JPH06314977A - Current output type d/a converter circuit - Google Patents

Abstract

PURPOSE:To set the variable range of the output current optional by connecting a reference current source to a 1st current mirror circuit, using an output of a 1st amplifier for a reference potential application point and giving 1st to n-th outputs of a 2nd current mirror circuit to n-sets of weighting terminals of an R-2R resistor ladder circuit. CONSTITUTION:A current of a reference current source 1 is given to a 1st current mirror circuit comprising transistors(TRs) 2-3 and resistors 5-7, its 1st output is inputted to a current-voltage circuit comprising a resistor 9 and an amplifier 8, in which the output is converted into a voltage and it is used for a reference voltage for an R-2R ladder circuit comprising resistors 24-29. A 2nd output of the 1st current mirror circuit is given to a 2nd current mirror circuit having n-sets of outputs comprising TRs 10-14 and resistors 15-19 and n-sets of outputs are given to n-sets of control current terminals to the R-2R resistor ladder circuit via switches 20-23 to decide an output voltage of the R-2R resistor ladder circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital / analog conversion circuit, and more particularly to a current output type digital / analog conversion circuit.

Generally, there is a circuit that controls an output current by a digital input signal based on a reference current input as a current output type digital / analog conversion circuit.

An example of a conventional current output type digital / analog conversion circuit is shown in FIG.

In the figure, transistors 36 and 37 of the same shape, resistors 38 and 39 of the same resistance value are formed, and one terminal of the resistors 38 and 39 is a first current mirror circuit connected to the power supply terminal. The input of the current mirror circuit is connected to the reference current source 1, and the output is connected to the input of the second current mirror circuit.

The second current mirror circuit is grounded through a transistor 40 and a resistor 45 which are inputs to the current mirror circuit, and a first output is the same transistor 41 as the transistor 40 and a resistor 46 having the same resistance value as the resistor 45. And the same current as the reference current source configured by the circuit grounded by the switch 20 controlled by the first bit of the digital input signal, and the second output of the transistor 42 and the resistor 45 having a double area of the transistor 40 and the resistor 45. It is composed of a circuit grounded by a resistor 47 having a value of ½ and a switch 21 controlled by the second bit of the digital input signal, and outputs a current twice as high as that of a reference current source. The third output is a transistor 40. A transistor 43 having a quadruple area and a resistor 4 having a resistance value ¼ that of the resistor 45
8 and a switch 22 which is controlled by the third bit of the digital input signal, which is grounded to output a current four times as high as that of the reference current source.
2 of transistor 44 and resistor 45 with ^n-1 times the area
It is composed of a circuit which is grounded by a resistor 49 having a resistance value of ¹ / ^n-1 and a switch 23 which is controlled by the n-th bit of a digital input signal, and which outputs a current of 2 ^n-1 times that of a reference current source. However, it has an output terminal for connecting all of the first output to the nth output of the second current mirror circuit.

Next, the operation of the conventional example will be described.

When a digital input signal composed of n bits is input to each switch and only the signal of the first bit becomes H, the switch 20 is turned on and the same current as the reference current source flows to the output terminal. When only the signal of the second bit becomes H, the switch 21 is turned on and the output terminal has the reference current source 2
A double current flows, and if the first bit and the second bit are turned on at the same time, a current three times that of the reference current source flows to the output terminal. Similarly, if all of the n bits are turned on, a current of 2 ⁿ -1 times flows in the output terminal, which is a digital / analog conversion circuit.

Therefore, the current flowing through the output terminal is represented by I _OUT
And the current value of the reference current source is Iref, it can be expressed by the following equation.

I _OUT = I _ref (Z ₁ + 2Z ₂ + 4Z ₃ +
... + 2 ^n-1 Z _n ) Z _n is substituted with 1 when the n-th bit is ON and 0 when it is OFF.

