JPH1131967A - Analog signal input device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To construct an analog signal input device by means of an A/D converter of a simple circuit constitution and with no use of a differential inlet circuit by connecting a resistance which converts a current signal into voltage to the common voltage. SOLUTION: One of both ends of a resistance R2 is connected to an end (source) of an NMOS FET Q2 with the other end connected to the common voltage respectively. In a current signal input mode, a plant monitoring device turns off a switch SW1 and turns a switch SW2 toward a buffer A4 to turn on the FET Q2. Then the current signal inputted to an input terminal 1 flows to the FET Q2 and the resistance R2 and then to the common voltage. Then the signal sent from one end of the resistance R2 is inputted to an A/D converter 5 via the buffer A4 and the switch SW2. The converter 5 converts the signal received from the buffer A4 into a digital signal. That is, the value that is converted into a voltage signal from a current signal by the resistance R2 is converted into a digital signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an analog signal input device for inputting a signal from a sensor provided in, for example, a plant monitoring apparatus. It concerns the device.

[0002]

2. Description of the Related Art There are two types of sensors used in a plant: a sensor for transmitting a measurement result as a current signal (4 to 20 mA) and a sensor for transmitting a measurement result as a voltage signal (1 to 5 V). The analog signal input device inputs these signals, converts them into digital signals, and supplies data to the plant monitoring device. The plant monitoring device monitors and controls the plant using the digital signal.

FIG. 3 shows such a conventional analog signal input device. In the figure, reference numeral 1 denotes an input terminal for inputting a measurement result (current signal or voltage signal) from a sensor (not shown), for example. A common terminal 2 is connected to a common voltage (0 [V]) inside the circuit. R1 is a resistor having one end connected to the input terminal 1 and converting a current signal into a voltage. Q1 is a switch (MOSFET), one end (drain) is connected to the other end of the resistor R1, the other end (source) is connected to the common terminal 2, and for example, V from a plant monitoring device (not shown).
The / I selection signal is received at the gate to perform on / off.
A1 is a buffer for inputting a signal from the input terminal 1.
A2 is a buffer for inputting a signal from the other end of the resistor R1. The switch SW selects and connects the output of the buffer A2 and the common terminal 2. Reference numeral 3 denotes an A / D converter, which has a differential circuit in an input stage, inputs the output of the buffer A1 and the output of the switch SW, and converts the difference between the output of the buffer A1 and the switch SW into a digital signal.

[0004] The operation of such an apparatus will be described. FIG. 4 is a diagram for explaining the operation of the device of FIG. (1) At the time of current signal input The plant monitoring device uses the V / I selection signal to switch Q
1 is turned on, and the switch SW is set to the buffer A2 side.
As a result, the current signal input to the input terminal 1 flows through the resistor R1 and the switch Q1, and flows to the common voltage. Then, the output of the buffer A1 and the output of the buffer A2 are A /
It is input to the D converter 3. The A / D converter 3 converts the difference between the buffers A1 and A2 into a digital signal. That is, the value converted from the current signal to the voltage signal by the resistor R1 is converted to a digital signal.

(2) At the time of voltage signal input The plant monitoring apparatus uses the V / I selection signal to switch Q
1 is turned off, and the switch SW is set to the common voltage side. Accordingly, the voltage signal input to the input terminal 1 is input to the A / D converter 3 via the buffer A1. A / D
The converter 3 converts the difference between the buffer A1 and the common voltage into a digital signal.

[0006]

In such a device, the measurement is made based on the difference between the voltages across the resistor R1 in order to eliminate the influence of the on-resistance of the switch Q1 when a current signal is input. For this reason, a differential circuit must be provided at the input stage of the A / D converter, which complicates the circuit configuration of the A / D converter.

An object of the present invention is to realize an analog signal input device which can be constituted by an A / D converter having a simple circuit configuration.

[0008]

SUMMARY OF THE INVENTION The present invention relates to an analog signal input device for inputting a current signal or a voltage signal to an input terminal and converting the signal into a digital signal. A resistor connecting the other end and one end of the switch, and connecting the other end to a common voltage; a switch for selecting a signal from the input terminal and a signal from one end of the resistor; a signal from the switch being a digital signal And an A / D converter for converting the current into a current signal.
The switch is configured to select a signal from one end of a resistor. When a voltage signal is input, the semiconductor switch is turned off, and the switch is caused to select a signal from an input terminal.

In the present invention, when a current signal is input, the semiconductor switch is turned on, and the switch selects a signal from one end of the resistor. Then, the current signal input to the input terminal flows to the common voltage via the semiconductor switch and the resistor. The A / D converter converts the voltage at one end of the resistor into a digital signal via a switch.

