JP2994649B2 - Voltage applied current measuring device with automatic calibration function - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Industrial application field" The present invention relates to a voltage applied current measuring device that can be used for measuring voltage-current characteristics of, for example, a high-resistance element, and particularly has an automatic calibration function. It is an object of the present invention to provide a voltage application current measurement device provided.

[Prior Art] FIG. 15 shows the structure of a conventional voltage applied current measuring device.
In the figure, reference numeral 10 denotes a microcomputer, reference numeral 20 denotes a DA converter that converts digital data output from the microcomputer 10 into an analog voltage and outputs an analog voltage, and reference numeral 30 denotes this DA converter.
Output terminal that is equivalent to the analog voltage given from 20
The output voltage generator is shown at 31.

The voltage generator 30 includes an operational amplifier 32 and a buffer amplifier 33 that detects the voltage output to the output terminal 31 and feeds back the voltage to the operational amplifier 32.

Reference numeral 40 denotes a current measuring device. The current measuring device 40 includes current detecting resistors R ₁ , R _2, ... R ₄ connected between the output of the operational amplifier 32 constituting the voltage generator 30 and the output terminal 31, and these current detecting resistors. R ₁ , R ₂ … R ₄ are inserted into and removed from the circuit, and the range changeover switches S ₁ , S ₂ , S ₃ for switching the current measurement range and the voltage generated in the current detection resistors R _{1 to} R ₄ are extracted. Differential amplifier
The detection voltage taken out by the differential amplifier 41 is AD-converted by the AD converter 50, and is taken into the microcomputer 10 as a current measurement value.

The microcomputer 10 is provided with a display 11 for displaying a measured current value or a generated voltage. Also memory 12
And stores the measured values and the like.

In a normal use state, a subject (not particularly shown) is connected to the output terminal 31, and a predetermined voltage is applied to the subject.

This applied voltage value is managed by the microcomputer 10 and changed, for example, in a stepwise manner, and a current is measured for each voltage value to measure a voltage-current characteristic or the like.

On the other hand, in this type of measuring instrument, it is necessary to perform calibration for inquiring whether the generated voltage and the output current are correct. Calibration includes calibration of the generated voltage and calibration of the output current. Calibration of the generated voltage can be performed relatively easily since it is sufficient to check whether the voltage output to the output terminal 31 matches the voltage value specified by the microcomputer 10.

On the other hand, for the calibration of the output current, a standard current source 60 that generates a standard current close to the full scale of each range is prepared, and the standard current source 60 is connected to the output terminal 31 to perform calibration.

[Problems to be solved by the invention] In the conventional voltage applied current measuring device, the current calibration is performed by preparing a current standard for each current measurement range and performing calibration individually. A constant current generation standard must be prepared. For this reason, the economic burden on the user is large.

Also, if the current value to be measured is a very small current of microamperes or less, if a calibration standard current is connected externally and calibrated, it takes time for the current to stabilize due to the stray capacitance of cables etc. There is. That is, there is a disadvantage that the time required for the calibration becomes long.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a voltage application current measurement device capable of minimizing the number of calibration standard current sources as much as possible and calibrating a current measurement circuit in a short time.

