Ohms Sensing - Keithley SourceMeter 2400 User Manual
Sourcemeter 2400 series source measure unit
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- Service manual (137 pages) ,
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2400 Series SourceMeter
User's Manual
Ohms sensing
Ohms measurements can be made using either 2-wire or 4-wire sensing. (See
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
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
I
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minimizes or eliminates the effects of lead resistance by measuring
Input, Output
R
LEAD
HI
Lead
V
V
M
M
Resistances
LO
R
LEAD
I
= Current sourced by SourceMeter
V
= Voltage measured by SourceMeter
M
V
= Voltage across resistor
R
Measured resistance =
Actual resistance =
Ohms Measurements
4-7
Section 2
for
Test Current (I)
Resistance
V
R
R
S
Under Test
V
+ (2 ×
M
=
R
)
R
S
LEAD
I
V
R
=
R
S
I
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