JPH044769B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an isolated amplifier, and more specifically, an isolated amplifier that receives an analog voltage and modulates the pulse width of on and off output pulses accordingly. Pulse width modulation circuit to output (therefore, duty ratio in on and off output pulses =
The off period/(on period + off period) is proportional to the input voltage), and the on/off pulse from the modulation circuit performs a switching operation to send on/off pulses of the same mode to the demodulation circuit at the next stage. The present invention relates to an isolated amplifier having a photocoupler for transmitting data, and a demodulation circuit that converts ON/OFF pulse input from the photocoupler into an analog voltage and outputs the analog voltage.

[Conventional technology]

Traditionally, transformer isolation has been the most common method for isolating analog signals, but it has drawbacks such as complicating the circuit, taking up a large amount of space, and being expensive, especially now that IC circuits are becoming more common. In this case, the transformer becomes a hindrance to converting the entire circuit into an IC.

Also, since the circuit uses an IC, it is normal to operate with two power supplies, positive and negative, but for example, in multi-point temperature measurement using many thermocouples, it is possible to use many analog input voltages obtained by measurement. A large number of isolation amplifiers are required to take in the signal in isolation, but in this case, the single power supply system is much more advantageous than the two power supply system.

However, such an isolated amplifier has not been proposed so far.

[Problem that the invention seeks to solve]

The problem to be solved by the present invention is that an isolation transformer is not used as an isolation amplifier, and two
It can be said that the goal is to achieve high performance with a single power supply system, regardless of the power supply system.

Therefore, it is an object of the present invention to provide a high-performance isolated amplifier as described above.

[Means and actions for solving problems]

In the present invention, in order to achieve the above object,
In an isolation amplifier comprising a pulse width modulation circuit, a photocoupler, and a demodulation circuit connected in cascade, the pulse width modulation circuit is connected to a first operational amplifier as an integrator that integrates an input analog voltage, and the integrator output is set to a certain reference. A second operational amplifier serves as a comparator that inverts the polarity of the output voltage depending on the magnitude relationship compared to the potential and outputs an on/off pulse, and a power source that operates both of the operational amplifiers. That is.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

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

As seen in the figure, an isolated amplifier as an embodiment of the present invention is composed of a pulse width modulation circuit 1, a photocoupler 2, and a demodulator 3, and the pulse width modulation circuit 1 has its characteristics.

The pulse width modulation circuit 1 basically consists of an integrator that integrates the analog input voltage _Vio , and compares the integrator output with a certain reference potential, inverts the polarity of the output voltage depending on the magnitude relationship, and outputs on and off pulses. It consists of a comparator that

Specifically, the integrator consists of a first operational amplifier Q ₁ , an input resistor R _io and a capacitor C.
Operational amplifier Q ₁ as an integrator with R ₃ and resistor R ₄
The reference potential applied to is shifted from 0V to the negative side.

The comparator consists of a second operational amplifier Q ₂ , Zener diodes ZD ₁ , ZD ₂ , ZD ₃ and a current limiting resistor R8. Resistance R5 and resistance R
6, the reference potential applied to the operational amplifier _Q2 as a comparator is shifted from 0V to the negative side. Note that the power source is 1 power source E.

The resistor RF is for current feedback of the output of the comparator to the integrator. Resistor R7 is connected in parallel to Zener diodes ZD ₂ and ZD ₃ . As a result, when the operational amplifier Q ₂ is saturated with the positive polarity, the output voltage VO2 of the comparator becomes a Zener diode.
It is maintained at a predetermined potential determined by the Zener voltage of ZD ₂ and ZD ₃ . Further, when the operational amplifier _Q2 is saturated at the negative pole, a feedback current flows from the integrator to the negative pole side of the DC power supply E via the resistor R7. zener diode
ZD ₁ operates as a diode when operational amplifier Q ₂ is saturated with the positive polarity, and as a Zener diode (blocks below a certain voltage) when saturated with the negative polarity.

Note that since ZD ₃ and ZD ₅ are for compensating the temperature characteristics of ZD ₂ and ZD ₄ , diodes may be used as long as that purpose can be achieved.

