JPH0690248B2 - Temperature compensation circuit for CV converter using inverting operational amplifier - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a temperature compensation circuit, and more particularly to a temperature compensation of a CV converter using an inverting operational amplifier (hereinafter abbreviated as operational amplifier) used in a digital capacitance meter. Regarding the circuit.

2. Description of the Related Art FIG. 3 (a) is a CV conversion circuit in which a resistor R _f is inserted in a feedback circuit of an inverting operational amplifier at a frequency of 1 MHz, and FIG. 1 (b) is an equivalent circuit diagram thereof.

In FIGS. 3 (a) and 3 (b), resistance R and reactance
X _L is A _O : DC open loop gain ω _O : 3dB lower angular frequency ω: Represented by angular frequency (working frequency 1MHz), when applied to the current highest frequency operational amplifier, when _{L f} = 8kΩ, XL = 100 ~ 200Ω R = 3~6Ω R _f = 800Ω at X _L = a 10~20Ω R = 0.3~0.6Ω, reactance X _L is a large resistor R is small.
The capacitance C _S is X _L compensation for eliminating the reactance X _L, setting the Hp terminal to a virtual ground (virtual ground), and establishing i = eωC _X.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, in the current operational amplifier, a change in the angular frequency ω _O and the DC open loop gain A _O due to a change in ambient temperature affects the performance at a high frequency of about 1 MHz. According to the experiment
+ 5 ° C. to change omega _O, A _O is changed X _L, and R are is confirmed to increase approximately + 1%. As a result, even if the reactance X _L is canceled by the value of the capacitance C _S at a certain temperature, if the ambient temperature changes by + 5 ° C, an inductive reactance of 1 to 2 Ω is generated due to the change of X _L , and R _f =
At 800Ω, the reactance increases by 0.1 to 0.2Ω. As a result, there is an error in the capacity meter reading, which varies depending on the C range, but +0.1 to + 0.2% for a change in ambient temperature of + 5 ° C.
Change occurs, and the performance deteriorates due to the influence of the capacitance between the Hp terminal and ground. The present invention relates to a compensation circuit for this temperature error.

The present invention has been made in view of the above-mentioned conventional circumstances,
Therefore, it is an object of the present invention to provide a novel temperature compensating circuit capable of solving the above-mentioned problems inherent in the conventional technique and realizing accurate and excellent temperature compensation.

Means for Solving the Problems In order to achieve the above object, a temperature compensation circuit of a CV converter using an inverting operational amplifier according to the present invention is a feedback circuit of an inverting operational amplifier including a first resistor R ₁ and Second in series with
A resistor R ₂ in parallel with the thermistor R _T is inserted, and the capacitance for eliminating the input reactance X _{L of the} operational amplifier.
It has a CV converter with C _S inserted and its reactance component Is the feedback resistance for the change of X _L caused by the change of ω _O A _{O of the} operational amplifier with temperature. The omega _O A but a change in the R _f due to the change of the thermistor R _T
By selecting the second resistor R ₂ so as to be equal to the change of _O, the reactance X _L is configured to be constant irrespective of temperature, and further, the output is the first resistor R ₁ Of the thermistor R _T so that it is irrelevant to the temperature change of the thermistor R _T , and as a result, the indicating error caused by the temperature change of the C meter and the indicating change caused by the capacitance between the Hp terminal and the ground. Is characterized by being able to make a small amount.

Embodiment Next, the present invention will be specifically described with reference to the drawings with respect to a preferred embodiment thereof.

Before explaining the embodiment of the present invention, the performance of the bead type thermistor will be briefly examined.

As a result of randomly selecting 30 bead type thermistors with a size of about 1 mm currently on the market and testing them at a high frequency of 1 MHz at 23 ° C., it was found that there was almost no error in the characteristics. The following three (1), (2), and (3) show the measurement results of the one with the largest error (| Z |: absolute value of impedance, θ; phase angle).

| Z | θ (1) 5.85KΩ −4.9 ° (2) 5.67KΩ −6.7 ° (3) 5.78KΩ −4.6 ° The values above are the constants of the parallel equivalent circuit shown in Fig. 2 (parallel capacitance: C _P , When converted to parallel resistance R _P ), Cp Rp (1) ……… 2.34pF 5872Ω (2) ……… 3.25pF 5708Ω (3) ……… 2.22pF 5798Ω. By the way, it was confirmed that the standard resistance value of direct current at 23 ℃ of this thermistor is 5.72 KΩ, which is almost equal to the DC resistance even at 1 MHz, and the reactance Cp is small and can be used as a temperature compensating thermistor.

