JPH0732364B2 - Digital-to-analog converter - Google Patents

Description

DETAILED DESCRIPTION [Overview] By combining a resistor string type D / A converter and a weighted resistor, a highly accurate D / A converter can be realized in a small area.

[Industrial application field]

The present invention relates to a digital-to-analog converter in which the number of elements used is reduced and an output analog amount is accurately proportional to an input digital value.

[Conventional technology]

As a basic configuration of the digital / analog (D / A) converter, there are a resistor string type and an R-2R type. An example of the former is shown in FIG. 2 (a) and an example of the latter is shown in FIG. 2 (b).

In the resistor string type shown in FIG. 2 (a), n resistors R (n is, for example, 16) having the same value are connected in series to the reference power source Vr.
It is connected between (-) and Vr (+), the power supply voltage is V, and this is divided into n equal parts, and each one is obtained from the series connection point of the resistors and input to the analog selector AS. Input the control bit (input digital value) CB to the analog selector AS,
One of the divided voltages is selected by CB and output through the buffer BUF. When n = 16, CB is 4 bits, and there are 16 types of 0000, 0001, 0010, ... 1111, 0, V / 16, 2V / 16,
…… Outputs 15V / 16.

In the R-2R type of FIG. 2 (b), the resistors having resistance values R and 2R (shown by the same reference numerals R and 2R) are connected together with the switches S ₁ , S ₂ , ... Sn as shown in the figure. . The switches S ₁ , S ₂ , ... Are closed to the 1 side or 0 side by the control bit CB. This 1,0 is CB
1 and 0, S ₁ is LSB side, Sn is MSB side switch. If n = 4, the switch is up to S ₄ , and if all of them are closed to 0 side, output (input of buffer BUF)
Is 0, only S ₁ is 1 side, and if S _{2 to} S ₄ are closed to 0 side, the output is V / 1
Only 6 and S ₂ are on 1 side, and if S ₁ , S ₃ , and S ₄ are closed on 1 side, the output is 2V / 16
Then, the following applies to this.

[Problems to be solved by the invention]

The resistance string type is monotonic, but the larger the number of bits, the larger the number of resistors and the larger the required area. On the other hand, the R-2R type has a small number of resistors, but it may not be monotonic. That is, the switches S ₁ , S ₂ , ... Sn are specifically transistors, but they need to be miniaturized in order to increase the degree of integration, and the small transistors have a large on-resistance. When the on-resistance of the switch becomes large, the on-resistance is added to the resistor 2R, so that it is not an R-2R ladder circuit, and the control bit CB is set to, for example, 0000,0001,001.
When the output analog voltage is changed to 0, ...
It does not show a monotonic increase as V / 16, 2V / 16, ..., and may even decrease temporarily on the way.

The analog selector AS is also specifically a transistor switch, which is provided with as many transistors as the number of taps of the resistance voltage divider circuit, and one of the transistors is turned on by the output of the decoder having CB as an input, or the switch is in a tree shape. The output voltage of the upper stage is more than the output voltage of the lower stage in the case where multiple switches are connected in series and each connection point is an output tap. It cannot be lowered, and even with the output voltage through the analog selector AS, the upper output voltage is unlikely to be lower than the lower output voltage. Therefore, the resistor string type has monotonicity, but the large number of resistors is a problem. For example, n = 8
If so, the number of resistors will be 2 ⁸ = 256, and the number of switches will increase accordingly.

The present invention is intended to provide a D / A conversion circuit which is monotonic and does not require so many switches.

The configuration of the present invention by which receives the upper n bits of the input digital value (D ₄ ~D _7), a digital-analog converter of the resistor string type that outputs an analog voltage corresponding steps thereto, said digital-to-analog converter The first variable resistance circuit and the second variable resistance circuit respectively inserted between the positive power source side and the positive power source (Vr (+)) and between the negative power source side and the negative power source (Vr (-)) of the resistor string And a weighting circuit that outputs a potential obtained by subdividing the voltage for one step of the conversion unit by 1 / m according to the lower m bits of the input digital value. Of the variable resistance circuit and the second variable resistance circuit, each of the m resistances (R / 2, R / 4, ...) Having different weights by one-half and each of the m resistances being the lower order. to the value of the corresponding bit of m bits A switch group consisting of m number of changeover switches cascaded in m stages for selecting and connecting to the series circuit or disconnecting from the connection of the series circuit. Via a switching transistor that connects the previous stage and the next stage via a switching transistor or a switching transistor that directly connects the previous stage and the next stage without using the resistor to perform switching selection, and a common bit The change-over switch of the first variable resistance circuit and the change-over switch of the second variable resistance circuit controlled by
The sum of the resistances of the changeover switches interposed between the input end and the output end of the first variable resistance circuit and the sum of the resistance values of the changeover switches interposed between the input end and the output end of the second variable resistance circuit are: It is characterized in that it is constant regardless of the changeover state of the changeover switch.

[Action]

As shown in FIG. 1, the present invention uses a resistor string type DAC and a weighting circuit. Resistance string is (n-1)
It consists of a series of resistors R, an analog selector AS for inputting the potential of each connection point and selecting one of them by the control bit CB, and a buffer BUF. This is the same type as in FIG. 2 (a). It is a thing. The weighting circuit consists of resistors R / 2, R / 4, R / 8, ... and their switches S _{11 to} S ₁₄ , S _{21 to} S.
₂₄ , which are provided in two groups as shown, one between the negative power supply Vr (-) and the lower end of the resistor string and the other between the positive power supply Vr (+) and the upper end of the resistor string. It is provided.

