JPH10112654A - Current segment system d/a converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain high speed by providing a regulator circuit between a current output terminal and a current summing point and outputting the level of the current summing point to a current output terminal to reduce the circuit scale of the converter. SOLUTION: Levels Voutpx, Voutnx of wires 5, 6 of current cells 1 connected in parallel are given to 1st and 2nd input terminals of a regulator circuit 9. Output terminals 7, 8 are connected to 1st and 2nd output terminals of the circuit 9, and Voutp, Voutn are outputted. The circuit 9 keeps the levels of the wires 5, 6 the same and currents given to the 1st and 2nd input terminals are outputted as they are to the 1st and 2nd output terminals. As a result, currents in response to digital signals are outputted to the 1st and 2nd output terminals of the circuit 9. Moreover, output terminals 7, 8 are connected to a point of a power supply voltage Vdd via a resistor R to convert an output current of the circuit 9 into a voltage. As a result, each level is made stable by adding the circuit 9 to the D/A converter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital-to-analog converter, and more particularly to a current segment type digital-to-analog converter used in a field requiring high speed.

[0002]

2. Description of the Related Art Current segment type digital-to-analog converters are often used for high-speed digital-to-analog conversion such as for video. FIG. 6 shows a conventional current segment type digital-to-analog converter. In FIG. 6, a digital signal composed of, for example, n-bit signals φ1 to φn is input to one group of current cells.

A current cell 1 comprises a current source 2, switches 3, 4
And wirings 5 and 6. One end of the current source 2 is grounded, and the other end, that is, the output terminal is connected to one end of the switches 3 and 4. This connection point will be referred to as point A. The other end of the switch 3 and the other end of the switch 4 are connected to a wiring 5 and a wiring 6, respectively. The switch 3 is controlled by a signal φ1. For example, the switch 3 is turned on when φ1 is at a high level, and is cut off when it is at a low level. The switch 4 is controlled by an inverted signal / φ1 of the signal φ (hereinafter, / represents an inverted signal). For example, the switch 4 is turned on when / φ1 is at a high level and cut off when / φ1 is at a low level. Hereinafter, the current cell having such a configuration will be referred to as a differential operation type current cell.

[0006] The n current cells 1 are connected in parallel by sharing the wirings 5 and 6. The wirings 5 and 6 are current addition points. One switch of each current cell has a signal φ
, Φn, and the other switch is opened and closed by their inverted signals. The ends of the wirings 5 and 6 are connected to output terminals 7 and 8, respectively, and the output terminals 7 and 8 are connected to the power supply potential Vdd via the resistors R, respectively. The output current value of the current source is the same in any of the current cells, for example. Output potentials at the output terminals 7 and 8 are represented as Voutp and Voutn, respectively.

In this digital / analog conversion circuit,
The switch 3 opens and closes according to the input digital signal, and the wiring 5
The current supplied to is changed. As a result, ΔI is
, The output potential Voutp is expressed as follows: Voutp = Vdd-R × ΣI In this way, digital-to-analog conversion is performed. On the other hand, the current source not connected to the wiring 5
Since no current flows when the circuit is kept open, the switch 4 is closed to connect to the inverted output potential Voutp or to Vdd.

[0006]

In the digital-to-analog converter shown in FIG. 6, the switch 3 is turned on and the switch 4 is turned off, and the switch 3 is turned off and the switch 4 is turned on, due to a change in the digital input signal. When the state is switched to the state of (A), the potential of the point A becomes (Voutn−Vunn).
It changes by the voltage of p).

[0007] Since the potential of the output terminal of the current source 2 fluctuates during switching as described above, it takes time for the current value at the output terminal of the current source 2 to stabilize.
In the digital-to-analog converter shown in FIG.
The output voltage Voutp or Vo is applied to the output terminal of the current source 2.
utn is added directly. Therefore, in order to improve the linearity of conversion in the digital-to-analog converter, the output impedance of the current source 2 must be increased.

