JP2605874B2 - DA converter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DA converter, and more particularly to a DA converter using a large number of constant current sources.

[Conventional technology]

Conventionally, various types of D / A converters have been considered, but in order to realize a high-precision D / A converter on a semiconductor substrate, a large number of constant current source circuits are used to obtain each current ratio accuracy. Is generally used.

FIG. 6 is a circuit diagram of a current addition type DA converter showing one example of such a conventional technique.

As shown in FIG. 6, the conventional current addition type DA converter comprises (2 ^m -1) constant current source circuits and current switches for 1 LSB of the DA output voltage (m is the number of bits and (Same number) connected in parallel, current switch according to digital data Is turned on to obtain an analog output.

For such D-A converter which requires the same number of current switch and (2 ^m -1) pieces of the constant current source circuit, there is a disadvantage that the circuit scale becomes large, (2 ^m -1) inputs Terminals, high conversion speed, and simple configuration
Because it can be converted, it is useful as a DA converter constituting a serial-parallel AD converter. This DA converter is:
For example, SSD in IEICE Technical Report Vol.84, No11
84-13 "10-bit 30MHz high-speed ACD for serial-parallel A / D converter
It has a well-known configuration as described in “Subsystem IC”.

FIG. 7 is a circuit diagram of a constant current source of the DA converter shown in FIG.

As shown in FIG. 7, (2 ^m -1) pieces of the constant current source circuit, a constant current source given a common bias potential V _B generated by the bias circuit 1 transistor Q ₁ ~Q _{n (n} = 2 ^m -1) and composed of a resistor R ₁ to R _n.

When used in the serial-parallel A / D converter, the first A-D converter is composed of (2 ^l -1) (1 is the first A-D converter of the serial-parallel A / D converter). The number of current switches and the number of constant current circuits are equal to the number of comparators and the number of constant current circuits, and the output of the comparator is compared with the output of the comparator.
In order to connect the input of the current switch of the converter, the current switch and the constant current circuit are often included in the comparator.

[Problems to be solved by the invention]

Conventional D-A converter described above, the common wiring tends to be long, such as a common bias voltage (terminal) V _B and ground line for the constant current source circuit is many. Since the ground wire has a large current due to the addition of the current of the constant current source circuit, if the wire length is long, the ground potential rises as the wire is separated from the bias circuit, and the current flowing to the constant current source circuit decreases. I do. This causes a differential error because the step size (voltage for 1 LSB) of the DA converter changes depending on the position. In addition, since there is monotonicity, there is a disadvantage that the integration is performed as an integration error and the DA conversion accuracy is reduced.

To prevent this, either eliminate the potential rise by the ground line may be a common bias V potential rise of the _B line and the potential rise of the ground wire to the same, the current of all of the constant current source circuit will be equal There is no decrease in accuracy due to wiring resistance.

However, in the former circuit, the width of the ground wiring must be significantly increased, and in the latter circuit, the common bias V _B
Since the current flowing between the line and the ground wiring differs by several tens to several hundreds and varies depending on the value of _hFE of the constant current source transistor, there is a drawback that it is difficult to realize.

An object of the present invention is to provide a DA converter capable of preventing the output current from being shifted due to the wiring resistance and improving the mounting density.

[Means for solving the problem]

The DA converter according to the present invention includes a plurality of unit current output circuits each including a current switch and a constant current source circuit arranged on a semiconductor substrate, and connects the plurality of unit current output circuits in parallel to add a current. In the A converter, each of the unit current output circuits includes a first and a second constant current source circuit having an output side commonly connected to a respective current switch, and a first and a second constant current source circuit. A bias line for supplying a bias voltage; a first ground line commonly connected to supply a ground potential to each of the first constant current source circuits; and a respective one of the second constant current source circuits A second ground line commonly connected to supply a ground potential to the first and second ground lines, and a power supply direction of at least the first and second ground lines of the bias line and the first and second ground lines. The opposite to the unit current The output circuit is arranged and wired.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram of a constant current source of a DA converter according to a first embodiment of the present invention.

As shown in FIG. 1, the DA converter according to the present embodiment has a unit in which a unit current output circuit is composed of a current switch and two constant current source circuits. Means for giving a potential, means for giving at least a ground potential among common potentials of one of the two constant current source circuits, means for giving a ground potential of the other constant current source circuit, and the m-bit array Means for supplying power to the blocks from opposite directions.

