Two-wire vs. four-wire resistance measurements
DMMs typically employ the constant-current method to measure resistance, which sources a constant current (ISOUR) to the device under test (DUT) and measures the voltage (VMEAS). Resistance (RDUT) is then calculated and displayed using the known current and measured voltage (RDUT = VMEAS/ISOUR). Figure 1 shows a simple diagram of the constant-current test.
The test current sourced to the DUT depends on the selected measurement range (Table 1). For example, for the 100Ω range, the test current is 1mA. Because the voltmeter of a typical DMM has very high input impedance, virtually all the test current (1mA) flows through the DUT.
Two-Wire Resistance Measurements
Figure 2 represents a two-wire resistance test configuration employing the constant current method.
The main measurement issue with the two-wire method, as applied to low resistance measurements, is that the total lead resistance (RLEAD) is added to the measurement. Because the test current (I) causes a small but significant voltage drop across the lead resistances, the voltage (VM) measured by the meter won’t be exactly the same as the voltage (VR) directly across the test resistance (R), and considerable error can result. Typical lead resistances range from 10mΩ to 1Ω, so it’s very difficult to obtain accurate two-wire resistance measurements when the resistance under test is lower than 100Ω because the resistance of interest will be completely swamped by the lead resistance. In fact, lead resistance will be the dominant source of error. For example, using test leads with a 100mΩ combined resistance to perform a two-wire resistance measurement on a 500mΩ resistor will result in a 20% measurement error in addition to that of the instrument.