JPH0740674B2 - Two-wire insulated transmission circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a two-wire insulation system suitable for transmitting, for example, an output signal of a sensor for measuring the pH of a solution or a dissolved oxygen concentration to a receiver provided at a remote location. The present invention relates to a transmission circuit.

Prior art Fig. 3 is a diagram showing a conventional two-wire insulation transmission circuit, in which T ₁ and T ₂ are signal input terminals, and pH or dissolved oxygen concentration is passed through an impedance conversion circuit or a current / voltage conversion circuit. The output signal voltage and the like of the measurement sensor of 1 are added. V / F is voltage
The frequency conversion circuit and PS are DC sources, and are composed of a rectifying circuit, a smoothing circuit, an output stabilizing circuit, and the like. TF ₁ and TF ₂ are isolation transformers, F / V is frequency / voltage conversion circuit, V / I is voltage / current conversion circuit, OSC is oscillator circuit, T ₃ and T ₄ are signal output terminals, L ₁
And L ₂ are transmission lines, T ₅ and T ₆ are input / output terminals on the receiving side, and R _X
Is a receiver provided with an indicator or a recorder, and E is a DC power supply provided on the receiving side.

The output voltage of the DC power source E provided on the receiving side is the receiver R _X , the input / output terminals T ₅ and T ₆ , the transmission lines L ₁ and L ₂ , and the signal output terminal T ₃
And T _4, applied to the oscillation circuit COC via a voltage-current exchange circuit V / I, allowed to oscillate it. The oscillation output of the oscillator circuit OSC is applied to the DC source PS via the isolation transformer TF ₂ , rectified and smoothed, and then stabilized, and the DC voltage insulated from the output voltage of the DC power source E is converted into the voltage / frequency conversion circuit V. Added to / F to activate this.

On the other hand, for example, an output signal of a sensor for measuring pH or dissolved oxygen concentration, etc. is added to the signal input terminals T ₁ and T ₂ via an impedance conversion circuit, a current / voltage circuit, etc., and input at a voltage / frequency circuit V / F. It is converted into an AC voltage with a frequency proportional to the magnitude of the signal voltage, and is applied to the frequency / voltage conversion circuit F / V via the insulating transformer TF ₁ . The AC voltage applied to the frequency / voltage conversion circuit F / V is converted to a DC voltage proportional to the magnitude of the input signal voltage, then converted to a DC current in the voltage / current conversion circuit V / I, and output terminal T ₃ and T ₄ , transmission lines L ₁ and L ₂ , input / output terminals T ₅ and T ₆ , and a direct current E are supplied to the receiver R _X.

Problems to be Solved by the Invention When remote indication or recording of pH or dissolved oxygen concentration, etc. is performed using the above-mentioned two-wire insulated transmission circuit, or when the indication or recording is performed and pH or dissolved oxygen concentration, etc. When performing process control, the lower limit of the output signal current is generally set to 4 mA and the upper limit is set to 20 mA, so if the output signal current is 4 mA, the voltage / frequency conversion circuit V / F and frequency / voltage conversion circuit It is necessary to use a circuit that can operate at a consumption current of 4 mA as a transmission circuit for F / V, etc., but a voltage / frequency conversion circuit and a frequency / voltage conversion circuit, etc. with a small consumption current in the past formed by an integrated circuit, etc. Has poor temperature characteristics and cannot avoid conversion errors.

In order to eliminate such a drawback, conventionally, by using a voltage / frequency conversion circuit and a frequency / voltage conversion circuit having the same temperature characteristics, the conversion errors due to temperature changes are canceled out to improve the temperature characteristics of the entire circuit. However,
It is extremely difficult to select a conversion circuit having the same temperature characteristics.

