JPH0344570A - Temperature compensation circuit of c-v converter using inverting operational amplifier - Google Patents

Abstract

PURPOSE:To execute temperature compensation accurately by a method wherein a parallel circuit of a resistance and a thermistor is inserted in series with a resistance into a feedback circuit of an amplifier, the resistance connected in series to the thermistor is selected so that an input reactance will be irrelevant to temperature and only a voltage drop of the other resistance is outputted. CONSTITUTION:An input impedance Z of an operational amplifier OP1 of a C-V converter 12 is expressed by R + jXL, and a reactance XL is expressed by omegaRf/omega0A0 (omega is an angular frequency to be used, omega0 a 3dB-lowered angular frequency, and A0 a DC open loop gain). Since the absolute value of a resistance R is small and the sum of vectors herein, the effect of a change in temperature thereon can be neglected. A change in the reactance XL is caused by a change in omega0A0 due to the temperature. However, by setting a feedback resistance Rf to be Rf = (R2RT/R2 + RT) + R1 and by selecting an appropriate resistance R2, a change in Rf can be made equal to omega0A0, since a thermistor resistance RT is designed to take part in the feedback resistance Rf. Accordingly, an error in indication due to a change in the temperature of a C meter and a change in the indication caused by a capacitance between a terminal Hp and the ground can be made very small.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a temperature compensation circuit, and in particular to a c-v converter using an inverting operational amplifier (hereinafter abbreviated as an operational amplifier) used in a digital capacitance meter. Regarding temperature compensation circuits.

Conventional technology FIG. 3(a) shows a c-v conversion circuit in which a resistor Rr is inserted into the feedback circuit of an inverting operational amplifier at a frequency of IMHz.
FIG. 1(b) is an equivalent circuit diagram thereof.

In Figures 3 (a) and (b), the resistance TAB and reactance XL are expressed as: Rf - ωQAO AO: DC open loop gain mo: 3 dB lower angular frequency ω: angular frequency 1% N□〉 , when applied to the current highest frequency operational amplifier, R = 0.3 to 0.6Ω, reactance XL is large but resistance R is small, capacitance CS eliminates reactance Xt, and connects the 41ρ terminal to virtual ground ( virtual ground) and 1=e4Ic
This is XL compensation so that , holds true.

Problems to be Solved by the Invention However, with current operational amplifiers, at high frequencies such as IMI (z), changes in the angular frequency "0 + DC open loop gain Ao due to changes in ambient temperature affect the performance. According to experiments, 10 for change, AO changes and X
It has been confirmed that R and R increase by approximately +1%. As a result, even if reactance XL is canceled by the value of capacitance Cs at a certain temperature, if the ambient temperature changes by +5°C, an inductance reactance of 1 to 2 Ω will be generated due to the change in XL. :, rtr・801)Ω is 0.1 to 0.
This results in an increase in reactance of 2Ω. As a result, an error in the reading of the capacitance meter occurs, and although it varies depending on the C range, the reading changes by +0.1 to +0.2% for a change in ambient temperature of +5°C, and the capacitance between the 11ρ 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 situation,
Accordingly, an object of the present invention is to provide a novel temperature compensation circuit that solves the above-mentioned problems inherent in the conventional technology and makes it possible to realize accurate adjacent temperature compensation.

Means for Solving the Problems In order to achieve the above object, the present invention provides a temperature compensation circuit for a C-V TR converter using an inverting operational amplifier. It has a C-V TR converter in which a parallel circuit consisting of a resistor R1, a second resistor R2, and a thermistor RA is inserted in series with the resistor R2, and a capacitor 1cs is further inserted to eliminate the input reactance XL of the operational amplifier. Then, by selecting the second resistor R2 so that the change in the feedback resistance R, +Rre is equal to the change in ω(IAO) with respect to the change in XL caused by the reactance change C, The reactance XL is configured to be substantially constant regardless of temperature, and the output is made to be only the voltage drop of the first resistor R1, so that it is independent of temperature changes of the thermistor R. As a result, it is possible to minimize indication errors caused by temperature changes of the C meter and indication changes caused by capacitance between the HP terminal and the ground.

Practical ItA Example Next, a preferred embodiment of the present invention will be specifically explained with reference to the drawings.

Before explaining the present invention in detail, let us briefly examine the performance of a bead-type thermistor.

As a result of randomly selecting 30 currently commercially available bead-type thermistors with a size of about 1 mar and testing them at 23° C. with a high frequency of IMH□, it was found that there was almost no error in the characteristics. The measurement results of the three with the largest errors are shown as (1), (2>, (3)) as follows (IZl: absolute value of impedance,
θ: phase angle).

