JP2000151404A - D/a conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a D/A conversion circuit with high precision where a stable voltage output can be acquired independently of a load circuit and to suppress interference among the D/A conversion circuits in the case of employing a plurality of the D/A conversion circuits. SOLUTION: A differential comparator circuit 50 compares a differential output voltage (Vout-Vxout) with a differential reference signal (VA-VB) and controls a count value cnt of an up-down counter 60 according to its comparison signal cmp. A switch S3 of a reference voltage correction 70 is controlled depending on the count value cnt to control a level of a reference voltage V1. A comparator circuit 10 compares the reference voltage V1 with a voltage V2, outputs a comparison signal V3 to control an output current of each of a current source 12 and a current source circuit 22. Then a maximum value of a differential output voltage is independent of a load circuit and kept to a level nearly equal to that of the differential reference signal and then a stable voltage output can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital / analog conversion circuit for converting a digital signal into an analog signal, and more particularly to a digital / analog conversion circuit capable of maintaining a constant output voltage without being affected by a change in load. .

[0002]

2. Description of the Related Art A current output type digital / analog conversion circuit controls a current input to a resistance element according to a digital signal, and obtains an analog signal by a voltage drop generated in the resistance element.

FIG. 4 shows an example of a conventional current output type digital / analog conversion circuit. Here, a digital / analog conversion circuit that converts an input 8-bit digital signal into an analog signal is illustrated. As shown, the conversion circuit includes a current source circuit 2 including a comparison circuit 10, a current source 12, a resistance element 14, and a plurality of current sources.
0, a switch circuit 22 including a plurality of changeover switches
And the resistance elements 30 and 32. Each current source constituting the current source circuit 20 supplies a unit current I,
The current source 12 supplies 16I of current. Further, assuming that the resistance values of resistance elements 30 and 32 are R, the resistance value of resistance element 14 is set to 16R.

[0006] The comparison circuit 10 includes a reference signal V 1 and a resistance element 1.
4 is compared with the generated voltage V2, and a signal V3 is output according to the comparison result. The supply current of the current source 12 is controlled by the signal V3, and the current source 12 outputs a current 16I that is 16 times the unit current I. The current of the current source 12 is input to the resistance element 14, and a voltage drop V2 occurs in the resistance element 14.

Each current source constituting the current source circuit 20 is controlled by an output signal V3 of the comparison circuit 10 and supplies a unit current I. Each switch constituting the switch circuit 22 outputs a current supplied from a current source of the current source circuit 20 to an output signal sdat of a decoder (not shown).
, And output to either the output terminal T _out or T _xout . In addition, the decoder responds to the input 8-bit digital signal, for example, by configuring the switch circuit 22 as 2.
A signal sdat consisting of 256 types of data for controlling 55 switches is output.

FIG. 5 is a circuit diagram showing a specific circuit configuration of the digital / analog conversion circuit shown in FIG. As shown, the current source 12 is constituted by a pMOS transistor P0, and the gate of the transistor P0 is
, The source is connected to the supply line of the power supply voltage V _CC , and the drain is connected to the resistance element 14.

Each of the current sources and the switch circuit 22 constituting the current source circuit 20 are each formed by a pMOS transistor. In FIG. 5, two current sources 2
0_1 and 20_2 and switches 22_1 and 22_2 are illustrated. The current sources 20_1 and 20_2 are respectively constituted by pMOS transistors P1 and P2,
The gates of these transistors are both connected to the output terminal of the comparison circuit 10. Switches 22_1 and 2
2_2 is composed of a pMOS transistor and an inverter, respectively. For example, in the switch 22_1, the signal sdat1 is output from the decoder to the transistor P1.
2, and the output terminal of the inverter INV1 is connected to the gate of the transistor P11. Therefore, the output signal s of the decoder
When dat1 is high, the transistor P11 is turned on, the current of the transistor P1 is output to the output terminal T _out, when the output signal sdat1 decoder is low conversely, the transistor P12 is turned on, the transistor P
1 is output to the output terminal _Txout .