"Means for solving the problem" The voltage applied current measuring device with an automatic calibration function of the present invention,
A DA converter that converts the applied voltage data into an analog voltage, and an output voltage of the DA converter and a voltage that is applied to the subject are input to the inverting input terminal via a resistor, respectively. A first operational amplifier for inputting to the inverting input terminal, a switching contact to which the output terminal of the first operational amplifier is connected, and a first switching switch for switching any one of the switching contacts to which ground is connected, A second plurality of changeover switches for switching any one of a changeover contact to which the common contact of the first changeover switch is connected, a changeover contact to which ground is connected, and a second plurality of changeover switches respectively A plurality of current detection resistors, one common contact of which is connected to one end of the range changeover switch and the other end of which is connected to one contact of each range changeover switch. The end is more than one A basic resistor connected to one contact of one of the range change switches, and a contact at the other end thereof are made common, and the contact of the other end made common is connected to the subject. A plurality of range changeover switches connected to the output terminal, a contact connected to a common contact of the first changeover switch, and a third changeover switch configured to switch between a contact connected to earth and the ground; A second operational amplifier having a common contact of a third changeover switch connected to a non-inverting input terminal and a common contact of a fourth changeover switch connected to an inverting input terminal, and three resistors are connected in series. One end of the series-connected resistor is connected to the output terminal of the second operational amplifier, the other end is connected to the output terminal to the subject, and two intermediate ends are respectively used for switching the fourth changeover switch. Amplification connected to contacts A switching resistor, an AD converter for converting the output of the second operational amplifier to a digital voltage, and a common contact for connecting the ground contact to a switching contact when measuring the amplification of the second operational amplifier. The first switch for switching, the second switch for switching a common contact to a switching contact to which ground is connected when measuring the resistance value of the resistor for current detection, and the resistor for current detection And a third contact for switching a common contact to a contact connected to earth when measuring the amplification of the second operational amplifier.
And the fourth changeover switch for changing a common contact to a changeover contact connected to two intermediate ends of the amplification degree changeover resistor when changing the amplification degree of the second operational amplifier. A microcomputer that gives and stores the voltage data to the DA converter, receives and stores the output data of the AD converter, and performs a predetermined calculation using the stored data. And

According to the present invention, the resistance value of the current detection resistor having the smallest resistance value is calibrated using the constant current generation standard device, and the resistance value of this resistor and the known value output by the voltage generator are calibrated. The resistance value of another current detecting resistor can be measured by using the voltage of (1).

As a result, since the resistance value of the current detecting resistor used in each measurement range can be known, calibration of each measurement range can be performed internally.

Therefore, according to the present invention, it is not necessary to prepare a current standard for calibration for each measurement range. Therefore, the economic burden can be reduced. In addition, since each measurement range can be calibrated internally, there is no influence of the stray capacitance of a cable or the like, so that the time required for calibration can be reduced.

FIG. 1 shows an embodiment of the present invention. In FIG. 1, reference numeral 10 denotes a microcomputer, reference numeral 20 denotes a DA converter, reference numeral 30 denotes a voltage generator, reference numeral 40 denotes a current measuring device, and reference numeral 50 denotes a microcomputer which converts the measurement result of the current measuring device 40 into an analog signal and converts it into a digital signal. Figure 10 shows an AD converter that inputs current measurement values.

In the present invention, one end of each of the current detection resistors R _{1 to} R ₅ is connected to the current detection circuit 40 from the output side of the operational amplifier 32 constituting the voltage generator 30 in addition to the range changeover switches S _{1 to} S _5. Changeover switch S ₆ that can be selectively connected to 42
The S ₉ is provided, configured to be used as a current load according resistors R ₁ to R ₅ for current detection necessary. That is, when the changeover switches S _{6 to} S ₉ select the contact 1, the current detection resistor R ₁
One end of the to R ₅ is connected to the output terminal of the operational amplifier 32, the contact 2
The except for the current detecting resistor R ₁ selected each end of the other of the current detecting resistor R ₂ to R ₅ is selectively connected to the common potential point 42.

Further in the present invention provide a changeover switch S ₁₀ to the output terminal of the operational amplifier 32. The output voltage of the operational amplifier 32 when the changeover switch S ₁₀ is to select the contact 1 is a current detecting resistor R ₁ ~
All of the current detecting resistors R _{1 to} R ₅ are connected to the common potential point 42 when applied to one end of R ₅ and the contact 2 is selected.

Further provided the switch S ₁₂ to one input terminal of the differential amplifier 41 for taking out a voltage generated in the current detection resistor R ₁ to R ₅ in this embodiment. The changeover switch S ₁₂ is in a state for taking a voltage generated in the current detection resistor R ₁ to R ₅ in the differential amplifier 41 when selecting contact 1, one of the differential amplifier 41 when selecting the contact 2 Are connected to a common potential point.

On the other hand, the differential amplifier 41 has two operational amplifiers 41A in this example.
And shows the case of a cascade connection structure 41B, shows the case of attaching a gain switching switch S ₁₁ to the operational amplifier 41A that is located before. Gain switching switch S ₁₁ is designed to be a overall gain, for example 3.3 times the differential amplifier 41 when selecting contact 1, when selecting the contact 2 is designed to be 33 times.

