JPH0634674A - Current measuring device - Google Patents

Abstract

PURPOSE:To obtain the resistance of a second resistor and to measure current accurately even if an accurate, stable, and expensive resistor is not used as a resistor for measuring current by selecting the connection state of switches and then using a known first resistor. CONSTITUTION:A first resistor 14 whose resistance is known and a second resistor 28 for measuring current and then a circuit net consisting of first, second, and third switches 24, 16, and 26 are connected between an inverted input terminal and an output terminal of an operational amplifier 10 where a known voltage is supplied to a non-inverted input terminal. A voltage between both terminals of the second resistor 28 for measuring current is detected by a differential amplifier 18, is converted to a digital value by an A/D converter, and then is processed by a CPU 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a current measuring device, in particular
The present invention relates to a minute current measuring device which does not require a highly accurate and expensive high resistor.

[0002]

2. Description of the Related Art Conventionally, the problems to be solved by the invention
When measuring the current value, supply the current to be measured to a measuring resistor with a known resistance value, measure the voltage across the measuring resistor, and divide the measured voltage value by the known resistance value. Thus, the measured current value is obtained. In particular, when the current to be measured is a minute current of the order of microampere or picoampere, the measuring resistor is required to have a high resistance value. However, normally, a high resistance resistor is inferior to a low resistance resistor in terms of resistance value accuracy and long-term stability. Further, a highly accurate resistor having a high resistance value is expensive, and the manufacturing cost of the current measuring device is increased.
Further, even if a highly accurate resistor is used, long-term accurate measurement is difficult due to the stability problem.

Therefore, an object of the present invention is to provide a current measuring device capable of performing accurate measurement without requiring an expensive resistor.

[0004]

In the current measuring device of the present invention, a central processing unit (hereinafter referred to as CPU) controls a digital-analog converter (hereinafter referred to as DAC),
Sets the voltage supplied to the non-inverting input terminal of the operational amplifier. The inverting input terminal of the operational amplifier is connected to the measurement terminal for connecting the device under test during current measurement. A first switch including a first switch connected in series and a first resistor having a known resistance value is provided between the output terminal and the measurement terminal of the operational amplifier.
The series circuit and the second series circuit including the second switch and the second resistor, which are serially connected in order from the output terminal of the operational amplifier toward the measurement terminal, are connected. The third switch is connected between the connection point of the second switch and the second resistor and the ground potential source. The voltage between the output terminal of the operational amplification means and the measuring terminal is detected by the voltage detection means, converted into a digital value by an analog-digital converter (hereinafter referred to as ADC), and then processed by the CPU.

[0005]

First, the first and third switches are closed and the second switch is opened. In this connection state, the first and second resistors are connected in series between the output terminal of the operational amplifier and the ground potential source, the voltage detector detects the voltage across the first resistor, and the first and second resistors are connected. The connection point, that is, the voltage at the measuring terminal becomes the voltage set at the non-inverting terminal of the operational amplifier. The current flowing through the first and second resistors is obtained by dividing the detected voltage by the resistance value of the first resistor, and by dividing the set voltage value by this current value, the resistance value of the second resistor is obtained. Can be asked. When performing current measurement, the device under test is connected to the measurement terminal, the first and third switches are opened, and the second switch is closed. In this connection state, the second resistor is connected in series with the device under test, and the voltage detector detects the voltage across the second resistor. The current flowing through the device under test is obtained by dividing the detected voltage by the resistance value obtained by the second resistor. In this way, the resistance of the second resistor can be obtained by selecting the connection state of the switch and using the known first resistor, so that an expensive resistor with high accuracy and high stability can be used as a current source. Accurate current measurement is possible without using it as a measuring resistor.

[0006]

FIG. 1 is a block diagram showing a current measuring device of the present invention. The CPU 2 generates data for controlling other components via the bus 4 as described later according to an input command from the host computer or the keyboard, and uses data sent from the other components, Make the necessary calculations for the measurement. The measurement result obtained by the CPU 2 is displayed on the display 6 such as a cathode ray tube.

The DAC 8 receives the digital voltage data output by the CPU 2 in accordance with the input command, converts the digital voltage data into an analog voltage, and outputs the analog voltage. The DAC 8 supplies the output analog voltage to the non-inverting input terminal of the operational amplifier 10. Operational amplifier 1
The output terminal of 0 is connected to the first contact of the double throw switch 12. The second contact of the double throw switch 12 is connected to one end of the resistor 14, one contact of the switch 16, the non-inverting input end of the differential amplifier 18, and one contact of the switch 20.
The third contact of the double throw switch 12 is connected to the inverting input terminal of the operational amplifier 10 via the resistor 22. The second and third contacts of the double throw switch 12 are indicated by X and Y in the figure. The other end of the resistor 14 is connected to one contact of the switch 24. The other contact of the switch 24 is directly connected, and the other contact of the switch 16 and the other contact of the switch 26 whose one contact is grounded are connected to the output terminal 30 via the resistor 28. The output terminal 30 is connected to one contact of the switch 32 and is also connected to the input terminal of a gain 1 buffer amplifier 35 having a high input impedance. The output terminal of the buffer amplifier 35 is connected to the operational amplifier 1 via the resistor 36 to negatively feed back the voltage of the output terminal.
It is connected to the inverting input terminal of 0 and is also connected to the inverting input terminal of the differential amplifier 18. The reference voltage source 34 generates voltages of + V1 / 2 and -V2 / 2 at the two output terminals, and supplies the voltages to the other contacts of the switches 20 and 32, respectively.

