Operating Considerations; Low-Ohms Measurements - Keithley 7710 User Manual

Operating considerations

Low-ohms measurements

For resistances in the normal range (>100 Ω ), the 2-wire method ( Ω 2) is typically used for ohms
measurements. For low-ohms ( ≤ 100 Ω ), the signal path resistance in series with the DUT could be
high enough to adversely affect the measurement. Therefore, the 4-wire method ( Ω 4) should be
used for low-ohms measurements. The following discussion explains the limitations of the 2-wire
method, and the advantages of the 4-wire method.
2-wire method
Resistance measurements in the normal range (>100 Ω ) are generally made using the 2-wire
method ( Ω 2 function). The test current is forced through the test leads and the resistance being
measured (R
The main problem with the 2-wire method, as applied to low-resistance measurements is the test
lead resistance (R
lies in the range of 1.5 to 2.5 Ω . Therefore, it is very difficult to obtain accurate 2-wire ohms mea-
surements below 100 Ω .
Due to this limitation, the 4-wire method should be used for resistance measurements ≤ 100 Ω .
This method is explained as follows:
4-wire method
The 4-wire (Kelvin) connection method using the Ω 4 function, as shown is
preferred for low-ohms measurements. The 4-wire method cancels the effects of channel and test
lead resistance.
These measurements can be made using the Model 2750 Digital Multimeter. With this configura-
tion, the test current (I
leads (R
sured through a second set of leads (R
With this configuration, the resistance of the DUT is calculated as follows:
R = V
DUT
where: I
As shown in
equations at the bottom of
celled out of the measurement process.
Maximum test lead resistance (Model 2750)
Table 1
that a larger test lead resistance can be tolerated with a smaller R
example:
Example:
If R
DUT
tance (R
10
). The meter then measures the voltage across the resistance value accordingly.
S
) and the channel resistance (R
LEAD
) is forced through the test resistance (R
TEST
and R ), while the voltage (V
LEAD2 LEAD3
/ I
M TEST
is the sourced test current and V
TEST
Figure 2, the measured voltage (V
Figure 2
l i sts the maximum test lead resistance (R
is 3Ω on the 10Ω range using the Ω4 (4-wire) function, then the maximum test lead resis-
) can be 4Ω.
LEAD
). The sum of these resistances typically
CH
) across the Device Under Test (DUT) is mea-
M
and R ) called the sense leads.
LEAD1 LEAD4
is the measured voltage.
M
) is the difference between V
M
show how test lead resistance and channel resistance is can-
), plus the resistance of the DUT (R
LEAD
Figure 2, is generally
) through one set of test
DUT
and V
SHI SLO
DUT
, as shown in the following
DUT
. The
). Note