Specifications

Section 7. Grounding
7-3
provide an adequate earth ground. For these situations, consult the literature on
lightning protection or contact a qualified lightning protection consultant.
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 challenging, but
still necessary. In older buildings, new AC receptacles on older AC wiring
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.
7.2 Common Mode Range
To make a differential measurement, voltage inputs must be within the CR1000
common mode range of ±5 V. The common mode range is the voltage range,
relative to CR1000 ground, within which both inputs of a differential
measurement must lie, in order for the differential measurement to be made.
For example, if the high side of a differential input is at 4 V and the low side is
at 3 V relative to CR1000 ground, there is no problem. A measurement made
on the ±5000 mV range will return 1000 mV. However, if the high input is at
5.8 V and the low input is at 4.8 V, the measurement can not be made because
the high input is outside of the ±5 V common mode range. The CR1000
indicates the overrange by returning NAN (not-a-number). Sensors that have a
floating output, or are not referenced to ground through a separate connection,
may need the CR1000 to use a voltage range “C” option to pull the sensor into
common mode range or to have the one side of the differential input (usually
the low input) connected to ground to ensure the signal remains within the
common mode range.
Common mode range can be exceeded when the CR1000 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 sensor’s power supply ground. If the
CR1000 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 CR1000 ground should be connected, creating
one ground for the system.
Problems with exceeding common mode range can be encountered when the
CR1000 is used to read the output of external signal conditioning circuitry if a
good ground connection does not exist between the external circuitry and the
CR1000. When operating where AC power is available, it is not always safe to
assume that a good ground connection exists through the AC wiring. If a
CR1000 is used to measure the output from a laboratory instrument (both
plugged into AC power and referencing ground to outlet ground), the best
practice is to run a ground wire between the CR1000 and the external circuitry.