Means for Solving the Problems In the present invention, a current corresponding to an input signal voltage is caused to flow into an integrating circuit, the output voltage of the integrating circuit is applied to a first comparator, and the output of the first comparator is added. The voltage is applied as a reset signal to the CMOS type reset set flip-flop circuit, the charging / discharging of the charging / discharging capacitor is controlled according to its inversion, and the second comparator is operated according to the charging voltage of this charging / discharging capacitor. The output voltage that is sent is applied as a set signal to the CMOS reset set flip-flop circuit, and the input signal voltage is converted into a rectangular wave voltage by reversing the direction of the current that flows into the integrating circuit according to its inversion. This rectangular wave voltage is applied to the third comparator via an insulating transformer, and the output voltage of the third comparator is converted into a current. Ri is obtained by realizing the two-wire insulated transmission circuit for obtaining Except above conventional drawbacks.

Action With the above configuration, the duty ratio of the rectangular wave voltage obtained by converting the input signal voltage is proportional to the magnitude of the input signal voltage, and the output current obtained by converting the rectangular wave voltage. Will be proportional to the Duoi's ratio.

Embodiment FIG. 1 is a view showing an embodiment of the present invention, in which T ₁ and T ₂ are signal input terminals, for example, through an impedance conversion circuit or a current / voltage conversion circuit (neither is shown). PH
Alternatively, an output signal voltage of a sensor for measuring dissolved oxygen concentration or the like (none of which is shown) is applied. R ₁ and R ₂ are input resistors, Q ₁ is a low power consumption operational amplifier, integrating capacitor
Form an integrating circuit with C ₁ . R ₃ is a charging resistor, C ₂ is a charging / discharging capacitor, Q ₂ and Q ₃ are comparators, each consisting of, for example, a low power consumption operational amplifier. R ₄ , R ₅ and R ₆ are voltage-dividing resistors, and their respective resistance values are, for example, substantially equal to each other, and are formed so that the respective divided voltages are applied as reference voltages to the comparators Q ₂ and Q ₃ . FF is a CMOS type reset set flip-flop circuit (hereinafter abbreviated as RS-flip-flop circuit FF), and TR ₁ is an opening / closing element, which is composed of, for example, an appropriate transistor. R ₇ is a base resistor, C ₃ is a DC blocking capacitor, TF ₁ is an insulating transformer, Q ₄ is a comparator consisting of a low power consumption operational amplifier, R ₈ and R ₉ are voltage dividing resistors, for example, both resistors. The values are made substantially equal to each other, and the voltage is applied to the comparator Q ₄ as a reference voltage.
Q ₅ is a CMOS type buffer amplifier, R ₁₀ is an input resistor, Q ₆ is a low power consumption type operational amplifier, and TR ₂ is a transistor, for example, an N-channel field effect transistor. TR ₃ is also a transistor, for example a PNP transistor. Z ₁ and Z ₂ are Zener diodes or reference diodes,
C ₄ is a smoothing capacitor, R ₁₁ is a smoothing resistor, R ₁₂ is a current limiting resistor, CC is a constant current circuit, OSC oscillator circuit, C ₅ is a coupling capacitor, TF ₂ is an insulating transformer, PS is a positive and negative DC source. , A rectifying circuit, a smoothing circuit, an output stabilizing circuit, and the like. T ₃ and T ₄ are signal output terminals, L ₁ and L ₂ are transmission lines, T ₅ and T ₆ are input / output terminals on the receiving side, R _X is a receiver equipped with an indicator or recorder, and E is a DC power supply. Is.

The output voltage of the DC power supply E on the receiving side is applied to the receiver R _X
Zener diodes Z ₁ and Z ₂ in addition to the constant current circuit CC via T ₅ and T ₆ , transmission lines L ₁ and L ₂ , and signal output terminals T ₃ and T _4.
When an operating current is supplied to the oscillator circuit, the Zener voltage (high level voltage is + V _DD , intermediate level voltage is + V ₃ and low level voltage is V ₀ ) generated in the zener diodes Z ₁ and Z ₂ is generated by the oscillation circuit OS.
It is added to C, and its oscillation output is applied to the DC source PS via the coupling capacitor C ₅ and the isolation transformer TF ₂ , rectified and smoothed, then stabilized, and insulated from the voltage of the DC power source E on the receiving side. Positive and negative DC voltage + V _CC and −V _EE
It is supplied as an operating voltage to each circuit element under Q ₁ . still,
As will be described later, the output of operational amplifier Q ₆
After TR ₃ is turned on, an operating current is supplied to the Zener diodes Z ₁ and Z ₂ via the transistor TR ₃ .

