JPH11186859A - Voltage-current conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a voltage-current conversion circuit with an offset where no complicated adjustment is required. SOLUTION: In the voltage-current conversion circuit that includes an operational amplifier X1 and converts a voltage received via an input resistor R3 connecting to the operational amplifier X1 into a current and provides an output of the current, an input circuit 11 that sums the input voltage V5 being a conversion object and an offset voltage V6 to provide an offset current is provided to a pre-stage of the input resistor R3. The resistance of the input circuit 11 is selected to be smaller than the resistance of the input resistor R3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage-current conversion circuit for outputting a current proportional to an input voltage, and more particularly to a circuit adjustment of a conversion circuit for outputting an offset current.

[0002]

2. Description of the Related Art FIG. 9 is a circuit diagram of a conventional voltage-current conversion circuit, and FIG. 10 is a characteristic diagram thereof. This conversion circuit is an example in which an input voltage of 0 to 5 V is converted into an output current of 0 to 20 mA. From the characteristic diagram of FIG.
It can be seen that a constant current characteristic is shown between 0.4 kΩ. However, for example, when the input voltage is 0 to 5 V and the output current 4-2
The conversion circuit for converting the output current to 0 mA has a minimum output current of 4 mA.
It is necessary to convert to a maximum of 20 mA, which is a conversion with an offset.

FIG. 11 is a circuit diagram of a conventional voltage-current conversion circuit with an offset, and FIG. 12 is a characteristic diagram thereof. In FIG. 11, a resistor R3 has an output current of 4-20 mA.
It is for adjusting the inclination when converting to
The resistor R4 is for adjusting to obtain an offset current of 4 mA, and the resistor R2 is for adjusting for minimizing a current change due to a load change. However, as shown in FIG. 13, when the resistance value of the resistor R3 having a gradient is changed, there is a convergence point where the input voltage is about 1.6 V. If it is not fixed to, the resistance R4 cannot be adjusted. That is, the input voltage is 0 V
Then, even if the resistance R4 is adjusted so that the converted output current becomes 4 mA, and the resistance R3 is adjusted so that the output current becomes 20 mA at an input voltage of 5 V, it operates as a 4-20 mA converter. It doesn't mean I could adjust it. Also,
As shown in FIG. 12, when the load resistance fluctuates, the current fluctuates, and the circuit does not operate normally as a constant current circuit. This is because the total resistance value of the resistors R3 and R4, that is, the value converted as the parallel resistance becomes smaller than the value of the resistor R1, so that the amplification degree of the circuit is changed, and thus such a phenomenon occurs. To correct this, for example, there is a method of changing the resistance value of the resistor R2, and as shown in FIG.
When R2 of 1 = 121 kΩ, constant current characteristics can be exhibited even if the load resistance changes.

[0004] However, in the conversion circuit of FIG.
It is necessary to adjust three resistances, and the adjustment cannot be easily adjusted because of the complicated relationship. At least, even if the resistor R3 can be a fixed resistor, it is necessary to correct the variation of the offset voltage V4, so that the resistors R4 and R2 need to be adjusted. further,
When the voltage of V4 is generated by resistance division or the like, the resistance value, that is, the impedance of V4 becomes a problem.

[0005]

Next, the location of the problem will be clarified by analyzing the relationship between the input and output in the conversion circuits shown in FIGS. 9 and 11.

FIG. 15 is a simplified circuit diagram of the conversion circuit of FIG. In the figure, resistors R7 and R10 are 0Ω.
Has been abbreviated. In this conversion circuit, the output voltage VOUT and the current (output current) i flowing through the resistor R6 are expressed by the following equations, respectively.

[0007]

(Equation 1)

Next, the location of the problem will be clarified by analyzing the relationship between the input and output in the conversion circuits shown in FIGS. 9 and 11.

FIG. 15 is a simplified circuit diagram of the conversion circuit of FIG. In the figure, resistors R7 and R10 are 0Ω.
Has been abbreviated. In this conversion circuit, the output voltage VOUT and the current (output current) i flowing through the resistor R6 are expressed by the following equations, respectively.