Or each selector switch has been described S ₁ to S ₅ and _{S 6 ~S 9, S 10,} S
_11, S ₁₂ are switched controlled individually by the microcomputer 10, respectively.

(Operation of Voltage Generator 30) A voltage generation command signal output from the microcomputer 10 is supplied to the DA converter 20. The DA converter 20 converts the voltage generation command signal supplied from the microcomputer 10 from DA to +10 to- Outputs analog voltage V ₀ in the range of 10V.

Analog voltage V ₀ is amplified by operational amplifier 32, a changeover switch S _10, are output to the output terminal 31 through the parallel connection of the range selector switch S ₁ to S current detecting resistor is selected by the ₅ R ₁ to R ₅ You.

The voltage signal output to the output terminal 31 is
Is fed back to the operational amplifier 32. Therefore the operational amplifier
Gain A ₁ in 32 is _{A 1 = R 10 / R 9} .

When the output voltage output to the output terminal 31 is V, Is output. Gain A ₄ normal buffer amplifier 33 A ₄ = 1
Set to. That operates to transmit to the output terminal 31 an input voltage V ₀ the operational amplifier 32 compensates the voltage drop in the middle of the current detecting resistor R ₁ to R ₅ by the feedback operation of the buffer amplifier 33.

In this way, the voltage applied to the subject connected to the output terminal 31 can be handled as a known value by the microcomputer 10, and the voltage applied to the subject can be determined from the digital value given to the DA converter 20. Can be.

(Operation of Current Measuring Device) Assuming that a subject is connected to the output terminal 31, a current flows through the subject. This current voltage signal generated flows, at both ends thereof through the current sensing resistor R ₁ to R _5, the voltage signal is inputted is taken out by the differential amplifier 41 to the AD converter 50,
The data is AD-converted by the AD converter 50 and taken into the microcomputer 10.

Note changeover switch S ₆ to S ₉ when operating the current measuring device 40 as a current measuring device is selected to the contact 1 side, the range selector switch S ₁ to S ₅ in accordance with the current flowing through selected.

The measurement range was 10 times the unit, when the resistance value r ₁ of the current detecting resistor R ₁ assumed as r ₁ = ₁ (MΩ) the resistance value of each current detection resistor _{r 1 = 1 (MΩ) r} 2 = become _{10 (MΩ) r 3 = 100} (MΩ) r 4 = 1 (GΩ) r 5 = 10 (GΩ).

In measuring the current, the current detection resistors R _{1 to} R ₅
The voltage between both ends of the resistor is 3.2 (Vmax), and each current detection resistor R _{1 to} R
Assuming that the current that can be measured at ₅ is I ₁ , I ₂ , I ₃ , I ₄ , I ₅ , Becomes

Also, if the measurement digit is assumed to be 5 digits in consideration of the accuracy of the AD converter 50, I ₁ = 3.2000 (μA) I ₂ = 320.00 (nA) I ₃ = 32.000 (nA) I ₄ = 3.2000 (nA) I ₅ = 320.00 (PA) range is configured.

The voltage across the current detection resistor R ₁ ~R ₅ (3.2Vmax) is amplified by the differential amplifier 41A. The gain g is determined by the resistors R ₆ , R ₇ , R ₈ , and the gain is changed by the gain switch S ₁₁ .

Gain measurement switch S ₁₁ during current measurement (normal use)
Is connected to the contact 1 side. Amplification factor g ₁ at this time of the differential amplifier 41A is designed to be 3.3 times.

Also, the gain switching switch S ₁₁ to a part of the automatic calibration is connected to the contact 2 side, the amplification degree g ₂ at this time of the operational amplifier 41A
Is designed to be 33 times.

The 33 times means usually 10 times of 3.3 times, which will be described when explaining the automatic calibration.

The relationship between the gain g and the resistors R ₆ , R ₇ , R ₈ is determined when the gain switch S ₁₁ is on the contact 1 side. When the gain switching switch S ₁₁ is in contact 2 side Becomes

The voltage amplified by the operational amplifier 41A is applied to the operational amplifier 41B.
The signal is differentially amplified by the A / D converter 50 and A / D converted by the A / D converter 50 to be taken into the microcomputer 10 as a measured current value.