When a voltage signal is input, the semiconductor switch is turned off, and the switch selects a signal from the input terminal. Then, the A / D converter converts the voltage signal from the input terminal into a digital signal.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram showing one embodiment of the present invention. The same components as those in FIG. 3 are denoted by the same reference numerals and description thereof is omitted. In the figure, Q2 is an N-type MOSFET which is a semiconductor switch.
At T, one end (drain) is connected to the input terminal 1. R2
Is a resistor, which connects one end to the other end (source) of the N-type MOSFET Q2 and the other end to a common voltage. R3 is a resistor having one end connected to the voltage V and the other end connected to the gate of the N-type MOSFET. SW1 is a switch, which is turned on / off by a signal from a plant monitoring device (not shown),
One end is connected to the other end of the resistor R3, and the other end is connected to a common voltage. A3 is a buffer constituted by an operational amplifier, and inputs a signal from the input terminal 1. A4 is a buffer composed of an operational amplifier, and inputs a signal from one end of the resistor R2. SW2 is a switch, a signal from the input terminal 1 via the buffer A3 and a buffer A4 from one end of the resistor R2.
Is selected by the signal of the plant monitoring device. An A / D converter 5 converts a signal from the switch SW2 into a digital signal.

The operation of such a device will be described below.
FIG. 2 is a diagram for explaining the operation of the device of FIG. (1) At the time of input of a current signal The plant monitoring device turns off the switch SW1 and sets the switch SW2 to the buffer A4 side. Thereby, N type M
OSFET Q2 is turned on. The current signal input to the input terminal 1 is composed of an N-type MOSFET Q2 and a resistor R2.
To the common voltage. Then, a signal from one end of the resistor R2 is input to the A / D converter 5 via the buffer A4 and the switch SW2. The A / D converter 5 converts the signal from the buffer A4 into a digital signal. That is, the value converted from the current signal to the voltage signal by the resistor R2 is converted into a digital signal.

(2) At the time of inputting a voltage signal The plant monitoring apparatus turns on the switch SW1 and sets the switch SW2 to the buffer A3 side. Thereby, N type M
OSFET Q2 is turned off. Then, the voltage signal input to the input terminal 1 is input to the A / D converter 5 via the buffer A3 and the switch SW2. A / D converter 5
Converts the voltage signal from the buffer A3 into a digital signal.

As described above, since the resistor for converting a current signal into a voltage is connected to the common voltage, an analog signal input device is constituted by the A / D converter 5 having a simple circuit configuration without using a differential input circuit. be able to. Thus, an analog signal input device can be manufactured at low cost.

The voltage V is expressed as V = I × R + Von (I: overcurrent limit value, R: resistance value of resistor, Von: MOSFE
The minimum voltage value at which T can be turned on (the voltage between the source and the gate). Thus, even if a current larger than the overcurrent limit value flows when a current signal is input, the MOSFE
TQ2 is turned off, and circuit protection against overcurrent can be performed.

The present invention is not limited to this. The buffers A3 and A4 are provided at the input stage of the switch SW2. A configuration may be provided.

Further, the semiconductor switch may be constituted by a bipolar transistor as shown in the example in which the semiconductor switch is constituted by a MOSFET. The lowest voltage value that can be turned on at this time is the voltage between the base and the emitter.

An attenuator is provided between the buffer A3 and the switch SW2, and the voltage signal is supplied to the A / D converter 5
The present invention also includes a configuration adapted to the scale of (1). By the way, the current signal can be adjusted by adjusting the resistance value of the resistor R2.
It can be adjusted to the scale of the / D converter.

[0019]

According to the present invention, the following effects can be obtained. According to the first to third aspects, since the resistor for converting the current signal to the voltage is connected to the common voltage, the analog signal input device is constituted by the A / D converter having a simple circuit configuration without using the differential input circuit. can do. Thus, an analog signal input device can be manufactured at low cost.

According to the third aspect, the voltage V is defined as V = I × R +
Von (I: overcurrent limit value, R: resistance value of resistor, Vo
n: the lowest voltage value at which the semiconductor switch can be turned on).
Thus, even if a current larger than the overcurrent limit value flows when a current signal is input, the semiconductor switch is turned off, and circuit protection against overcurrent can be performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the device of FIG. 1;

FIG. 3 is a diagram showing a configuration of a conventional analog signal input device.

FIG. 4 is a diagram for explaining the operation of the device in FIG. 3;

[Explanation of symbols]

 1 input terminal 5 A / D converter A3, A4 buffer Q2 NMOSFET R2 resistor SW2 switch

Claims (3)

[Claims] 1. An analog signal input device for inputting a current signal or a voltage signal to an input terminal and converting the signal into a digital signal, comprising: a semiconductor switch having one end connected to the input terminal; and the other end connected to one end of the semiconductor switch. A resistor for connecting the other end to a common voltage; a switch for selecting a signal from the input terminal and a signal from one end of the resistor; and an A / A for converting a signal from the switch into a digital signal.
And a D converter, wherein the semiconductor switch is turned on when the current signal is input, the signal is selected from one end of the resistor by the switch, and the semiconductor switch is turned off when a voltage signal is input. An analog signal input device, wherein a signal from an input terminal is selected. 2. The analog signal input device according to claim 1, wherein a buffer is provided before or after the switch. 3. The voltage V for turning on / off the semiconductor switch is set to V = I × R + Von (I: overcurrent limit value, R: resistance value of resistor, Von: minimum voltage value at which the semiconductor switch can be turned on). The analog signal input device according to claim 1 or 2, wherein: