Specifications

3.1.3 Four-terminal configuration
The four-terminal (4T) configuration can reduce the effects of lead impedances (wL
L
and R
L
) and
contact resistances (Rc) because the signal current path and the voltage sensing leads are indepen-
dent, as shown in Figures 3-5 (a) and (b). The voltage sensing leads do not detect the voltage drop
caused by the R
L
, L
L
, and Rc on the current leads. The impedances on the voltage sensing leads do
not affect measurement because signal current scarcely flows through these leads. Measurement
errors due to the lead impedances and contact resistances are thereby eliminated. Accuracy for the
lower impedance measurement range is thus improved typically down to 10 mΩ. Measurement accu-
racy on the higher impedance range is not improved because the stray capacitances between the
leads still remain. The 4T configuration is also called Kelvin connection configuration.
When the DUT’s impedance is below 10 mΩ, large signal current flows through the current leads,
generating external magnetic fields around the leads. The magnetic fields induce error voltages in
the adjacent voltage sensing leads. The effect of mutual coupling (M) between the current and volt-
age leads is illustrated in Figure 3-5 (e). The induced error voltages in the voltage sensing leads
cause a measurement error in very low impedance measurements.
Figure 3-5. Four-terminal (4T) configuration
(a) Schematic diagram
DU
T
V
Null
detector
H
p
H
c
L
c
L
p
R
s
R
r
(b) Connection image
H
p
H
c
L
c
L
p
(d) Typical impedance
measurement range
(c) Residual parameters
H
p
H
c
L
c
L
p
DUT
R
L
L
L
C
O
M
M
R
C
R
C
Test current
(e) Mutual coupling
Magnetic flux generated
by test current
H
c
or L
c
cable
H
p
or L
p
cable
Voltage is induced
1 m 10 m 100 m 1 10 100 1 K 10 K 100 K 1 M 10 M (Ω)
3-4