VTI Instruments Metek EX1202 User Manual page 89

Minimizing Lead Wire Resistance Errors
While the best way to minimize lead wire resistance is to use the 4-wire method, this may not be
practical for all applications, especially when the wiring density is high. When 4-wire
measurements are impractical, the user can offset the lead resistance of the 2-wire resistance to
obtain a more accurate value. Typically, this is done by shorting the test leads, measuring the lead
resistance, then performing the actual measurement. Knowing the lead resistance allows the user
to remove this error source from all subsequent measurements using the application software.
Test Current Selection
The test current amplitude is user-configurable for 2-wire and 4-wire resistance measurements.
Table 4-5 shows the available test currents for the range and resolution selected.
Selecting a higher test current generates a higher voltage drop for a given resistance, and, as such,
improves the signal to noise ratio, improving measurement accuracy. Higher test currents are
especially helpful when measuring low resistances. Increasing the test current, however, can lead
to self-heating errors which are discussed later in this section.
4-Wire Resistance Measurements
The 4-wire resistance measurement method is the most accurate way to measure low resistances.
Lead wire and contact resistance errors are reduced significantly using this method, as the voltage
is measured through the sense lines, leaving R
The 4-wire resistance method is often used in automated test applications where long cable
lengths, numerous connections, or switches exist between the multimeter and the DUT. The
recommended connections for 4-wire resistance measurements is shown in Figure 4-13.
F
The desired resistance is calculated as:
V

R
DUT
Basic EX1200 DMM Operation
T 4-5: S
ABLE
Input HI
Sense HI
+V
MEAS
Sense LO
-V
MEAS
Input LO
EX1200 DMM
4-13: DMM C
IGURE ONNECTIONS FOR
MEAS
I
TEST
Range Test Current
100 Ω
1 mA
1 kΩ
1 mA
10 kΩ
100 µA, 500 µA
100 kΩ
10 µA, 50 µA
1 MΩ
5 µA
10 MΩ
700 nA, 10 MΩ
100 MΩ 700 nA, 10 MΩ
T C
ELECTABLE EST URRENTS
from the HI and LO inputs essentially ignored.
LEAD
R
LEAD
R
LEAD
4-W R
IRE ESISTANCE
www.vtiinstruments.com
R
DUT
Resistance Source
M
EASUREMENTS
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