Appendix E - Testing & Troubleshooting the LJTIA

Testing Basic Operation

The following is a procedure for testing basic operation of the LJTick-InAmp.

For T-series devices, use the Register Matrix tab to read the analog input voltages from the LabJack and to also control the analog output.

For the U3/U6/UE9, use the test panel in LJControlPanel to read the analog input voltages from the LabJack and to also control the analog output.

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.  The output voltage of the LJTIA is Vout = (Vin * Gain) + Voffset, so you should see that both channels measured by the LabJack measure about 0.4 volts.

Leave INA- & INB- jumpered to GND, but change INA+ & INB+ so they are both jumpered to DAC0.  Set DAC0 to 1.5 volts, and the output should read about 1.5*1 + 0.4 = 1.9 volts.

Turn on switches 2 and 8 (in addition to 5), to give a gain of x11 and offset of 0.4 volts.  Set DAC0 to 0.1 volts, and the output should read about 0.1*11 + 0.4 = 1.5 volts.

You can do similar to test the gains of x51 and x201, but the analog output on your LabJack might not go low enough.  The limit for Vin at G=x51 is 0.024 volts, and the limit for Vin at G=x201 is 0.0059 volts.  Set DAC0 & DAC1 to 0.0, use a DMM to measure the actual voltage output by both, and if either is low enough use that for testing.

Troubleshooting

Signals Not Referred to GND

You can't just connect 2 floating voltages to IN+ and IN- of an instrumentation amp.  See the Differential Readings app note.

Common-Mode Voltage Out of Range

This is a common issue when trying to measure individual cell voltages in a battery pack with multiple cells in series.  See the Differential Readings app note, and see the Signal Range Tables in Appendix A.

Source Impedance Too High

The LJTick-InAmp has high input impedance and low input bias currents, but some signal sources are particularly sensitive (e.g. some sources called "electrodes").  Such sources need an amp with very high input impedance, but usually more importantly ultra low input bias currents.

Don't forget about Voffset

Vout = (Vin * Gain) + Voffset.  If your Vout is 0.4 or 1.25 volts higher than you expect, perhaps you have not accounted for Voffset.