That is, the present embodiment the transistors _{Q 1, Q 2, ......,} Q n and a resistor R _1, R _2, ......, a first constant current source circuit which is formed by R _n, transistor Q _1A, Q _2A , ……, Q _nA and resistance R _1A , R
_2A ,..., R _nA and common biases V _B1 and V _B2 and ground potentials V _G1 and V _{G2, respectively.}
Are given. Collectors of transistors Q ₁ and Q _1A , collectors of transistors Q ₂ and Q _2A , transistors Q _n and Q
The collector of _nA are respectively connected, the output current I ₁ Together, the current flowing through the respective transistors, I _2, ......, to form a I _n. Features of embodiments and that I take the output current I ₁ ~I _n by two constant current source circuit further first constant current source circuit bias potential V _B1 and the ground potential V _G1 from the output current I ₁ side it is and second constant current source circuit given is the point that a bias potential V _B2 and a ground potential V _G2 is supplied from the output current I _n side.

Therefore, the ground wiring and the common bias wiring of the first and second constant current source circuits are made the same, and the arrangement and wiring are performed so that the connection method between each unit current source and the common wiring and the spacing on the layout are the same. If it is, the bias potentials V _B1 and V _B1
As long as providing the _B2 and a ground potential V _G1 and V _G2 are each the same potential, increase and decrease potential by wiring resistance of the bias wiring of the ground potential by wiring resistance of the ground wire, if the distance from the bias source is the same, The potentials at the corresponding portions of the first and second constant current circuits are the same. That is, the base potential of the transistor Q _1, the transistor Q _nA, the base potential of the transistor Q _1A and the transistor Q _n, resistor R ₁ and the ground potential of the resistor R _nA, the ground potential of the resistor R _1A resistor R _n is the respective same potential Become. Thus, the transistor Q _1, the output current of the transistor Q _nA is equal, the transistor Q _1A and the transistor Q _n
Because also the output current equal, the output current I ₁ and I _n is the current value is obtained equal. For the same reason, the output current I ₂ and I _n-1, I ₃
I _{n-2 is} also equal, resulting in output currents I ₁ , I ₂ ,.
..., I _n is everyone equally.

FIG. 2 is a plan view of the layout and wiring of the constant current source circuit shown in FIG.

As shown in FIG. 2, block B ₁ , block B ₂ ,.
Block _Bn indicates a block including the first and second constant current circuits described above. Block B ₁ represents, includes a connection terminal of the transistor Q _1, the transistor Q _1A and resistors R ₁ and R _1A and the common line, the block B _2, ......, block B _n
Has a similar configuration. These block trains and the first
Of the bias line _WB1 , the ground line _WG1 and the second
Arranged and wired by folding the common wiring of the ground wire W _G2 like the bias line W _B2 of the constant current circuit in the middle of the block column. As shown in FIG. 1, the bias line W _B1 and the ground line W _G1 of the first constant current circuit is connected from the block B ₁ side to a predetermined terminal 2 and 3 of the bias circuit block 1, a second constant ground line and the bias line W _B2 of the current circuit W _G2 block
_Bn is connected to predetermined terminals 2 and 3 of the bias circuit block 1 from the _Bn side.

Thus, by arranging and wiring blocks,
The output current of each constant current output circuit can be made equal as described above.

FIG. 3 is a circuit diagram of a constant current source of a DA converter according to a second embodiment of the present invention.

As shown in FIG. 3, in the present embodiment, the bias voltages (terminals) V _B1 and V _B2 of the first and second constant current circuits are shared in the first embodiment described above. Normal,
Current flowing through the bias line, the transistor Q _1, Q ₂ constituting a constant current circuit, ......, Q _n and the transistor Q _1A,
Q _2A, ......, is the base current of Q _nA, the base current is 1 of the transistor h _FE minute compared to the current in the ground line. Therefore, the change in the potential of the bias line is significantly smaller than the change in the potential of the ground line. Therefore, even with the above-described configuration, the deviation of the output current of the current output circuit can be improved as compared with the conventional example. The arrangement of the blocks and the common wiring in this embodiment is the same as the block arrangement shown in FIG. 2, and one of the bias wiring _WB2 or the bias wiring _WB1 is removed, and the other of the transistors constituting all the constant current circuits is removed. Just connect the base. Further, by providing a bias line for each of the left and right rows of blocks may be connected to a predetermined terminal 2 and 3 of each bias block 1 than the block B ₁ and block B _n side.

In this embodiment, the point that only one bias wiring is required and the bases of the transistors Q ₁ and Q _1A of the first and second constant current circuits that constitute the current output circuit are common, so that they are in the same base region. This is advantageous in that two emitter regions can be provided, and a DA converter can be formed with a smaller area than in the first embodiment.

