GEN7iB
257
INPUT CARDS
INPUT CARDS
12
12.6.8 Resistance mode
The resistance measurement is based on the bridge of Wheatstone. This measurement
can be implemented using two modes:
● Half bridge
● Quarter bridge
Theimpedanceofthewiringcaninuencethemeasurement.Thehalfbridgesetup
gives the best trade-off between simple schematics and performance. The quarter
bridgeschematicsareanotheralternative,butcaninuencetheresistancevaluesof
thebridgesignicantly.
The details of the bridge setup are described in a separate chapter, see chapter „Bridge
mode and cabling“ on page 249.
Theaccuracyoftheresistancemeasurementcanbeinuencedbychoosingthecom-
pletion resistor (R2) optimally. Accuracy improves if value of the completion resistor is
chosen near the resistance to measure. The relation between UR and R is asymptotic,
seeformulas(Fig.12.78)andgures(Fig.12.77/Fig.12.78).
TheGN840B/GN1640Bsupportstwointernalcompletionresistors,respectively350Ω
and120Ω.Thecompletionresistorcanalsobesetupexternallybytheuserinhalf
bridge mode.
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