The resistor 28 is a resistor having a high resistance value of, for example, several giga Ω as used in a conventional current measuring device, and as described above, the accuracy and the long-term stability are not good.
Therefore, it is necessary to measure the resistance value of the resistor 28 before performing the current measurement. On the other hand, resistor 14 is resistor 28
It has a known resistance value of, for example, several mega Ω, which is lower than that of, and has good accuracy and long-term stability, and can be obtained at low cost. Switches 12, 16, 20, 24, 26
And 32 are preferably electronic switches,
The ON / OFF state is selected by the control signal sent from the.

The differential amplifier 18 detects the difference voltage between the output terminal of the operational amplifier 10 and the output terminal 30 and supplies it to the variable gain circuit 38. The gain of the variable gain circuit 38 is the CPU
It is adjustable according to the control data from 2. An analog / digital converter (hereinafter referred to as ADC) 40 converts the voltage adjusted by the variable gain circuit 38 into digital voltage data and sends it to the CPU 2.

Next, the operation of the current measuring device of the present invention in FIG. 1 will be described with reference to the flow chart including a plurality of blocks shown in FIGS. The CPU 2 receives an adjustment command from the host computer or the keyboard and performs a process of adjusting the variable gain circuit 38. In the adjustment procedure, first,
In the block 100, the CPU 2 opens the switches 16, 24 and 26, connects the switch 12 to the Y side, and closes the switches 20 and 32 to the connected state a.
As a result, the voltages + V1 / 2 and -V1 / 2 are supplied to both input terminals of the differential amplifier 18, respectively.
Output voltage is V1. The ADC 40 converts the voltage amplified by the gain currently set in the variable gain circuit 38 into a digital value and sends it to the CPU 2 via the bus 4. CP
U2 checks whether the received digital value is a value indicating the voltage V1 and, according to the result, the variable gain circuit 38
Of the output voltage of the ADC 40 is adjusted to the voltage V
Be sure to indicate exactly 1. If there is no error in the operation of the buffer amplifier 35, the differential amplifier 18, the variable gain circuit 38, and the ADC 40, the gain of the variable gain circuit 38 is set to 1. However, normally, there is an error, and the error is comprehensively compensated by setting the gain of the variable gain circuit 38 to a value close to 1 in accordance with the error. When the gain of the variable gain circuit 38, that is, the gain A at which the output digital value of the ADC 40 becomes the value indicating the voltage V1 is determined, the CPU 2 stores the control data corresponding to this gain A and maintains the gain A state (block 102). ).

Next, in block 104, the CPU 2 closes the switches 24 and 26, maintains the switch 16 in the open state, opens the switches 20 and 32, and connects the switch 12 to the X-side contact. , Connection state b. As a result, the output voltage of the operational amplifier 10 is
Resistors 14 and 28 connected in series between 0 and ground potential
The voltage across the resistor 14 is supplied to the differential amplifier 18
It is supplied between both input terminals. The gain of the variable gain circuit 38 is maintained at the gain A. The CPU 2 supplies control data to the DAC 8 and sets its output voltage to V2.
The operational amplifier 10, the resistor 14, the buffer amplifier 35, and the resistor 36 constitute a negative feedback type amplifier circuit, and the voltage of the output terminal 30 becomes equal to the output voltage V2 of the DAC 8.

For purposes of illustration, the known resistance of resistor 14 is R1, the nominal resistance of resistor 28 is R2, and the actual resistance is R.
2 '. The CPU 2 previously uses the nominal resistance value R2 of the resistor 28 to calculate the voltage Vx across the resistor 14 by the following formula, and stores the result. Vx = (V2 / R2) × R1 (1) Next, in block 105, the actual voltage across the resistor 14 V
Measure x '. This Vx 'can be expressed by the following equation. Vx '= (V2 / R2') * R1 (2) From the formulas (1) and (2), the block 106 calculates the ratio of R2 and R2 'using the following formula. R2 / R2 '= Vx' / Vx (3)

To make the current measurement, leave switch 12 connected to the X-side contact and switch 16 closed.
All the other switches are opened, the device under test 42 is connected to the output terminal 30, and the connection state c is established. Output terminal 4
The voltage of 0 is equal to the voltage of the non-inverting terminal of the operational amplifier 10, and the measured current determined by the impedance of the device under test flows through the operational amplifier 10, the switch 12, the resistor 28 and the device under test. Since the switch 26 is connected to the operational amplifier 10 side of the resistor 28, the leakage current of the switch 26 does not affect the current flowing through the resistor 28. The differential amplifier 18 detects the voltage V3 across the resistor 28 caused by the measured current. The current to be measured is obtained by dividing the measured voltage by R2 'obtained from the equation (2). As described above, since the first resistor having a resistance value lower than that of the second resistor and having high accuracy and high stability is used, and the resistance value of the second resistor is obtained, the current measurement is performed, the high accuracy and Accurate current measurement without the use of expensive high stability resistors

