Text preview for : 2Wire_4Wire Resistance Article.pdf part of Keithley 2Wire 4Wire Resistance Article Keithley Appnotes 2Wire_4Wire Resistance Article.pdf
Back to : 2Wire_4Wire Resistance Ar | Home
Two-Wire Resistance
Measurements
A Greater Measure of Confidence
Figure 2 represents a two-wire resistance test
configuration employing the constant cur-
rent method.
The main measurement issue with the
two-wire method, as applied to low resist-
ance measurements, is that the total lead
resistance (R LEAD) is added to the measure-
ment. Because the test current (I) causes a
small but significant voltage drop across the
lead resistances, the voltage (VM) measured
two-Wire vs. Four-Wire
by the meter won't be exactly the same as the
voltage (VR) directly across the test resist-
ance (R), and considerable error can result.
Resistance Measurements:
Typical lead resistances range from 10m
to 1, so it's very difficult to obtain accurate
two-wire resistance measurements when
Which Configuration Makes the resistance under test is lower than 100
because the resistance of interest will be
sense for Your Application?
completely swamped by the lead resistance.
In fact, lead resistance will be the dominant
source of error. For example, using test leads
with a 100m combined resistance to per-
form a two-wire resistance measurement
on a 500m resistor will result in a 20%
Jerry Janesch measurement error in addition to that of the
Keithley Instruments, Inc. instrument.
M
ost precision digital multi- impedance, virtually all the test current Four-Wire (Kelvin) Resistance
meters (DMMs) and many (1mA) flows through the DUT. Measurements
Source Measurement Units Due to the limitations of the two-wire
(SMUs) offer both two-wire Table 1. Typical DMM ranges and test currents method, a different approach is used for low
and four-wire resistance Measurement resistance measurements that reduce the
measurement capabilities. However, these Range Test Current effect of test lead resistance. For measuring
two techniques are not equally well suited 100 1 mA DUTs with resistances equal to or less than
for all resistance measurement applications. 1 k 1 mA 1k, test engineers may use the four-wire
This article offers a quick overview of how to 10 k 100 A (Kelvin) connection shown in Figure 3.
determine the most appropriate technique 100 k 10 A Because the voltage is measured at the DUT,
for a specific application. 1 M 1 A voltage drop in the test leads is eliminated
DMMs typically employ the constant- 10 M 0.1 A (this voltage could be significant when meas-
current method to measure resistance, 100 M 0.1 A uring low-resistance devices).
which sources a constant current (ISOUR) to With this configuration, the test current
the device under test (DUT) and measures (Source Keithley Model 2110) (I) is forced through the test resistance (R)
the voltage (VMEAS). Resistance (R DUT) is
then calculated and displayed using the
known current and measured voltage (R DUT Input HI
= VMEAS/ISOUR). Figure 1 shows a simple dia-
gram of the constant-current test.
The test current sourced to the DUT VMEAS V ISOUR DUT
depends on the selected measurement range
(Table 1). For example, for the 100 range,
the test current is 1mA. Because the volt-
meter of a typical DMM has very high input
Figure 1. The constant-current method of resistance measurement, in a two-wire test configuration.
Two-Wire vs. Four-Wire Resistance Measurements: Which Configuration Makes Sense for Your Application? May 2013 1
via one set of test leads, while the voltage
DMM
RLEAD Test Current (I) (VM) across the DUT is measured through a
HI
second set of leads (sense leads).
Although some small current (typically
less than 100pA) may flow through the sense
Lead
I VM VM Resistances VR R Resistance leads, it is usually negligible and can gener-
Under Test
ally be ignored for all practical purposes.
RLEAD Therefore the voltage measured by the meter
LO
(VM) is essentially the same as the voltage
(VR) across the resistance (R). As a result, the
VM = Voltage measured by meter
resistance value can be determined much
VR = Voltage across resistor
more accurately than with the two-wire
VM
Measured Resistance = = R + (2