JPS6122886B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a resistance value output circuit that converts voltage or current into a resistance value, and particularly to a resistance value output circuit that has a differential configuration to improve conversion accuracy.

Resistance bridge circuits are widely used as sensors for temperature control and other various control aspects.
At this time, the resistance value of one side of the resistive bridge circuit changes correspondingly depending on the voltage and current. A resistance value output circuit is the means for obtaining this resistance value. It goes without saying that such a resistance value output circuit is used for purposes other than the bridge circuit described above. Hereinafter, a conventional example of a resistance value output circuit will be described without going into details regarding usage.

FIG. 1 shows a resistance value output circuit that utilizes the on/off state of a switch. In the figure, variable input voltage
Ei is input to the AD converter 100 and converted into a digital signal, then decoded by the decoder 200, and connected to the series resistors 400a, 400b, 400.
Each switch 30 is connected in parallel to c,...
0a, 300b, 300c, . . . are turned on and off to obtain a predetermined resistance value between output terminals 1 and 2. Such a configuration has the disadvantage that resistance values can only be set discontinuously.

FIG. 2 shows a configuration diagram in which analog resistance values are changed using a servo system. That is, the variable input voltage Ei is input to the operational amplifier 20, and the output of the amplifier 20 drives the servo motor 500. The input to the servo motor 500 changes the resistance values of potentiometers 410 and 411 which are interlocked with each other. This set resistance value is negatively fed back to the operational amplifier 20. Note that the voltage E _S of the potentiometer 410 is a reference voltage. Such a configuration has the disadvantage of slow response time.

FIG. 3 shows a resistance value output circuit that is more advanced than the above two cases. The operational amplifier 20 includes:
A variable input voltage Ei is inputted via an impedance element 420 made of an FET element. Furthermore,
The input of this operational amplifier 20 is biased by a resistor 15 and a reference voltage E _S . The impedance element 420 is turned on and off by feeding back the output of the operational amplifier 20. Further, the output of the operational amplifier 20 controls the gate of a second impedance element 421 made of an FET element. Impedance elements 420 and 421 are the fourth
As shown in the figure, materials with matching characteristics are used.

Therefore, by controlling the resistance value of one impedance element 420 with respect to the input voltage Ei, a similar resistance value can be obtained for the other impedance element 421. However, since such a method requires grounding one end of the other impedance element 421 (because one end of one impedance element 420 is virtually grounded at the input of the operational amplifier 20), insulation cannot be achieved. do not have. In addition, a pair of impedance elements 420, 4
21 uses a composite FET (field effect transistor), but as shown in Figure 4, the FET control voltage V _G
As can be seen from the relationship of on-resistance R _DS to _S , even if the characteristics match, there are differences like A and B. This difference directly becomes an error in the output resistance value and does not improve accuracy. Therefore, the use of a conventional resistance value output circuit has the disadvantage that accuracy cannot be improved despite the differential configuration.

An object of the present invention is to provide a resistance value output circuit with improved accuracy.

The gist of the present invention is not to use the output of the operational amplifier directly for control, but to first convert it into a time width signal, then take out the output via an isolated buffer such as a pulse transformer, and When controlling the input and output using the buffer, the semiconductor switch in the section consisting of the resistor and the semiconductor switch is turned on and off by the buffer output. Hereinafter, the present invention will be explained in detail with reference to the drawings.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 5 is a schematic configuration diagram showing an embodiment of the resistance value output circuit of the present invention, in which the same reference numerals as in FIGS. 1 to 3 indicate corresponding parts. In FIG. 5, the input summing point of the operational amplifier 20 has an input voltage of
Ei is connected through an impedance element 450 consisting of a resistor 11 and a semiconductor switch 10, and a negative reference voltage -E _S is connected through a resistor 15. The input of the operational amplifier 20 is a voltage one-time conversion circuit 30,
That is, after passing through the V/T conversion circuit, one side controls the semiconductor switch 10 and the other side controls the semiconductor switch 40. Further, an impedance element 451 consisting of a resistor 41 and a semiconductor switch 40 is connected to the output terminals 1 and 2.

