2400 Series SourceMeter
®
User’s Manual Ohms Measurements 4-7
Ohms sensing
Ohms measurements can be made using either 2-wire or 4-wire sensing. (See Section 2 for
information on connections and sensing methods.) Note that resistance measurement
accuracy specifications are based on using 4-wire sensing.
The 2-wire sensing method has the advantage of requiring only two test leads. However, as
shown in Figure 4-2, test lead resistance can seriously affect the accuracy of 2-wire resis-
tance measurements, particularly with lower resistance values. The 4-wire sensing method
shown in Figure 4-3 minimizes or eliminates the effects of lead resistance by measuring
the voltage across the resistor under test with a second set of test leads. Because of the
high input impedance of the SourceMeter voltmeter, the current through the sense leads is
negligible, and the measured voltage is essentially the same as the voltage across the resis-
tor under test.
Figure 4-2
2-wire resistance sensing
SourceMeter
HI
LO
I
V
M
Input, Output
V
M
V
R
Test Current (I)
Resistance
Under Test
Lead
Resistances
R
S
R
LEAD
R
LEAD
= Current sourced by SourceMeter
= Voltage measured by SourceMeter
= Voltage across resistor
I
V
M
V
R
Measured resistance = = + (2 × )
Actual resistance = =
V
M
I
R
S
R
LEAD
I
V
R
R
S
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