[0007]

(Equation 1)

This means that the current i is determined only by the relationship between the input voltage V5 and the resistor R6, and a current conversion circuit of 0 to 20 mA is established in which the output current is proportional to the input voltage. I understand.

FIG. 16 is a simplified circuit diagram of the conversion circuit of FIG. In the figure, resistors R7 and R10 are 0
Abbreviated as Ω. In this circuit, the output voltage VOUT and the current (output current) i flowing through the resistor R6 are represented by the following equations, respectively.

[0010]

(Equation 2)

The above equation includes the relationship between the resistances R1 and R4. For example, when V5 = 0V, i = 4mA When V5 = 5V, i = 20mA is adjusted by adjusting the resistance in the following manner. It is inevitable that it will come out. Therefore, it has been extremely difficult to adjust the resistance as described above.

The present invention has been made to solve such a problem, and an object of the present invention is to provide a voltage-current conversion circuit with an offset which does not require the complicated adjustment as described above. .

[0013]

(1) A voltage-current conversion circuit according to one aspect of the present invention includes a differential amplifier, and an input is made via an input resistor connected to the differential amplifier. In a voltage-current converter that converts a voltage into a current and outputs the same, an input circuit for adding an input voltage to be converted and an offset voltage for giving an offset current is provided at a stage preceding the input resistor, Then, the resistance value of the input circuit is set to a value smaller than the resistance value of the input resistor. (2) A voltage-current conversion circuit according to another aspect of the present invention includes:
In the voltage-current conversion circuit according to the above (1), a differential amplifier constituting a voltage follower circuit is inserted between the input circuit and the input resistor. (3) A voltage-current conversion circuit according to another aspect of the present invention includes:
In a voltage-current converter including a differential amplifier, which converts a voltage input to the differential amplifier into a current and outputs the current, an input voltage to be converted and an offset voltage for providing an offset current are added. Input circuit for outputting to the non-inverting input side of the differential amplifier, and an amplification feedback circuit for detecting and amplifying the output current of the differential amplifier and for negatively feeding back to the inverting input side of the differential amplifier It is.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. FIG. 1 is a circuit diagram of a voltage-current conversion circuit according to one embodiment of the present invention. In the figure, the resistance values shown in the figure are set for the resistors R1 to R3 and R5 to R10, and the resistors R7 and R10 correspond to the load resistors. The voltage-current conversion circuit of FIG.
And an input circuit 11 is added to the circuit 10 of the voltage-current conversion circuit. Here, the parallel resistance value of the resistors R8 and R9 of the input circuit 11, (R8 · R9) / (R8 + R
If the value of 9) is sufficiently smaller than the resistance R3, the influence on the voltage-current conversion circuit 10 of FIG. 9 is reduced, and the output voltage Vout and the output current i are reduced as in the case of the conversion circuit of FIG. Since the resistance constant is not included, there is no need to adjust.

Here, the input voltage Vin in FIG. 1 is as follows: Vin = V5 + (V6-V5) .R8 / (R8 + R9) The following values are set in the conversion circuit of FIG. V5 = 0-5V V6 = 15V R8 = 1.3KΩ R9 = 15KΩ

The values of the resistors R8 and R9 correspond to the input voltage V5.
It is assumed that R8 and R9 are adjusted so that the voltage at the connection point "P" becomes about 1 V when = 0V. These resistors R8 and R9
Has a parallel resistance of about 1.2 KΩ, and R3 = 100 KΩ.
It can be said that the value is sufficiently smaller than the above. Then, when about 1.2 kΩ is added to the resistance value of the resistor R3, the resistance R1
It can be seen that this corresponds to + 1.2% of From this, it can be understood that the error rate is improved by 10 times or more as compared with the parallel resistance value of the resistors R3 and R4 in FIG.