(Calibration operation) Before the automatic calibration operation is performed, the voltage generator 30 is calibrated as a preliminary step and the maximum current range (3.2 μ
Calibrate A) by manually measuring the resistance value r ₁ of the most a small resistance value R ₁ in the current detection resistor R ₁ to R _5, measures a differential amplifier 41A, a gain of 41B manually .

Calibration of the maximum current range The calibration state of the maximum current range is shown in FIG. The maximum current range is set by turning on all the range selector switch S ₁ to S _5. Further, all the changeover switches S _{6 to} S ₁₂ are connected to the contact 1. Further, a constant current generating standard device 60 is connected to the output terminal 31.

Here, r ₁ ′: a parallel combined resistance value of R _{1 to} R ₅ D ₁ : an AD conversion output value I _SS : an output current value of a constant current generating standard device g ₁ : a gain of the differential amplifier 41 (3.3 times) × ₁ : Calibration coefficient D ₁ = g ₁ · r ₁ ′ · I _SS (1) In order for the AD conversion value D ₁ to match the output current I _{SS of the} constant current generating standard device 60, the calibration coefficient x ₁ is x ₁ = g ₁ · r ₁ ′ (2)

Standard current I _SS of Naojo current generator standards 60 full scale to generate 3.2μA so near the 3.0Myuei. The value of this time I _SS is treated as numeric 30000.

Measuring the resistance value r ₁ of the most a small resistance value R ₁ in the current detection resistor R ₁ of the resistance value measured then the current detection resistor R ₁ to R _5.

A state of measuring the resistance value r ₁ of the current detecting resistor R ₁ 3
Shown in the figure. This state is realized by connecting a range changeover switch S ₁ to S ₄ off, the S ₅ ON, all the switch S ₆ to S ₁₂ contacts 1.

r ₁ : 1 (MΩ) D ₂ : AD conversion value.

The relationship at this time is: D ₂ = g ₁ · r ₁ · I _SS (3) From equation (3), r ₁ = D ₂ / g ₁ · I _SS (4) The resistance value r ₁ and the second (2) storing calibration coefficients x ₁ obtained by the formula by the microcomputer 10 in the memory 12.

Measurement of the Gain of the Differential Amplifier To measure the gain of the differential amplifier 41, the state is switched to the state shown in FIG. Therefore the range selector switch S ₁ to S ₄ off, connects the S ₅ ON, all the switch S ₆ to S ₉ contacts 1, the changeover switch S ₁₀ and S ₁₂ are connected to the contacts 2.

Here, D ₃ : AD conversion value D ₄ : AD conversion value g ₁ : True gain when the gain of the differential amplifier 41 is set to 3.3 times g ₂ : The gain of the differential amplifier 41 is set to 33 times The true gain at the time V _SS1 : The voltage of the constant voltage generating standard 70 (3.0 V) V _SS2 : The voltage of the constant voltage generating standard 70 (0.3 V) The differential amplifier 41 is a gain changeover switch S shown in FIG. ₁₁
Is connected to the contact 1 to switch the gain to 3.3 times, and to the contact 2 to switch the gain to 33 times.

Applying a gain switching switch S ₁₁ to the output terminal 31 V _SS1 to (3.0 V) from the constant voltage generator standard 70 while connected to the contact 1.

The relation at this time is: D ₃ = g ₁ · V _SS1 (5) Applying a gain switching switch S ₁₁ to the output terminal 31 V _SS2 to (0.3V) from the constant voltage generator standard 70 while connected to the contact 2.

The relation at this time is D ₄ = g ₂ · V _SS2 … (7) Here, V _SS1 = 3.0000 (V) and V _SS2 = 0.30000 (V).

To end the calibration and measurement of R ₁ and g _1, g ₂ by manually or more.

The following moves to the automatic calibration operation. This automatic calibration operation is executed according to a program stored in the microcomputer 10.

The parallel combined resistance value r ₂ ′ of the current detection resistors R _{2 to} R ₅ used in the current measurement range 320 (nA) is measured.