[0010]

In this conventional current output type digital / analog conversion circuit, only the current value of an integral multiple can be controlled based on the current value of the reference current source, and the variable range cannot be arbitrarily selected. Further, there is a problem that the variable current value is limited by the current value of the reference current source.

Further, in order to obtain the linearity of the output current, the area ratio of the transistors forming the current mirror circuit is important, and when the number of bits of the digital input signal increases, the transistors forming the current mirror circuit exponentially increase. Further, in order to change the minute range, it is necessary to reduce the current value of the reference current source, which causes a problem that the resistance value forming the current mirror circuit becomes large.

[0012] Next, the actual value will be described in detail.
Output current value is set to 1 by digital input signal of 4 bits
Consider a current input-current output type digital / analog conversion circuit that is variable between 5 μA and 30 μA.

Since the 4-bit digital input signal is set in 16 steps, if the variable current value is divided by the variable step, a change of 1 μA is required per step, and this is taken as the current value of the reference current source. However, if this is left, 0 μA
It is necessary to connect a current source of 15μA to the output terminal in addition to the reference current source because it is only possible to change from 15μA to 15μA. By using this second current source,
It is possible to change from 1 to 30 μA.

Considering the resistance value of the current mirror circuit, when used inside the IC, it is necessary to set the voltage generated at both ends of the resistor to about 0.3 V in consideration of variations in transistors and resistors. If the current value of the reference current source is 1 μA, the first current mirror circuit requires two 300 KΩ resistors.

In addition, the second current mirror circuit also has 300
A large number of resistors having high resistance values, such as two KΩ resistors and one 150 KΩ resistor, are required, and it is impossible to use them in ICs.

[0016]

In the current output type digital-analog converter circuit of the present invention, a reference current source is connected to a collector of a first transistor which is an input of a first current mirror circuit having two outputs. , The emitters of the three transistors forming the first current mirror circuit are connected to a power supply terminal via a resistor, and the first output is a current-voltage composed of a first amplifier and a first resistor. The first resistor is connected to the input of the conversion circuit and is grounded to generate a voltage obtained by multiplying the reference current and the resistance value. The first resistor serves as an equipotential terminal of the R-2R resistance ladder circuit through the first amplifier having a gain of 1. Is entered.

The second output of the first current mirror circuit is connected to the collector of the second transistor which is the input of the second current mirror circuit having n outputs, and the second output of the second current mirror circuit is connected. The emitter of the second transistor forming the input is grounded via the second resistor, and the third transistor forming the first output is controlled by the third resistor and the first bit of the digital input signal. First
The fourth transistor which constitutes the second output is grounded by the fourth switch and the second switch which is controlled by the second bit of the digital input signal.
Is connected to the fifth resistor and the n-th switch controlled by the n-th bit of the digital input signal to ground and the first transistor of the second current mirror circuit.
From the output to the nth output are input to the n weighting terminals of the R-2R resistance ladder circuit.

The output of the R-2R resistance ladder circuit is the second
Connected to the non-inverting input of an amplifier of
The output is input to the base of the sixth transistor,
The emitter of the transistor is connected to the inverting input of the second amplifier and is grounded via a sixth resistor,
A current conversion circuit is provided, and the collector of the transistor is provided with a voltage-current conversion circuit connected to the output terminal.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

The current of the reference current source 1 is supplied to the transistors 2 and 2.
3 and 4 and resistors 5, 6 and 7 are connected to the input of a first current mirror circuit having two outputs, and a resistor 9 and an amplifier 8 are connected from the first output of the first current mirror circuit. The voltage is input to the current-voltage circuit to be converted and converted into a voltage, which is used as the reference voltage of the R-2R resistance ladder circuit composed of the resistors 24 to 29.

[0022] The current value of the reference current source 1 and I _ref, current - becomes the voltage of the output of the voltage circuit to V _ref and V _ref = R9 × I _ref.