Next, regarding the value of Vin, when V5 = 0V, Vin = 1.1963V When V5 = 5V, Vin = 5.797V. Since Vin changes linearly between V5 and 0 to 5 V, if the resistance R6 is 0.29 KΩ, the current i becomes 4.
It can be 1 mA to 19.98 mA. That is,
There are no places to make adjustments.

FIG. 2 is a characteristic diagram of the conversion circuit shown in FIG. 1, in which the resistance R7 is changed as a parameter in the range of 0 to 1 KΩ in steps of 0.1 KΩ. From the figure, the output current changes linearly with respect to the input voltage.
It can be seen that proper conversion processing has been performed.

FIG. 3 is a characteristic diagram of the conversion circuit shown in FIG. 1, in which the resistance R9 is varied as a parameter in a range of 10 KΩ to 20 KΩ in steps of 1 KΩ. Even if the offset amount is adjusted by the resistor R9, the output current changes in parallel with the change in the resistance value of the resistor R9. For example, when the input voltage V5 is 0 V, the output current of the resistor R9 is set to 4 mA. Adjust the value.

Embodiment 2 FIG. 4 is a circuit diagram of a voltage-current conversion circuit according to another embodiment of the present invention. This voltage-
In the current conversion circuit, an operational amplifier (voltage follower) X2 having a high input impedance is inserted on the output side of the input circuit 11 of the voltage-current conversion circuit in FIG.
Therefore, the parallel resistance value of the resistor R8 and the resistor R9 becomes
The conversion characteristic of the output current with respect to the input voltage is more excellent without affecting the ratio of 3 to the resistor R5.

That is, in this embodiment, the voltage-
The voltage input of the current conversion circuit is configured to be a high input impedance input of the operational amplifier circuit X1, and an offset voltage is applied on the input side. Since the offset voltage is on the high impedance side, other circuit constants are not affected, and the performance of voltage-current conversion is improved.

FIG. 5 is a characteristic diagram of the conversion circuit of FIG. 4, in which the resistance R7 is varied as a parameter in the range of 0 to 1 KΩ in steps of 0.1 KΩ. From the figure, the output current changes linearly with respect to the input voltage.
It can be seen that proper conversion processing has been performed.

FIG. 6 is a characteristic diagram of the conversion circuit shown in FIG. 4, in which the resistance R9 is changed as a parameter in the range of 10 KΩ to 20 KΩ in steps of 1 KΩ. Even if the offset amount is adjusted by the resistor R9, the output current changes in parallel with the change in the resistance value of the resistor R9. For example, when the input voltage V5 is 0 V, the output value of the resistor R9 becomes 4 mA so that the output current becomes 4 mA. Can be adjusted.

In the first and second embodiments, the case where the resistance value of the input circuit 11 (the parallel resistance value of the resistors R8 and R9) is about 1.2% of the input resistor R3 has been described. The present invention is not limited to such an example, and the resistance value of the input circuit 11 is set to, for example, 10% or less, 5% or less, or preferably 3% or less with respect to the input resistor R3. I do.

Embodiment 3 FIG. FIG. 7 is a circuit diagram of a voltage-current conversion circuit according to another embodiment of the present invention. This voltage-
In the current conversion circuit, an operational amplifier X3 as shown is used for the feedback circuit of the operational amplifier X1 in FIG.

Next, the operation of this circuit will be described.
The input Vin to the non-inverting terminal of the operational amplifier X1 is represented by Vin = V5 + R8 (Vcc-V5) / (R8 + R9). Here, Vcc = 15V, resistance R8 = 1.3.
Assuming that KΩ and the resistance R9 = 15KΩ, when V5 = 0V, Vin = 1.2V When V5 = 5V, Vin = 5.8V. The current i flowing through the resistor R6 is represented by i = Vin / R6. When Vin = 1.2V, i = 4.1mA When Vin = 5.8V, i = 20.0mA. Here, since R21 = R22 = R23 = R24, the output V 'of the operational amplifier X3 is represented by V' = V2-V1.

The operational amplifier X1 determines V2 such that the voltages Vin and V 'are equal. Vin and (V2-V
1) is added to the output V2 of the operational amplifier X1, the output of the operational amplifier X1 is V2 + {Vin- (V2-V1)} = Vin + V1, and if V1 is almost 0V, V2 = Vin, thus the circuit functions so that Vin = V2 and the output current is determined.

Also in this embodiment, the voltage input of the voltage-current conversion circuit is configured to be the high input impedance input of the operational amplifier circuit X1, and an offset voltage is applied to the input side. Since the offset voltage is on the high impedance side, other circuit constants are not affected, and the performance of voltage-current conversion is improved.

FIG. 8 is a characteristic diagram of the conversion circuit of FIG. 7, in which the resistance R7 is changed as a parameter in the range of 0 to 1 KΩ in steps of 0.1 KΩ. From the figure, the output current changes linearly with respect to the input voltage.
It can be seen that proper conversion processing has been performed.

[0030]

As described above, according to the present invention, an input circuit for adding an input voltage to be converted and an offset voltage for giving an offset current is provided at a stage preceding the input resistor; The input circuit is configured so that its resistance value is smaller than the resistance value of the input resistor, or the input voltage to be converted and the offset voltage for giving an offset current are added. By providing an input circuit for outputting to the non-inverting input side of the differential amplifier, and an amplification feedback circuit for detecting and amplifying the output current of the differential amplifier and negatively feeding back to the inverting input side of the differential amplifier In addition, even when an offset current is output, complicated circuit adjustment is not required.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a voltage-current conversion circuit according to one embodiment of the present invention.

FIG. 2 is a characteristic diagram (part 1) of the conversion circuit of FIG.

FIG. 3 is a characteristic diagram (part 2) of the conversion circuit of FIG. 1;

FIG. 4 is a circuit diagram of a voltage-current conversion circuit according to another embodiment of the present invention.

FIG. 5 is a characteristic diagram (part 1) of the conversion circuit of FIG. 4;

FIG. 6 is a characteristic diagram (part 2) of the conversion circuit of FIG. 4;

FIG. 7 is a circuit diagram of a voltage-current conversion circuit according to another embodiment of the present invention.

FIG. 8 is a characteristic diagram of the conversion circuit of FIG. 7;

FIG. 9 is a circuit diagram of a conventional voltage-current conversion circuit.

FIG. 10 is a characteristic diagram of the conversion circuit of FIG. 9;

FIG. 11 is a circuit diagram of a conventional voltage-current conversion circuit with an offset.

12 is a characteristic diagram (part 1) of the conversion circuit of FIG.

FIG. 13 is a characteristic diagram (part 2) of the conversion circuit of FIG. 11;

FIG. 14 is a characteristic diagram (3) of the conversion circuit of FIG. 11;

FIG. 15 is a simplified circuit diagram of the conversion circuit of FIG. 9;

FIG. 16 is a simplified circuit diagram of the conversion circuit of FIG. 11;

Claims (3)

[Claims] 1. A voltage-current converter including a differential amplifier and converting a voltage input through an input resistor connected to the differential amplifier into a current and outputting the current, an input voltage to be converted And an input circuit for adding an offset voltage for giving an offset current is provided at a preceding stage of the input resistor, and a resistance value of the input circuit is smaller than a resistance value of the input resistor. A voltage-current conversion circuit, wherein: 2. The voltage-current conversion circuit according to claim 1, wherein a differential amplifier forming a voltage follower circuit is inserted between the input circuit and the input resistor. 3. A voltage-current converter including a differential amplifier for converting a voltage input to the differential amplifier into a current and outputting the current, wherein an input voltage to be converted and an offset for providing an offset current An input circuit for adding a voltage and outputting the result to the non-inverting input side of the preceding differential amplifier; detecting and amplifying an output current of the differential amplifier to negatively feed back to the inverting input side of the differential amplifier A voltage-current conversion circuit comprising an amplification feedback circuit.