 # Three high-precision methods of measuring resistance with digital multimeter

In the process of measuring resistance with a multimeter, engineers sometimes need to accurately measure small resistances less than 100Ω, which often requires the help of some techniques that can improve the measurement accuracy. This article summarizes three common multimeter resistance measurement techniques for the technicians here. Let’s take a look.

Four-wire measurement method

In the process of measuring resistance with a digital multimeter, technicians often use the four-wire measurement method in order to improve the accurate test of small resistances less than 100Ω. The so-called four-wire measurement method is to separate the two current lines of the constant current source current flowing into the resistance R under test and the two voltage lines of the voltage measuring end of the digital multimeter, so that the voltage of the measuring end of the digital multimeter is no longer between the two ends of the constant current source. Direct voltage, its operating principle is shown in Figure 1. figure 1

As can be seen from the test schematic diagram given in Figure 1, in the process of using the four-wire measurement method to complete the accurate test of the resistance of the digital multimeter, this method has two more feeders than the usual measurement method, which disconnects the voltage Connect the measuring end to both ends of the constant current source. Since the voltage measurement terminal is disconnected from the constant current source terminal, the constant current source forms a loop with the measured resistance Rx, feeder lines RL1 and RL2. The voltage sent to the voltage measurement terminal is only the voltage at both ends of Rx, and the voltages of the feeders RL1 and RL2 are not sent to the voltage measurement terminal. Therefore, the feeder resistances RL1 and RL2 have no effect on the measurement results. The feeder resistances RL3 and RL4 have influence on the measurement, but the influence is very small. Since the input impedance of the digital multimeter is much larger than the feeder resistance, the four-wire measurement method has high accuracy for measuring small resistances.

Four-wire measurement plus constant current source measurement

The four-wire measurement method mentioned above can certainly help engineers to complete the high-precision multimeter resistance measurement work. However, in the four-wire measurement process, the accuracy of the constant current source current is very critical. It is recommended to use an external more stable constant current source current.

It should be noted that the magnitude of the applied constant current source current should be equal to the magnitude of the constant current source current of the digital multimeter. The external constant current source current we use is composed of a high-precision reference voltage source MAX6250, an operational amplifier and a current-expanding composite tube, as shown in Figure 2. The temperature drift of the voltage source MAX6250 is ≤2ppm/℃, and the time drift ΔVout/t=20ppm/1000h. In this measurement process, the current I should be 800μA ~ 1mA, R is the extremely low temperature drift wirewound resistance (if I=1mA, R=5kΩ), the temperature drift and time drift of I are equivalent to the level of MAX6250.

Feeder Resistance Compensation Measurement Method

The feeder resistance compensation method is another common high-precision measurement method for multimeters to measure resistance. In the industrial field, if a high-precision resistance test is required, the three-wire connection method is often chosen, so that the measured resistance is connected to the grounding line. connected. The principle of this test method is shown in Figure 3. When using this technique for measurement, the current I is 800μA ~ 1mA, R is the extremely low temperature drift wirewound resistance (if I=1mA, R=5kΩ), the temperature drift and time drift of the current I are equivalent to the MAX6250 Level.

In the operation schematic diagram of this feeder resistance compensation measurement method shown in the figure above, when the constant current source current I flows into the measured resistance through feeder 2, feeder 1 is grounded, and feeders 2 and 3 are connected to the op amps A1 and A2 respectively. input. If the gain of both op amps is 1, the output voltages V1 and V2 can be calculated as: At this time, the output voltage V0 of the differential amplifier A3 can be calculated as follows: It can be seen from the above formula that in the process of feeder resistance compensation measurement, the additional voltage on the feeder resistance is added to the input end of A3, and is eliminated by calculating the difference. The output voltage is only related to the measured resistance and has a linear relationship. Errors are fully compensated regardless of the size of the resistance being measured. This is why this measurement method can ensure that the resistance measurement results of the digital multimeter remain highly accurate.

In this feeder resistance compensation method to measure the small resistance circuit, the measurement accuracy mainly depends on the accuracy of the constant current source current I and whether the size of the feeder resistance RL is equal. The operational amplifier in the circuit can choose four general-purpose single operational amplifier (LM324). When using this method to measure resistances with a resistance value of less than 0.5Ω, make sure that the RLs are exactly equal.

The above is the summary and sharing of the three common high-precision techniques for measuring resistance by digital multimeters in this article. I hope it will be helpful to the daily work of all technicians.

