LJ U3 HV- Issue with Input Resistance (EIO1) | LabJack

LJ U3 HV- Issue with Input Resistance (EIO1)

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mgrubenmann's picture
LJ U3 HV- Issue with Input Resistance (EIO1)

Hi LJ Team

My coworker is using 3 LabJack U3 HV for a board test fixture (All OEM Versions connected to an Interface board).
Until recently, everything was working fine. But since last week we have a bad voltage measurement on Channel EIO1.

If the LabJack U3 is connected to the DUT/UUT testpoint, the voltage is pulled down to 0.3V instead of nominal 0.8V.
If we Remove the LabJack, the testpoint goes back up to 0.8V. If we replace the LabJack, the error is gone.

If we connect a reference voltage without a load to the EIO1, the voltage measurement is working perfectly fine.

No visual damage is visible. Any clue what could have taken damage?

HW Revision 1.3, newest Firmware. Error can be viewed in the official LabJack Tools as well.

Many thanks.

LabJack Support
labjack support's picture
We would recommend testing

We would recommend testing the input as described in our application note. We recommend removing all external connections while testing:


If the test results look good, that would indicate some issue with the source signal. Could something have happened to where the U3 is now seeing a very high source impedance on that port? The flexible IO should work with a source impedance up to 10kohms normally, and up to 200kohms with "long settling" enabled:


mgrubenmann's picture
Many thanks for the quick

Many thanks for the quick response. Will update as soon as I had a chance to test the input.

Best regards