JP2870764B2 - Current measuring device with automatic calibration function - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current measuring device used for measuring, for example, current-voltage characteristics of a high resistance element, and more particularly to a current measuring device having an automatic calibration function. It is an invention.

[Prior Art] FIG. 5 shows a conventional current measuring device. In the figure, reference numeral 10 indicates the entire current measuring device. The current measuring device 10 includes an operational amplifier 11,
A range switch for selectively connecting the current detection resistors R ₁ , R ₂ , and R ₃ and the current detection resistors R ₁ , R ₂ , and R ₃ between the input terminal and the output terminal of the operational amplifier 11. S ₁ , S ₂ , S ₃ ,
An AD converter 15 that converts the voltage signal output from the operational amplifier 11 into a digital signal, and controls the AD converter 15, performs a correction operation, displays a measurement result on a display 16, and the like. And a memory 18 attached to the microcomputer 17.

The current to be measured _IX is supplied to the current detection resistors R ₁ , R ₂ , and R ₃ selected by the range change switches S _{1 to} S ₃ through the input terminal 19.
Flow or to any of R _3, causes a voltage drop in any of the current detecting resistor _{R 1, R 2, R 3} . A voltage corresponding to this voltage drop is output as a voltage signal, AD converted, and displayed on the display 16.

That is, the operational amplifier 11 and the current detection resistors R _{1 to} R ₃ constitute a current-voltage converter. The operational amplifier 11 operates as an impedance converter that realizes a high input impedance-low output impedance.

The measurement accuracy of this current measuring device 10 is a current detecting resistor R ₁
It is determined by the resistance value of the to R _3. Therefore, in order to calibrate the current measuring device 10 conventionally, the current detecting resistors R _{1 to} R ₃
Connect a variable resistor for resistance adjustment in series with
Or stores the calibration coefficient of the current detecting resistor R ₁ to R ₃ in the memory 18 which is attached to the microcomputer 17, the calibration coefficient measurement result or the current detection resistor R ₁ to R ₃ in every measurement
Is corrected by multiplying the resistance value.

Connect the variable resistor for resistance adjustment to each of the current detecting resistor R ₁ to R ₃ "0005" in series, or the current detection resistor R ₁ to R ₃ In any of the methods of storing the calibration coefficients in the memory 18, the calibration must be performed for each range.

For this reason, a standard current generator that generates a current corresponding to the full scale of each range is prepared, and each range is calibrated using the standard current generator.

When the measurement current range is micro current, such as micro, nano or pico order, the current given by the standard current generator is affected by the capacitance of the cable connected between the input terminal 19 and the standard current generator. There is also a disadvantage that the calibration takes time because the time until stabilization is long.

SUMMARY OF THE INVENTION An object of the present invention is to provide a current measuring apparatus having a calibration function for automating the calibration in a minute current region by solving these disadvantages.