FIG. 5 shows a state where the parallel combined resistance value r ₂ ′ of the current detection resistors R _{2 to} R ₅ is measured. This state is realized by connecting a range changeover switch S ₁ to S ₅ on, the switch S ₆ to S ₉ connected to the contact 2, the switch S ₁₀ to S ₁₂ in contact 1.

r ₂ ′: Parallel combined resistance value of resistors R _{2 to} R ₅ D ₅ : AD conversion value I ₁ : True value of current V _S : 30.000 (V) Set the generated voltage V _S of the voltage generator 30 to 30.000 V .

The relationship at this time is D ₅ = r ₁ · I ₁ · g ₁ (10) From equations (4), (6) and (10) Substituting equation (11) into equation (9) Calibration of 320 (nA) range Fig. 6 shows the state of calibrating the 320 (nA) range. This condition can be achieved by connecting range switching switch S ₁ to S ₄ on the S ₅ off, the switch S ₆ to S ₁₂ to the contact 1.

D ₆ : AD conversion value I ₂ : True value of output current x ₂ : Calibration coefficient The relationship at this time is D ₆ = I ₂ · r ₂ '· g ₁ ... (13) From equation (13), I ₂ = D ₆ • 1 / r ₂ ′ • g ₁ … (14) From equations (6) and (12) Therefore, the calibration coefficient x ₂ of r ₂ ′ is Here, V _S = 30.000 (V) is set to 3000 and handled as a numerical value.

Measure the combined resistance value r ₃ ′ of the current detection resistors R ₃ , R ₄ and R ₅ used in the 32 (nA) range.

This measurement state is shown in FIG. The connection state range switching switch S ₁ to S ₃ and S ₅ ON, range selector switch
Off S _4, the changeover switch S ₆ ~S _10, S ₁₂ and connected to the contact 1, switches the gain switching switch S ₁₁ to the contact 2, sets the gain of the differential amplifier 41 to 33 times.

D ₇ : AD converted value r ₃ ′: Parallel combined resistance value of current detection resistors R ₃ , R ₄ , R ₅ I ₃ : True value of current V _S : 30.000 (V) Generated voltage V _{S of} voltage generator 30 Is set to 30.000 (V).

The relationship at this time is D ₇ = R ₁ · I ₃ · g ₂ ... (18) From equations (4), (8) and (18) Substituting equation (19) into equation (17) Calibration of measurement range 32 (nA) The circuit configuration in this case is shown in FIG. The Figure 8 states on the range selector switch _{S 1 ~S 3, S 4,} S 5 off,
This is realized by connecting the changeover switches S _{6 to} S ₁₂ to the contact 1.

D ₈ : AD conversion value I ₄ : True value of current x ₃ : Calibration coefficient The relationship at this time is: D ₈ = I ₄ · r ₃ '· g ₁ …… (21) From formula (21) From equations (6) and (20) Therefore, the calibration coefficient x of the combined resistance value r 'is Here, V _S : 30.000 (V) is treated as a numerical value with 30000.

When the current I ₄ of the combined resistance value r ₃ ′ is measured with V _S : 30 V and the gain g ₁ of the differential amplifier 41, the AD conversion output value D ₇ becomes 1/10 full scale, and the accuracy drops by one digit. in the embodiment and the gain of the differential amplifier 41 to 33 times 10 times the g _1, AD conversion output value D ₇ is made to be full scale. For this reason, g ₂ is provided for the gain of the differential amplifier 41 only during calibration.

Measure the resistance of the current detection resistor R ₃ (100 MΩ).

In the future, when calibrating the current detection resistors R ₄ (1 GΩ) and R ₅ (10 GΩ), when measuring 10 (GΩ) and 1 (GΩ) based on 1 MΩ of the current detection resistor R ₁ Since the accuracy is worse, here the current detection resistor R ₃ (100 MΩ)
Calibrated only, to measuring the resistance value of 10 with the resistance value r ₃ = 100 M.OMEGA the resistor R ₃ (GΩ) and 1 (G [Omega]).

The measurement conditions of the resistor R ₃ shown in FIG. 9. The connection state range switching switch S _1, S _2, S ₄ off, S _3, connect the S ₅ ON, the switch S ₆ ~S _10, S ₁₂ to the contact 1, contact gain selector switch S ₁₁ 2 and the gain of the differential amplifier 41 is 33
Set to double.

