Difference between Constant Voltage System and Constant Current System in Bridge Excitation
Excitation Signal conditioners using DC bridge voltage are available in 2 types: CDV series having the bridge circuit excited on constant voltage and CDA series having the bridge circuit excited on constant current. Generally, the constant voltage system is used but if the cable is extended between the signal conditioner and strain gage transducer, the cable resistance lowers the voltage to be applied to the transducer, thereby resulting in decreased sensitivity (approx. –6%/100 m, 120Ω). On the contrary, the constant current system can keep the current constant against increased cable resistance, and thus the voltage applied to the transducer is always constant and remains unaffected by cable extension. With the constant current system,however, the bridge resistance should be compensated if it differs from the nominal bridge resistance.
Note that the CDV-900A has a built-in remote-sensing circuit, and thus use of a 6-conductor (0.5 mm2 ) shielded cable prevents the sensitivity from decreasing up to approximately 2 km.