In the process of measuring resistance with a multimeter, engineers sometimes need to accurately measure small resistances less than 100Ω, which often requires the help of some techniques that can improve the measurement accuracy. This article summarizes three common multimeter resistance measurement techniques for the technicians here. Let’s take a look.

Four-wire measurement method

In the process of measuring resistance with a digital multimeter, technicians often use the four-wire measurement method in order to improve the accurate test of small resistances less than 100Ω. The so-called four-wire measurement method is to separate the two current lines of the constant current source current flowing into the resistance R under test and the two voltage lines of the voltage measuring end of the digital multimeter, so that the voltage of the measuring end of the digital multimeter is no longer between the two ends of the constant current source. Direct voltage, its operating principle is shown in Figure 1. figure 1

As can be seen from the test schematic diagram given in Figure 1, in the process of using the four-wire measurement method to complete the accurate test of the resistance of the digital multimeter, this method has two more feeders than the usual measurement method, which disconnects the voltage Connect the measuring end to both ends of the constant current source. Since the voltage measurement terminal is disconnected from the constant current source terminal, the constant current source forms a loop with the measured resistance Rx, feeder lines RL1 and RL2. The voltage sent to the voltage measurement terminal is only the voltage at both ends of Rx, and the voltages of the feeders RL1 and RL2 are not sent to the voltage measurement terminal. Therefore, the feeder resistances RL1 and RL2 have no effect on the measurement results. The feeder resistances RL3 and RL4 have influence on the measurement, but the influence is very small. Since the input impedance of the digital multimeter is much larger than the feeder resistance, the four-wire measurement method has high accuracy for measuring small resistances.

Four-wire measurement plus constant current source measurement

The four-wire measurement method mentioned above can certainly help engineers to complete the high-precision multimeter resistance measurement work. However, in the four-wire measurement process, the accuracy of the constant current source current is very critical. It is recommended to use an external more stable constant current source current.

It should be noted that the magnitude of the applied constant current source current should be equal to the magnitude of the constant current source current of the digital multimeter. The external constant current source current we use is composed of a high-precision reference voltage source MAX6250, an operational amplifier and a current-expanding composite tube, as shown in Figure 2. The temperature drift of the voltage source MAX6250 is ≤2ppm/℃, and the time drift ΔVout/t=20ppm/1000h. In this measurement process, the current I should be 800μA ~ 1mA, R is the extremely low temperature drift wirewound resistance (if I=1mA, R=5kΩ), the temperature drift and time drift of I are equivalent to the level of MAX6250.

Feeder Resistance Compensation Measurement Method

The feeder resistance compensation method is another common high-precision measurement method for multimeters to measure resistance. In the industrial field, if a high-precision resistance test is required, the three-wire connection method is often chosen, so that the measured resistance is connected to the grounding line. connected. The principle of this test method is shown in Figure 3. When using this technique for measurement, the current I is 800μA ~ 1mA, R is the extremely low temperature drift wirewound resistance (if I=1mA, R=5kΩ), the temperature drift and time drift of the current I are equivalent to the MAX6250 Level.

In the operation schematic diagram of this feeder resistance compensation measurement method shown in the figure above, when the constant current source current I flows into the measured resistance through feeder 2, feeder 1 is grounded, and feeders 2 and 3 are connected to the op amps A1 and A2 respectively. input. If the gain of both op amps is 1, the output voltages V1 and V2 can be calculated as: At this time, the output voltage V0 of the differential amplifier A3 can be calculated as follows: It can be seen from the above formula that in the process of feeder resistance compensation measurement, the additional voltage on the feeder resistance is added to the input end of A3, and is eliminated by calculating the difference. The output voltage is only related to the measured resistance and has a linear relationship. Errors are fully compensated regardless of the size of the resistance being measured. This is why this measurement method can ensure that the resistance measurement results of the digital multimeter remain highly accurate.

In this feeder resistance compensation method to measure the small resistance circuit, the measurement accuracy mainly depends on the accuracy of the constant current source current I and whether the size of the feeder resistance RL is equal. The operational amplifier in the circuit can choose four general-purpose single operational amplifier (LM324). When using this method to measure resistances with a resistance value of less than 0.5Ω, make sure that the RLs are exactly equal.

The above is the summary and sharing of the three common high-precision techniques for measuring resistance by digital multimeters in this article. I hope it will be helpful to the daily work of all technicians.