VTI Instruments Ametek EX1200A Series, User Manual

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EX1200A Triggering 93
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.
Range Test Current
10
Ω
10 mA
100
Ω
1 mA
,
10 mA
1
kΩ
1 mA
10
kΩ
100 µA , 1 mA
100
kΩ
10 µA
1
MΩ
1 µA, 10 µA
10
MΩ
100 nA, 1 µA
100
MΩ
100 nA
T
ABLE
4-5: S
ELECTABLE
T
EST
C
URRENTS
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
LEAD
from the HI and LO inputs essentially ignored.
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.
Input HI
Sense HI
Sense LO
Input LO
EX1200 DMM
-V
MEAS
+V
MEAS
Resistance Source
R
DUT
R
LEAD
R
LEAD
F
IGURE
4-13: DMM C
ONNECTIONS FOR
4-W
IRE
R
ESISTANCE
M
EASUREMENTS
The desired resistance is calculated as:
TEST
MEAS
DUT
I
V
R
