The output of the Vernier Power Amplifier, order code PAMP, $225, is bipolar. The negative terminal is not quite at ground, because of the presence of a small current sense resistor. Current through this resistor is used to report the current delivered to the PAMP load.
Per the documentation that comes with the PAMP, the current readings will be correct only if no other point in the circuit is grounded.
This means that one must use great caution when measuring potentials in the load circuit. The Voltage Probe, order code VP-BTA, $12, is a poor choice because the black terminal is grounded to the interface. Instead, one should use the Differential Voltage Probe, order code DVP-BTA, $39, or Go Direct® Voltage Probe, order code GDX-VOLT, $69, because its leads are floating.
For example, consider a very simple Ohm's law experiment. Connect a 10 Ω resistor across the output terminals of the PAMP. Connect the VP-BTA across the resistor, black lead on the black terminal of the PAMP.
Connect the PAMP current output to a LabQuest. Connect the VP-BTA to the LabQuest. Both will be detected by the LabQuest App. Zero both sensors. Configure a selected events data collection run. Use the Power Amp application to set the output potential to a variety of values, keeping the current and voltage readings each time.
Now graph potential vs. current. The slope should be the resistance. You'll get about 12 Ω, instead of 10, because not all of the current returned through the sense resistor. Instead, some current returned through the black lead of the VP-BTA. This configuration will never work because of the bipolar, non-differential nature of the VP-BTA.
The experiment works as expected with the DVP-BTA or GDX-VOLT.