Table Of Contents

Connecting Strain/Bridge Signals

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    Last Modified: August 1, 2019

    You can connect full-bridge, half-bridge, and quarter-bridge signals to the FieldDAQ device. Refer to the FieldDAQ Physical Channels topic in the NI-DAQmx Help for more information about how to use the NI-DAQmx API to take bridge measurements.

    If your device also features TEDS sensors, refer to Connecting TEDS Sensors for additional TEDS connection information.

    • Full-Bridge Signals—You can connect a ground referenced or floating full-bridge signal to the FieldDAQ device. When connecting floating signals to the FieldDAQ device, NI recommends connecting the EX- signal (pin 8) to the earth ground or shield for better noise rejection. Remote sense connections are recommended; refer to Remote Sensing and Shunt Calibration for remote sense use cases.

      Figure 1. Connecting a Full-Bridge Signal to the FieldDAQ Device
    • Half-Bridge Signals—You can connect a ground referenced or floating half-bridge signal to the FieldDAQ device. When connecting floating signals to the FieldDAQ device, NI recommends connecting the EX- signal (pin 8) to the earth ground or shield for better noise rejection. Remote sense connections are recommended; refer to Remote Sensing and Shunt Calibration for remote sense use cases.

      Figure 2. Connecting a Half-Bridge Signal to the FieldDAQ Device
      spd-note-note
      Note  

      When you insert a sensor in to the FieldDAQ device, slight changes in the excitation voltages can cause a mismatch between the internal half-bridge completion resistors and the half-bridge sensors, which results in a change in the measurement offsets. NI recommends performing offset nulls of quarter- or half-bridge sensors after attaching any sensor to the FieldDAQ device. Use the DAQmx Perform Bridge Offset Nulling Calibration VI/function or the DAQ Assistant with the Software Compensation (Initial Bridge Voltage) method to perform an offset nulling calibration. Refer to the Offset Nulling (Bridge Balancing) topic of the NI-DAQmx Help for more information.

    • Quarter-Bridge Signals—You can connect a ground referenced or floating quarter-bridge signal to the FieldDAQ device by adding a resistor externally to create a half bridge, as shown in the following figure. Internal shunt calibration allows for quarter-bridge connections. The FieldDAQ device supports internal quarter-bridge completion with resistances of 120 Ω and 350 Ω. Shunt calibration can correct for errors caused by wire resistance within an arm of the quarter bridge; refer to Remote Sensing and Shunt Calibration for more information.

      Figure 3. Connecting a Quarter-Bridge Signal to the FieldDAQ Device
      spd-note-note
      Note  

      When you insert a sensor in to the FieldDAQ device, slight changes in the excitation voltages can cause a mismatch between the internal half-bridge completion resistors and the half-bridge sensors, which results in a change in the measurement offsets. NI recommends performing offset nulls of quarter- or half-bridge sensors after attaching any sensor to the FieldDAQ device. Use the DAQmx Perform Bridge Offset Nulling Calibration VI/function or the DAQ Assistant with the Software Compensation (Initial Bridge Voltage) method to perform an offset nulling calibration. Refer to the Offset Nulling (Bridge Balancing) topic of the NI-DAQmx Help for more information.


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