D ₉ : AD conversion value r ₃ : Resistance value of current detection resistor R ₃ (100 MΩ) V _S : 30.000 (V) I ₅ : True value of current Generated voltage V _S of voltage generator 30 is 30.000 (V) Set to.

The relationship at this time is D ₉ = r ₁ · I ₅ · g ₂ (26) From equations (4), (8) and (25) The current detection resistor R ₄ used in the measurement range 3.2 (nA)
The combined resistance value r ₄ 'current detection resistor the resistance value r ₃ of the measurement target on the obtained current detection resistor R ₃ as the reference resistor R _4, a parallel combined resistance value r ₄ of R _5' R _5, and Measure. FIG. 10 shows the state. The connection state on the range selector switch S ₁ to S _3, connect the S _4, S ₅ off, the switch S _6, S ₇ to the contact 2, connects the changeover switch S ₈ to S ₁₂ in contact 1 Can be realized.

In this connection state, I ₆ : true value V _S : 30.000 (V) D ₁₀ : AD conversion value r ₄ ′: parallel combined resistance value of current detection resistors R ₄ and R ₅ Generated voltage V of voltage generator 30 Set _S to 30.000 (V).

The relationship at this time is D ₁₀ = r ₃ · I ₆ · g ₁ ... (29) From equations (6), (27) and (29) Substituting equation (30) into equation (28), Get.

Measurement range 3.2 (nA) calibration Figure 11 shows the connection status during this calibration.

In this connection state, the range changeover switches S ₁ and S ₂ are turned on and S ₃
To S ₅ off, the switch S ₆ to S ₁₂ is implemented by connecting the contact 1.

D ₁₁ : AD conversion value I ₇ : True value x ₄ : Calibration coefficient The relationship at this time is D ₁₁ = I ₇ · r ₄ '· g ₁ …… (32) From formula (32) From equations (6) and (31) Therefore, the calibration coefficient x is Here, V _S : 30.000 (V) is assumed to be 30,000 and treated as a numerical value.

Measurement range 320 current detection resistor used in (PA) R ₅
To measure the resistance value r ₅ of.

Showing a measurement state of the current detecting resistor R ₅ in FIG. 12. In this connection state, the range changeover switches S ₁ and S ₃ are turned on, and S ₂ , S ₄ and S ₅
Off, connecting the changeover switch S _6, S ₁₁ to the contact 2, is realized by connecting the changeover switch S ₇ ~S _10, S ₁₂ to the contact 1.

D ₁₂ : AD conversion value I ₈ : True value V _S : 30.000 (V) r ₅ : Resistance value of current detection resistor R ₅ (10 GΩ) Set the generated voltage V _S of the voltage generator 30 to 30.000 (V) .

The relationship at this time is D ₁₂ = r ₃ · I ₈ · g ₂ … (37) From equations (8), (25) and (37) Substituting equation (38) into equation (36) Calibration of measurement range 320 (PA) Fig. 13 shows the calibration state of measurement range 320 (PA). The connection state, on the range selector switch S _1, off S ₂ to S _5, is realized by connecting the switch S ₆ to S ₁₂ to the contact 1.

D ₁₃ : AD conversion value I ₉ : True value x ₅ : Calibration coefficient The coefficient at this time is D ₁₃ = I ₉ · r ₅ · g ₁ …… (40) From formula (40) From equations (6) and (39) Therefore the calibration coefficient x ₅ is Here, V _S : 30.000 (V) is treated as a numerical value as 30,000.

The above calibration operation is shown in the flowchart of FIG. Prior to performing the calibration of the measurement range, the resistance value of the current detection resistor used in the measurement range is measured, seeking calibration coefficient x ₁ ~x ₅ based on the resistance value thereof measured. This automatic calibration operation is executed according to a program stored in the microcomputer 10.

