Specifications

Guard Voltage

The Guard Voltage diagnostic accessed through internal channel 14

measures the guard voltage. This measurement verifies that the guard on

the relay strain gage multiplexers is driven to the necessary potential by the

Wagner Ground. The proper potential minimizes leakage currents caused

by low gage isolation resistance. Figure C-5 shows how the guard is driven

to a known voltage, and where that voltage is measured.

Specification The guard voltage should be

⁄

the bridge excitation voltage ± 2%.

Set Up To perform the guard voltage diagnostic, your system should be configured

as follows:

1. If the bridge excitation voltage is externally supplied, connect the

supply to the terminal module as shown in Figure C-1. Set the

voltage to 5.0V.

2. If desired, connect the bridge arrangement to the bridge completion

channels. Strain gages do not have to be connected for this

diagnostic.

CAUTION Use care when externally supplying the bridge excitation

voltage. Voltages greater than 5.4V may damage the bridge

completion resistors.

Figure C-5. Driving the Card

116 Strain Gage Diagnostics Appendix C

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