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12-
OPERATING RECOMMENDATIONS
Noise Reduction
Use twisted pair #16 to #22 AWG wire
for power to the LCA unit to minimize
EMI (electromagnetic interference) pickup.
Use twisted pair #18 to #22 AWG wire
for the voltage or current outputs of the
LCA unit to minimize EMI pickup and generation.
Use shielded twisted pair wire
for the excitation leads, sense leads, and signal
leads for cable lengths above 3 m (10 ft). Suitable cables with an outer shield for
multiple inner twisted pairs are available from cable manufacturers and can be used
for distances up to 1 km (3,300 ft). Tie the shield to the -EXC terminal of the LCA
unit (Pin 7 of J1). For cable lengths less than 3 m (10 ft), shielded cable may not be
required, but twisted pairs are advised.
Do not place signal and sense cables
in the same conduit or cable tray that holds
power cables or cables to motors and solenoids, as these are known EMI genera-
tors.
Thermal Stability
Minimize ambient temperature variations
since low-level signal measurements
can be corrupted by thermal emf’s generated by the electronics and signal input
connections. Minimize changing air flows around the LCA unit.
Allow the LCA to warm up before taking measurements.
Allow 15 minutes if the
LCA does not provide sensor excitation, is powered by 12V, and a voltage output is
used. Under these conditions, power dissipation inside the LCA unit is minimized
and is only around 120 mW. Otherwise allow 30 minutes of warmup time.
Apply a maximum 500
Ω load
when a current output is used so that up to 200 mW
is dissipated in the external load, not inside the LCA unit. If a precision 100
Ω resis-
tor is used to monitor the output current, a 400
Ω max non-precision resistor should
be placed in series with it.
Potentiometer Operation
Precision 25-turn potentiometers
are used to make zero and span adjustments
following the warmup period. These are electromechanical devices subject to
stiction (or static friction) between the wiper and resistive surface. Stiction causes
hysteresis, or slightly different outcomes if the rotation is clockwise or counter-
clockwise.
To minimize stiction and hysteresis,
rotate the potentiometer screw in one
direction and then back off in the opposite direction by a fraction of a degree. Then
gently tap the potentiometer body with the adjustment screwdriver. Both actions will
decrease the angular tension between the wiper and resistive surface.