On the other hand, when the output signal of the sensor for measuring pH or dissolved oxygen concentration is applied to the signal input terminals T and T ₂ via the impedance conversion circuit or the current / voltage conversion circuit, E _i
/ R ₁ (E _i is the magnitude of the positive input signal voltage, R ₁ is the input resistance R ₁
In the following, the resistance value of each resistor and the capacitance of each capacitor are represented by the same symbols as the reference symbols of each resistor and each capacitor, etc.), and a current flows through the integrating capacitor C ₁ circuit, and the operational amplifier Q ₁ As shown in the waveform diagram of FIG. 2 (a), the output voltage of is applied to the comparator Q ₃ while decreasing linearly toward the low level voltage −V _EE . The output voltage of the operational amplifier Q ₁ applied to the comparator Q ₃ is, when down below the comparator Q is determined by the reference voltage -V ₁ applied to the ₃ threshold level, the output of the comparator Q ₃ is a high level voltage RS-
It is applied to the reset signal input terminal R of the flip-flop circuit FF, and the output voltage of the Q terminal of the RS-flip-flop circuit FF becomes the low level voltage -V _EE .

The relationship between the magnitude of the input signal voltage E _i, the resistance values of the input resistors R ₁ and R ₂ , and the magnitude of the low-level voltage −V _EE satisfies (E _i / R ₁ −V _EE / R ₂ ) <0 During this period, a current of E _i / R ₁ −V _EE / R ₂ flows through the integrating capacitor C ₁ circuit, and the output voltage of the operational amplifier Q ₁ linearly increases toward the high level voltage + V _CC .

{FIG. 2 (a)} As described above, when the output voltage of the Q terminal in the RS-flip-flop circuit FF becomes a low level voltage, the switching element TR ₁ opens, and charging / discharging is performed via the common terminal and the charging resistor R _3. The charging capacitor C ₂ is charged, and its charging voltage, that is, the potential V _C2 of the non-inverting input terminal of the comparator Q ₂ rises as in the following equation {Fig. 2 (b)}. In the above equation, t is the elapsed time (seconds) from the opening of the switching element TR ₁ .

Comparator Q potentials V _C2 of the non-inverting input terminal of the _2, at the threshold level or higher which is determined by the reference voltage -V ₂ of the comparator Q _2, the output voltage of the comparator Q ₂ is turned to a high level voltage
It is applied to the set signal input terminal S of the RS-flip-flop circuit FF, the Q-terminal output of the RS-flip-flop circuit FF becomes a high level voltage, the switching element TR ₁ is closed, and the charging / discharging capacitor C ₂ is discharged. At the same time, {Fig. 2 (b)}, the current E _i / R ₁ flows again in the integrating capacitor C ₁ circuit, and the output voltage of the operational amplifier Q ₁ starts to fall again as shown in Fig. 2 (a). After that, the above operation is repeated, and the output of the Q terminal in the RS-flip-flop circuit FF changes as shown in FIG.

When the time period during which the Q output terminal voltage of the RS-flip-flop circuit FF maintains the low level state is t ₁ and the time period during which the Q output terminal voltage maintains the high level state is t ₂ , And again Was established, Becomes

Therefore, the cycle of the level change of the Q terminal output voltage of the RS-flip-flop circuit FF, and therefore the level change of the terminal output voltage, is Frequency F of Q terminal and level change of each output voltage of terminal
Is Becomes

As is clear from the equation (6), the frequency F of the level change of the Q terminal and the output voltage of each terminal is proportional to the magnitude of the input signal voltage E _i. Therefore, the operational amplifier Q _{1 to} RS-flip-flop circuit The circuit made up of FFs constitutes a voltage / frequency conversion circuit.