IZI θ (1) 5.85 to Ω -4,9° (2)
5.67Ω -6,7° (3) 5
．． 78 and the constants of the parallel equivalent circuit shown in Figure 2 (parallel capacitance: Cp, parallel resistance: 1ζ
P), Cp Rp (1)...2.34pF 5872
Ω(2)・・・・・−・3.25PF 570
80 (3n...2.22pF
It becomes 5798Ω. Incidentally, the standard DC resistance value of this thermistor at 23° C. is 5.72Ω, which is approximately equal to the DC resistance even at 1M112, and the reactance Cp is small, so it was confirmed that it can be used as a temperature compensation thermistor.

FIG. 1(a) shows a block configuration of an embodiment of the temperature compensation circuit according to the present invention using the above-mentioned thermistor.
b) is an equivalent circuit diagram of FIG. 1(a).

Referring to FIGS. 1(a) and (b), as mentioned above,
The input impedance Z of the operational amplifier oP1 of the C-■ converter 12 is expressed as Z = R + j However, since the absolute value is small and it is a vector sum with respect to the impedance of the capacitance to be measured CX, the influence of temperature change can be ignored.

Therefore, changes in reactance XL due to temperature directly affect the indication. @l0AO due to temperature change of operational amplifier OPI
The change in is in the negative direction, increasing the reactance XL in response to a positive temperature change, and since a thermistor is included in the feedback resistor Rr, the change in reactance XL is in the negative direction, increasing the reactance XL in response to a positive temperature change. Resistance B
It is clear from the above equation that if the resistor R2 inserted in parallel with T is appropriately carried, it is possible to equalize the change in -◎AO of the amplifier. That is, it is possible to keep the actance XL constant within a certain temperature range, and therefore, by using this compensation circuit, the current ix can be kept constant almost independently of the temperature. However, if the e1 terminal of the operational amplifier OPl is used as an output like a normal operational amplifier terminal, it is clear that the output will change due to temperature changes due to the change in Rr due to the thermistor, but the output will be changed as shown in the figure. If taken from the terminal, the thermistor can be used only as a factor for keeping the reactance XL, that is, the current i constant, without being affected by changes in the thermistor.

In this circuit, the resistor R is designed so that e=eQ at the full scale of the capacitance to be measured ex, and while maintaining this, it is possible to arbitrarily change only the temperature characteristics by selecting the resistor R2.

The operational amplifier OP2 shown in FIG. 1(a) has a resistor R.

It is a voltage follower with a gain of 1 to receive the voltage at high impedance.

It goes without saying that care must be taken to ensure that the thermistor is brought into close contact with the case surface of the operational amplifier OP1 or the heat sink so that the temperature of the thermistor is always the same as the temperature change of the operational amplifier OPI.

As described in detail of the invention, according to the temperature compensation circuit according to the present invention, the IIP terminal can be made into a virtual ground regardless of temperature changes, and as a result, the change in indication due to changes in ambient temperature can be reduced, The influence of the capacitance between the Hp terminal and the ground can also be minimized.

[Brief explanation of drawings]

FIGS. 1(a) and (b) are circuit configuration diagrams and equivalent circuit diagrams showing one embodiment of the present invention, FIG. 2 is a diagram for explaining the present invention, and FIGS. 3(a) and (b). ) is a circuit diagram for explaining the prior art and its equivalent circuit diagram. 11...Power supply, 12.12'/C-V conversion circuit, O
P, ,op2・,. Operational amplifier, RT...thermistor

Claims (2)

[Claims] (1), the first resistor R_ in the feedback circuit of the inverting operational amplifier
1, a parallel circuit consisting of a second resistor R_2 and a thermistor R_T is inserted in series with this first resistor R_1, and a capacitor C for eliminating the input reactance X_L of the operational amplifier is further added.
s is connected to the input end, and the reactance X_L=ωR_f/ω_0A_0 (however, ω:
Angular frequency used, ω_0: Angular frequency of 3 dB reduction, A_0
ω_0A_
The second resistor R_2 is selected so that the change is equal to 0, the reactance X_L is made almost independent of temperature, and only the voltage drop of the first resistor R_1 is output, so that the thermistor R_T 1. A temperature compensation circuit for a C-V converter using an inverting operational amplifier, characterized in that it is independent of temperature changes. (2) The inverting operational amplifier according to claim (1), further characterized in that a voltage follower type second operational amplifier is inserted at the connection point between the first resistor R_1 and the second resistor R_2. A temperature compensation circuit for a C-V converter using