FIG. 7 shows an example of a current source circuit using MOS transistors and an equivalent circuit thereof. FIG. 7A shows a current source configured with one transistor and an equivalent circuit thereof. This current source comprises, for example, an n-channel transistor 23 whose source is grounded and whose drain is connected to the output terminal. A voltage that turns on the transistor 23 is applied to the base of the transistor 23. In this configuration, as shown in the equivalent circuit, only the output impedance of rds can be obtained. Here, rds is rds = ΔVds / ΔId, where ΔVds is a change in voltage between the source and the drain, and ΔId is a change in drain current.
Is the value defined by

Therefore, generally, as shown in FIG. 7B, it is necessary to form a current source with two or more transistors to increase the output impedance.
For example, when a current source is composed of the two-stage transistors 24 and 25, the output impedance is rds × (gm × r
ds + 1). Here, gm is transconductance, and is usually gm × rds >> 1.

However, if the output impedance is increased, the circuit of the current source becomes complicated, and a plurality of current sources are required in the digital-to-analog converter of this type.
The problem that a circuit scale becomes large arises.

In the circuit shown in FIG. 6, load capacitances CL of the wirings 5 and 6 are added to the output terminals 7 and 8. The settling time of the current segment type digital-to-analog converter, that is, the time required for the output potential to reach the target voltage, is calculated by R ×
It is proportional to the time constant of CL. Therefore, as the CL increases, the settling time increases. As described above, there is a problem that a load capacitance is easily attached to the output terminal, which is not suitable for high-speed operation.

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and has as its object to simplify a current source and enable high-speed operation in a current segment type digital-to-analog converter.

[0013]

To solve the above problems, a current segment type digital-to-analog converter according to the present invention comprises a current source having one end connected to a constant potential and one end connected to the other end of the current source. A current cell having at least one current cell having a switch connected at the other end to the current summing point and opened and closed by a digital input signal, a current value output terminal, and a current value provided between the current summing point and the current value output terminal; A regulator circuit for stabilizing the potential at the addition point so as not to depend on the potential at the current value output terminal, and outputting the current at the current addition point to the current value output terminal as it is.

According to another aspect of the present invention, there is provided a current segment type digital-to-analog converter having one end connected to a constant potential and one end connected to the other end of the current source. Is connected to a first current summing point and is opened and closed by a digital input signal; one end is connected to the other end of the current source; the other end is connected to the second current summing point; At least one current cell having a second switch that is opened and closed by an inverted signal of the signal, first and second current value output terminals, first and second current summing points, and first and second current summing points; The first and second current summing points are provided between the current value output terminals and stabilized at the same potential regardless of the potentials at the first and second current value output terminals; And the current at the second current addition point Comprising a regulator circuit to be output to the respectively first and second current value output terminal.

Further, in order to solve the above problems, a current segment type digital-to-analog converter according to the present invention comprises:
A current value output terminal, a current source having one end connected to a constant potential, one end connected to the other end of the current source, the other end connected to a first current addition point, and a switch opened / closed by a digital input signal And at least one current cell having: a first current addition point provided between the first current addition point and the current value output terminal; stabilizing the potential of the first current addition point so as not to depend on the potential at the current value output terminal; A first digital-to-analog conversion circuit comprising a first regulator circuit for directly outputting the current at the first current addition point to the current value output terminal; a current source having one end connected to the constant potential; The other end of the current source is connected, the other end is connected to the second current summing point,
At least one current cell having a switch which is opened and closed by a digital input signal, provided between the second current addition point and the current value output terminal, and the potential of the second current addition point is set to the potential at the current value output terminal. A second regulator circuit that stabilizes the potential at the first current addition point to a potential different from the stabilized potential at the first current addition point and outputs the current at the second current addition point to the current value output terminal as it is. And two digital / analog conversion circuits.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention.
The same components as those in FIG. 6 are denoted by the same reference numerals, and description thereof will be omitted.

