Keithley Source Meter answers your questions: What is the four-wire test method?

Keithley source meters and multimeters have always been favored by customers. During use, people often ask: What is the four-wire test method? Today, Antai Test will simply share with you what the four-wire test method is.

The four-wire test method is the best test solution so far to eliminate the error introduced by the lead resistance (or minimize it). Two-wire measurement method
:
Traditional resistance measurement usually uses the two-wire measurement method for measurement, such as our most commonly used handheld multimeter. When measuring, you only need to touch the red and black test leads to the left and right ends of the resistor to be measured, and the multimeter will automatically add an excitation current or excitation voltage (the automatic excitation size is related to the selected gear, and the excitation size in the multimeter is not adjustable). While adding the excitation voltage, test the current at both ends of the device under test; or while adding the excitation current, test the voltage at both ends of the device under test. Then use Ohm's law R=U/I to get the resistance value. As shown in Figure 1,

Among them:
I is the excitation current (test current)
VM is the voltage measured by the multimeter
RLEAD is the lead resistance
R is the measured resistance
HI and LO are the output and input terminals of the multimeter
The measurement result is R=VM/I, From the working circuit diagram of Figure 1, we can know that this test result actually contains two parts: the measured resistance R and the two lead resistances 2*RLEAD. The typical lead resistance value is roughly 1 milliohm to 10 milliohms. Of course, when the measured resistance value is large, the influence of the lead resistance can be ignored; but when the measured resistance is small or the test accuracy requirement is high, the additional error source of the lead resistance cannot be ignored.

Rel option:

Some new handheld multimeters on the market are designed with rel options (bench multimeters generally have them), such as Keithley bench multimeters. The working principle is: before testing and measuring, short-circuit the red and black test leads to obtain the lead resistance value and record this data. The resistance value measured later is directly subtracted from the recorded lead resistance value. This mathematical method is used to reduce errors and improve measurement accuracy. The result is: R=(VM/I) -2*RLEAD. In general, this measurement method is easy to use and has higher accuracy than the two-wire measurement method. However, in the actual measurement process, the lead resistance RLEAD is not a fixed value. As the temperature environment changes, the resistance value will also change. In the mathematical calculation process of the Rel function, the lead resistance at the moment when the rel option is pressed is used for calculation. If the tester has higher requirements for test measurement accuracy, it is recommended to use the four-wire method for measurement.

· Four-wire test method:
The four-wire test method is the best test method to eliminate the error introduced by the lead resistance (or minimize it). Its principle is: add a set of sampling leads on the basis of the two-wire method, and use the test results of the sampling leads to replace the measurement results of the test leads for calculation, so as to obtain a more accurate resistance value. As shown in Figure 2,

Where:
I is the excitation current,
VM is the voltage measured by the multimeter,
RLEAD is the lead resistance,
R is the resistance to be measured,
source HI, source LO is the output excitation current terminal of the multimeter,
sampling HI, and sampling LO are the sampling leads, that is, the input voltage VM terminal
does not produce a voltage drop on the sampling lead during the entire test process, so the lead resistance 2*RLEAD has no effect on the test. The measured voltage VM is basically the same as the actual voltage on the measured resistor R, that is, the measurement result is still R=VM/I. Although there is still a small current flowing through the sampling lead, it can be ignored in actual measurements. Compared with the two-wire test method, the resistance value measured by the four-wire test method does not contain the lead resistance, so the accuracy is higher than the two-wire test method.
Take the Keithley 2000 desktop multimeter as an example:

When using the two-wire test method, you only need to connect the HI and LO terminals of the input. As shown in Figure 3,

When using the four-wire test method, you need to connect the input HI, LO, Sense HI, and Sense LO ports (Sense HI and Sense LO are sampling lead interfaces). As shown in Figure 4,

It is especially important to note that the sampling lead ends (Sense HI and Sense LO) should be as close to the two ends of the resistor being measured as possible. The farther the distance, the more test lead resistance is introduced. As shown in Figure 5, there is still a long lead between the contact point of the sampling lead and the resistor being measured. This connection method will increase the error.