JPH08297143A - Current measuring device - Google Patents

Abstract

PURPOSE: To reduce hum noises when the current flowing in a measured element is measured. CONSTITUTION: A current detecting resistor Rs is grounded at the first terminal and converts the current flowing in a measured element 10. A differential amplifier 14 detects the voltage between both ends of the current detecting resistor Rs. The current flowing in the measured element 10 can be measured. The input terminal and output terminal of a phase inverter 16 are connected to the second terminal of the current detecting resistor Rs. Hum noises are transferred to the current detecting resistor Rs from a power source 12 via the parasitic capacity, however the AC-coupled phase inverter 16 reduces the hum noises.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current measuring device for measuring a current flowing through a device under test, and more particularly to a current measuring device in which adverse effects of hum noise are reduced.

[0002]

2. Description of the Related Art FIG. 4 is a basic circuit diagram of a conventional current measuring device for measuring a current flowing through a device under test (DUT). The DUT 10 is, for example, a transistor or a diode. If the DUT 10 is a transistor,
Further, means for supplying voltage or current to the control terminal is provided. A predetermined voltage is supplied to the DUT 10 from the DC power supply 12. The voltage across the DUT 10 is detected by the differential amplifier 13. Further, at that time, the current flowing through the DUT 10 is converted into a voltage by the current detection resistor Rs, and the voltage across this is detected by the differential amplifier 14. Since the voltage across the current detection resistor Rs is proportional to the magnitude of the current flowing through the DUT 10, it is possible to obtain the current value flowing through the DUT 10 from the known resistance value of the current detection resistor Rs by measuring this voltage. it can. If the DUT 10 is a bipolar transistor, for example, the current flowing between the collector and the emitter is measured. The voltages detected by these measuring circuits (not shown) are converted into digital data by, for example, an analog-digital converter and processed by a system using a microprocessor.

[0003]

The output voltage of the DC power supply 12 is often generated by rectifying an AC commercial (AC) power supply. In this case, there is a parasitic capacitance between the primary and secondary coils of the transformer, and the vibration signal of 50 or 60 Hz of the AC power source on the primary side or twice the frequency is transmitted to the secondary side through the parasitic capacitance. Hum noise that mixes in may occur. If a noise current due to such hum noise flows through the current detection resistor Rs, it adversely affects the measurement of the current flowing through the DUT 10. FIG. 5 is an equivalent circuit showing that the noise current Inoise flows from the AC power supply to the current detection resistor Rs.

Although not shown, the resistance value of the current detection resistor Rs can be switched depending on the measurement range. If the current detection resistor Rs having a large resistance value is used, the same current is converted into a larger voltage, so that it becomes possible to measure a small current. By the way, in order to reduce the noise voltage generated in the current detection resistor Rs due to the noise current to the extent that it falls within the allowable range of the measuring instrument, the current detection resistor Rs
The resistance value of s should be lowered. However, the current detection resistor R
If the resistance value of s is simply lowered, the minute current cannot be measured.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a current measuring circuit with reduced noise during current measurement.

[0006]

SUMMARY OF THE INVENTION The present invention is a current measuring device for measuring a current flowing through a device under test when a voltage is supplied to the device under test, wherein a first terminal is grounded and the device under test is connected to the device under test. A current detection resistor that converts the flowing current into a voltage,
It is characterized by comprising a voltage detecting means for detecting a voltage between both ends of the current detecting resistor, and a phase inverting circuit in which an input terminal and an output terminal are connected to a second terminal of the current detecting resistor. This phase inversion circuit reduces hum noise.

[0007]

1 is a circuit diagram of one preferred embodiment of the present invention. The power supply 12 supplies a voltage to the device under test (DUT) 10, and the current flowing at that time is converted into a voltage by the current detection resistor Rs. One terminal (first terminal) of the current detection resistor Rs is grounded, and the differential amplifier 14 detects the voltage across the current detection resistor Rs with respect to the ground (reference) potential. The voltage across the DUT 10 is the voltage detection differential amplifier 1
Detected in 3. These detected voltages are converted into digital data by, for example, an analog-digital converter and processed in a system using a microprocessor.

The other terminal (second terminal) of the current detection resistor Rs is connected to the noise elimination circuit 16. This includes
A well-known phase inversion circuit is used. Here, a circuit using an operational amplifier is shown. The noise elimination circuit 16 is connected to its input / output terminal, receives the noise current Inoise at the same terminal and outputs the noise compensation current Icomp from the same terminal. The noise removing circuit 16 is AC-coupled to the current measuring circuit by a capacitor and only the noise component is input to the noise removing circuit 16. The frequency of the hum noise is 50 or 60 Hz or about twice thereof, which is sufficiently low compared with the slew rate of the operational amplifier. Therefore, the noise current Ino
The noise compensation current Icomp whose phase is inverted with respect to ise is
It is generated without delay with respect to the noise current Inoise. As a result, the noise current Inoise can be largely removed. FIG. 2 shows the noise current Inoise and the noise compensation current Ic.
It is an equivalent circuit showing the relationship between omp and the current detection resistor Rs.

FIG. 3 is a circuit diagram of another preferred embodiment of the present invention. Compared to FIG. 1, the resistor Rd is connected in series with the current detection resistor Rs. The resistor Rd is a voltage dividing resistor that divides the voltage generated in the current detection resistor Rs. The ratio of the resistance values of the current detection resistor Rs and the voltage dividing resistor Rd may be arbitrary, but may be 1: 9, for example. According to this example, the voltage generated in the current detection resistor Rs is 1/10 of that in the case where the voltage dividing resistor Rd is not provided. The fraction of the voltage generated in the current detection resistor Rs may be determined in consideration of the allowable range of the performance of the measuring instrument.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the embodiments described herein, and various modifications and changes can be made as necessary without departing from the gist of the present invention. It will be apparent to those skilled in the art that changes can be made. For example, the current detection resistor Rs may receive a current obtained by shunting the current flowing through the device under test.

[0011]

According to the current measuring circuit of the present invention, the hum noise can be reduced when measuring the current flowing through the element to be measured, so that a smaller current value can be measured.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of a current measuring device according to the present invention.

FIG. 2 is an equivalent circuit for hum noise and compensation current.

FIG. 3 is a circuit diagram of another embodiment of the current measuring device according to the present invention.

FIG. 4 is a circuit diagram of an example of a conventional current measuring device.

FIG. 5 is an equivalent circuit for hum noise.

[Explanation of symbols]

 10 device under test (DUT) 12 power supply 13 voltage detection means 14 current detection means 16 noise elimination circuit (phase inversion circuit)

Claims (1)

[Claims] 1. A current measuring device for measuring a current flowing through a device under test when a voltage is supplied to the device under test, wherein a first terminal is grounded and the current flowing through the device under test is converted into a voltage. A current detecting resistor, a voltage detecting means for detecting a voltage across the current detecting resistor, and a phase inverting circuit having an input terminal and an output terminal connected to the second terminal of the current detecting resistor. A current measuring device characterized in that