In FIG. 1, the wiring 5 and the wiring 6 of the current cell 1 connected in parallel are connected to the first and second input terminals of the regulator circuit 9, respectively. The potentials of the wirings 5 and 6 are represented as Voutpx and Voutnx, respectively. First and second output terminals of the regulator circuit 9 are connected to output terminals 7 and 8, respectively. The potentials at the output terminals 7 and 8 are Voutp and Voutn, respectively.
It expresses. The regulator circuit 9 keeps the potentials of the wirings 5 and 6 the same (hereinafter, referred to as a first condition), and outputs the currents input to the first and second input terminals to the first and second input terminals as they are, respectively. (Hereinafter, referred to as a second condition).

Therefore, a current value corresponding to the digital signal is output to the first and second output terminals of the regulator circuit. Further, the output terminals 7 and 8 are connected to the power supply potential Vdd via the resistors R, respectively.
Is converted to a voltage value.

FIG. 2 shows a specific circuit example of the regulator circuit 9. FIG. 2A shows the first circuit of the regulator circuit 9.
The following shows an example of the circuit. This circuit comprises transistors 11, 12
It consists of. Wiring 5 to the source of transistor 11
Is connected, the output terminal 7 is connected to the drain, and a constant voltage VB for operating the transistor 11 in a saturated state is supplied to the gate. Similarly, the wiring 6 is connected to the source of the transistor 12, the output terminal 8 is connected to the drain, and the voltage VB is supplied to the gate.

In the regulator circuit shown in FIG. 2A, since the source currents of the transistors 11 and 12 flow directly to the drains, the above-described second condition is satisfied. The change in the output current of the transistor 11 is represented by ΔI, and the transconductance of the transistor 11 is represented by gm.
Assuming that the voltage change of Voutpx is ΔV, ΔV = ΔI / gm. The change in the output voltage in the conventional example is ΔVconv
Then, ΔVconv = ΔI × R. Therefore, when 1 / gm <R, the voltage fluctuation can be made smaller than in the conventional example, and the first condition is satisfied. The same can be said for the transistor 12.

By adding this regulator circuit, the potential of Voutpx or Voutnx becomes Vo
Stabilization can be achieved without being affected by changes in the potential of outp or Voutn. Therefore, the change in potential at the output terminal of the current source is reduced, and it is not necessary to increase the output impedance of the current source. Therefore, the current source can be simplified. For example, a single-stage transistor like the current source shown in FIG. As a result, the size of the digital-to-analog converter can be reduced.

Further, since the regulator circuit 9 is interposed, the capacitances of the wirings 5 and 6 cannot be seen from the output terminals 7 and 8. Therefore, the stray capacitance at the output terminals 7 and 8 can be greatly reduced, and the speed can be increased.

Furthermore, when the potential difference between Voutpx and Voutnx is reduced by appropriately setting the regulator circuit, the fluctuation of the output voltage of the current source, that is, the voltage at the point A during the switching of the current source is reduced, and the switching is performed at high speed. Can be

In this embodiment, since the resistor R is used to convert the output current value of the regulator circuit 9 into a voltage value, the current value can be converted into a voltage value at high speed. In the above-described embodiment, instead of the differential operation type current cell, a current source 2 having one end grounded, one end connected to the other end of the current source 2, the other end connected to the wiring 5, and a digital A current cell 1 is constituted by a switch 3 controlled by an input signal φ, and a switch 4, a wiring 6, a transistor 1
2. The output terminal 8 may be omitted.

In this case, the effect of reducing the fluctuation of the output voltage of the current source 2 at the time of switching cannot be obtained, but the change in the output potential of the current source 2 is reduced by the regulator circuit 9 as in the above-described embodiment. Therefore, a simple current source can be used. Further, similarly to the above-described embodiment, the output terminal 7 does not have the stray capacitance based on the wiring 5, so that high-speed operation is possible.