FIG. 1A is a block diagram showing an embodiment of a temperature compensation circuit according to the present invention using the thermistor, and FIG. 1B is an equivalent circuit diagram of FIG. 1A.

Referring to FIGS. 1 (a) and 1 (b), as described above,
The input impedance Z of the operational amplifier OP ₁ of the CV converter 12 is represented by Z = R + jX _L , and the resistance and reactance components are It is represented by. In Fig. (A), The resistance R of the input impedance Z has a small absolute value even if it changes with temperature, and since it is a vector sum with respect to the impedance of the measured capacitance C _X , the effect of temperature change can be ignored. .

Reactance X _L is Changes in the reactance X _L with temperature directly affect the indication, since it affects the in-phase with. The change of ω _O A _O due to the temperature change of the operational amplifier OP ₁ is in the negative direction and the direction of increasing the reactance X _L with respect to the positive temperature change.
Since R _f contains a thermistor, it tends to decrease the reactance X _L against a positive temperature change, and if the resistor R ₂ inserted in parallel with the thermistor resistance R _T is selected appropriately, the amplifier ω _O A _O It is clear from the above equation that it can be made equal to the change of That is, it is possible to make the reactance X _L constant in a certain temperature range, and therefore the current i _X can be made constant irrespective of temperature by this compensation circuit. However, if the e ₁ terminal of the operational amplifier OP ₁ is used as an output like a normal operational amplifier terminal, it is clear that the output change due to the temperature change occurs due to the change of R _f by the thermistor, but the output is shown in the figure. Thus, if it is taken from the R ₁ terminal, the thermistor can be a factor only for keeping the reactance X _{L, that} is, the current i _X constant, without being affected by the change of the thermistor.

In this circuit, the resistance R ₁ is designed so that e = e _o when the measured capacitance C _X is full scale, However, it is possible to arbitrarily change only the temperature characteristic by selecting the resistor R ₂ while maintaining this condition.

The operational amplifier OP ₂ shown in FIG. 1 (a) is a voltage follower with a gain of 1 for receiving the resistor R ₁ with high impedance.

It is needless to say that the thermistor should be closely attached to the case surface of the operational amplifier OP ₁ or a heat dissipation plate so that the temperature change of the operational amplifier OP ₁ and the thermistor are always at the same temperature.

EFFECTS OF THE INVENTION As described above, according to the temperature compensation circuit of the present invention, the Hp terminal can be used as the virtual ground irrespective of the temperature change, and as a result, the instruction change is reduced with respect to the change of the ambient temperature, and It is possible to minimize the influence of the capacitance between the Hp terminal and ground.

[Brief description of drawings]

1 (a) and 1 (b) are circuit configuration diagrams showing an embodiment of the present invention, an equivalent circuit diagram thereof, FIG. 2 is a diagram for explaining the present invention, and FIGS. 3 (a) and 3 (b). ) Is a circuit diagram for explaining the conventional technique and its equivalent circuit diagram. 11 ... Power supply, 12, 12 '... CV conversion circuit, OP ₁ , OP ₂ ... Operational amplifier, _RT ... Thermistor

Claims (2)

[Claims] 1. A first resistor in a feedback circuit of an inverting type operational amplifier.
The parallel circuit of a second resistor R ₂ and the thermistor R _T is inserted into the first resistor R ₁ in series with R ₁ Toko, further input capacitance C _S for erasing the input reactance X _L of the operational amplifier A CV converter connected to the end, wherein the reactance X _L = ωR _f / ω _O
The reactance X _L of A _O (where ω: working angular frequency, ω _O : angular frequency of 3 dB decrease, A _O : DC open loop gain) causes the reactance X _L of ω _O A _{O of the} operational amplifier to change with temperature. Feedback resistance Rf = (R ₂ R _{T
/ R 2 + R T) +} R 1 of the thermistor the changes due to a change in the R _T ω _O A _O changes equal manner the second resistor R ₂
Is selected so that the reactance X _L is substantially independent of the temperature, and only the voltage drop of the first resistor R ₁ is output so that it is independent of the temperature change of the thermistor R _T. A temperature compensation circuit for a CV converter using an inverting operational amplifier. 2. The inverting circuit according to claim 1, further comprising a voltage follower type second operational amplifier inserted at a connection point between the _first resistor R ₁ and the second resistor R _2. Compensation Circuit for CV Converters Using Type Operational Amplifier.