JPH04302221A - Digital/analog converter - Google Patents

Abstract

PURPOSE:To remove the influence of a power supply voltage upon an output by obtaining an output voltage fluctuating the output of a current/voltage converting means only at a prescribed level from a space between the collector of a PNP transistor and a collector resistor. CONSTITUTION:The output current of a current output type D/A converter 12 is converted to a voltage signal by a resistor R1 as the current/voltage converting means and afterwards outputted from an output terminal 14 through a transistor (Tr) Q1. At such a time, power supply noise applied to the output resistor R1 as the current/voltage converting means is made equal to power supply noise applied to a resistor R2 as the emitter resistor of the TrQ1. As the result, the power supply noise is canceled at the TrQ1 and no power supply noise such as the fluctuation of a power supply voltage Vcc is superimposed to the voltage signal obtained at the output terminal 14. Therefore, a satisfactory analog signal having no noise can be obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a D/A converter using a current output type digital/analog converter (hereinafter referred to as a D/A converter).

0002

2. Description of the Related Art Conventionally, a D/A converter using a current output type (current addition type) D/A converter has been developed as a D/A converter for converting digital data into an analog voltage signal. FIG. 2 is a diagram showing the configuration of a D/A converter using this conventional current output type D/A converter. In the figure, 1 indicates a digital data output terminal. The current output type D
/A converter 2. This current output type D/A converter 2 has a constant current source inside, is supplied with a power supply voltage VDD from a power supply terminal 3, and outputs an output current of the constant current source based on digital data supplied from an output terminal 1. Value controlled. That is, the output current value is changed in response to changes in digital data.

Then, the power supply terminal 3 from which the power supply voltage VDD is obtained is connected to one end of the resistor R0, and a current output type D/D
The output current of the A converter 2 is supplied to the other end of this resistor R0. By doing so, current-voltage conversion is performed by the resistor R0, and a voltage signal corresponding to the output current of the D/A converter 2 is obtained at the other end of the resistor R0. Therefore, a voltage value corresponding to the change in digital data is obtained at the other end of the resistor R0, and the voltage signal obtained at the other end of the resistor R0 can be used as the output of the D/A converter.

0004

[Problem to be solved by the invention] However, the resistor R0
A DC component close to the power supply voltage VDD is superimposed on the voltage signal obtained at the other end, which has the disadvantage that the lowest level ("0" level) does not become 0V. In particular, when converting a digital video signal to an analog video signal, it is often desired to set the blanking level of the video signal to 0V, and it is necessary to remove DC components close to the power supply voltage VDD. Therefore, in practice, as shown in FIG. 2, a DC shift circuit 4 is connected to the other end of the resistor R0, and this DC shift circuit 4 shifts the DC level so that the lowest level becomes 0V. The signal thus obtained is supplied to the output terminal 5 of the D/A converter. This DC shift circuit 4
had a relatively complex circuit configuration.

[0005] Furthermore, there is a problem in that power supply noise is mixed into the voltage signal obtained at the other end of the resistor R0. That is, since current-voltage conversion is performed by the resistor R0 to which the power supply voltage VDD is supplied, fluctuations in the power supply voltage VDD appear directly in the converted voltage value, and power supply noise is mixed into the output of the D/A converter. Resulting in.

In view of these points, the present invention provides a current output type D/D device that can eliminate the influence of power supply voltage on the output with a simple circuit.
An object of the present invention is to provide a D/A converter using an A converter.

[0007]

[Means for Solving the Problems] The present invention, as shown in FIG. an output resistor R1 connected to a power supply voltage supply means having a voltage;
A current-voltage converter whose other end is supplied with an output voltage, a PNP transistor Q1 whose base is supplied with the output voltage of the current-voltage converter, and this PNP.
An emitter resistor R2 is connected between the emitter of the PNP transistor Q1 and the power supply voltage supply means, and an emitter resistor R2 is connected between the emitter of the PNP transistor Q1 and the emitter resistor R2 to forcibly supply current to the PNP transistor Q1. current source 15 and PNP transistor Q1
and a collector resistor R3 connected between the collector of the PNP and a ground voltage supply means having a ground voltage.
collector of type transistor Q1 and collector resistor R3
The output voltage is obtained by varying the output voltage of the current-voltage converting means by a predetermined level between the two.

[0008]

[Operation] By doing this, the power supply noise applied to the output resistor R1 as the current-voltage conversion means becomes equal to the power supply noise applied to the emitter resistor R2 of the PNP transistor Q1, and the power supply noise is canceled out by the transistor Q1. Power supply noise is no longer superimposed on the output voltage. Furthermore, although the above-described configuration constitutes an emitter-grounded circuit using the transistor Q1, since the collector resistor R3 is grounded, the DC level is automatically shifted to the lower side, eliminating the DC shift close to the power supply voltage. Can be done.