The operation of FIG. 5 configured in this way will be explained. When the input voltage Ei is applied to the operational amplifier 20 via the impedance element 450, the operational amplifier 2
The resistance value of the impedance element 450 corresponds to the input voltage by converting the output voltage into a pulse signal in the V/T conversion circuit 30 and switching the semiconductor switch 10 so that the potential at the input addition point becomes 0. Becomes a value. That is, at this time, current Ii flowing through Z _{1 with impedance element 450 as Z 1 =}
Ei/Z ₁ and resistor 15 as R, current flowing through _R =
When negative feedback control is performed via the operational amplifier 20 so that Es/R becomes equal (so that the potential at the input addition point of the operational amplifier 20 becomes 0), the following equation holds true.

Z ₁ = KEi, (K = R/Es) ...(1) ∴Z ₁ ∝Ei ...(2) On the other hand, the pulse signal controlled by the V/T conversion circuit 30 is also applied to the impedance element 451, and the impedance element 450, the semiconductor switch 40 is switched by this pulse signal, so
If the impedance element 451 is Z ₂ , then Z ₂ =Z ₁ (3). That is, output resistance values proportional to the input voltage Ei are obtained at the output terminals 1 and 2.

Next, as described above, the impedance element 45
0 and 451 as a pair, and these pair of impedance elements 450 and 451 are used as having substantially the same characteristics, and the semiconductor switch 10
Let us consider the dispersion of a pair of impedance elements in a differential circuit configuration in which the voltages and 40 are controlled by the output signal of the voltage-to-time conversion circuit 30. The impedance element 450 is Z ₁ , the resistor 11 is r, the on-resistance of the semiconductor switch 10 is r _po , the impedance element 451 is Z ₂ , the resistor 41 is r (the same value as the resistor 11 is used), the semiconductor switch 40 is turned on If the resistance is r _po (1 - Δ) (the same element as the semiconductor switch 10 is used, but the characteristic variation Δ is taken into account), and the duty of the control pulse signal of the semiconductor switches 10 and 40 is α, then Z ₁ = r + r _po /α...(4) Z ₂ = r+r _po (1-Δ)/α...(5) Taking the ratio, Z ₂ /Z ₁ = r+r _po (1-Δ)/r+r _po (6) Become. Therefore, Δ is usually around 10%, so
If r=r _po , then the variation in the resistance values of the pair of impedance elements will be 5%, and if r=10r _po , then it will be 0.9%. In this way, since the impedance element is configured such that a resistor is connected in addition to the semiconductor switch to absorb and compensate for variations in the on-resistance of the semiconductor switch, the above-mentioned effects can be obtained. Therefore, in order to improve characteristics such as the nonlinear shape of the input/output characteristics of the impedance elements or the influence on temperature, a pair of impedance elements with the same characteristics is used as a differential configuration. By controlling the resistance value of one impedance element with a circuit configuration that uses the similarity of the characteristics of the elements to improve the characteristics, it is possible to accurately obtain the resistance value of the other impedance element.

FIG. 6 is a detailed circuit configuration diagram of the resistance value output circuit of the present invention shown in FIG. 5, and the same reference numerals as in FIG. 5 indicate corresponding parts. Regarding the configuration in FIG. 6, only the parts different from the circuit in FIG. 5 will be described. The output of the V/T conversion circuit 30 is branched via a pulse transformer 50, one being distributed to the semiconductor switch 10 via a resistor 13, and the other to the semiconductor switch 40 via a resistor 43. Further, a capacitor 12 is connected in parallel to the series circuit of the resistor 11 and the semiconductor switch 10. Also, resistance 4
A capacitor 42 is connected in parallel to the series circuit of the semiconductor switch 1 and the semiconductor switch 40. Here, a three-winding transformer is used as the pulse transformer 50. Further, bipolar transistors are used for the semiconductor switches 10 and 40, and are connected as shown in FIG. 6 to reduce on-resistance.

