JPH1117547A - Digital-to-analog converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an accurate output even to the multibit input by preparing a 1st D/A conversion part which flows a constant current to a resistor ladder part of an R-2R system in response to the input bit and a 2nd D/A conversion part which flows plural constant currents after the conversion of bit number by defining the MSB side of the resistor ladder part as the load and obtaining the analog output from one of both ends of the load of the MSB side. SOLUTION: A 1st D/A conversion circuit 2 of an ordinary R-2R system consists of a resistor ladder 3 including the LSB through 6 bits, the bit switches 4A to 4F and the constant current sources 5A to 5F which flow a constant current I respectively. A 2nd D/A conversion circuit 6 consists of the bit switches 7A to 7C, the constant current sources 8A to 8C which flow a constant current 2I respectively and a bit converter 20. The converter 20 converts the input of higher two bits into the value of three bits, controls the switches 7A to 7C and adds together the currents 2I in sequence. As the numbers of bits in charge of both circuits 2 and 6 can be freely changed, the high conversion accuracy is secured.

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 (D / A) converter, and more particularly to a digital-to-analog (D / A) converter suitable for use in an IC that can obtain an accurate analog output even for a digital signal input having a large number of bits.
/ A converter.

[0002]

2. Description of the Related Art As a D / A converter utilizing a constant current source, a D / A converter as shown in FIG. 2 constituted by an R-2R ladder circuit can be considered. A digital signal is applied to the input terminals (1) to (8) of FIG. 2, and an analog signal is generated at the output terminal (9). The input terminal (1) becomes the most LSB,
The input terminal (8) is the most significant bit.

When a digital signal (0, 0, 0... 0) is applied to input terminals (1) to (8), a constant current source (1) is applied.
0) to (17) are turned off, and no current flows through each resistor. Therefore, a reference voltage Vref which is the maximum voltage is generated at the output terminal (9). Next, when the digital signal “1” is applied to the input terminal (8) and the digital signal “0” is applied to all other input terminals, current flows only to the constant current source (17). Then, the output voltage becomes Vref−R · I / 2.

Where I is a constant current source (10) to (17)
And R is a reference resistance value. Further, when a current flows only through the constant current source (16), the output voltage becomes Vre.
f−R · I / 4. FIG. 3 shows a D / A converter configured by connecting weighted resistors in series, which also uses a constant current source. Each resistance is weighted by (1/2) n (n is a natural number of 2 or more). FIG.
Is different from FIG. 2 only in the configuration of the resistor, and the other configurations and operations are the same as those in FIG.

According to the D / A converter shown in FIGS. 2 and 3, 8
256 types of analog signals corresponding to the bit digital signal are obtained.

[0006]

However, the D / A converter shown in FIGS. 2 and 3 generally has an input digital signal of 7 bits.
If the number of bits is greater than or equal to the bit, the analog output potential may become abnormal due to the variation of the constituent elements (for example, the resistance value of the IC, the current value of the constant current source). When the variation becomes severe, the potential of the analog output potential, which should increase in accordance with the input bit, conversely decreases, and the value does not change even if the input bit signal is changed. This becomes more significant as the number of digital bits increases.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and provides a resistor ladder section of an R-2R system and a constant current I to the resistor ladder circuit in accordance with a digital signal of a plurality of bits. A first D / A conversion circuit including a first switch unit for performing the operation, and a second switch unit that supplies a plurality of constant currents 2I in response to a plurality of bits of a digital signal using the most MSB side of the resistor ladder unit as a load. A 2D / A conversion circuit, wherein an analog output is obtained from one end of the load on the most MSB side of the resistor ladder section.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a D / A converter according to the present invention. (1) is a power supply, (2) is an R-2R type resistor ladder section (3) and a multi-bit digital ladder section. A first switch unit (4) including a plurality of switches (4A) to (4F) that are turned on and off according to signals, and the resistance ladder according to each switch (4A) to (4F) of the first switch unit (4). A first D / A conversion circuit including a plurality of constant current sources (5A) to (5F) for flowing a constant current I to the section (3); and (6) a plurality of switches (ON / OFF) according to a plurality of bits of a digital signal. 7A) to (7C) and the respective switches (7A) of the second switch unit (7)
To (7C), the maximum M of the resistance ladder portion (3)
A second D / A conversion circuit including a plurality of constant current sources (8A) to (8C) for flowing a constant current 2I to the SB side, and (9) is an analog signal output terminal.

[0009] The digital signal is applied in 8 bits.
Of the 8 bits, the 6 bits on the LSB side are simply the first D / A
It is applied to the switches (4A) to (4F) of the conversion circuit (2). The remaining 2 bits are converted to 3-bit values, and the switches (7A) to (7A) of the second D / A conversion circuit (6) are converted.
C). Most MS of the first D / A conversion circuit (2)
The left side (LSB) from the connection point A between the resistor R on the B side and the resistor 2R.
2 has the same circuit configuration as the output terminal (9) shown in FIG. 2 on the left side, and operates similarly.