However, it is clear from the equation (6) that the frequency F also changes when the capacity of the charging / discharging capacitor C ₂ and the magnitude of the reference voltage −V ₂ of the comparator Q ₃ change due to fluctuations in the ambient temperature. , When calculating the duty ratio D when the Q terminal output in the RS-flip-flop circuit FF is a low level voltage, Becomes

As is clear from the equation (7), the duty ratio D is proportional to the magnitude of the input signal voltage E _i , and the capacitance change of the integrating capacitor C ₁ and the charging / discharging capacitor C ₂ or the comparator Q ₃ and
The duty ratio D is kept constant even when the times t ₁ and t ₂ change due to changes in the magnitudes of the reference voltages −V ₁ and −V ₂ of Q ₂ , so the duty ratio D is always It is proportional to the magnitude of the input signal voltage E _i .

The terminal output voltage of the RS-flip-flop circuit FF is applied to the inverting input terminal of the comparator Q ₄ via the DC blocking capacitor C ₃ and the isolation transformer TF _1, and the voltage of the inverting input terminal of the comparator Q ₄ { 2 (d)} falls below the threshold level of the comparator Q ₄ determined by the reference voltage V ₃ , the output voltage of the comparator Q ₄ becomes a high level voltage and the buffer amplifier Q ₅
Output voltage is high level voltage +
It becomes V _DD . Conversely, when the input voltage at the inverting input terminal of the comparator Q ₄ is higher than the threshold level, the output voltage of the buffer amplifier Q ₅ becomes the low level voltage + V ₀ .

Incidentally, FIG. 1 exemplifies a case where the voltage dividing resistors R ₈ and R ₉ having resistance values substantially equal to each other are provided, and the voltage V ₃ is applied as a reference voltage to the comparator Q ₄ by way of example. Alternatively, the voltage dividing resistors R ₈ and R ₉ may be omitted, and the intermediate level voltage V ₃ at the connection point of the Zener diodes Z ₁ and Z ₂ may be applied to the non-inverting input terminal of the comparator Q ₄ .

Also, if the output voltage is a low level voltage as the comparator Q ₄ , +
When using a comparator that can be + V _{DD in} the case of V ₀ and a high level voltage, the buffer amplifier Q ₅ can be omitted.

The output voltage of the buffer amplifier Q ₅ is applied to the inverting input terminal of the operational amplifier Q ₆ which is operated by a single power source via the input resistor R ₁₀ , and the output voltage is applied to the level shift transistor TR.
_The collector current is applied to the transistor TR ₃ via ₂ to change the collector current, and the control operation is performed so that the voltage drop in the smoothing resistor R _{11 and} the voltage drop in the current limiting resistor R ₁₂ become equal.

That is, the output voltage of the buffer amplifier Q ₅ includes an operational amplifier Q _6, although the be converted to a current in the voltage-current converting circuit comprising the level shift transistor TR ₂ and the transistor TR _3, the converted output current i ₀ The size of And is proportional to the duty ratio D of the Q terminal output voltage in the RS-flip-flop circuit FF.

Substituting equation (7) into equation (8), Therefore, the magnitude of the conversion output current i ₀ is proportional to the magnitude of the input signal voltage E _i , and the proportional coefficient is the voltage of the Zener diodes Z ₁ and Z ₂ + V _DD and the output voltage of the DC source PS −V _EE . Since the size and the resistance values of the resistors R ₁ , R ₂ , R ₁₀ , R ₁₁ and R ₁₂ are included, the change of the proportional coefficient due to the fluctuation of the ambient temperature is extremely small.

The converted output current i ₀ is received by the receiver through the signal output terminals T ₃ and T ₄ , the transmission lines L ₁ and L ₂ , the input / output terminals T ₅ and T ₆ , and the DC power source E.
It is transmitted to R _X for instruction or recording.