The DC power supply E creates a bias potential from the DC voltage E using Zener diodes ZD ₄ and ZD ₅ and a resistor R8, and uses this as a reference potential of 0V.

The transistor T _r1 operates to turn on the light emitting part of the photocoupler 2 when the positive electrode of the comparator is saturated. Photocoupler PC ON-OFF i.e.
It operates so that duty ratio=T _OFF /T _ON +T _OFF is proportional to the input voltage V _io . The demodulator 3 turns on and off a constant voltage in proportion to the duty ratio, smoothes it, and outputs a DC voltage output _Vput .

FIG. 2 shows the comparator output waveform V02 and the integrator output waveform V01 in FIG. 1. Comparator output voltage V02 is positive polarity
The output voltage V01 of the integrator when switching from VP to negative polarity VN and then from negative polarity VN to positive polarity VP is
They are shown in FIG. 2b as V01A and V01B. That is, V01A=-R1/R2VP+(1+R1/R2)VC (1) V01B=-R1/R2VN+(1+R1/R2)VC (2) Next, the circuit operation will be explained. Comparator output
When V02 is saturated with positive polarity, current IC1 flows through capacitor C at VP.
flows.

IC1=V _io −VB/R _io +VP−VB/RF =V _io /R _io +VP/RF−(1/R _io +1/RF)VB…(3) Here, V _io −VB/R _io = input current Let VP-VB/RF=feedback current 1/R _io +1/RF=IB.

When the comparator output V02 is in negative saturation, the current of IC2 flows through the capacitor C at VN.

IC2=-( _Vio- VB)/ _Rio- (VN-VB)/RF= _-Vio /Rio _- VN/RF+(1/ _Rio +1/RF)VB…(
4) Also, when the comparator output V02 is +VP, T _ON
(IC1 flows) between VN becomes T _OFF (IC2 flows).

T _ON = C/IC1 (VP-VN) …(5) T _OFF = C/IC2 (VP-VN) …(6) Obtain the following formula (7) from the above formulas (5) and (6) ε=T _ON /T _ON +T _OFF =1/IC1/1/IC1+1/IC2 =1/1+IC1/IC2 …(7) From the above formula (7) 1+IC1/IC2=1/ε IC1/IC2=1/ε−1=1 −ε/ε εIC1=(1−ε)IC2 ε(IC1+IC2)−IC2=0 By substituting equations (3) and (4) into the above equation, equation (8) is obtained.

ε{VP/RF-VN/RF}+Vio/ _Rio +VN/ _RF -IB=0Vio/ _Rio =ε(VN/RF-VP/ _RF )-VN/RF+IB=(1-ε)(VP /RF-VN/RF)-VP/RF+IB...(8) Here, (1-ε)=T _OFF /T _ON +T _OFF =ρ.

From equation (8), V _io = ρR _io /RF (VP-VN) - R _io /RFVP + (1+R _io /RF)VB...(9) In equation (9), to operate with one power supply, VP=
It needs to be 0. Now, by substituting VP=0, we obtain _Vio = _ρRio /RF(-VN)+(1+ _Rio /RF)VB...(10) Equation (10) is obtained.

Setting VP=0 can be achieved using the Zener diode ZD ₁ in Fig. 1.

That is, it can be seen that the duty ratio ρ of the pulse width modulation circuit 1 shown in FIG. 1, which operates with one power supply, is proportional to the input voltage.

FIG. 3 is a circuit diagram showing another embodiment of the present invention.

The difference between the embodiment shown in FIG. 1 and that shown in FIG. 1 is that a transistor is used instead of the resistor R7.
The point is that T _r2 is connected in parallel to the Zener diodes ZD ₂ and ZD ₃ , and an operational amplifier Q ₃ is newly connected as shown.

That is, when the input at one terminal of the operational amplifier _Q3 is of negative polarity, its output becomes positive, turning on the transistor T _r2 and short-circuiting the series circuit of the Zener diodes ZD ₂ and ZD ₃ . Also, when the input at one terminal of operational amplifier _Q3 is positive polarity,
Its output becomes negative and turns off transistor T _r2 .

