You can program the FieldDAQ device to supply 3 V, 5 V, or 10 V of excitation voltage. 10 V excitation is only supported in full-bridge and half-bridge modes. In quarter-bridge mode, the FieldDAQ device supports connection of a single external resistance or strain gage of either 120 Ω or 350 Ω. In full- and half-bridge modes, the minimum allowable load resistance between the EX+ and EX- pins is 225 Ω with 10 V excitation and 108 Ω with 5 V or 3 V excitation.
The FieldDAQ device goes into a current limited mode to protect the excitation supply from short circuits or load resistance values lower than those listed above. In this current limited mode, the Overcurrent status is set in NI-DAQmx and the Strain/Bridge LED lights red.
The FieldDAQ device also has an open circuit detection mode to indicate whether a sensor is connected. In all modes and excitation voltages, an open circuit is detected if the total load resistance is greater than 1.2 kΩ. When an open circuit is detected, the Open Channel status is set in NI-DAQmx and the Strain/Bridge LED turns off.
If a connected sensor applies a total resistance above the minimum values and below 1.2 kΩ, the Strain/Bridge LED lights green.
The following table summarizes the supported excitation voltages for each element of most common bridge resistance values.
|120 Ω||3 V, 5 V||3 V, 5 V, 10 V||3 V, 5 V|
|350 Ω||3 V, 5 V||3 V, 5 V, 10 V||3 V, 5 V, 10 V|
|1 kΩ||Not supported||Refer to the note||3 V, 5 V, 10 V|
|Note: The total resistance of a half-bridge with 1 kΩ elements is 2 kΩ, which exceeds the open channel detection threshold of the FieldDAQ device. The FieldDAQ device operates and measures such half bridges correctly, but they may not result in the Strain/Bridge LED lighting and may cause the Open Channel flags to be set in NI-DAQmx.|