VTI Instruments Ametek EX1200A Series, User Manual

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VTI Instruments Corp.
30 EX1200A Series Introduction
M
AXIMIZING
M
EASUREMENT
P
ERFORMANCE
This section discusses tips and procedures that can help maximize the actual performance realized
with the EX1200A DMM and aid the user in avoiding some common pitfalls associated with
making measurements.
Warm-up Time
For the most accurate DMM measurements, the EX1200A should be warmed up for 30 minutes.
Auto-zero
As is true with most test equipment, measurements are affected by temperature changes in the
environment as well as component drift. The auto-zero feature of the EX1200A DMM
compensates for these changes by using internal references to adjust the gain and offset of reading
to ensure accuracy. For this reason, it is recommended that the auto-zero feature always be
enabled. The additional operations, however, decrease measurement speed. In cases where
measurement speed is more important than accuracy, the auto-zero feature can be disabled which
will nearly double the measurement speed. For more information on auto-zero, refer to
Auto-Zero
in Section 4 .
Cabling
In most circumstances, a shielded cable should be used to obtain the most accurate measurement.
The shield of the cable should be connected to chassis ground at the EX1200A and left floating at
the DUT. However, if the device under test (DUT) is in an environment with a substantial amount
of noise, such as near a motor, the shield should be attached to the ground of the device that is
generating the noise to provide the most direct path to ground.
In 4-wire measurements, if all four wires are enclosed with a common shield, the above practices
should be followed. If there is a shield around both the Sense pair of wires and the Current Source
wires, both shields may be connected to chassis ground. If only one pair of wires is shielded these
should be the sense lines and the above practices should be observed.
The user should consider they type of cable being use when taking measurements. Transient errors
may occur when initial contact is made if the input connector and mating connector are at different
temperatures and are made of dissimilar metals. This can create a thermoelectric voltage which is
a common error source. If both the EX1200A DMM and the mating connector are at the same
ambient temperature, this error typically dissipates to less than 0.1 ºC within 3 minutes. However,
if a mating connector at room temperature is plugged into a DMM at a substantially different
ambient temperature, a significant initial error may appear. This error will also take longer to
decay to a level of insignificance. This error mechanism is not affected by calibration and is
avoided by allowing for a thermal equilibrium delay after initial insertion. To reduce the effect of
thermocouple voltages and temperature variations in low-level voltage measurements, it is
recommended that connections be made using copper-to-copper crimped connections.
In addition to ensuring thermal stability, to ensure the highest degree of reading reliability, cables
used with the mating connector should be insulated with dithiolate.
High-Resistance Measurements
When making high-resistance measurements, it is important to take test cable leakage due to
insulation into account. Cable jackets with poor insulators can allow voltage to leak from the test
system and can create a significant measurement error. This error increases if this measurement is
performed in a humid environment as this will increase the conductance of the insulation.
Contamination of the cabling or input connector with high-resistance surface films, such as the oil
present on fingertips, should also be avoided. To maximize high resistance measurement readings,
care should be taken to maintain a “clean” test e
nvironment.