FIG. 2B shows a second example of the regulator circuit. In this circuit, the wiring 5 is connected to the source of the transistor 14, and the drain of the transistor 14 is connected to the output terminal 7. Further, Voutpx is supplied to the inverting input terminal of the inverting amplifier 13 having a gain of A,
The reference potential VR is supplied to the non-inverting input terminal of the inverting amplifier 13, and the output terminal of the inverting amplifier 13 is connected to the gate of the transistor 14. Similarly, Voutnx is supplied to the source of the transistor 15, and the drain of the transistor 15 is connected to the output terminal Voutn. Further, Voutn is connected to the inverting input terminal of the inverting amplifier 16 having a gain of A.
x is supplied, the reference potential VR is supplied to the non-inverting input terminal, and the output terminal is connected to the gate of the transistor 15.

In the circuit shown in FIG. 2B, since the source currents of the transistors 14 and 15 become drain currents as they are, it is clear that the second condition is satisfied. The change in the output current of the regulator circuit is represented by ΔI, and the transconductance of the transistor 14 is represented by gm.
Assuming that the voltage change of Voutpx is ΔV, ΔV = ΔI / gm / (A + 1). Thus, if 1 / gm / (A + 1) <R, the above first condition can be satisfied.

In this circuit, the same effect as that of the circuit shown in FIG. 2A can be obtained, and the voltage change is made smaller by the gain A of the inverting amplifier than in the circuit shown in FIG. 2A. It becomes possible. Further, by adjusting the reference potential VR, Voutpx and Voutnx can be easily set to desired values.

FIG. 2C shows a third example of the regulator circuit. This circuit comprises a transistor 18,
19, 20, and 22, and current sources 17 and 21.
The wiring 5 is connected to the source of the transistor 19, and the drain of the transistor 19 is connected to the output terminal 7. The gate of the transistor 18 is connected to the source of the transistor 19, the source of the transistor 18 is grounded, and the drain of the transistor 18 is connected to one end of the current source 17 and the gate of the transistor 19. The other end of the current source 17
Connected to power supply potential.

Similarly, the wiring 6 is connected to the source of the transistor 20, and the drain is connected to the output terminal 8. The source of the transistor 20 is the transistor 2
2 is connected to the second gate. The source of the transistor 22 is grounded, and the drain is connected to one end of the current source 21 and the gate of the transistor 20. The other end of the current source 21 is connected to a power supply potential.

In this circuit, transistors 19, 2
Since the source current of 0 becomes the drain current as it is, the second condition can be satisfied. Further, the gain of the amplifier circuit composed of the current source 17 and the transistor 18 is set to A
The change in the output current of the regulator circuit, that is, the change in the output current at the drain of the transistor 19 is ΔI, the transconductance of the transistor 19 is gm, and V
Assuming that the voltage change of outpx is ΔV, ΔV = ΔI / gm / (A + 1). Therefore, if 1 / gm / (A + 1) << R, the first condition described above can be satisfied.

As described above, by using this circuit, the same effect as that of the circuit shown in FIG. 2A can be obtained, and the gain A is smaller than that of the circuit shown in FIG. It is possible to further reduce the voltage change.
Further, since one transistor and a current source are used instead of the inverting amplifier, the scale can be made smaller than that of the regulator circuit shown in FIG.

In the embodiment shown in FIG. 1, a resistor may be used instead of the current source as shown in FIG. In FIG. 3A, a resistor R is used in place of the current source 2.
In this case, all the current cells output the same current value. In FIG. 3B, resistors having resistance values R and 2R are arranged in a ladder shape. In this case, each current cell outputs half the current of the output current value of the current cell on the left.

FIG. 4 shows a second embodiment of the present invention. In this embodiment, two or more current segment type digital-to-analog converters shown in the first embodiment are prepared, and the output terminals of the respective regulator circuits are connected to a common output terminal.

As shown in FIG. 4, the first current segment type digital-to-analog converter 31 includes switches 3a and 4a and a current source 2a as in the embodiment shown in FIG.
And a plurality of current cells connected in parallel via wirings 5a and 6a and a regulator circuit 9a. The digital input signals of the digital / analog converter 31 are a1, a2,.

