Keithley 7710, User Manual

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10
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
S
). The meter then measures the voltage across the resistance value accordingly.
The main problem with the 2-wire method, as applied to low-resistance measurements is the test
lead resistance (R
LEAD
) and the channel resistance (R
CH
). The sum of these resistances typically
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 Figure 2 , is generally
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
TEST
) is forced through the test resistance (R
DUT
) through one set of test
leads (R
LEAD2
and R
LEAD3
), while the voltage (V
M
) across the Device Under Test (DUT) is mea-
sured through a second set of leads (R
LEAD1
and R
LEAD4
) called the sense leads.
With this configuration, the resistance of the DUT is calculated as follows:
R
DUT
= V
M
/ I
TEST
where: I
TEST
is the sourced test current and V
M
is the measured voltage.
As shown in Figure 2 , the measured voltage (V
M
) is the difference between V
SHI
and V
SLO
. The
equations at the bottom of Figure 2 sho w how test lead resistance and channel resistance is can-
celled out of the measurement process.
Maximum test lead resistance (Model 2750)
Table 1 l i sts the maximum test lead resistance (R
LEAD
), plus the resistance of the DUT (R
DUT
). Note
that a larger test lead resistance can be tolerated with a smaller R
DUT
, as shown in the following
example:
Example:
If R
DUT
is 3
Ω
on the 10
Ω
range using the
Ω
4 (4-wire) function, then the maximum test lead resis-
tance (R
LEAD
) can be 4
Ω
.
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