The operation in the configuration shown in FIG. 6 is explained in FIG. 5, so it will not be repeated here.
The resistance of the pair of impedance elements Z ₁ and
The relationship of Z ₂ is shown in Figure 7. Here, Z ₁ is the resistance 11
_Z2 is the resistance value of the impedance element consisting of the resistor 41 and the semiconductor switch 4.
This is the resistance value of the impedance element consisting of 0.
In Figure 7, the resistance value is proportional to the input voltage Ei.
This shows that Z ₁ and Z ₂ have been obtained. However, the reason why there is a difference between the solid line and the dotted line is that among the resistance values Z ₁ and Z ₂ of the two impedance elements, resistance value Z ₁ is in the control loop and is not accurately controlled with respect to the input voltage Ei. However, the resistance value Z ₂ can only be changed by the amount that the resistance value Z ₁ is controlled, and the resistance value cannot be changed.
This is due to variations in the on-resistance of the semiconductor switches 10 and 40 used for Z ₁ and Z ₂ . However, in the present invention, in order to reduce the difference due to this variation, the piezoelectric transistors used in the semiconductor switches 10 and 40 are connected as shown in FIG. 6, and the saturation voltage value is lowered. As can be seen, a resistor 11 is connected in series with the semiconductor switches 10 and 40.
and 41 are connected. Therefore, the resistance value
The difference between Z ₁ and Z ₂ can be set to a value that poses almost no problem in practice, so in a resistance value output circuit with a differential configuration in which a pair of impedance elements 450 and 451 with the same characteristics are used to improve the characteristics, , it is possible to obtain an output resistance value proportional to the input voltage with high precision.

FIG. 8 shows another embodiment of the resistance value output circuit of the present invention. The difference from FIG. The voltage -Es is connected to the resistor 11 via an impedance element consisting of a semiconductor switch 10. By configuring in this way, the following formula can be obtained.

Z ₁ = K/Ei (K=Es/R...(7) Z ₂ = Z ₁ ∝1/Ei...(8) where Z ₁ is the resistance value of the impedance element consisting of the resistor 11 and the semiconductor switch 10.

Z ₂ ; resistance value of the impedance element consisting of the resistor 41 and the semiconductor switch 40;

R; resistance value of resistor 15.

Therefore, it becomes possible to accurately obtain the output resistance value _Z2 which is inversely proportional to the input voltage Ei, and the characteristics shown in FIG. 9 are obtained. In this example as well, the sixth
The same effect as the embodiment shown in the figure can be obtained.

According to this embodiment, it is possible to operate the impedance element consisting of a pair of resistors and a semiconductor switch in the differential circuit configuration as described above, so that by accurately controlling only one impedance element with respect to the input voltage, the other The impedance element can also be changed in the same way as the other impedance element, making it possible to obtain a highly accurate output resistance value corresponding to the input voltage.

In the above embodiments, bipolar transistors were used as semiconductor switches, but similar effects can be achieved using FETs.
Furthermore, a photo coupler can be used instead of a transformer.

According to the present invention, a highly accurate output resistance value could be obtained.

[Brief explanation of the drawing]

1, 2, and 3 are conventional examples, FIG. 4 is a characteristic diagram of an impedance element, FIG. 5 is an embodiment of the present invention, and FIG. 6 is a more specific embodiment of the present invention. FIG. 7 is an explanatory diagram of the present invention, FIG. 8 is a diagram of another embodiment of the present invention, and FIG. 9 is an explanatory diagram thereof. 20... operational amplifier, 30... V/T conversion circuit, 450, 451... impedance element.

Claims (1)

[Claims] 1. An operational amplifier to which an input signal consisting of an input voltage or current and a reference signal are commonly applied through a first resistor and a second resistor, respectively;
a first semiconductor switch provided in series with either the first resistor or the second resistor;
signal-to-time width converting means which takes the output signal of the arithmetic amplifier as an input and outputs a signal with a time width corresponding to the magnitude of the input; An insulated buffer electrically insulated from the output of the switch, a second semiconductor switch, and a third resistor that compensates for the on-resistance of the second semiconductor switch are connected in series. a resistance value output circuit comprising: an output section that generates an output to the outside from both sides of the switch; and means that controls on/off of the first and second semiconductor switches based on the output signal of the buffer. . 2. The resistance value output circuit according to claim 1, wherein the insulated buffer comprises a pulse transformer. 3. The resistance value output circuit according to claim 2, wherein the insulated buffer is a photocoupler.