Next, the second output of the first current mirror circuit is connected to the input of a second current mirror circuit having n outputs composed of transistors 10 to 14 and resistors 15 to 19. , The n outputs are input to the n control current terminals to the R-2R resistance ladder circuit from the switch 20 controlled by the digital input signal by the switch 23 to determine the output voltage of the R-2R resistance ladder circuit. .

The reference resistance value of the R-2R resistance ladder circuit is R
When the output voltage V _R-2R of R-2R resistor ladder circuit is _{V R-2R = V ref -R} × I ref × (Z n + ... + Z 2/2
_{^{n-1 + Z 1/2}} n) Z n substitutes bit time ON 1, OFF time 0 in the n. Becomes

The output of the R-2R resistance ladder circuit is the amplifier 3
0, connected to the input of the voltage-current circuit composed of the transistor 31 and the resistor 32, and the collector of the transistor 31 is connected to the output terminal to form a current output type digital / analog conversion circuit.

The output current I _out of this circuit is obtained by the following equation.

I _OUT = V _R-2R / R32 = (R9 / R32) I _ref- (R / R32) I _ref (Z
_{n + ... + Z 2/2} n-1 + Z 1/2 n) described in the conventional example, the four-bit digital input signal, the current output type digital / analog variable between the output current value from 15μA to 30μA Consider a conversion circuit.

First, the maximum current value 30 μA of the output current is obtained by the following equation.

I _out = (R9 / R32) I _{ref If} the current value of the reference current source is 60 μA and the output voltage of the current-voltage conversion circuit is 3.6 V, then R9 = 60 KΩ and R32 = 120 KΩ.

Next, the minimum current value 15 μA of the output current is obtained by the following equation.

I _out = (R9 / R32) I _ref − (R /
R32) I _ref (8/15) R = 16 KΩ. Also, the resistance that constitutes the current mirror circuit is 5K.
Ω, which is a value that can be used in an IC without any problem.

[0032]

As described above, according to the present invention, the current value of the output terminal can be determined regardless of the current value of the reference current source. Therefore, the variable range of the output current can be set arbitrarily. Moreover, even if the number of bits of the digital input signal is increased, the number of transistors and the resistance value can be reduced even if the range is changed in a minute range, which is effective for reducing the size of a chip in an IC. Have the result that there is.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional example.

[Explanation of symbols]

1 Reference current source 2 to 4, 10 to 14, 31, 36 to 37, 40 to 44
Transistors 5-7, 9, 15-19, 24-29, 32, 38, 3
9,45-49 Resistance 8,30 Amplifier 20-23 Switch 33 Input terminal 34 Power supply terminal 35 Output terminal

Claims (1)

[Claims] 1. A reference current source is connected to an input of a first current mirror circuit having two outputs, and a first output of the first current mirror circuit has a first resistor whose other end is grounded. Connected to one end of the first amplifier and the input of the first amplifier, the output of the first amplifier serves as a reference potential supply point, the equipotential terminal of the R-2R resistance ladder circuit is connected to the reference potential supply point, and The second output of the first current mirror circuit is connected to the input of a second current mirror circuit having n outputs, the input of which is a second transistor whose emitter is grounded through a second resistor. And a third transistor constituting the first output of the second current mirror circuit is controlled by a third resistor and a first bit of the digital input signal.
Grounded via the switch of the 4th, which constitutes the 2nd output
Transistor is grounded via a fourth resistor and a second switch controlled by the second bit of the digital input signal, and a fifth transistor which similarly constitutes an nth output is connected to the fifth resistor and the fifth resistor. It is grounded via an nth switch controlled by the nth bit of the digital input signal, and n pieces of the R-2R resistance ladder circuit are provided from the first output to the nth output of the second current mirror circuit. Of the R-2R resistor ladder circuit, the output of the R-2R resistor ladder circuit is connected to the output, and the emitter is connected to the inverting input together with one end of the sixth resistor whose other end is grounded. Current output type digital / analog conversion circuit connected to the non-inverting input of the second amplifier, and the collector of the sixth transistor serves as a current output terminal.