[Means for Solving the Problems] A current to be measured is applied to an inverting input terminal, and a current detecting resistor R ₁ ,
R ₂ and R ₃ are connected, and an operational amplifier that outputs a voltage corresponding to the value of the current to be measured, and current detection resistors R ₁ , R ₂ , and R ₃ are selectively connected to the output terminal of the operational amplifier and the inverting input. And a range changeover switch that is connected between the input terminal and the input terminal of the operational amplifier to supply a measured current or a standard current externally to the inverted input terminal of the operational amplifier. An input terminal, a standard battery connection terminal to which a standard voltage is supplied from a standard battery during calibration, a calibration voltage source for generating a calibration voltage Vr, an output voltage of an operational amplifier, a standard voltage supplied to the standard battery connection terminal, The calibration voltage Vr and the voltage of the common potential are selectively inputted, and an AD converter for AD-converting each of these voltages, and a resistor having the smallest resistance value among the current detection resistors R ₁ , R ₂ and R ₃ defined as a reference resistor a current detecting resistor R _1, the The reference resistor R ₁ by applying a calibration voltage Vr through the other end to the range selector switch of the current detecting resistor R ₂ or R ₃ in a state of being connected between the inverting input terminal and the output terminal of the operational amplifier, the current A mode changeover switch for changing to a state in which a current determined by the calibration voltage Vr and each resistance value of each current detection resistor R ₂ and R ₃ is supplied to the inverting input terminal of the operational amplifier through the detection resistor R ₂ or R ₃ ; The standard voltage applied to the standard battery connection terminal is directly input to the AD converter, and the value K = S _F −S _Z of the difference between the AD converted value S _F and the value S ₂ obtained by AD converting the common potential voltage, and difference calibration voltage Vr inputted directly to the AD converter, and the AD converted value E _F a common potential value E _Z a voltage of the AD conversion of the P =
E _F -E _Z and the voltage displayed on the standard cell value D and the standard current I to the input terminal while connecting the reference resistor R ₁ between the inverting input terminal of the operational amplifier and the output terminal _S
Enter a, the operational amplifier value a ₁ and the AD converting the voltage to be output, the current value is displayed in the standard current source enter the standard current I _S
Current detection with the A _1, a memory for storing as an initial calibration coefficient, a reference resistor R ₁ is greater than the resistance value in a state of connecting the reference resistor R ₁ between the inverting input terminal of the operational amplifier and the output terminal given calibration voltage Vr to one end of the use resistors R _2,
The voltage operational amplifier in a state in which type the current determined by the calibration voltage Vr and the current resistance of the detecting resistor R ₂ to the inverting input terminal of the operational amplifier through the current detection resistor R ₂ is calibrated AD
The converted value b ₂ and the current detecting resistor R ₃ having a resistance value larger than the current detecting resistor R ₂ with the reference resistor R ₁ connected between the inverting input terminal and the output terminal of the operational amplifier. one end giving calibration voltage Vr of, is an operational amplifier in a state in which type the current determined by the resistance value of the calibration voltage Vr and the current detection resistor R ₃ to the inverting input terminal of the operational amplifier through its current detecting resistor R ₃ a calibration operation control means for measuring the value b ₃ to the voltage output to the AD converter, and the memory further stores a value b _2, b ₃ obtained in this calibration operation control means, an initial calibration coefficient K stored in the memory , P, D, a ₁ , A ₁ and the value b ₂ ,
b ₃ and by a calibration factor G ₂ of the current detecting resistor R ₂ G ₂ = A ₁ ·
Determined by _{K · b 2 / a 1 ·} P · D, operations for storing the calibration coefficient G ₃ of the current detecting resistor _{R 3 G 3 = A 1 ·} K · b 3 / a 1 · P · found through D storage means, the measured current applied to the input terminal during practical input to the operational amplifier, and Amasan the calibration coefficient G ₂ or G ₃ is read in accordance with the measurement range to the voltage output, current to be measured A current measuring device with an automatic calibration function is constituted by the calculating means for calibrating the value.

[Operation] In the configuration of the present invention, first, a standard battery is prepared outside, and the standard voltage of the standard battery is directly supplied to the AD converter, and whether or not the AD conversion output has a value corresponding to the standard voltage is determined. If the value is out of the value corresponding to the standard voltage, a calibration coefficient for correcting the AD conversion value to a value corresponding to the standard voltage is obtained, and the calibration coefficient is stored in the memory.
Finish the calibration of the AD converter.

Using the AD converter calibrated in this way, the calibration voltage of the calibration voltage source provided inside is measured, and the true value of the calibration voltage is obtained.

Next, the resistance value of the first current detection resistor used in the range having the smallest resistance value among the plurality of current detection resistors, that is, the largest current value is calibrated.

This calibration is performed by connecting a standard current source to the input terminal of the operational amplifier with the first current detecting resistor connected between the input and output terminals of the operational amplifier.