[0009]

Embodiment An embodiment of the present invention will be described below with reference to FIG.

In FIG. 1, reference numeral 11 indicates a digital data output terminal, and the digital data outputted from this output terminal 11 is supplied to a current output type D/A converter 12. The D/A converter 12 converts the digital data into a current signal using an internal constant current source. In this case, the power supply voltage VDD is supplied from the power supply terminal 13 to the D/A converter 12. In addition, the power supply voltage VDD obtained at the power supply terminal 13
is supplied to one end of a resistor R1 serving as current-voltage conversion means, and a current signal converted and output by the current output type D/A converter 12 is supplied to the other end of the resistor R1. The other end of this resistor R1 is connected to the base of a PNP type transistor Q1.

Furthermore, the power supply voltage V obtained at the power supply terminal 13
DD is supplied to one end of a resistor R2 as an emitter resistance of the transistor Q1, and the other end of this resistor R2 is connected to the emitter of the transistor Q1. Then, the collector of transistor Q1 is grounded via resistor R3 as a collector resistance, and this transistor Q1
The analog signal output terminal 14 is drawn out from the connection point between the collector of the resistor R3 and the resistor R3.

Furthermore, a current signal output from a constant current source 15 is supplied to the midpoint between the emitter of the transistor Q1 and the resistor R2. That is, the power supply terminal 17 from which the power supply voltage VCC is obtained is connected to one end of the resistor R4, the resistor R5, and the capacitor C1, and the other end of the resistor R4 is connected to the PNP terminal.
connected to the emitter of transistor Q2 of type Q2, and connected to the resistor R
5 and the other end of the capacitor C1 are connected to the base of the transistor Q2. Also, bias control terminal 1
6 is applied to the base of transistor Q2 via resistor R6. Then, the current signal obtained at the collector of the transistor Q2 is supplied to the emitter of the transistor Q1 as the output of the constant current source 15.

With this configuration, digital data is converted into an analog voltage signal and output from the analog signal output terminal 14. That is, current output type D/A
After the output current of the converter 12 is converted into a voltage signal by the resistor R1 as current-voltage conversion means, the transistor Q
This voltage signal is outputted from the output terminal 14 via 1. In this case, the power supply noise applied to the output resistor R1 as the current-voltage conversion means is equal to the power supply noise applied to the resistor R2 as the emitter resistance of the PNP transistor Q1, so the power supply noise is canceled out by the transistor Q1. Therefore, power supply noise such as fluctuations in the power supply voltage VCC is not superimposed on the voltage signal obtained at the output terminal 14, and a good noise-free analog signal can be obtained.

Furthermore, in the circuit of this example, the transistor Q1
However, since the resistor R3 serving as the collector resistor is grounded, the DC level is automatically shifted to the lower side, eliminating the DC shift close to the power supply voltage. For example, The lowest level of the output signal can be set to 0V. Therefore, there is no need to connect a DC shift circuit to the D/A converter 12, and the circuit configuration of the D/A converter can be simplified accordingly.

Note that since the bias current is forcibly supplied from the constant current source 15 to the emitter of the transistor Q1, an accident in which the bias current of the transistor Q1 runs out and the transistor Q1 stops operating can be prevented.

[0016]

Effects of the Invention: According to the D/A converter using the current output type D/A converter of the present invention, the influence of power supply noise on the output signal can be eliminated with a simple circuit configuration, and the DC level can be automatically adjusted. It has the advantage of not requiring a DC shift circuit.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing a D/A conversion device according to an embodiment of the present invention.

FIG. 2 is a configuration diagram showing an example of a conventional D/A conversion device.

[Explanation of symbols]

11 Digital data output terminal 12 Current output type digital/analog converter 13
Power supply terminal 14 Analog signal output terminal 15 Constant current source 16 Bias control terminal 17 Power supply terminal

Claims (1)

[Claims] 1. A current output type digital/analog converter, and an output current of the current output type digital/analog converter is supplied to the other end of an output resistor whose one end is connected to power supply voltage supply means having a predetermined voltage. a PNP type transistor whose base is supplied with the output voltage of the current-voltage converter;
an emitter resistor connected between the emitter of the PNP transistor and the power supply voltage supply means; and an emitter resistor connected between the emitter of the PNP transistor and the emitter resistor to force current to the PNP transistor. a collector resistor connected between the collector of the PNP transistor and a ground voltage supply means having a ground voltage; A digital/analog conversion device characterized in that an output voltage is obtained by varying the output voltage of a current-voltage conversion means by a predetermined level.