Campbell Scientific CR23X Micrologger - Manual (Page 235)

SECTION 14. INSTALLATION AND MAINTENANCE
14-9
For these situations, consult the literature on
lightning protection or contact a qualified
lightning protection consultant. An excellent
source of information on lightning protection can
be located via the web at
http://www.polyphaser.com.
In vehicle applications, the earth ground lug
should be firmly attached to the vehicle chassis
with 12 AWG wire or larger.
In laboratory applications, locating a stable
earth ground is not always obvious. In older
buildings, new cover plates on old AC sockets
may indicate that a safety ground exists when in
fact the socket is not grounded. If a safety
ground does exist, it is good practice to verify
that it carries no current. If the integrity of the
AC power ground is in doubt, also ground the
system through the buildings, plumbing or
another connection to earth ground.
14.7.2 EFFECT OF GROUNDING ON
MEASUREMENTS: COMMON MODE RANGE
The common mode range is the voltage range,
relative to the CR23X ground, within which both
inputs of a differential measurement must lie in
order for the differential measurement to be
made. Common mode range for the CR23X is
±5.0 V. For example, if the high side of a
differential input is at 2 V and the low side is at
0.5 V relative to CR23X ground, a measurement
made on the ±5.0 V range would indicate a signal
of 1.5 V. However, if the high input changed to 6
V, the common mode range is exceeded and the
measurement may be in error.
Common mode range may be exceeded when
the CR23X is measuring the output from a
sensor which has its own grounded power
supply and the low side of the signal is
referenced to the sensors power supply ground.
If the CR23X ground and the sensor ground are
at sufficiently different potentials, the signal will
exceed the common mode range. To solve this
problem, the sensor power ground and the
CR23X ground should be connected, creating
one ground for the system.
In a laboratory application, where more than
one AC socket may be used to power various
sensors, it is not safe to assume that the power
grounds are at the same potential. To be safe,
the ground of all the AC sockets in use should
be tied together with a 12 AWG wire.
14.7.3 EFFECT OF GROUNDING ON SINGLE-
ENDED MEASUREMENTS
Single-ended analog voltage measurements
(Instructions P1, P4, P5, P7, P11, P12, and
P13) are referenced to the CR23X ground. This
makes single-ended measurements particularly
sensitive to fluctuations in the CR23X ground
potential. The ground potential will fluctuate
proportional to how much power is being used
by, and being sourced from, the CR23X.
Peripheral devices such as multiplexers and
many sensors source power from the CR23X.
The grounding scheme in the CR23X has been
designed to minimize the effects of ground
potential fluctuations. To take advantage of this
design, observe the following grounding rule:
NOTE: Always connect a device’s ground
next to the active terminal associated with
that ground.
Examples:
1. Connect 5 Volt, 12 Volt, and control
grounds to G terminals.
2. Connect excitation grounds to the closest
terminal on the upper right CR23X
terminal block.
3. Connect the low side of single-ended
sensors to the nearest terminal on the
upper or lower left terminal blocks.
4. Connect shield wires to the nearest
terminal on the upper or lower left terminal
blocks.
If offset problems occur because of shield or
ground leads with large current flow, tying the
problem leads into the terminals next to the
excitation, CAO, and pulse-counter channels
should help. Problem leads can also be tied
directly to the ground lug to minimize induced
single-ended offset voltages.
14.8 POWERING SENSORS AND
PERIPHERALS
The CR23X is a convenient source of power for
sensors and peripherals requiring a continuous
or semi-continuous 5 VDC or 12 VDC source.
The CR23X has 2 continuous 12 Volt (12V)
supply terminals, 1 switched 12 Volt (SW12)