The size of the transistors constituting each current source of the current source circuit 20 is all equal, and the size of the transistors constituting the current source 12 is set to 16 times the size. Therefore, assuming that the unit current I is output from each current source of the current source circuit 20, the current source 12 supplies 16I current. The current source circuit 2 thus configured
The 0 and the switching circuit 22, in accordance with the output signal of the decoder, and output a current of up to 255I to an output terminal T _out or T _xout. The output current of each current source of the current source 12 and the current source circuit 20 is controlled by the output signal V3 of the comparison circuit 10.

In the above-mentioned digital / analog conversion circuit, the output signal sdat of the decoder is set according to the input digital signal, and each switch of the switch circuit 22 supplies the output current of the 255 current source to the terminal T in response to this. _out or _Txout . Therefore, a voltage drop occurs in the resistance element 30 or 32 according to the input current, and is output from the terminals T _out and T _xout as a differential signal. That is, a differential analog signal is output according to the digital signal input to the decoder.

[0010] The maximum output voltage V _max of the digital / analog converter circuit is maintained by the configured control circuit by the comparison circuit 10, a current source 12 and resistance element 14, to a predetermined level set by the reference voltage V1. For example, a current of 16I is supplied from the current source 12 to the resistance element 14, and a voltage drop of (V2 = 256IR) is obtained in the resistance element 14. By the comparison 10, the supply current of the current source 12 is controlled such that the voltage drop V2 is always equal to the reference voltage V1. Thereby, the output terminal T _out or T _xout
, A maximum (255 IR) voltage output is obtained.
Further, by setting the reference voltage V1, the maximum output voltage can be controlled. Unless the reference voltage V1 fluctuates, the output current I and the maximum output voltage of each current source constituting the current source circuit 20 can be controlled. It is always kept at a certain level.

[0011]

In the above-mentioned conventional digital / analog conversion circuit, the resistance elements for converting an input current into a voltage, for example, the resistance elements 14, 30, and 32 in FIG. Is desirably built in a chip in order to reduce the number of points. For this reason, variations occur in these resistance elements. As shown in FIG. 4, when a load is connected to the output terminal T _out or T _xout , the input impedance Z _i of the load is connected in parallel with the resistance element 30 or 32. That is, the current - voltage conversion is performed by a parallel circuit of the input impedance Z _i of the resistance element R load. When the input impedance Z _{i of the} load is much larger than the resistance R, the influence of the input impedance of the load is negligible. However, when the input impedance Z _{i of the} load is not negligible compared to the resistance R. , the maximum voltage drop that occurred in the parallel resistance R _L of the resistor R and the impedance Z _i becomes (255IR _L), and the reference voltage V1 smaller value. That is, a deviation occurs between the maximum output voltage of the digital / analog conversion circuit and the reference voltage V1.

When a plurality of the above-mentioned digital / analog conversion circuits are incorporated, the resistance value R of the current-to-voltage conversion resistance element may vary depending on manufacturing variations and load conditions. Will be different. This manufacturing variation can be reduced by arranging all the resistance elements and the MOS transistors constituting the current source at one place on the substrate, but this disadvantageously causes an interference problem between the digital / analog conversion circuits. There is.

The present invention has been made in view of the above circumstances, and an object of the present invention is to obtain a stable voltage output without depending on a load circuit, and to provide a plurality of conversion circuits. An object of the present invention is to provide a high-precision digital / analog conversion circuit which can suppress interference between signals.

[0014]

In order to achieve the above object, a digital / analog conversion circuit according to the present invention comprises: a current supply circuit for supplying a current based on a digital signal; and a voltage signal output from the current supply circuit. A digital / analog conversion circuit having a resistance element for converting the voltage signal to an output terminal, wherein the level of the voltage signal varies according to the impedance of a load connected to an output terminal from which the voltage signal is output; A comparison circuit that compares the voltage signal with a reference signal and outputs a comparison signal according to a comparison result; a counter that increases or decreases a count value according to a comparison signal from the comparison circuit; and a level that corresponds to the count value. And a correction signal generating circuit for controlling the output current of the current supply circuit according to the correction signal.

