Both the Instrumentation Amplifier and the Differential Voltage Probe are true differential voltage measuring devices. Neither terminal is at ground potential, which is sometimes important in making connections to external circuits. However, this can cause noise problems under certain circumstances.

The noise will present in various ways, depending on the data collection rate. At data rates over 500 Hz or so a clear 120 Hz power curve can be seen. If a slower (1 Hz, for example) is used, the noise will be aliased and may present as a very slow variation in the reading, with periods of several minutes or longer.

If the LabQuest is being operated by its AC adapter (and not just its internal battery) the local LabQuest ground is weakly coupled to the power mains through the switching power supply (AC adapter). This will introduce noise, sometimes a considerable amount of noise, into the measurements if there is no ground reference at all in the external circuit.

There are three workarounds:

1) Use LabQuest on battery only. This allows LabQuest to float and the noise is eliminated.
2) Ground some part of the circuit to LabQuest ground. This can be done with the black lead of the standard voltage probe (VP-BTA). Connect the VP-BTA to LabQuest and the black lead to a part of the circuit that can be grounded. This need not be the same as either terminal of the differential voltage sensor. Only the black lead needs to connected. LabQuest will detect this sensor, but you don’t need to use the data collected from that channel.
3) Ground the grounding pin on the back of the INA-BTA to a part of the circuit that can be grounded. In this case you do not need to use a separate input line.