The sum of the upper weighted resistance group is equal to one resistance R of the resistance string. That is, the upper weighting resistance group is R / 2 + R / 4 + R / 8 + R / 16 + R / 16 = R. The lower resistance group is R / 2, R / 4, R / 8, R / 16, and the sum total is 15R / 16. The lower resistance group is inserted and removed in the opposite manner to the upper resistance group. Ie switch
When S ₁₁ is closed to the 0 side (except R / 16), switch S ₂₁ is closed to the 1 side (the side where R / 16 is inserted), and R
The same applies to / 8, R / 4, .... In this way, the sum of the weighting resistor groups is always R, since the same one is inserted in the other except the one in the other.

In this circuit, 2n steps can be obtained with the resistor string (n-bit D / A conversion is possible), and each step is 2m steps with the weighting resistor group (m is a weighting resistor R / 2, R / 4, ...
..), so that steps of 2m ⁺ n can be obtained as a whole, and m / n-bit D / A conversion can be performed.

〔Example〕

An embodiment of the 8-bit D / A converter is shown in FIG. The analog selector AS uses a switch tree circuit.
The buffer BUF connects the operational amplifier + input terminal to the output terminal of the analog selector AS, and connects the output terminal to the − input terminal.
A voltage follower with a gain of 1 is obtained by performing 0% negative feedback. The capacitor C at the input end is for stabilizing the input voltage when switching the switch.

The resistor string is formed by connecting 15 resistors having a resistance value R, and the weighting resistor group is R / 2, R / 4, R / 8, R / 16, R / 16. These weighting resistor insertion / removal switches are switched by the lower 4 bits D0, D1, D2, D3 of the 8-bit digital input, and the resistor string is switched by the upper 4 bits D4 to D7.

Ignoring the on resistance of the switch and the input current of the buffer BUF, the total resistance between the reference power supplies Vr (+) and Vr (-) is 16R constant no matter how the switch is switched.
The potential at the lower point A of the resistor string is (Here, V = Vr (+) − Vr (−), D is the bit data 1,0), D _{0 to} D ₃ is 0000, _0, 0001 is V / 25
6 and 0010 will be 2V / 256, …… 1111 will be 15V / 256. The resistance string changes by V / 16 in one step, and the output voltage is changed by the weighting resistance group and the resistance string. become. That is, an 8-bit D / A conversion output can be obtained. An example is shown in the following table. In this case, Vr (+) = 3.78V, Vr (−) =
1.22V, and the analog output voltage V ₀ is It is represented by. 0.01 in one step in weighting circuit
V, 1 step in the resistor string part is 0.16V.

In an 8-bit D / A converter, 256 resistors are required if it is realized by only a resistor string, but in the present invention, 15 + 9 = 2
Only 4 pieces are needed, and the area can be reduced. Also, monotonicity can be obtained.

That is, the monotonicity of the resistor string portion of this circuit is the same as that of FIG. 2 (a), and the weighting resistor portion only has to insert and remove the resistors in series. However, after inserting the resistance, the resistance is certainly higher than before insertion, and there is no potential reversal. In particular, when the changeover switches S ₁₁ , S ₁₂ , ... are used as shown in the figure, the ON resistance of the switch is set to R ₀ , and for example, for R / 16, R ₀ before insertion,
After insertion, R / 16 + R ₀ , and R / 16 is securely inserted and removed. The same applies to other resistors.

The weighting resistor group is for subdividing one step of the resistor string into a plurality of stages, and the subdivision number may be any of 2, 4, 8, ....

〔The invention's effect〕

As described above, according to the present invention, it is possible to provide an A / D converter that can perform multi-bit D / A conversion with a small number of resistors and switches and that does not lose monotonicity.

[Brief description of drawings]

FIG. 1 is an explanatory view of the principle of the present invention, FIG. 2 is an explanatory view of a conventional example, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. In FIG. 1, R, R, ... And AS are resistor string type D / A converters, resistors R / 2, R / 4, ... And switches S _{11 to} S ₁₄ , S _{21 to} S _24.
Is a weighting circuit.

Continued Front Page (72) Toru Mizutani Toru Mizutani, 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa, Fujitsu Limited (56) References JP 58-133031 (JP, A) JP 60-191522 (JP, A) JP-A-59-44125 (JP, A) JP-A-56-132815 (JP, A)

Claims (1)

[Claims] 1. Upper n bits (D _{4 to} D ₇ ) of an input digital value
A resistor string type digital-analog converter for receiving an analog voltage at a step corresponding thereto, and a positive power source (Vr (+)) and a negative power source of the resistor string of the digital-analog converter. Side and a negative power source (Vr (-)), which are respectively inserted between a first variable resistance circuit and a second variable resistance circuit, and one step of the conversion unit according to the lower m bits of the input digital value. A weighting circuit that outputs a potential obtained by subdividing the voltage of 1 minute into 1 / m, and the first variable resistance circuit and the second variable resistance circuit of the weighting circuit each have a different weight by 1/2. Resistors (R / 2, R / 4, ...) And each of the m resistors are selected according to the value of the corresponding bit of the lower m bits to connect to a series circuit or a series circuit. Disconnect from the connection And a switch group consisting of m changeover switches that are cascaded in m stages, and the changeover switch of each stage is a switching transistor that connects the previous stage and the next stage via the resistor, or the previous stage and the next stage. The changeover switch and the second variable resistance of the first variable resistance circuit, which are configured to operate and select one of the switching transistors directly connected to the wind through the resistor, and which are controlled by a common bit. The changeover switch of the circuit performs the changeover operation complementarily,
The sum of the resistances of the changeover switches interposed between the input end and the output end of the first variable resistance circuit and the sum of the resistance values of the changeover switches interposed between the input end and the output end of the second variable resistance circuit are: A digital-to-analog converter, which is constant regardless of the changeover state of a changeover switch.