In other words, a standard current having a known value is given to the first current detection resistor from the outside, and the voltage generated there is subjected to AD conversion by the AD converter.
Convert and convert the AD conversion value to the first connected
Find the true resistance of the current detection resistor.

When the resistance value of the first current detection resistor is determined, a second current detection resistor having the next highest resistance value is connected between the operational amplifier and the calibration voltage source provided in the operation amplifier. 2
A current is supplied to the first current detection resistor through the current detection resistor. The voltage generated in this state is R ₁ , the resistance value of the first current detection resistor, and the resistance value of the second current detection resistor.
Assuming that R ₂ and the voltage of the calibration voltage source are V _r , it is given by R ₁ / R ₂ · V _r . As a result the measured voltage and the R ₁ and V _r can be obtained resistance value R ₂ of the second current detection resistor because it is known.

In this manner, the resistance value of the first current detection resistor is determined by:
The resistance values of the second, third, and fourth current detection resistors are measured using the voltage of the calibration voltage source, and a calibration coefficient for correcting the deviation is calculated by a microcomputer.

Therefore, according to the present invention, measurement for calibrating the resistance values of the second, third, fourth,... Current detecting resistors is performed using the calibration voltage generated by the calibration voltage source provided therein. Therefore, there is an advantage that a standard current source for calibration need not be prepared for each range.

Also, since each range is calibrated using the internal calibration voltage source, there is no influence of the cable. For this reason, there is also obtained an advantage that calibration can be performed in a short time.

FIG. 1 shows an embodiment of the present invention. In the figure, reference numeral 10 denotes a current measuring device with an automatic calibration function according to the present invention.

In addition to a standard voltage input terminal 21 of the automatic calibration function with the current measuring device 10 measures the input terminal 19 according to the present invention, to connect the standard cells 22 as required to the standard voltage input terminal 21, the switch S ₇ It is configured so that the standard voltage can be supplied from the standard battery 22 to the AD converter 15 by converting to the contact c.

Further, in the present invention, a calibration voltage source 23 is provided inside, and a calibration voltage _Vr is generated from the calibration voltage source 23, and the calibration voltage _Vr
The configured to be supplied to the AD converter 15 through the contact a of the switch S _7. Note the switch S ₈ indicates a changeover switch for selectively inputting the common potential and the calibration voltage V _r to the AD converter 15.

On the other hand, one ends of the current detection resistors R ₁ , R ₂ , R ₃ , and R ₄ are connected to the inverting input terminal of the operational amplifier 11. Now the first current detecting resistor and R ₁ which defines the largest current range, the resistor R ₁ is connected to the output terminal of the operational amplifier 11 through the range change switch S _0.

The other current detection resistors R ₂ , R ₃ , and R ₄ are connected to the mode selector switches S ₄ , S ₅ , and S ₄ through the range selector switches S ₁ , S ₂ , and S ₃ .
Connect to the movable contact of S _6.

One contact a of the mode changeover switches S ₄ , S ₅ , S ₆ is connected to the output terminal of the operational amplifier 11, and the other contact b is connected to the calibration voltage source 23.

Range switching switch S ₀ ~S ₃ and the mode selector switch S ₄ ~
S ₆ and the changeover switch S _7, S ₈ is automatically switched controlled according independently programmed by the microcomputer 17.

In the normal measurement mode, the mode changeover switches S ₄ , S ₅ , S ₆
Is connected to the contact a, also the changeover switch S ₇ is connected to the contact b, the range selector switch S ₀ to S ₃ is any one operation to turn on is selected in accordance with the size of the measured current input The measurement is performed.

 On the other hand, the calibration operation is performed in the following order.

Calibrate the AD converter 15.

Connects the switch S ₇ to the contact c, the standard battery to the input terminal 21
Connect 22. The AD converter 15 AD converts the known voltage D supplied from the standard battery 22 and stores the AD converted value _SF in the memory 18. In this case enter the 0V common potential to the AD converter 15, an AD conversion value S _Z at that time is subtracted from S _F, initial calibration of K = S _F -S _Z removing the offset of the zero point in the memory 18 Store as one of the coefficients. The true voltage D (shown on the battery 22) of the standard battery 22 used at this time is stored in the memory 18 as one of the initial calibration coefficients.

