Data Acquisition

32 Practical Data Acquisition for Instrumentation and Control Systems2.8.4 Full bridge configurationIn circumstances where it is possible to place strain gauges, which have equal, andopposite strain (i.e. on opposite sides of a bending beam), it is possible to make all armsof the bridge active and get four times the sensitivity. This configuration, shown in Figure2.19, is referred to as a full bridge.Figure 2.19Full bridge circuit2.8.5 Wiring connectionsAs well as providing a choice of voltage or current excitation for the bridge circuit, signalconditioning equipment used to measure the output from a Wheatstone bridge, oftenprovides two of the precision trimmed compensation resistors as part of its own circuitry.This provides flexibility in configuring quarter bridge, half bridge or full bridge circuits,but requires the user to add the active element(s) and any required matchingcompensation resistors. Any compensation resistors added by the user, and external to thesignal conditioning equipment, should be precision-trimmed, with high accuracy andstability, especially with regard to temperature.As the voltage output sensitivity from the Wheatstone bridge is proportional to the inputexcitation voltage, it is possible that cable and connector resistance voltage drops mayreduce the excitation voltage seen at the bridge circuit and lead to inaccuracies in themeasured output. Consider the three-wire half bridge configuration of Figure 2.20.Figure 2.20Three-wire half bridge circuit wiring configuration