Cable shielding is used primarily to minimize or eliminate capacitively coupledinterference from electric fields. When properly implemented, it can also be usedto minimize inductive coupling from magnetic fields. Shielding is only effectiveagainst electric fields if it provides a low impedance path to ground. A floatingshield provides no protection against interference. Grounding of shields can bea controversial subject because there are several ways to do it. The correctplace to connect an electrostatic shield is at the reference potential of the circuitrycontained within the shield. This point will vary depending upon whether thesource and receiver are both grounded or whether one or the other is floating.

Block diagrams of the SCM5B modules, found in the Product Catalog, show thereference potential for the input signal (i.e. IN). This point is generally also thereference potential of the field-side circuitry (shown with the ground symbol).Because all of the SCM5B modules have a high level of isolation between thefield-side and system-side circuitry, the field-side connections are effectivelydifferential inputs or outputs.

When using sensors with no shield connection at the sensor, connect the signalline shield to the SCM5B input signal reference potential (Figure 1). Some dataacquisition systems require the sensor to be grounded. This might be foundwhen using thermocouple or RTD probes which are designed for insertion intothermowells. In this configuration, the SCM5B module provides the isolationnecessary to eliminate signal degradation from differences in ground potentialsand ground loop currents. If a cable shield is present, it should be tied to groundat the sensor (Figure 2). Make the shield connection to ground as close aspossible to the sensor connection to ground to avoid a difference in potentialbetween signal and shield grounds. This potential difference can induce noiseon the signal lines.

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