_According to this embodiment, unlike the embodiment shown in _FIG . The output waveform becomes sharper, and the accuracy of the output can be increased accordingly.

In addition, in this embodiment, compared to the embodiment shown in Fig. 1, the number of parts increases due to the addition of operational amplifier _Q3 , etc., but in general, ICs with four operational amplifiers, etc. are commercially available as general-purpose products, so if such IC parts are used, even if the number of operational amplifiers used increases within the range of four, the cost will not increase significantly.

〔Effect of the invention〕

As explained above, the present invention has the advantage of being able to provide an isolated amplifier with stable performance without using an isolation transformer (therefore requiring less space) and using a single power supply system. .

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 is a circuit diagram showing an embodiment of the present invention.
The figure is a waveform diagram showing the output waveforms of the main parts in FIG. 1, and FIG. 3 is a circuit diagram showing another embodiment of the present invention.
It is. Description of symbols: 1... Pulse width modulation circuit, 2... Photocoupler, 3... Demodulator.

Claims (1)

[Claims] 1 Duty ratio of on and off pulses as output {off period/(on period + off period)}
a pulse width modulation circuit that modulates and outputs the pulse width of the on and off pulses according to the input voltage so that the on and off pulses are proportional to the input voltage; and a pulse width modulation circuit that performs a switching operation using the on and off pulses from the modulation circuit and demodulates the next stage. In an isolated amplifier comprising a photocoupler that transmits on and off pulses of the same mode to the circuit, the pulse width modulation circuit includes an integrator that integrates a deviation between an input voltage and a first reference potential; a comparator that compares the integrated output with a second reference potential and inverts the polarity of the output voltage depending on the magnitude relationship and outputs the on/off pulse; a first operational amplifier forming the integrator; and a first operational amplifier forming the comparator. a power supply for operating two operational amplifiers; and a power supply connected between the negative terminal of the first power supply and a zero reference potential terminal for applying the first and second reference potentials to the integrator and the comparator. A first Zener diode is connected to the output terminal of the comparator to eliminate the influence of residual voltage when the negative electrode of the comparator is saturated. The output side of the first Zener diode is connected to the integrator via a feedback resistor, and the positive polarity saturation of the comparator is connected between the output side of the first Zener diode and the negative terminal of the first power supply. A second Zener diode that maintains the output side voltage of the first Zener diode at a predetermined potential is connected at the time, and a resistor is connected that causes a small feedback current to flow from the integrator to the negative pole of the first power supply when the negative pole of the comparator is saturated. An isolation amplifier characterized in that it is connected in parallel to the second Zener diode. 2 Duty ratio of on and off pulses as output {off period/(on period + off period)}
a pulse width modulation circuit that modulates and outputs the pulse width of the on and off pulses according to the input voltage so that the on and off pulses are proportional to the input voltage; and a pulse width modulation circuit that performs a switching operation using the on and off pulses from the modulation circuit and demodulates the next stage. In an isolated amplifier comprising a photocoupler that transmits on and off pulses of the same mode to the circuit, the pulse width modulation circuit includes an integrator that integrates a deviation between an input voltage and a first reference potential; a comparator that compares the integrated output with a second reference potential and inverts the polarity of the output voltage depending on the magnitude relationship and outputs the on/off pulse; a first operational amplifier forming the integrator; and a first operational amplifier forming the comparator. a power supply for operating two operational amplifiers; At the same time, a first Zener diode is connected to the output terminal of the comparator to eliminate the influence of residual voltage when the negative electrode of the comparator is saturated. The output side of the Zener diode is connected to the integrator via a feedback resistor, and the first Zener diode is connected between the output side of the first Zener diode and the negative terminal of the first power supply when the positive electrode of the comparator is saturated. A second Zener diode that maintains the output side potential of the diode at a predetermined potential is connected, and a switching element that causes a minute feedback current to flow from the integrator to the negative pole side of the first power supply when the negative pole of the comparator is saturated is connected to the second Zener diode. An isolation amplifier characterized in that the switching element is connected in parallel to a diode and is turned on when the negative electrode of the comparator is saturated.