The voltage of the calibration voltage source 23 is measured.

The changeover switch S ₇ is connected to the contact a, it gives the common potential 0V and the calibration voltage Vr to the AD converter 15 to convert the change-over switch S _8. The AD conversion value E _Z when given 0V from the AD conversion values E _F of the calibration voltage Vr by subtracting to obtain the P = E _F -E _Z, and stores as one of the initial calibration coefficient P in the memory 18.

Calibrating the reference resistor R _1.

When the first current detecting resistor for defining the maximum range and R _1, the resistor R ₁ is most resistance value smaller in the current detecting resistor. In other words, the current measurement range is 1,1 / 1
Assuming that the relationship is set to 0, 1/100, 1/1000, R ₁ , R ₂ ,
If the resistance value of R ₃ and R ₄ is ₁ , the resistance value of R ₁ is ₁ , R ₂ is 10 times, and R ₃
Is 100 times and R ₄ is 1000 times the resistance.

Therefore, here the current detection resistor with the smallest resistance value
Set a reference resistor R _1, to measure the true resistance of the reference resistor R _1.

For this purpose, connect the switches S ₇ to the contact b, to control the range selector switch S ₀ on and off S ₁ to S _3. In this state, the standard current source 24 is connected to the input terminal 19, and the known standard current _IS is input. In this case the voltage generated at the output of the operational amplifier 11 and AD conversion, and stores the measured value as one of the initial calibration coefficient memory 18 as a _1. Also stored as one of the initial calibration coefficient the true value of the standard current I _S that have entered at this time (the value that is displayed in the standard current source 24) in the memory 18 as A _1. When these initial calibration coefficients K, P, D, a ₁ and A ₁ are stored, the standard current source 24 is disconnected from the input terminal 19.

Here the voltage value a ₁ of the operational amplifier 11 outputs can be determined by _{a 1 = R 1 · A 1} . When the calibration coefficient of the reference resistor R ₁ and G _1,
The current value A ₁ which is input, A ₁ = G ₁ a ₁ holds between the output voltage a ₁ of the operational amplifier 11.

Thus calibration coefficients G ₁ reference resistor R ₁ determined by calculating the _{G 1 = A 1 / a 1} , and stores the value in G ₁ in the memory 18, and terminates the initial calibration operation.

Measuring the resistance value of the current detection resistor R _2.

Connect the changeover switch S ₇ to b, the range selector switch
S ₀ and S ₁ are turned on, S ₂ and S ₃ are turned off, and the mode switch S ₄ is connected to the contact b.

Therefore, in this state, as shown in FIG.
There is connected to the inverting input terminal of the operational amplifier 11 through a current detecting resistor R _2, providing a current I _b to the current detection resistor R ₁ through a current detecting resistor R _2. The time for storing the voltage at the output side of the AD converter 15 as b ₂ in the memory 18.

Determining calibration coefficients of the current detecting resistor R _2.

Now where the connection state of the measurement range by the current detecting resistor R ₂ is as shown in Figure 3.

In this measuring condition, the current to be measured if the X, its the AD conversion data and _{c 1, c 1 = R 2} X ...... because it is (1) (hereinafter current detection resistor R ₁ to R ₄ When this is calibrated, the data must be X. That is, when the calibration coefficient and _{G 2, X = G 2 ·} c 1 ...... (2) (1), (2) calibration coefficients G ₂ from equation Should be obtained.

In FIG. In the calibration data _{a 1 = R 1 · A 1} ...... (4), the AD converter 15 in the calibration of the current detecting resistor R ₁ obtained, V by measurement of the calibration voltage V _{r r:} P = D : K (5) From equation (5), Substituting equations (4) and (6) into equation (3) gives As a result, the calibration coefficient G ₂ of the range of the current detection resistor R ₂ is obtained.
Is found. Storing the calibration coefficients G ₂ in the memory 18.

