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Datasheets and User Guides

Software & Driver

 

Bridge Circuits (App Note)

A Wheatstone bridge is a circuit used to measure small differences in resistance.  Such bridge circuits are common with various types of sensors such as load cells, pressure sensors and strain gauges.  Sometimes these sensors are provided raw and sometimes they have added signal-conditioning making them much easier to use.

Load Cell - Most load cells are raw bridge circuits without signal conditioning.  If the output is specified as something like 2 mV/V it is a raw bridge.  If the output is specified as something high level like 0-5 volts, +/-10 volts, or 4-20 mA, then the load cell has signal conditioning and this app not does not apply.

Pressure Sensor - Some pressure sensors are raw bridge circuits while many have signal conditioning.  See the Pressure Sensors App Note.

Strain Gauge - A strain gauge is simply a resistive element whose resistance changes with strain.  The change in resistance is small so a bridge circuit is commonly used.  If you only plan to have 1 or 2 active strain gauges in your measurement you must add 3 or 2 resistors or dummy (inactive) strain gauges to form a complete bridge (quarter bridge and half bridge).

 

The LabJack U6 and T7 series of devices can acquire the small differential bridge signals directly, although the high gain (x100 is typical) required to do this reduces the max sample rates possible.  See Section 3 of the U6 Datasheet or Appendix A-1 of the T7 Datasheet.

The U3 and T4 require signal-conditioning between a raw bridge circuit and the LabJack such as the LJTick-InAmp.

For more information see old forum topic 4376, and in particular post #13 is a tutorial for the U6/T7 series of devices.