FIG. 4 is a plan view of the electrodes of the transistors constituting the constant current circuit shown in FIG.

As shown in FIG. 4, such a transistor includes a base 5, a collector 6 and the same base region 5 in a semiconductor substrate 4.
And emitters 7A and 7B formed therein.

FIG. 5 is a circuit diagram of a constant current source of a DA converter according to a third embodiment of the present invention.

As shown in FIG. 5, the present embodiment is a circuit applied to a weighted constant current source to improve the DA conversion accuracy. That is, the transistors Q ₁ , Q ₂ , Q ₃ , Q ₄ and the resistors R ₁ , R ₂ , R ₃ , R ₄ form a first constant current source circuit, and the transistors Q _1A ,
Q _2A , Q _3A , Q _4A and the resistors R _1A , R _2A , R _3A , R _4A form a second constant current source circuit. The first as in the second embodiment described above, the base bias V _B of the second constant current source circuit is shared, given the ground potential V _G1, V _G2 from the opposite direction to each separated only ground line Have been. In the present embodiment, the current ratio between the output current I ₁ obtained from the transistors Q ₁ and Q _1A , the output current I ₂ obtained from the transistors Q ₂ and Q _2A , and the output currents I ₃ and I ₄ similarly obtained is To make I ₁ : I ₂ : I ₃ : I ₄ = 1: 2: 4: 8, the emitter area of the transistor is A _Q1 : A _Q2 : A _Q3 : A
_Q4 = 1: 2: 4: 8 and AQ _1A : AQ _2A : AQ _3A : AQ _4A = 1: 2:
Designed for 3: 4. The resistance is age, Designed for Further, since the rise in the ground potential due to the wiring resistance r of the ground wiring is weighted to the output current of each constant current source circuit, even if the distance from the bias source is the same, the increase in the ground potential will occur.
1. The electric potential at the relevant portion of the second constant current source circuit is not the same. Therefore, a deviation occurs in the output current ratio, but the deviation is smaller in this embodiment than in the conventional example. Also, the constant current source circuit with the larger ground potential rise, here a transistor
The potential at the ground point of the emitter resistors R ₁ , R ₂ , R ₃ and R ₄ of the _first constant current source transistor of the first constant current source circuit composed of Q _{1 to} Q ₄ and the resistors R _{1 to} R ₄ and the rise of the ground potential Increase the ground wiring resistance of the constant current source circuit with the smaller ground potential rise so that the ground potentials of the emitter resistances R _1A , R _2A , R _3A , R _4A of the smaller constant current source circuit become the same. Thus, it is possible to prevent the output current ratio of each current output circuit from being shifted.

 This will be described below using specific numerical values.

First, assuming that the ground wiring resistance between the emitter resistors of the respective constant current source transistors is r, the ground potential V _{4 at} the end of the first constant current source circuit (the ground point of the resistor R ₄ ) is the ground point V of the resistor R _{1. On the} basis of ₁ , it can be expressed as follows.

_{V 4 = (I 2A + I} 3A + I 4A) r + (I 3A + I 4A) r + I 4A r = 34I 1A r _{However, I 2A = 2I 1A, I} 3A = 4I 1A, an I _4A = 8I _1A.

Here, assuming that the wiring resistance r = 0.5Ω and I _1A = 1mA, V ₄
= 17mV. Similarly, V ₃ = 13 mV and V ₂ = 7 mV. Further, when the ground point V _4A resistor R _4A based on determining the respective ground point potential of the second constant current source _{circuit, V 3A = (I 1B +} I 2B + I 3B) r = 7I 1B × r = 3.5mV V _2A = V _3A + (I _1B + I _2B ) r = 10I _1B × r = 5mV V _1A = V _2A + I _1B r = 11I _1B × r = 5.5mV (However, I _2B = 2I _1B , I _3B = 4I _1B Do). However, the base potential V _B is the 1 / h _FE than the ground potential, when designing all of the transistors of the potential V _R of the same and to and emitter resistance potential 400 mV, the output current by an increase in the ground potential I ₁ and I ₄ are Becomes Is obtained.

However, in the case of the above-described conventional example, the output current is equal to the configuration of only the first constant current circuit shown in FIG.
I ₁ = I _1A = ₂ mA, I ₂ = I _2A = 4 mA, I ₃ = I _3A = 8 mA, I ₄ = I _4A = 16
by obtaining a ground potential V ₄ as _{mA, V 4 = (I 2} + I 3 + I 4) r + (I 3 + I 4) r + I 4 r = 34I 1 r = 34mA ( _{where, I 2 = 2I 1, I} 3 = 4I ₁ , I ₄ = 8I ₁ ). Also, the ratio of the output currents I ₁ and I ₄ is The current ratio is doubled, and the ratio accuracy is lower than that of 7.88 of the present embodiment described above.

