D.1 - Bridge Troubleshooting | LabJack
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D.1 - Bridge Troubleshooting

Following is a basic procedure for troubleshooting and testing a bridge connected to an LJTick-InAmp.  We will look at the voltages using the test panel in LJControlPanel.  This takes software complications out of the loop.  Make sure all other LabJack related software is closed.

Start by removing all signals from the LJTIA, and jumper all 4 inputs (INA+/INA-/INB+/INB-) to GND.  Turn off all switches on the LJTIA, except #5, to give you a gain of x1 and offset of 0.4 volts.  You should see that both channels measured by the LabJack measure about 0.4 volts.

Now turn on switches 3 and 9 (in addition to 5), to give a gain of x51 and offset of 0.4 volts.  The voltages measured by the LabJack will likely change a little, but should both still be near 0.4 volts.

Now remove the jumpers.  Connect Vexc+/Vexc- on the bridge to VREF/GND on the LJTIA.  Connect Vout+/Vout- on the bridge to INA+/INA- on the LJTIA.  If the bridge is reasonably balanced and is in the zero condition, the bridge output should be near 0.0 mV and the LJTIA output should still be near 0.4 volts.  If not, use a DMM to confirm that the raw bridge output voltage is near 0.0 mV.

A bridge has an output specification such as 2.0 mV/V which we will use for this example.  Since we are using VREF for excitation, and it is about 2.5 volts, then we will get about 5mV of differential output (Vsig+ versus Vsig-) at full scale (rated load/strain/pressure/etc.).

Change the bridge so it is at some known condition.  For example, if rated load is 100 pounds, and we are at 50 pounds, then the bridge should give about 2.5 mV of output, and thus the LJTIA will give about (0.0025*51)+0.4 = 0.528 volts of output.  Use a DMM to confirm those voltages at the input (INA+ versus INA-) and output (OUTA versus GND) of the LJTIA, and also confirm that the test panel reports about 0.528 volts.

In general, the troubleshooting technique is to:

  • use a DMM to measure the LJTIA input voltage (INA+ versus INA-)
  • use a DMM and the test panel to measure the LJTIA output voltage (OUTA versus GND)
  • confirm the LJTIA output is (input*gain)+offset
  • confirm that the DMM reading of the LJTIA output and the test panel reading both agree