WO2018179372A1

WO2018179372A1 - Analog input unit and reference voltage stabilizing circuit

Info

Publication number: WO2018179372A1
Application number: PCT/JP2017/013706
Authority: WO
Inventors: 有記浩奥田; 健太渡邉; 圭治二宮
Original assignee: 三菱電機株式会社
Priority date: 2017-03-31
Filing date: 2017-03-31
Publication date: 2018-10-04
Also published as: JPWO2018179372A1; CN110114638A; JP6342100B1; CN110114638B

Abstract

An analog input unit (1) is provided with: input terminals (2, 3, 4, 5) to which analog values from analog sensors can be inputted; switches (8, 9) with which selection or non-selection of the analog sensors can be made; an AD converter (6) which converts an analog value from a selected analog sensor to a digital value; a reference resistor (7) which generates a reference voltage to which the AD converter (6) refers; and a reference voltage stabilizing circuit (10) connected in parallel to the reference resistor (7).

Description

Analog input unit and reference voltage stabilization circuit

The present invention provides an analog input unit that can be connected to a resistance temperature detector, connected to a current output sensor, and connected to a voltage output sensor by a switch, and can be applied to the analog input unit. The present invention relates to a reference voltage stabilization circuit.

The sensor connected to the input terminal of the analog input unit includes a resistance temperature detector that detects temperature, a current output sensor that detects physical quantities such as pressure and flow rate, or a voltage output sensor. Conventionally, there is an analog input unit that can connect these sensors and can selectively connect various sensors to input terminals by incorporating a circuit that executes input processing according to the sensor input specifications. (For example, the following patent document 1).

In addition, the conversion value by actual analog digital (Analog-to-Digital: hereinafter referred to as “AD”) conversion is compared with the AD conversion value with no fluctuation, and the fluctuation is calculated by arithmetic processing, and the constant current value There is also one that corrects (for example, Patent Document 2 below).

JP 2008-304203 A JP 2013-19738 A

However, in the conventional analog input units represented by

Patent Documents

1 and 2, when an input is selected by switching an internal switch according to a connected sensor, a leakage current is generated in a circuit not selected through the switch. It will flow. Due to this leakage current, there is a problem that a current necessary for a circuit that should flow originally is not supplied.

Also, when a three-wire RTD is connected, it is necessary to generate a reference voltage. The reference voltage is generated by passing a current through the reference resistor. However, the current that should originally flow is reduced by the leakage current. As the current decreases, the reference voltage varies. As the reference voltage fluctuates, the resolution and accuracy required for AD conversion are reduced. From the above viewpoint, when a sensor that needs to generate a reference voltage is connected to an analog input unit, a mechanism for keeping the reference voltage constant with high accuracy is required.

The present invention has been made in view of the above, and even when a sensor is connected to an analog input unit that needs to generate a reference voltage, a reference caused by a leak current that can occur when switching the sensor input An object of the present invention is to provide a reference voltage stabilization circuit capable of reducing voltage fluctuation.

In order to solve the above-described problems and achieve the object, an analog input unit according to the present invention includes an input terminal that can input an analog value from an analog sensor, a switch that enables selection or non-selection of the analog sensor, and selection An AD converter that converts an analog value from the analog sensor into a digital value, a reference resistor that generates a reference voltage that is referenced by the AD converter, and a reference voltage stabilization circuit that is connected in parallel to the reference resistor.

According to the present invention, even when a sensor is connected to an analog input unit that needs to generate a reference voltage, it is possible to reduce fluctuations in the reference voltage caused by a leakage current that may occur when the sensor input is switched. Play.

The figure which shows the structure of the input process part in the analog input unit which concerns on embodiment The figure which shows the state by which the voltage output sensor was connected to the analog input unit shown in FIG. The figure which shows the state in which the current output sensor was connected to the analog input unit shown in FIG. The figure which shows the state by which the three-wire type resistance thermometer was connected to the analog input unit shown in FIG. 1 is a circuit diagram showing the internal configuration of the reference voltage stabilization circuit shown in FIGS.