In the present invention, preferably, the correction signal generation circuit includes a plurality of resistance elements connected in series between a first voltage supply terminal and a second voltage supply terminal; And a switching element for connecting any one of the connection midpoints of the resistance elements to the correction signal output terminal in accordance with the count value from.

Further, in the present invention, preferably, the current supply circuit is constituted by a transistor whose control terminal receives the correction signal.

According to the present invention, the count value of the counter is determined according to the comparison signal obtained by comparing the output signal of the digital / analog conversion circuit with the predetermined reference signal.
The level of the correction signal generated by the correction signal generation circuit is controlled according to the count value. The output current of the current supply circuit is controlled according to the correction signal, and the output current is input to the resistance element and converted to a voltage signal. The level of the correction signal is controlled according to the impedance of the load circuit connected to the output terminal of the voltage signal, and a stable voltage signal is always output based on the digital signal without being affected by the load impedance.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.
FIG. 2 is a circuit diagram illustrating an embodiment of an analog conversion circuit. Here, a digital / analog conversion circuit for converting an 8-bit digital signal into an analog signal is shown as an example. As shown, the digital / analog conversion circuit of the present embodiment includes a comparison circuit 10, a current source 12, a resistance element 1
4, a current source circuit 20 including a plurality of current sources, a switch circuit 22 including a plurality of switches, resistance elements 30 and 32,
It comprises a switch 40, a differential voltage comparison circuit 50, an up / down counter 60, a reference voltage correction circuit 70, and a reference voltage generation circuit 80.

The comparison circuit 10 compares the reference voltage V1 with a voltage drop V2 generated in the resistance element 14, and outputs a signal V3 according to the comparison result. Current source 12 and current source circuit 2
0 is the output signal V3 of the comparison circuit 10.
Controls the respective output currents. Current source circuit 2
0 each supply a unit current I and a current source 1
2 supplies a current (16I) 16 times the unit current.

Each switch constituting the switch circuit 22 receives a digital signal sdat from a decoder (not shown).
And outputs the output current of each current source to an output terminal T _out
Or output to either _Txout . The resistance element 30 is connected to the output terminal _Tout , converts the current output by the switch circuit 22 into a voltage, and outputs the voltage to the terminal _Tout . The resistance element 32 is connected to the output terminal _Txout , converts the current output from the switch 22 into a voltage, and outputs the voltage to the terminal _Txout . Here, assuming that the resistance values of the resistance elements 30 and 32 are R, the resistance value of the resistance element 14 that generates the voltage drop V2 is 1
6R.

The switch 40 has terminals T _out and T
_It comprises two switches S1 and S2 provided between _xout and the input terminal of the differential voltage comparison circuit 50.
These switches S1 and S2 to OFF during normal operation, and turned on when the correction operation, and inputs the output voltage of the output terminal T _out and T _xout the differential voltage comparator 50.

The differential voltage comparison circuit 50 is constituted by, for example, a differential chopper comparison circuit. The differential voltage comparator circuit 50 compares the output voltage input and the reference voltage V _A and V _B through the switch 40, and outputs a comparison signal cmp according to the comparison result. The up / down counter 60 counts up or down according to the comparison signal cmp from the differential voltage comparison circuit 50 to increase or decrease the count value cnt.

The reference voltage correction circuit 70 calculates a reference voltage V based on the count value cnt of the up / down counter 60.
1 level is controlled. As shown, the reference voltage correction circuit 70 includes a plurality of voltage-dividing resistance elements connected in series between two reference voltage input terminals T ₁ and T ₂ . Any one of a connection point of a plurality of voltage dividing resistive element is connected to the output terminal T ₃ through the switch S3. The switch S3 is controlled by the count value cnt. Reference voltages V _ref1 and V _ref2 are input to reference voltage input terminals T ₁ and T ₂ , respectively. Note that the reference voltage V
_ref1 and V _ref2 are generated according to the reference voltage V _A and V _B. For example, V _ref1 = (V _A −V _B ), V _ref2 = − (V
_A− V _B ).