In the range of the current detection resistor R ₃ and R ₄ may be changed current detection resistor R ₂ to R ₃ and R ₄ in Figure 2.

However, in the range of the current detection resistor R ₄
Assuming that changes by 10 times, the ratio of the resistance value of R ₄ and R ₁ is
It becomes 1/1000, and the dynamic range of the AD converter 15 becomes insufficient. Necessary valid data cannot be obtained from this restriction. Therefore it is conceivable to use a current detection resistor R ₃ as a reference resistor in place of R _1.

(7) Calibration Factor G ₃ of the current detecting resistor R ₃ from the equation Becomes

From equation (3) Substituting equations (8) and (6) into equation (9), Becomes

Thus, the reference resistor R ₁ and each current detection resistor R ₂ ,
R _3, calibration coefficients _{R 4 G 1, G 2,} G 3, G 4 is Given by

In the actual measurement, the calibration coefficients G ₁ , G ₂ , G ₃ , G ₃ are set according to the measurement range (the current detection resistors R ₁ , R ₂ , R ₃ ... Connected to the operational amplifier 11). ₄ ... is selected and read out, and the calibration coefficients G ₁ , G ₂ , G ₃ , G ₄ ... Are converted into the measured voltage value V _x (the value obtained by AD-converting the output voltage of the operational amplifier 11). By performing addition, calibration can be performed for each measurement range.

The above-described calibration operation is automatically executed according to a program stored in a ROM built in the microcomputer 17. Therefore, it can be seen that the calibration control means is calibrated by the microcomputer 17 and the program stored in the ROM.

"Variant" is in the above described case of the range selector switch S ₀ to S ₆ is Ru switched is operated on sequentially only one range structure, connected in parallel all the plurality of the current detecting resistor It is also possible to set a range in which the current value is the largest, and to separate the current detection resistors one by one from that state to lower the range of the measured current.

It is also possible to connect the reference resistor R _X as shown in FIG. 4 to the input side of the operational amplifier 11.

[Effects of the Invention] As described above, according to the present invention, each range is calibrated using the calibration voltage source 23 provided therein, so that it is not necessary to prepare a standard current source for each range. Therefore, the economic burden on the user can be reduced.

In addition, since the calibration is performed using the calibration voltage source provided inside, the calibration can be performed without being affected by the capacitance of the cable. As a result, calibration can be completed in a short time.

Further, according to the present invention, a resistor having the smallest resistance value among the current detection resistors R _{1 to} R ₄ is determined as a reference resistor, and the calibration of each range is performed using the resistance value of the reference resistor. Thus, highly accurate calibration can be performed.

In other words, the change with time of the resistor shifts greatly as the resistance value increases. For this reason, by selecting the reference resistor as the resistor having the smallest resistance value, the influence due to aging can be minimized, and in this regard, the accuracy of calibration can be ensured.

[Brief description of the drawings]

FIG. 1 is a connection diagram showing one embodiment of the present invention, FIGS. 2 and 3 are connection diagrams for explaining a calibration operation of the present invention,
FIG. 4 is a connection diagram showing a modified embodiment of the present invention, and FIG. 5 is a connection diagram for explaining a conventional technique. 10: current measuring device, 11: operational amplifier, R ₁ to R _4: the current detecting resistor, S ₀ to S _3: Range selector switch, S ₄ to S _6: mode switch, S _7, S _8: Switch Switch, 15: AD converter, 1
6: display, 17: microcomputer, 18: memory, 22:
External standard battery, 23: Internal calibration voltage source, 2
4: Standard current source provided externally.

