Applications - Keithley 2182 User Manual

Applications

Low-resistance measurements
The Model 2182 can be used with a current source to measure resistances at levels well below
the capabilities of most conventional instruments. The following paragraphs discuss low-
resistance measurement techniques and include some applications to test switches.
Measurement techniques
Techniques used to measure resistances in the normal range are not generally suitable for
making low-resistance measurements because of errors caused by voltage drops across the test
leads. To overcome these limitations, low resistance measurements are usually made using the
4-wire (Kelvin) connections shown in
through an unknown resistance, developing a voltage across that device. Even though the test
lead resistance, R
assumed to be a constant current source with high output impedance. Also, since the voltmeter
has a very high input resistance (very low leakage current), the current through the sense leads
will be negligible, and the voltage drop across R
measured by the meter will be essentially the same as the voltage across the unknown resistance,
R .
DUT
Figure 2-10
4-Wire low-resistance measurement technique
Current
Source
Since the current through the measured resistance and the voltage across the device are both
known, the value of that resistance can easily be determined from Ohm's law:
R
DUT
, is present, it does not affect the current through R
LEAD
I
R
LEAD
R
LEAD
R
DUT
= V /I
M
Voltage and Temperature Measurements
Figure 2-10. A current source forces the current (I)
will be essentially zero. Thus, the voltage
LEAD
V
OFFSET
R
LEAD
R
DUT
R
LEAD
V
M
=
I
because I is
DUT
V Voltmeter
M
2-23