NI 5114/5122/5124/5142 AC/DC/GND Coupling

You can select AC, DC, or GND input coupling for the 1 MΩ input path, and DC or GND input coupling for the 50 Ω input path. Select AC-coupling if the input signal has a DC component that you want to reject, provided that you are not concerned about low-frequency flatness. A DC input offset adjustment is available if the signal you want to measure is below this limit, or if you are using the 50 Ω path.

Ground coupling disconnects the input channel from the signal connected and internally connects the channel to ground to provide a ground reference.