Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 32)

Section 1. Installation and Maintenance
1-8
A good earth (chassis) ground will minimize damage to the datalogger and
sensors by providing a low resistance path around the system to a point of low
potential. Campbell Scientific recommends that all dataloggers be earth
(chassis) grounded. All components of the system (dataloggers, sensors,
external power supplies, mounts, housings, etc.) should be referenced to one
common earth (chassis) ground.
In the field, at a minimum, a proper earth ground will consist of a 6 to 8 foot
copper sheathed grounding rod driven into the earth and connected to the
CR5000 Ground Lug with a 12 AWG wire. In low conductive substrates, such
as sand, very dry soil, ice, or rock, a single ground rod will probably not
provide an adequate earth ground. 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.
1.7.2 Effect of Grounding on Measurements: Common Mode
Range
The common mode range is the voltage range, relative to the CR5000 ground,
within which both inputs of a differential measurement must lie in order for the
differential measurement to be made correctly. Common mode range for the
CR5000 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 CR5000 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 CR5000 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
CR5000 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 CR5000 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.