Similarly, the second current segment type digital-to-analog converter 32 also includes a plurality of current cells including switches 3b, 4b and a current source 2b, which are connected in parallel via wires 5b, 6b. Regulator circuit 9b
It is composed of The digital input signals of the converter 32 are b1, b2,. It is assumed that the current cells used in the first and second digital-to-analog converters 31 and 32 are the same.

Further, the first output terminals of the regulator circuits 9a and 9b are both connected to the first current value output terminal 7, and the second output terminals of the regulator circuits 9a and 9b are both connected to the second current value. It is connected to the value output terminal 8. Output currents at the first and second current value output terminals 7 and 8 are represented as Iout and / Iout, respectively.

In this embodiment, the regulator circuit 9
By appropriately setting a and 9b, the potentials of the wirings 5a and 6a are maintained at Va, and the potentials of the wirings 5b and 6b are maintained at Vb. Assuming that the current sources 2a and 2b in the first and second digital / analog converters 31 and 32 are constituted by transistors as shown in FIG. 7, the voltage applied to the output terminals of the current sources 2a and 2b and The relationship with the output current is represented by a graph shown in FIG. First and second digital-to-analog converters 31,
32, the output currents of the current sources 2a and 2b are I
Assuming that a and Ib, Iout = (a1 + a2 +...) × Ia + (b1 + b2 +
...) × Ib.

Thus, the output voltage V of the regulator
By controlling a and Vb to change the currents Ia and Ib, weighted addition of digital input signals can be easily performed.

The current source 2 of the current cell may be replaced with a resistor R as shown in FIG. In this case, the voltage applied to the resistor and the output current of the current cell are as shown in FIG.
Is proportional as shown. Therefore, the control of Ia and Ib can be easily performed, and the control of the weighting described above can be simplified.

A resistor may be provided between each of the output terminals 7 and 8 and the power supply potential, and the result of converting the digital signal into analog may be output to the output terminals 7 and 8 as a voltage value instead of a current value.

In the above-described embodiment, the differential signal Vou
Although the case where tp and Voutn are output is shown, the same effect can be obtained by connecting one of the output terminals of the regulator circuit to the power supply potential Vdd.

In the above embodiment, one end of the current source in the current cell is connected to Vss, but this may be connected to Vdd. In this case, the resistor R is provided between the output terminals 7 and 8 and Vss.

Further, only the case where MOS transistors are used for the current source and the regulator circuit has been described. However, the present invention is not limited to this, and bipolar transistors may be used instead of MOS transistors.

[0045]

As described above, according to the present invention,
The addition of the regulator circuit reduces the fluctuation of the potential at the current addition point, so that the output impedance of the current source may be low, and the circuit scale of the digital-to-analog converter can be reduced.

Further, according to the present invention, the capacity of the wiring is not added to the current value output terminal due to the interposition of the regulator circuit. Operation can be accelerated.

Further, according to the present invention, in the differential operation type current cell, the change in the potential at the output terminal of the current source during switching is reduced, so that the operation can be performed at a higher speed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a regulator circuit used in the present invention.

FIG. 3 is a diagram showing an embodiment of the present invention using a resistor as a current source.

FIG. 4 is a diagram showing a second embodiment of the present invention.

FIG. 5 is a diagram showing a relationship between a voltage applied to an output terminal of a current source and an output current.

FIG. 6 is a diagram showing a conventional example.

FIG. 7 shows a current source.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Current cell, 2 ... Current source, 3, 4 ... Switch, 5, 6 ... Wiring, 7, 8 ... Output terminal, 9 ... Regulator circuit, 11, 12, 14, 15, 18, 19, 20, 22 ... M
OS transistor, 13, 16 ... inverting amplifier, 17, 20 ... current source.

