Campbell CR800 Spécifications télécharger pdf (Page 60)

Section 1. Installation and Maintenance

1-8

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 CR800 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

CR800 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 CR800 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 CR800 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

CR800 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 CR800 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.

1.7.3 Effect of Grounding on Single-Ended Measurements

Low-level single-ended voltage measurements can be problematic because of

ground potential fluctuations. The grounding scheme in the CR800 has been

designed to eliminate ground potential fluctuations due to changing return

currents from 12 V, SW-12, 5 V, and the control ports. This is accomplished

by utilizing separate signal grounds (

) and power grounds (G). To take

advantage of this design, observe the following grounding rule:

Always connect a device’s ground next to the active terminal

associated with that ground. Several ground wires can be

connected to the same ground terminal.

NOTE

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Campbell CR800 Spécifications Page 60

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