In the above description, the voltage generated by the voltage generator 30 is calibrated in advance, and the calibration of each current measurement range is performed using the voltage value generated from this voltage generator and the resistance value of the calibrated current detection resistor. Although the case where the current generation system is used has been described, the current measurement is performed while generating the current using the current measurement system of the calibrated measurement range, and the calibration coefficient of the generation system is automatically obtained by calculating the calibration coefficient of the generation system from the error. It may be configured to perform calibration.

In the above description, a plurality of current detection resistors R _{1 to} R ₅ are connected in parallel to define each measurement range. However, each measurement range may be defined by one current detection resistor. .

[Effects of the Invention] As described above, according to the present invention, the resistance value of one of the current detection resistors and the gain of the differential amplifier 41 constituting the current measurement system are measured and made known. Using the current measurement system with a known gain based on the resistance value of the resistor, measure the resistance value of the current detection resistor used in the other current measurement range, and use the measurement result. Since each measurement range is calibrated, there is no need to prepare a standard current source for each measurement range externally.

Therefore, the economic burden on the user can be reduced.

Furthermore, since the calibration of each measurement range is performed by applying a calibrated voltage from the voltage generator 30 and measuring the resistance value of the current detection resistor that defines each measurement range, no external cable is connected. Therefore, since the calibration can be performed without being affected by the stray capacitance of the cable, the calibration can be completed in a short time.

Further, in the present invention, a resistor having the smallest resistance value among the current detection resistors R _{1 to} R ₅ is determined as a reference resistor, and a calibration is performed for each range 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 an embodiment of the present invention, and FIGS. 2 to 13 are connection diagrams for explaining a connection structure corresponding to each calibration state for explaining the operation of the present invention. FIG. 14 is a flowchart for explaining the operation of the present invention, and FIG.
FIG. 1 is a connection diagram for explaining a conventional technique. 10: microcomputer, 11: display, 12: memory, 20:
DA converter, 30: voltage generator, 40: current measuring device, 50: AD converter, R ₁ to R _5: current detecting resistor, S ₁ to S _5: Range selector switch, S ₆ to S _9: Changeover switch, S ₁₁ : Gain changeover switch.

Claims (1)

(57) [Claims] 1. A DA converter for converting given voltage data into an analog voltage, and an output voltage of the DA converter and a voltage given to a subject are input to an inverting input terminal via a resistor. A first operational amplifier for inputting ground to a non-inverting input terminal, a switching contact to which an output terminal of the first operational amplifier is connected, and a first contact for switching any of a switching contact to which ground is connected. A second changeover switch for switching any one of a changeover contact to which a common contact of the first changeover switch is connected, and a changeover contact to which ground is connected; and the second plurality of changeover switches. A plurality of current detection resistors each having a common contact connected to one end of the changeover switch and the other end connected to one contact of each range changeover switch; and a common contact of the first changeover switch having one end Is entered into
A basic resistor having the other end connected to one contact of one of the plurality of range change switches, and a contact at the other end being common, and a contact at the other end being shared A plurality of range changeover switches connected to an output terminal to the subject, a third contact switch connected to a common contact of the first changeover switch, and a third contact switch connected to ground.
And a second operational amplifier having a common contact of the third changeover switch connected to the non-inverting input terminal and a common contact of the fourth changeover switch connected to the inverting input terminal. One end of the series-connected resistor is connected to the output terminal of the second operational amplifier, the other end is connected to the output terminal to the subject, and two intermediate ends are respectively connected to the second terminal. An amplification degree switching resistor connected to the changeover contact of the changeover switch of No. 4 and the output of the second operational amplifier to a digital voltage
An AD converter, the first changeover switch for switching a common contact to a changeover contact connected to earth when measuring the amplification degree of the second operational amplifier, and the resistance value of the current detection resistor When measuring, the second changeover switch for changing a common contact to a changeover contact to which ground is connected, the range changeover switch for changing a resistance value of the current detecting resistor, and an amplification of the second operational amplifier When measuring the degree, the third contact switch switches the common contact to the switching contact to which the earth is connected, and when changing the amplification degree of the second operational amplifier, the common contact is connected to the amplification degree switching resistor. Controlling the fourth changeover switch for switching to the changeover contact connected to the two intermediate terminals, giving and storing the voltage data to the DA converter, and receiving the output data of the AD converter With this And a microcomputer for performing a predetermined calculation using these stored data.