The above has illustrated the case where the terminal output voltage of the FF of the RS-flip-flop circuit is applied to the comparator Q ₄ via the DC blocking capacitor C ₃ and the insulating transformer TF _1. However, the Q terminal, the input resistance R ₂ and the base Connection point of resistor R ₇ and DC blocking capacitor
Between the C _3, for example, a buffer amplifier allowed intervention, Q terminal output voltage buffer amplifier, also be applied to the comparator Q ₄ via a capacitor C ₃ and the insulating transformer TF ₁ DC blocking the invention embodiment You can do it.

According to the present invention, the input signal voltage is converted into a rectangular wave in the voltage / frequency conversion circuit, and this rectangular wave is introduced into the frequency / voltage conversion circuit via the DC blocking capacitor and the insulating transformer. The conversion output of the frequency / voltage conversion circuit is introduced into the voltage / current conversion circuit, and the magnitude of the conversion output current is made proportional to the duty ratio of the converted output rectangular wave voltage in the voltage / frequency conversion circuit. Therefore, it is possible to convert the input signal voltage into a current with good temperature characteristics and linearity, and the circuit configuration is relatively simple, and it is effective when used for transmitting a measurement signal such as pH or dissolved oxygen concentration to a remote point. Extremely large

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a waveform diagram for explaining the operation thereof, and FIG. 3 is a diagram showing a conventional two-wire insulated transmission circuit. T ₁ and T ₂ : Signal input terminals, R ₁ , R ₂ and
R ₁₀ : Input resistance, Q ₁ and Q ₆ : Operational amplifier, C ₁ : Integrating capacitor, R ₃ : Charging resistor, C ₂ : Charging / discharging capacitor, Q ₂ , Q ₃ and
Q _4: comparator, R ₄ to R _6, R ₈ and R _9: dividing resistors, FF: flip-flop circuit, TR _1: switching element, R _7: base resistance, C _3: DC blocking capacitor, TF ₁ and TF _2: insulation transformer, Q ₅ buffer amplifier, TR ₂ and TR _3: Taranjisuta, Z ₁ and Z _2: the Zener diode, C _4: smoothing capacitor, R _11: smoothing resistance, R
₁₂ : Current limiting resistor, CC: Constant current circuit, OSC: Oscillation circuit, C ₅ :
Coupling capacitor, PS: DC source, T ₃ and T _4: signal output terminal,
L ₁ and L ₂ : Transmission line, T ₅ and T ₆ : Input / output terminals, R _X : Receiver, E: DC power supply, V / F: Voltage / frequency conversion circuit, F / V: Frequency / voltage conversion circuit, V / I: A voltage / current conversion circuit.

Claims (1)

[Claims] 1. A circuit for causing a current corresponding to an input signal voltage to flow into an integrating circuit, a first comparator to which an output voltage of the integrating circuit is applied, and an output voltage of the first comparator is a reset signal input. The CMOS type reset set flip-flop circuit that is applied to the pin and the output voltage of the pin that outputs the low level output voltage when the reset signal is applied to this CMOS type reset set set flip-flop circuit is low level. At the time of applying a reset signal to the CMOS type reset set flip-flop circuit, and a circuit for causing a combined current of a current corresponding to the input signal voltage and a current opposite thereto to flow into the integrating circuit. An opening / closing element for opening / closing the discharge circuit of a charging / discharging capacitor, the switching of which is controlled by the output voltage of a terminal that outputs a low-level output voltage; A charging circuit of capacitor, said when the charging voltage of the charging and discharging capacitor reaches the set value
A second comparator for applying a set signal to the CMOS reset set flip-flop circuit;
A third comparator to which the output voltage of the set flip-flop circuit is applied via a first isolation transformer;
The voltage / current conversion circuit to which the voltage corresponding to the output of the comparator is applied, the transmission line to which the output current of this voltage / current conversion circuit is supplied, and the operating current by the DC voltage applied via this transmission line. A Zener diode that is supplied and supplies an operating voltage to the secondary side circuit element of the first isolation transformer, an oscillation circuit that is operated by the Zener voltage of the Zener diode, and a Zener diode that is applied via a second isolation transformer. A two-wire insulated transmission circuit consisting of a DC source that generates a positive and negative DC voltage from the oscillation output of the oscillator circuit.