Hereinafter, an analog input unit and a reference voltage stabilization circuit according to an embodiment of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the following embodiments.

Embodiment.
FIG. 1 is a diagram illustrating a configuration of an input processing unit in the analog input unit according to the embodiment. In FIG. 1, an analog input unit 1 according to the embodiment includes

input terminals

2, 3, 4, and 5 used for at least one of a three-wire resistance thermometer, a voltage output sensor, and a current output sensor. More specifically, the

input terminals

2 and 3 are voltage input terminals for a three-wire resistance thermometer. The input terminal 4 is a current input terminal for the three-wire resistance thermometer, and is also used as a voltage input terminal for the voltage output sensor and a current input terminal for the current output sensor. The input terminal 5 is a voltage input terminal of the voltage output sensor and a current input terminal of the current output sensor. Note that the three-wire resistance thermometer, voltage output sensor, and current output sensor mentioned here are examples of analog sensors that output analog values. Further, the

input terminals

2, 3, 4, and 5 are examples, and any terminals may be used as long as at least one of the voltage value and the current value can be input.

In addition, the analog input unit 1 can select or deselect an AD converter 6 that converts an input analog value into a digital value, a reference resistor 7 that generates a reference voltage referenced by the AD converter 6, and an analog sensor. The

switches

8 and 9 are further provided. More specifically, the switch 8 selects an input of a current supplied from the three-wire temperature measuring resistor, an applied voltage from the voltage output sensor, or a supply current from the current output sensor. The switch 9 selects an input of an applied voltage from the voltage output sensor or a supply current from the current output sensor.

The analog input unit 1 further includes a reference voltage stabilization circuit 10 that suppresses fluctuations in the reference voltage generated by the reference resistor 7 and a current-voltage conversion resistor 11. Here, as shown in FIG. 1, the resistance value of the reference resistor 7 is R _ref, and the resistance value of the current-voltage conversion resistor 11 is R.

FIG. 2 is a diagram showing a state where the voltage output sensor 12 is connected to the analog input unit 1 shown in FIG. As shown in FIG. 2, the voltage output sensor 12 is connected between the input terminal 4 and the input terminal 5. When the voltage output sensor 12 is connected between the

input terminals

4 and 5, the switch 8 is controlled to be turned off by software inside the analog input unit 1. In addition, the switch 9 is switched to a side where the voltage applied from the input terminal 4 can be directly input to the AD converter 6 by software inside the analog input unit 1. As a result, the voltage applied from the input terminal 4 is input to the AD converter 6 via the switch 9, and the AD converter 6 measures the potential difference between the input terminal 4 and the input terminal 5.

FIG. 3 is a diagram showing a state where the current output sensor 13 is connected to the analog input unit 1 shown in FIG. The current output sensor 13 is connected between the input terminal 4 and the input terminal 5. When the current output sensor 13 is connected between the

input terminals

4 and 5, the switch 8 is controlled to be turned off by software inside the analog input unit 1. The switch 9 is switched to the side where the current supplied from the input terminal 4 flows to the current-voltage conversion resistor 11 by software inside the analog input unit 1. At this time, the current is converted into a voltage by the current-voltage conversion resistor 11. That is, the current supplied from the input terminal 4 flows to the current-voltage conversion resistor 11 via the switch 9, and the current value is converted into a voltage value in the current-voltage conversion resistor 11. The voltage generated at both ends of the current-voltage conversion resistor 11 is measured by the AD converter 6, and the current value of the current supplied from the input terminal 4 is calculated by the AD converter 6.

FIG. 4 is a diagram showing a state in which a three-wire resistance thermometer 14 is connected to the analog input unit 1 shown in FIG. The three-wire resistance thermometer 14 is connected to the

input terminals

2, 3, and 4. When the three-wire RTD 14 is connected to the

input terminals

2, 3, and 4, the switch 8 is turned on by software inside the analog input unit 1. The switch 9 is controlled to be turned off by software inside the analog input unit 1.

When the three-wire resistance thermometer 14 is connected to the

input terminals

2, 3, and 4, the

input terminals

2 and 3 operate as voltage input terminals and current input terminals. Specifically, as shown in FIG. 4, a constant current I is supplied from the AD converter 6 toward the three-wire temperature measuring resistor 14. As a result, the current 2I directed from the

input terminals

2 and 3 to the three-wire temperature measuring resistor 14 flows into the reference resistor 7 via the input terminal 4 and the switch 8, and generates the reference voltage R _ref · 2I. Here, a part of the current 2I flows to the switch 9 as the leakage current I _leak . For this reason, the current 2I flowing into the reference resistor 7 is reduced by the amount of the leakage current I _leak . As a result, the reference voltage decreases by R _ref · I _leak .

The reference voltage stabilization circuit 10 is connected to the reference resistor 7 in parallel. With this connection, the reference voltage stabilization circuit 10 can detect a decrease in the reference voltage. The reference voltage stabilizing circuit 10 detects R _ref · I _leak that is a reduced voltage component from the voltage of the reference resistor 7, and generates a compensation current I _comp that is the same as the leakage current I _leak from the detected voltage. Returning the compensation current I _comp to the reference resistor 7 cancels out the leakage current I _leak which is a decrease of the current 2I.

FIG. 5 is a circuit diagram showing an internal configuration of the reference voltage stabilizing circuit 10 shown in FIGS. As shown in the figure, a resistor 16, which is a first resistor, is connected between the output terminal of the operational amplifier 15 and a positive terminal (+ side) on the input side of the operational amplifier 15. A resistor 17, which is a second resistor, is connected between the negative terminal and the negative terminal (−) terminal of the reference voltage stabilizing circuit 10, and the output terminal of the operational amplifier 15 and the input side of the operational amplifier 15. A resistor 18, which is a third resistor, is connected between the negative terminal (− side) of the terminal. Further, the positive side (+ side) terminal of the reference voltage stabilizing circuit 10 is connected to the reference voltage input terminal 20 of the AD converter 6 to which the reference resistor 7 is connected as shown in FIG.

Here, the voltage of the reference resistor 7 is V _ref . At this time, taking the polarity of the compensation current I _comp in the drawing direction, also, the resistance value R ₂ of the resistor 17 and the resistance R ₃ of the resistor 18 is set to the same value, the relational expression is obtained.

V _ref = −R ₁ · I _comp (1)
Where R ₁ is the resistance value of the resistor 16

On the right side of the above equation (1), a sign “−” is attached. When the reference voltage V _ref increases, the compensation current I _comp is drawn into the reference voltage stabilization circuit 10 and the reference voltage V _ref decreases. This means that the compensation current I _comp flows out of the reference voltage stabilization circuit 10. Therefore, when the leak current I _leak occurs, the reference voltage V _ref tends to decrease. The reference voltage stabilization circuit 10 that detects this decrease generates the compensation current I _comp and positively feeds it back to the reference resistor 7. As a result, the leakage current I _leak is canceled out, and the decrease in the reference voltage V _ref is suppressed.

As described above, the reference voltage stabilizing circuit 10 is given a function of positive current feedback by the resistor and the operational amplifier. By providing the reference voltage stabilization circuit 10 having this function, the current flowing through the reference resistor 7 can be made constant, and the reference voltage V _ref can be maintained with high accuracy.

In the above description, taken with the resistance value R ₂ of the resistor 17, although the resistance value R ₃ of the resistor 18 has been described as set to the same value, and the resistance value R ₂ and the resistance R ₃ is necessarily the same value It does not have to be. Specifically, it may be a numerical value satisfying the following relationship.

