Bridge Circuits (App Note) | LabJack
 
« Close

Datasheets and User Guides

App Notes

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 (or gage) is simply a resistive element whose resistance changes with strain.  The change in resistance is small so a bridge circuit is commonly used.  Look for signal conditioning hardware that includes bridge completion, or complete the bridge yourself using one of these methods:

  • Use 4 strain gauges to make a full bridge.  This generally requires having access to opposite strains so the effects compound.  For example, a beam where you can mount gauges on opposite sides.
  • Add dummy resistors to complete the bridge (quarter bridge and half bridge).
  • Add inactive strain gauges to complete the bridge (quarter bridge and half bridge).  This is considered superior because the inactive gauges should have the same tempco as the active gauges and thus temperature effects will cancel out in the 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.  Consider using an LJTick-Vref-41 as your excitation source, as discussed in forum topic 4376 linked below.

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

The LJTick-InAmp (LJTIA) is an external per-channel instrumentation amplifier add on. It (or similar) is required for bridge measurements with the U3/T4. It is seldom used with the U6/T7 as they have a similar amp already built-in, and in fact the built-in amp is superior (noise, resolution, accuracy). Reasons to add LJTIAs to the U6/T7:

  • When using the internal amp with Gain >1, acquisition speed decreases substantially when scanning more than 1 channel.  A per-channel amp, such as the LJTIA, is the solution to this.
  • The LJTIA can be used in addition to the internal amp to stack the gains and get extremely high amplification. Noise is also amplified, so it is questionable whether this technique will actually result in an increase of the signal-to-noise ratio.

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