The reference voltage source 80 generates reference voltages V _A and V _B , supplies them to the differential voltage comparison circuit 60, and further supplies the reference voltages V _ref1 and T _ref1 to the terminals T ₁ and T ₂ of the reference voltage correction circuit 70. Supply V _ref2 .

As described above, the digital / analog conversion circuit according to the present embodiment includes a portion constituted by the comparison circuit 10, the current source 12, the resistance element 14, the current source circuit 20, the switch circuit 22, and the resistance elements 30 and 32. Has almost the same configuration as the conventional digital / analog conversion circuit, but in the present embodiment, the reference voltage V1 supplied to the comparison circuit 10 is not a fixed voltage but a reference voltage generated by the reference voltage correction circuit 70. is there. The reference voltage V1 is equal to the output terminal _Tout or Tout of the digital / analog conversion circuit.
Input impedance Z of the load circuit connected to _xout
i , the maximum differential voltage (V _out -V _xout ) at the voltages V _out and V _xout output from the output terminals T _out and T _xout is _independent of the impedance of the load circuit. Can always be kept at a certain level.

Hereinafter, the operation of the digital / analog conversion circuit of the present embodiment will be described. The correction operation is performed when the power of the digital / analog conversion circuit is turned on or when the digital / analog conversion circuit enters the operating state from the standby state. At this time, first, all bits of the output signal sdat of the decoder are set to “1”. That is, the signal of each bit is set to a high level. In response, the switch circuit 22 outputs all the output current of the current source circuit 20 to the output terminal _Tout .

Therefore, from the output terminal T _out (V _out =
Voltage 255IR) is output, 0 from the output terminal T _xout
A voltage _Vxout of V is output. During the correction operation, the switches S1 and S2 constituting the switch 40 are turned on, and the output voltages _Vout and _Vxout are input to the differential voltage comparison circuit 50, respectively. The differential voltage comparator circuit, the input differential voltage (V _out -V _xout) the differential reference voltage (V _A -V _B)
Are compared, and a high-level or low-level signal cmp is output according to the comparison result.

The up / down counter 60 counts up or down according to the signal cmp to control the count value cnt. The count value c
switch S in the reference voltage correction circuit 70 according to the
3, the level of the reference voltage V1 output from the reference voltage correction circuit 70 is controlled. For example, the differential voltage (V _out -V
_xout ) is higher than the differential reference voltage (V _A -V _B ), the high-level signal cmp is output by the differential voltage comparison circuit 50, the up / down counter 60 counts up, and the count value cnt increases. . In response to this, the switch 3 at the reference voltage correction circuit 70 approaches the terminal T ₂ side, since the level of the reference voltage V1 output decreases, the level of the output signal V3 of the comparator circuit 10 becomes high.
The current source circuit 20 is, for example, a pMOS as shown in FIG.
In the case of using transistors, the output current of each current source decreases as the level of the signal V3 increases, so that the output voltage _{Vout at} the output terminal _Tout decreases. In the opposite case, when the differential voltage (V _out -V _xout ) input to the differential voltage comparison circuit 50 becomes lower than the differential reference voltage (V _A -V _B ), the signal is output by the reference voltage correction circuit 70. The level of reference voltage V1 is controlled to be high. Therefore, decreases the level of the output signal V3 of the comparator circuit 10, the output current of each current source of the current source circuit 20 is increased, the level of the output voltage V _out of _the terminal T _out rises.

By the above-described correction operation, the maximum differential voltage (V _out -V _xout ) of the output voltages V _out and V _xout of the digital / analog conversion circuit is changed to the differential reference voltage (V _A-
The level of the reference voltage V1 output by the reference voltage correction circuit 70 is corrected so as to be substantially equal to V _B ). As a result of the correction operation, the output differential voltage (V _out −V _xout ) is not affected by the input impedance Z _i of the load circuit connected to the output terminal T _out or T _xout , and the reference voltages _VA and V _B. The differential reference voltage (V _A −
V _B ).

