VTI Instruments EX1401, User Manual

Page: 36 / 114

VTI Instruments Corp.
36 EX1401 Basic Operation
within acceptable bounds. In general, best results will be achieved with the shortest and thickest
wire that the physical requirements of the application can support.
The EX1401 offers excellent noise rejection through its high common mode rejection and selectable
bandwidth limiting, which allows for high integrity, noise-free measurements with even less-than-
ideal wiring setups. However, some common instrumentation wiring practices can be used to
achieve or ensure maximum performance.
Shielded thermocouple wire can be used to raise the system’s rejection of electrical interference.
Shielded wire encloses the two thermocouple wires with a low impedance conductor that should be
terminated by the user to a convenient earth ground. The EX1401 provides an external ground stud
that can be used for this purpose, but any earth ground point is acceptable.
Magnetic interference, which is present wherever high currents are flowing, is conversely decreased
by minimizing the loop area represented by the two thermocouple wires. That is, the wires should
be run closely together from the thermocouple junction to the connections in the thermocouple jack.
Fortunately, most thermocouple wire comes with a sheath that covers the two thermocouple
conductors, inherently creating a small loop area.
Many test applications involve the monitoring of a test article in a chamber, requiring the routing of
numerous types of signals through the chamber’s cable access ports. It is recommended that the
thermocouple wires be run through a separate port and conduit from cables carrying power or high
frequency signals.
Voltage Connections
Each input channel of EX1401 can be independently configured to measure differential voltage
signals. Since the EX1401 employs an uncompensated (Cu-Cu) mini-thermocouple female jack as
its input connector, measurements are not affected by the thermocouple effect. Users are
recommended to use suitable Cu-Cu mini thermocouple male plugs, with proper polarity. A popular
source is the SMPW series from Omega Engineering.
Note: The Cu-Cu type uncompensated thermocouple hardware is sometimes called as Type-U or
Calibration type. They will be generally available in White Color. The wider pin represents the
negative polarity, while the narrow pin is marked for positive polarity.
F
IGURE
2-9: V
OLTAGE
C
ONNECTOR
P
OLARITY
I
DENTIFICATION