Next, in order to further improve the current ratio accuracy, the ground potentials V _1A , V _2A , and V _3A of the second constant current source circuit are changed to V _1A = V ₄ , V _2A = V ₃ , V _3A
= The Maze ground wiring resistance such that V _2, i.e. resistance
If you choose 2r between R _4A and R _3A , 4r between resistors R _3A and R _2A, and 8r between resistors R _2A and R _1A , V _1A = V ₄ , V _2A = V ₃ , V _3A = V ₂ Because
Each output current I ₁ , I ₂ , I ₃ , I ₄ Therefore Also It can be.

As is clear from the above description, in the present embodiment, by applying the present invention to a constant current source circuit having a weight on the output current, the current ratio accuracy can be improved as compared with the related art. In addition, the output current ratio could not be accurately obtained unless the wiring resistance of the conventional ground wiring was significantly reduced, but according to the present embodiment, a simple configuration in which the wiring resistance of the ground wiring of one constant current source circuit was increased. As a result, the output current ratio can be made more accurate.

〔The invention's effect〕

As described above, in the DA converter of the present invention, the unit current output circuit is composed of two constant current source circuits, which are arranged for m bits, and which supplies the ground potential to one constant current circuit. By supplying the current from the other constant current circuit in a direction opposite to that of the other constant current circuit on the layout arranged for m bits, it is possible to prevent the output current of the current output circuit from shifting due to the rise of the ground potential due to the wiring resistance. effective.

Further, according to the present invention, the output current of the current output circuit does not shift even if the ground potential rises, so that the ground wiring connecting the constant current circuits can be connected by a thinner wiring than before, and the substrate area can be reduced. is there. That is, when the present invention is applied to the current addition type DA converter, there is an effect that a DA converter with higher precision than before can be realized with a smaller area.

The present invention is not limited to the DA converter, but may be applied to a relatively large-scale constant current circuit to prevent the output current from shifting. The same effect can be obtained by applying the invention not only to the NPN transistor but also to a constant current circuit constituted by a PNP transistor.

[Brief description of the drawings]

FIG. 1 is a diagram of a constant current source circuit of a DA converter according to a first embodiment of the present invention, FIG. 2 is a plan view of a layout arrangement and wiring of the constant current source circuit shown in FIG. 1, and FIG. Second embodiment of the present invention
FIG. 4 is a constant current source circuit diagram of a DA converter according to an embodiment, FIG. 4 is a plan view of electrode arrangement of transistors constituting the constant current circuit shown in FIG. 3, and FIG. -A
FIG. 6 is a circuit diagram of a current addition type DA converter showing an example of a conventional converter, and FIG. 7 is a circuit diagram of a constant current source circuit of the DA converter shown in FIG. It is. V _B1 , V _B2 ... common bias voltage (terminal), V _G1 , V _G2 ... ground potential (terminal), Q _{1 to} Q _n ... transistors for the first constant current source circuit, Q _{1A to} Q _nA ... ... second constant current source circuit _{transistor, R 1 ~R n, R 1A} ~R nA ...... resistance, r ...... wiring resistance, I _{1
~I n, I 1A, I 1B} ~I 4A, I 4B ...... output current, I ...... bias circuit block, 2 ...... V _B terminal, 3 ...... V _G terminals, B ₁ ~B _n ...
… Block, _WG1 , _WG2 … Ground wiring, _WB1 , _WB2 … Bias wiring, 4… Board, 5… Base, 6… Collector
7A, 7B …… Emitter.

Claims (1)

(57) [Claims] 1. A DA which comprises a plurality of unit current output circuits each comprising a current switch and a constant current source circuit arranged on a semiconductor substrate, and connecting the plurality of unit current output circuits in parallel to add a current.
In the converter, each of the unit current output circuits has first and second constant current source circuits whose output sides are commonly connected to respective current switches, and a bias is applied to the first and second constant current source circuits. A bias line for supplying a voltage, a first ground line commonly connected to supply a ground potential to each of the first constant current source circuits, and a bias line for supplying a voltage to each of the second constant current source circuits. A second ground wiring commonly connected to supply a ground potential, wherein at least the second ground wiring of the bias wiring and the first and second ground wirings is provided.
1. A DA converter characterized in that the unit current output circuits are arranged and wired such that the power supply direction of the second ground wiring is reversed.