After the correction operation is completed, the switch S3 of the reference voltage correction circuit 70 is fixed as it is, and the connection does not change until the next correction operation. For this reason, the reference voltage V1 output from the reference voltage correction circuit 70 is set and corrected according to the impedance of the load circuit connected to the output terminals T _out and T _xout of the digital / analog conversion circuit and other operating conditions. maximum differential output voltage of the rear digital / analog converter circuit (V _out -V _xout) is controlled to be approximately equal to the differential reference voltage (V _a -V _B).

FIG. 2 shows the waveform of the output voltage of the digital / analog conversion circuit during the correction operation. In FIG. 2, the vertical axis indicates the differential output voltage ( _Vout - _Vxout ) of the digital / analog conversion circuit, and the horizontal axis indicates time t. As shown in the figure, the circuit is set up after the power is turned on, and the setup is completed after the time t1 has elapsed.
The correction operation starts. At this time, each bit of the output signal of the decoder is set to “1”, the count value cnt of the up / down counter is set according to the output signal cmp of the differential voltage comparison circuit 50, and the reference voltage correction circuit is set accordingly. The level of the output signal V1 at 70 is controlled. Result of the correction operation, the digital / analog conversion circuit of the differential output voltage (V _out -V _xout) approaches the differential reference voltage (V _A -V _B). When the time t2 has elapsed from the start of the correction, the differential output voltage (V _out -V _xout ) becomes substantially equal to the differential reference voltage (V _A -V
_B ) reach a level equal to.

As a result of the correction, the differential output voltage (V _out -V
When _xout) is substantially equal to the differential reference voltage (V _A -V _B), the output signal having a specific pattern from the differential voltage comparator circuit, for example, a signal consisting of repetition of "0" and "1" is output Therefore, the correction ends at this point. Also,
Even if this specific pattern does not appear, the correction may be terminated after a certain time has elapsed from the start of the correction. When the correction is completed, the switches S1 and S2
Is turned off, and a normal conversion operation is performed by the digital / analog conversion circuit. At this time, according to the reference voltage V1 output from the reference voltage correction circuit 70,
The output current of each current source constituting the current source 12 and the current source circuit 20 is controlled. Each bit of the output signal sdat of the decoder in accordance with the digital signal to be input is set, the voltage signal V _out and V _xout corresponding to the digital signal is outputted from the output terminal T _out and T _xout.

FIG. 3 is a circuit diagram showing an application example of the above-described digital / analog conversion circuit. For example, a two-channel differential output type digital / analog conversion circuit mounted on an LSI for a mobile communication device such as a mobile phone. It is a block diagram of a circuit. This conversion circuit converts the input two-channel digital signals Idat and Qdat into a voltage signal Id, respectively.
_out , _Ixout and a differential voltage signal and a voltage signal Q
_out , _Qxout .

In FIG. 3, 8-bit digital / analog converters (D / A converters) 100_1 and 100_
2 has the same configuration as the comparison 10, the current source 12, the resistance element 14, the current source circuit 20, the switch circuit 22, and the partial circuit including the resistance elements 30 and 32 in FIG. Switch 40, differential voltage comparison circuit 50, up / down counter (UP / DOWN counter) 60
Respectively correspond to the partial circuits shown in FIG. 8-bit R
The -D / A converters 70_1 and 70_2 have the same configuration as the reference voltage correction circuit 70 shown in FIG. That is, these D / A converters are composed of a plurality of voltage dividing resistance elements and switches connected between two reference voltage input terminals, and the switches are controlled according to the count value of the UP / DOWN counter 60. The level of the output reference voltage is corrected.

In the digital / analog conversion circuit of FIG. 3, the switch 40, the differential voltage comparison circuit 50 and the UP
The / DOWN counter 60 is shared by two digital / analog converters. During the correction operation, for example,
First, the output signals I _out and I of the D / A converter 100_1
_xout is selected by the switch 40 and input to the differential voltage comparison circuit 50. The count value of the UP / DOWN counter 60 is set according to the output signal of the differential voltage comparison circuit 50, and is input to the 8-bit RD / A converter 70_1. A reference voltage corresponding to this is generated, and an 8-bit D / A
It is supplied to the converter 100_1.