That is, it is assumed that only the switch (4F) is closed and all the remaining switches are opened. Then, the output voltage becomes Vref-RI. Further, when a current is passed only to the constant current source (5E), the output voltage becomes Vref-RI / 2. Therefore, D / A conversion of 6 bits can be performed by the R-2R method by opening and closing the switches (4A) to (4F).

Next, the upper two bits of the D / A converter of FIG. 1 will be described. The upper two bits are subjected to current addition type D / A conversion in order to reduce errors. Each of the constant current sources (8A) to (8C) passes a current 2I.
The impedance when the point A is viewed from the switch (7A) is R. Therefore, assuming that the switch (7A) is closed and all other switches are opened, the output voltage becomes
Vref-2R · I, which is 1 compared to the switch (4F) side
It can be seen that the bit is on the MAB side.

The constant current sources (8A) to (8C) are of a type in which a current 2I flows and the currents are sequentially added. Therefore, errors in the resistance value and the current value can be reduced. In the case of an IC circuit, if the resistance and the current have the same value as much as possible, errors between the resistance and the current can be reduced. Therefore, the overall error is reduced. In FIG. 1, since equal current sources are used, a 3-bit switch and a current source are required to create a state corresponding to 2 bits.
Therefore, a bit number converter (20) is provided to generate a 3-bit output signal according to a 2-bit input signal. This is shown in FIG. The left two bits in FIG. 4 indicate an input signal, and the central three bits indicate an output signal. "0" of the output signal indicates that the switch is open, and "1" of the output signal indicates that the switch is closed. Assuming that the switches (4A) to (4F) are all open, the output voltage at that time becomes the “output voltage” shown in FIG. That is,
When the output bit is "100" in FIG. 4, only the switch (7A) is closed among the switches (7A) to (7C). Therefore, the output voltage becomes Vref−2R · I. FIG. 4 shows a case where 2 bits are converted into 3 bits, but other methods are also possible. For example, the D / A converter may be configured by using the upper 3 bits of the 8 bits as a current addition type. In this case, a switch and a current source for 7 bits are required. This general formula is expressed by 2 to the power of n minus 1 when n bits are required.

In the description of FIG. 1, the second D / A conversion circuit (6)
Has been described on the MSB side, but this is
It may be provided on the SB side. In the case of the lower 2 bits, the current value is changed to 2I and set to I / 4. In the case of the lower three bits, the current value is set to I / 8. That is, D /
To perform A conversion, I / (2 n). Even in the case where it is provided on the LSB side, an accurate analog output signal can be obtained even if the number of bits is high (7 or more) as in the case of FIG.

[0014]

As described above, according to the present invention, it is possible to provide a D / A converter suitable for integration into an IC that can obtain an accurate analog output even for a digital signal input having a large number of bits. In particular, according to the present invention, the number of bits assigned to the first D / A conversion circuit and the second D / A conversion circuit can be freely selected. Is obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a D / A converter according to the present invention.

FIG. 2 is a circuit diagram showing a conventional D / A converter.

FIG. 3 is a circuit diagram showing a conventional D / A converter.

FIG. 4 is a correspondence diagram of an input signal and an output signal used for explaining FIG. 1;

[Explanation of symbols]

 (2) First D / A conversion circuit (6) Second D / A conversion circuit (9) Output terminal

Claims (3)

[Claims] 1. A first D / A including an R-2R type resistor ladder unit and a first switch unit that supplies a constant current I to each resistor of the resistor ladder unit according to a digital signal of a plurality of bits.
And a second D / A conversion circuit including a second switch unit that supplies a plurality of constant currents 2I in accordance with a digital signal of a plurality of bits by using a most significant bit of the resistance ladder unit as a load. A D / A converter characterized in that an analog output is obtained from one end of a load on the most MSB side of the section. 2. A first switch unit comprising an R-2R type resistor ladder unit and a plurality of switches which are turned on / off in response to a digital signal of a plurality of bits, and the resistor ladder unit according to each switch of the first switch unit. A first D / A conversion circuit including a plurality of constant current sources for supplying a constant current I; a second switch unit including a plurality of switches that are turned on and off in accordance with a digital signal of a plurality of bits; and each switch of the second switch unit. A second D / A conversion circuit including a plurality of constant current sources for supplying a constant current 2I to the most MSB side of the resistance ladder section, and an analog output from one end of the most MSB side load of the resistance ladder section. A D / A converter characterized by: 3. A first D / A conversion circuit including an R-2R type resistor ladder unit and a first switch unit that supplies a constant current I to the resistor ladder circuit in accordance with a digital signal of a plurality of bits; A plurality of constant currents: I / (2 n
And a second D / A conversion circuit including a second switch unit for supplying the power to the power supply (where n is the number of bits output by the second D / A conversion circuit). A D / A converter characterized in that an analog output is obtained from a digital signal.