JPS61292422A - Digital-analog converter - Google Patents

Abstract

PURPOSE:To obtain a converter with high accuracy by using a signal with an opposite logic to that of a switching control signal of a switching circuit so as to control a current source being the same kind of a current source of a conventional circuit by means of another switching circuit provided in pairs with the said switching control signal circuit thereby blocking the effect of a current and switching circuit onto output noise. CONSTITUTION:Figure shows a 2-bit switching circuit. Transistors (TRs) Q1 and Q2 among NPN TRs Q1-Q4 and the TRs Q1-Q4 constitute the other pair. A signal 1 is a signal turning on/off the TR Q1 and a signal 2 is a signal having an inverted logic to the signal 1. Signals 3, 4 have the similar relation with the signals 1, 2. The pair switch circuits are constituted in this way so as to decrease the current fluctuation from the voltage source, that is, to decrease the fluctuation of the voltage source.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a digital analog (hereinafter abbreviated as D/A) that outputs an analog voltage corresponding to an input digital signal.
It concerns a converter.

(Prior Art) In recent years, graphic display terminals using raster, scan, and CRT display monitors have become popular for CAD/
It is used in fields such as CAM. In particular, color graphic display terminals are used to display shading figures, etc., which make the display screen resemble the real thing, due to the richness of its expressive colors. A color graphic screen displays graphics by D/A converting a multi-bit video signal obtained from a plurality of frame memories in a terminal and applying the analog video signal to a CRT display monitor.

A conventional transducer will be explained with reference to FIG. In the figure, numerals 11, 12, 13, . . . are current circuits connected in parallel, and both ends of which are connected to the input terminals of the summing amplifier. °I, IIi is a current source that flows a current of size ■ into the current circuit 11, 112 is a current source that flows a current of size I/2 into the current circuit 12, and 15 is a current source that flows through the current circuit 13.
This is a current source that causes a current of magnitude IA to flow through. Although not shown, in the case of an n-bit D/A converter, there is a 5, n-th current circuit n, and a size/2n.
-1 current source I is provided. 811
t 812. #S, 3, . . . are switching circuits that respectively turn on and off each current circuit. AMP is a summing amplifier that adds the currents flowing through each current circuit, R2 is a feedback resistor of the amplifier, and Eout is an analog output of the summing amplifier AMP, that is, the side converter.

In this way, the current flowing through each current circuit 11, 12, 13... is weighted with I t' I/2, I/4-..., so each switching circuit S111 S1□,
By selectively turning on S13..., the digital signal determined by the combination is converted into an analog signal and can be taken out from the summing amplifier AMP.

(Problems to be Solved by the Invention) In the D/A converter having the above configuration, the switching circuit and the current source are directly connected, and each current source is connected at point A. As a result, the current flowing through point A changes depending on the on/off state of the switching circuits S11 # S12 j S13 . . . , so that there is a drawback that noise appears in the analog output.

The purpose of the present invention is to eliminate the conventional drawbacks, focus on the fact that the output noise in one converter is in the current circuit and the switching circuit, and prevent the influence of the current and switching circuit on the output noise to achieve high accuracy. An object of the present invention is to provide a D/A converter.

(Means for Solving Problems) One converter of the present invention has current sources each having a load corresponding to each bit of a digital signal connected in parallel via a switching circuit whose opening/closing is controlled by the digital signal of each bit. In a D/A converter that outputs an analog voltage corresponding to an input digital signal, a current source of the same type as the current source is connected to the opening/closing control signal using a logic Ω signal that is opposite to the opening/closing control signal of the switching circuit. This is controlled by a separate switching circuit provided in pair with the control circuit.

Further, the current source may be a resistor, or the current source may be a resistor, each resistor may be connected with another resistor, and the terminal may be the resistor.