Claims (12)

[Claims] 1. A current source having one end connected to a constant potential, and a switch having one end connected to the other end of the current source, the other end connected to a current addition point, and opened and closed by a digital input signal. At least one current cell, a current value output terminal, and a current value output terminal; provided between the current addition point and the current value output terminal, for stabilizing the potential of the current addition point so as not to depend on the potential at the current value output terminal. And a regulator circuit for outputting the current at the current addition point to the current value output terminal as it is. 2. The regulator circuit according to claim 1, wherein a source is connected to the current summing point, a drain is connected to the current output terminal, and a gate is supplied with a voltage exceeding a threshold voltage. Claim 1
A current-segment digital-to-analog converter as described. 3. One end is connected to the current value output terminal,
2. The current segment type digital-to-analog converter according to claim 1, further comprising a resistor having the other end connected to a constant potential different from the constant potential. 4. The current segment type digital-to-analog converter according to claim 1, further comprising a resistor in place of said current source. 5. A current source having one end connected to a constant potential, one end connected to the other end of the current source, the other end connected to a first current addition point, and opened and closed by a digital input signal. At least one switch having one end connected to the other end of the current source, the other end connected to a second current summing point, and opened and closed by an inverted signal of the digital input signal. Two current cells; first and second current value output terminals; and first and second current value output terminals, provided between the first and second current addition points and the first and second current value output terminals. Are stabilized at the same potential independently of the potentials at the first and second current value output terminals, respectively, and the currents at the first and second current addition points are respectively left unchanged. Output to the first and second current value output terminals. Current segment system digital-to-analog converter characterized by comprising a regulator circuit. 6. The regulator circuit, wherein a source is connected to the first current addition point, a drain is connected to the first current value output terminal, and a voltage exceeding a threshold voltage is supplied to a gate. A second transistor having a source connected to the second current addition point, a drain connected to the second current output terminal, and a gate supplied with a voltage exceeding a threshold voltage. 6. The current segment type digital-to-analog converter according to claim 5, wherein: 7. A first resistor having one end connected to the first current value output terminal, the other end connected to a constant potential different from the constant potential, and one end connected to the second current value output terminal. 6. The current segment type digital-to-analog converter according to claim 5, further comprising a second resistor connected to the other end and connected to a constant potential different from the constant potential. 8. The current segment type digital / analog converter according to claim 5, further comprising a resistor in place of said current source. 9. A current value output terminal, a current source having one end connected to a constant potential, one end connected to the other end of the current source, the other end connected to a first current summing point, and a digital input terminal. At least one current cell having a switch that is opened and closed by a signal; provided between the first current addition point and the current value output terminal, and the potential of the first current addition point at the current value output terminal A first digital-to-analog conversion circuit comprising a first regulator circuit for stabilizing the current at the first current addition point to the current value output terminal as it is without being dependent on the potential; At least one current having a current source connected to a constant potential and a switch connected at one end to the other end of the current source, the other end connected to a second current summing point, and opened and closed by a digital input signal. And the second current addition point is provided between the second current addition point and the current value output terminal, and the potential of the second current addition point is independent of the potential at the current value output terminal and the potential of the first current addition point is A second digital-to-analog conversion circuit comprising a second regulator circuit for stabilizing a potential different from the stabilized potential and outputting the current at the second current addition point to the current value output terminal as it is. A current segment type digital-to-analog converter, comprising: 10. The first regulator circuit, wherein a source is connected to the first current addition point, a drain is connected to the current value output terminal, and a voltage exceeding a threshold voltage is supplied to a gate. In the second regulator circuit, a source is connected to the second current addition point, a drain is connected to the current value output terminal, and a voltage exceeding a threshold voltage is supplied to a gate. 10. The current segment type digital-to-analog converter according to claim 9, comprising a second transistor. 11. The current segment type digital circuit according to claim 9, further comprising a resistor having one end connected to the current value output terminal and the other end connected to a constant potential different from the constant potential. Analog converter. 12. The current segment type digital-to-analog converter according to claim 9, wherein the first and second digital-to-analog conversion circuits include a resistor instead of a current source.