You can configure the NI 5112 input channels to be either AC or DC-coupled. When you need to measure a small AC signal on top of a large DC component, you can use AC coupling. AC coupling rejects any DC component in your signal before it enters the PGA. Activating AC coupling inserts a capacitor in series with the input. You can select input coupling through software. The boundary between DC and AC signals is called the AC coupling cutoff frequency. Frequencies above this cutoff pass through to the PGA, while frequencies below it are blocked. As shown in the following figure, adding an attenuator probe lowers this cutoff point.
DC coupling allows DC and low-frequency components of a signal to pass through without attenuation, and also allows high-frequency signals to pass. Refer to the NI PCI/PXI-5112 Specifications for input limits that must be observed regardless of coupling.