0.95 · R ₂ ≦ R ₃ ≦ R ₂ (2)

That is, when the resistance value R ₂ and the resistance value R ₃ satisfy the above equation (2), the resistance value R ₂ and the resistance value R ₃ may be regarded as the same value.

Further, the above equation (1) means that the error of the reference voltage V _ref can be reduced by selecting the resistance value R ₁ of the resistor 16. That is, if the method of the present embodiment is used, the error of the reference voltage V _ref can be reduced regardless of the resistance value R _ref of the reference resistor 7.

When the analog input unit 1 includes a terminal that can be connected in parallel to an element corresponding to the reference resistor 7, the reference voltage stabilization circuit 10 can be externally attached to an external terminal of the analog input unit 1. . According to this configuration, the reference voltage stabilization circuit 10 may be retrofitted when needed, and it is not necessary to incorporate the reference voltage stabilization circuit 10, thereby suppressing an increase in cost of the analog input unit 1. Is possible.

1 to 4, the reference voltage stabilizing circuit 10 is connected to the outside of the AD converter 6, but may be built in the AD converter 6. When the AD converter 6 is made into an IC, the analog input unit 1 can be downsized by making an IC together with the AD converter 6.

In the above description, a case where a three-wire resistance thermometer is used has been described as an example, but the present invention can also be applied to a two-wire resistance thermometer or a four-wire resistance thermometer.

In the above description, the application to a circuit configuration in which a reference voltage is generated by flowing a current through a reference resistor has been described. However, application to a circuit configuration in which a reference voltage is generated by a current is also possible, and the reference voltage can be accurately The effect which can be hold | maintained can be acquired.

Note that the configurations shown in the above embodiments are examples of the contents of the present invention, and can be combined with other known techniques, and can be combined without departing from the gist of the present invention. It is also possible to omit or change a part of.

1 analog input unit, 2, 3, 4, 5 input terminal, 6 AD converter, 7 reference resistor, 8, 9 switch, 10 reference voltage stabilization circuit, 11 current voltage conversion resistor, 12 voltage output sensor, 13 current output Sensor, 14 3-wire resistance thermometer, 15 operational amplifier, 16, 17, 18 resistance, 20 reference voltage input terminal.

Claims

An input terminal that can input analog values from analog sensors,
A switch that enables selection or non-selection of the analog sensor;
An AD converter for converting an analog value from the selected analog sensor into a digital value;
A reference resistor that generates a reference voltage referenced by the AD converter;
A reference voltage stabilizing circuit connected in parallel to the reference resistor;
An analog input unit characterized by comprising
2. The analog input unit according to claim 1, wherein the reference voltage stabilizing circuit is provided with a function of positive current feedback by a resistor and an operational amplifier.
3. The analog input unit according to claim 1, wherein the reference voltage stabilizing circuit is mounted inside the AD converter.
4. The analog input unit according to claim 1, wherein the reference voltage stabilization circuit is configured to be retrofitted outside or inside the analog input unit. 5.
An input terminal capable of inputting an analog value from the analog sensor; a switch that enables selection or non-selection of the analog sensor; an AD converter that converts the analog value from the selected analog sensor into a digital value; A reference voltage stabilizing circuit applied to an analog input unit comprising: a reference resistor that generates a reference voltage that is referred to when converting the analog value to a digital value;
A resistor and an operational amplifier, which are connected in parallel to the reference resistor, and operate so that a compensation current for compensating the leakage current flows through the reference resistor when the leakage current flows inside the analog input unit A reference voltage stabilizing circuit.
The resistor includes a first resistor, a second resistor, and a third resistor,
The first resistor is connected between an output terminal of the operational amplifier and a positive terminal on the input side of the operational amplifier, and a negative terminal on the input side of the operational amplifier and a negative terminal in the reference voltage stabilizing circuit. The second resistor is connected between the terminal and a third resistor is connected between the output terminal of the operational amplifier and the negative terminal on the input side of the operational amplifier. Reference voltage stabilization circuit described in 1.