15
4-Wire Resistance Measurement
The four-terminal configuration of the 4021 eliminates errors normally caused by test lead and
contact resistances. In many applications the contact resistance can exceed the value of the load
by several orders of magnitude. The 4021 bypasses this potential error source by providing two
terminals of constant current and an additional two terminals for high impedance voltage
measurement. The result is a fast, accurate resistance measurement of the load, independent of
the resistance of the current carrying leads.
Figure 1 Error Sources in Resistance Measurements
Figure 1 illustrates how the 4-wire principle is used to eliminate lead, wire and contact
resistances as potential error sources. The internal current source inherently overcomes all series
resistance (within compliance voltage limits) and delivers a precise constant current.
The internal high-impedance DVM senses the voltage drop across the load. There is negligible
contact and lead resistance error created by the voltage measurement because the high input
impedance of the DVM limits current flow in the voltage leads.