Strictly speaking, however, at this time, R2 '
By digitally calculating each value of V3 and V3,
An error occurs due to truncation. Further, the nominal resistance value R2 of the resistor 28 is, for example, 5 GΩ, but the measured value R2 ′ is 4.
When it is 95 GΩ, the voltage V3 is 5 × 10 * 9 (N * n
Represents Nth power of n. ) Divided by 4.95 compared
Dividing by × 10 * 9 requires a larger number of bits for calculation. The larger the number of bits, the longer the calculation time, and the more limited the number of bits, the larger the calculation error due to truncation.

In order to solve this problem, in the present invention,
Further, by properly adjusting the gain of the variable gain circuit 38, the number of digital calculations is reduced during the current measurement, and the output digital value of the ADC 40 is the voltage generated in the resistor 28 having the nominal resistance value R2. Is shown. Therefore, the ratio of the nominal resistance value R2 and the actual resistance value R2 'to the voltage across the resistor 28 during current measurement, R2 /
The gain of the variable gain circuit 38 is adjusted so that the value obtained by multiplying Vx '/ Vx equal to R2' appears at the output of the ADC 40.

In order to adjust the variable gain circuit 38 in this manner, in the connection state b, the CPU 2 calculates and stores (Vx '* Vx') / Vx in the block 108 following the block 106. Next, in block 110, the CPU 2
Indicates that the voltage indicated by the output digital value of the ADC 40 is (V
x '× Vx') / Vx. At present, the variable gain circuit 38 is set to the gain A which is approximately 1, so that the output digital value of the ADC 40 is approximately Vx '.
Equivalent to. Therefore, if the actual resistance value R2 'is larger than the nominal resistance value R2, Vx' / Vx <1 and AD
The output digital value of C40 is determined to be greater than (Vx '* Vx') / Vx and the process proceeds to block 112, where the gain of the variable gain circuit 38 is decreased. If it is not determined to be large, the process moves to block 114. If the actual resistance value R2 'is smaller than the nominal resistance value R2, then Vx' / Vx> 1,
The output digital value of ADC 40 is (Vx '× Vx') / Vx
If it is determined to be smaller, the process proceeds to block 116, and the gain of the variable gain circuit 38 is increased.

After the gain is increased or decreased in the block 112 or the block 116, the magnitude relationship between the output digital value of the ADC 40 and (Vx '* Vx') / Vx is checked again, and the gain is increased or decreased as necessary. After repeating such processing, the voltage indicated by the output digital value of the ADC 40 becomes (Vx '
× Vx ') / Vx, block 11
8, the control data for setting the gain B of the variable gain circuit 38 at that time is stored in the CPU 2, and the gain B state is maintained. This gain setting is accurate because it takes into account the inherent errors of the buffer amplifier 32, the differential amplifier 18, the variable gain circuit 38, and the ADC 40.

To perform the current measurement, block 120 sets the connection state c as described above. The voltage V3 across the resistor 28 is exactly Vx '/ Vx times at the output of the ADC 40,
It is assumed that the resistor 28 has a nominal resistance value. C
The PU 2 can measure the measured current by dividing the output digital value of the ADC 40 by the nominal resistance value which is stored in advance and is 5.0 MΩ, for example. In this measurement, the input voltage of the ADC 40 is adjusted in an analog manner, and the output digital value of the ADC 40 can be digitally calculated only once with a nominal resistance value having a small number of significant figures, so that the current value is accurately calculated. A measurement can be made.

[0019]

In the current measuring device of the present invention, the resistance value of the current measuring resistor is lower than that of the current measuring resistor, and the resistance value of the current measuring resistor is obtained after the resistance value of the current measuring resistor is calculated. Since current is measured, accurate current can be measured without using expensive current measuring resistors with high accuracy and high stability.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a current measuring device of the present invention.

2 is a flow chart for explaining the operation of the apparatus of FIG.

FIG. 3 is a flowchart for explaining the operation of the apparatus of FIG.

[Explanation of symbols]

 10 Operational Amplifier Means 14 First Resistor 16 Second Switch Means 18 Voltage Detector 24 First Switch Means 26 Third Switch Means 28 Second Resistor

Claims (1)

[Claims] 1. A measurement terminal, a non-inverting input terminal to which a known voltage is supplied, and an inverting input terminal connected to the measurement terminal, an operational amplifier means, an output terminal of the operational amplifier means and the measurement terminal. A first series circuit including first switch means connected in series between terminals and a first resistor having a known resistance value; and a first series circuit connected in series between the output terminal of the operational amplification means and the measurement terminal in order. A second series circuit including two switch means and a second resistor; a third switch means connected between a connection point of the second switch means and the second resistor and a reference potential source; A current measuring device comprising a voltage detecting means for detecting a voltage between an output terminal and the measuring terminal.