing an exemplary schematic configuration of a DA converter  11  according to the present exemplary embodiment. It should be noted that substantially the same configurations and operations in the present exemplary embodiment to that of the first exemplary embodiment are allocated the same reference numerals and detailed explanation thereof is omitted. 
     The DA converter  11  of the present exemplary embodiment has a resistor Rx inserted into the current path x 1  between a current cell  20  and a current-voltage converting section  28 , and a resistor Ry inserted into the current path y 1  between the current cell  20  and the current-voltage converting section  28 . A current Ix flows to the output side of an IV conversion amplifier A 1  via the resistor Rx and the feedback resistor R 2 . A current Iy flows to the output side of the IV conversion amplifier A 1  via the resistor Ry. Since a current Ix+Iy is drawn in the amplifier stage  44  of the IV conversion amplifier A 1  the current amount is fixed. 
     The parasitic capacitance occurring in the PMOS transistors Px of the differential switch section  26  does not now appear as output capacitance load of the TV conversion amplifier A 1  due to the resistor Rx. In a similar manner, the parasitic capacitance occurring in the PMOS transistors Py does not now appear as output capacitance load of the IV conversion amplifier A 1  due to the resistor Ry. Influence of the parasitic capacitance of the PMOS transistors Px and Py on the output of the IV conversion amplifier A 1  can be suppressed. Consequently, any reduction in phase margin of the IV conversion amplifier A 1  can be suppressed. 
     As explained above, in the DA converter  11  of the present exemplary embodiment, since the current path x 1  and the current path y 1  are connected to the output side of the IV conversion amplifier A 1 , the current amount drawn in the amplifier stage of the IV conversion amplifier A 1  is fixed and not related to the decode signals SPx and SPy. There is therefore no detriment to the linearity of the output voltage of the IV conversion amplifier A 1 , and distortion of the output voltage waveform can be suppressed. Consequently the THD characteristics can be improved. As a specific example, when the current Ix+Iy is from 0 to 300 μA (when the current Ix and the current Iy change between 0 and 300 μA), the THD characteristics of the output voltage of the IV conversion amplifier A 1  of the DA converter  11  of the present exemplary embodiment in such a case is −108.5 dB, and the TBD characteristics can be improved. 
     In the DA converter  11  of the present exemplary embodiment there is the resistor Rx inserted into the current path x 1 , and the current Ix flows via the resistor Rx into the current-voltage converting section  28 . Influence of the parasitic capacitance of the PMOS transistors Px of the differential switch section  26  on the output of the IV conversion amplifier A 1  can thereby be suppressed. In a similar manner, the resistor Ry is inserted into the current path y 1 , and the current Iy flows via the resistor Ry into the current-voltage converting section  28 . Influence of the parasitic capacitance of the PMOS transistors Py of the differential switch section  26  on the output of the IV conversion amplifier A 1  can thereby be suppressed. Consequently, any reduction in phase margin can be suppressed.