(Function) In the present invention, a current circuit and a switching circuit are provided in pairs, and when one switching circuit of the pair is on,
The above problem is solved by turning off the other switching circuit and eliminating the change in current at the point where the current circuits are commonly connected.

(Example) An example of the present invention will be described based on FIGS. 1 and 2.

FIG. 1 is a circuit diagram of a D/A converter according to the present invention, showing a switching circuit for 2 bits.

In the figure, Q to Q4 are NPN transistors, of which transistor Q and transistor Q2 form a pair for the first bit. This corresponds to the switching circuit S11 shown in FIG. Further, transistor Q3 and transistor Q4 constitute one pair. This corresponds to S,2 shown in FIG.

1 in FIG. 1 is a signal that turns on and off the transistor Q1, and signal 2 is a signal whose logic is opposite to 1. , signals 3 and 4 have the same relationship as signals 1 and 2.

The operation will be explained using the above configuration.

When signal 1 turns on transistor Q1, signal 2 turns off transistor Q2, signal 3 turns off transistor Q3, and signal 4 turns on transistor Q4, the voltage at point B, V, is also the current flowing at point A. (where R is the resistance value of resistor R) Next, when signal 1 turns on transistor Q, t- and signal 3 turns on transistor Q3, the voltage at point B, V, also flows through point A. The current I is summarized in the table below.

Here, ■' is the current at point A when transistors Q2*Q4 of FIG. 1 are not used, rather than the pair structure of the present invention.

FIG. 2 is a circuit diagram of a D/A converter according to a second embodiment of the present invention.

In the figure, switching circuits S, , S2.
I have S3. Switch-7 circuits S and S are switch γ
When the tink°° circuit S is on, the switching circuit S
, is a switch that has the opposite logic so that it is turned off.

With this configuration, each current source 11. I2. Since the current at point A, where I, -, etc. are connected, is constant at 7/4 regardless of the state of the switch, a stable /A converter can be constructed.

Furthermore, in a ladder type/A conversion circuit in which resistor 2R and resistor R are assembled in a ladder shape, a ladder circuit that is paired with the ladder circuit is provided, and the terminal end of the ladder resistor and the paired resistor are connected. By doing so, the current at the terminal point becomes constant, and a more stable /A converter can be constructed.

(Effects of the Invention) According to the present invention, by configuring a pair of switch circuits,
Fluctuations in the current from the voltage source can be reduced, thereby making it possible to suppress fluctuations in the voltage source. Therefore, there is an effect that fluctuations in the output signal of the D/A converter can be kept small.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a D/A converter according to the first embodiment of the present invention, FIG. 2 is a circuit diagram of a VA converter according to the second embodiment, and FIG. 3 is a conventional D/A converter. FIG. 1.2, 3.4...Signal, 11.12.13...Current circuit, AMP-...Summing amplifier, E...Voltage source, E
out...Analog output, I, 1. I, I...
Current source, Ql p Q2 s Q3 h Q4...Transistor, R, 2R...Resistor, s, s,
s, s, s, s2. s3゜S14. S, S
...Switching circuit. Figure 1 Figure 2 Figure 3

Claims (3)

[Claims] (1) Each current source having a load corresponding to each bit of the digital signal is connected in parallel via a switching circuit whose opening/closing is controlled by the digital signal of each bit, and outputs an analog voltage corresponding to the input digital signal. In the digital-to-analog converter, a current source of the same type as the current source is controlled by a second switching circuit provided in a pair with the switching circuit, using a signal having a logic opposite to the opening/closing control signal of the switching circuit. A digital-to-analog converter featuring: (2) When the unit resistance value is R, the current source is a resistor whose resistance value is R, 2R, 4R, etc. analog converter. (3) When the unit resistance value is R, the current source is a resistor with a resistance value of 2R, 2R, 2R, etc., each resistor is connected with a resistor with a resistance value of R, and the termination is The digital-to-analog converter according to claim 1, characterized in that the resistor has a resistance value of 2R.