Then, the D / A converter 100_2 is corrected. Output signal Q of D / A converter 100_2
_out and Q _xout are selected by the switch 40 and input to the differential voltage comparison circuit 50. Differential voltage comparison circuit 50
, The count value of the UP / DOWN counter 60 is set, and the 8-bit RD / A converter 70_2
Is input to A reference voltage corresponding to this is generated and supplied to the 8-bit D / A converter 100_2.

In the digital / analog conversion circuit shown in FIG. 3, the differential voltage comparison circuit 50 is configured using, for example, a differential chopper comparison circuit. Therefore, the offset voltage error is reduced, and the two D / A converters 100
In the case where the differential voltage comparison circuit 50 is shared in time division between _1 and 100_2, an output error between channels can be reduced. Also, as described above, the two D / A converters 100_
By arranging 1 and 100_2 apart from each other on the substrate, mutual interference hardly occurs.

In the above description, a digital / analog conversion circuit for converting a digital signal into a differential voltage and outputting the converted signal has been described as an example. However, the present invention is not limited to this. The present invention can also be applied to a digital / analog conversion circuit that changes to a single-ended voltage signal. Further, in the above embodiment, the circuit example in which the current source and the switch are configured by the p-channel MOS transistors is described. However, it is needless to say that the current source and the switch can be configured by the n-channel MOS transistor or the bipolar transistor.

[0039]

As described above, according to the digital / analog conversion circuit of the present invention, the dependence on the load circuit can be reduced, and a stable output voltage can always be obtained. In addition, by sharing one correction circuit with a plurality of digital / analog converters, the relative error between the converters can be reduced. Furthermore, since a plurality of digital / analog converters can be arranged apart when relative accuracy is required, there is an advantage that interference between them is hard to occur.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of a digital / analog conversion circuit according to the present invention.

FIG. 2 is a waveform chart showing waveforms during a correction operation according to the embodiment.

FIG. 3 is a block diagram showing an application example of the embodiment.

FIG. 4 is a circuit diagram showing an example of a conventional digital / analog conversion circuit.

FIG. 5 is a circuit diagram showing a specific configuration example of the digital / analog conversion circuit of FIG. 4;

[Explanation of symbols]

Reference numeral 10: comparison circuit, 12: current source, 14: resistor element, 20
... current source circuit, 22 ... switch circuit, 30, 32 ... resistance element, 40 ... switch, 50 ... differential voltage comparison circuit, 60
... Up / down counter, 70.70_1, 70_2
... Reference voltage correction circuit, 80 ... Reference voltage source, 100_1,
100_2: digital / analog converter, V _cc : power supply voltage, GND: ground potential.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tadashi Maekawa 6-35, Kita-Shinagawa, Shinagawa-ku, Tokyo Sony Corporation F-term (reference) 5J022 AB06 BA01 CB01 CB07 CD01 CE06 CF01 CF02 CF04 CF05 CF07 CG01

Claims (3)

[Claims] A current supply circuit for supplying a current based on a digital signal; and a resistance element for converting an output current of the current supply circuit into a voltage signal and outputting the voltage signal. A digital / analog conversion circuit in which the level of the voltage signal varies according to the impedance of a load connected to a terminal, compares the voltage signal with a reference signal, and outputs a comparison signal according to the comparison result. A comparison circuit, a counter for increasing or decreasing the count value according to a comparison signal from the comparison circuit, a correction signal having a level corresponding to the count value, and an output current of the current supply circuit according to the correction signal. A digital / analog conversion circuit having a correction signal generation circuit for controlling the operation of the digital / analog converter. 2. The correction signal generating circuit according to claim 1, wherein: a plurality of resistance elements connected in series between a first voltage supply terminal and a second voltage supply terminal; and a count value from the counter. 2. The digital / analog conversion circuit according to claim 1, further comprising: a switching element for connecting any one of the connection middle points of said resistance elements to a correction signal output terminal. 3. The digital / analog conversion circuit according to claim 1, wherein said current supply circuit comprises a transistor to which said correction signal is input to a control terminal.