Claims (1)

(57) [Claims] A. A current to be measured is supplied to an inverting input terminal.
B. the operational amplifier that selectively connects current detection resistors R ₁ , R ₂ , and R ₃ between the output terminal and the inverting input terminal and outputs a voltage corresponding to the value of the current to be measured; C. a range switch for selectively connecting the current detection resistors R ₁ , R ₂ , and R ₃ between the output terminal and the inverting input terminal of the operational amplifier, and switching a measurement range of the current to be measured; An input terminal derived from the inverting input terminal of the operational amplifier and externally supplying a measured current or a standard current to the inverting input terminal of the operational amplifier; D. a standard battery connection terminal to which a standard voltage is supplied from a standard battery during calibration E. a calibration voltage source that generates a calibration voltage Vr; F. an output voltage of the operational amplifier, a standard voltage supplied to the standard battery connection terminal, and a calibration voltage Vr generated by the calibration voltage source. And the potential voltage are selectively input. Defined an AD converter for converting the G. most for small current detection resistance value resistor R ₁ in the current detecting resistor and the reference resistor, the reference resistor R ₁
Was applying the calibration voltage Vr through the range change switch to one end of the other of the current detecting resistor R ₂ or R ₃ in a state of being connected between the inverting input terminal and the output terminal of the operational amplifier, said current sensing A mode changeover switch for switching to a state in which a current determined by the calibration voltage Vr and the respective resistances of the current detection resistors R ₂ and R ₃ is supplied to the inverting input terminal of the operational amplifier through the resistor R ₂ or R _3. H. The standard voltage given to the standard battery connection terminal
Directly input to the transducer, the AD conversion value S _F to a common potential voltage difference value K = S _F -S _Z and the AD converted value S _Z of, directly the calibration voltage Vr to the AD converter type, the difference P of the voltage of the common potential value E _F that was AD converted AD converted value E _Z
= E _F -E _Z , the voltage value D displayed on the standard battery, and the standard current I connected to the input terminal with the reference resistor connected between the inverting input terminal and the output terminal of the operational amplifier. _S
A memory type stores the operational amplifier value a ₁ and the AD converting the voltage to be output, and a current value A ₁ which is displayed in the standard current source inputs the standard current I _S as an initial calibration coefficient, I. above the reference resistor one end of the current detection resistor R ₂ in a state of connecting the R ₁ with the reference resistor R ₁ is greater than the resistance between the inverting input terminal and the output terminal of the operational amplifier given calibration voltage Vr, the operational amplifier in a state in which type the current determined by the resistance value of the calibration voltage Vr to the inverting input terminal and the current detection resistor R ₂ of the operational amplifier through the current detection resistor R ₂ the value b ₂ obtained by AD converting the voltage but the output, the current detecting resistor while connected to the reference resistor R ₁ between the inverting input terminal and the output terminal of the operational amplifier R ₂
One end of the current detection resistor R ₃ with larger resistance value gives the calibration voltage Vr, the calibration voltage Vr and the current detection resistor to the inverting input terminal of the operational amplifier through its current detecting resistor R ₃ the value b of the voltage the operational amplifier in a state in which type the current determined by the resistance value is output vessels R ₃ and AD conversion
₃ , calibration operation control means for measuring, J. the memory for further storing the values b ₂ and b ₃ obtained by the calibration operation control means, and K. the initial calibration coefficients K and P stored in the memory. , D, a ₁ , A ₁ and value
By and b _2, b ₃ a calibration factor G ₂ of the current detecting resistor R ₂
Determined by _{G 2 = A 1 · K ·} b 2 / a 1 · P · D, the calibration factor G ₃ of the current detecting resistor _{R 3 G 3 = A 1 ·} K · b 3 / a 1 · P · D
A calculation memory means for storing determined by, the measured current applied to the input terminal and the input to the operational amplifier in L. practical, the calibration coefficient G ₂ or G are read in accordance with the measurement range to the voltage output _{3. A} current measuring device with an automatic calibration function, comprising: a calculating means for calculating the current value to be measured by dividing ₃ ;