The FFT analyzer can select both an AC coupling and a DC coupling as the input coupling, same as a normal oscilloscope. A DC coupling lets input signals pass through and an AC coupling lets input signals pass by through by cutting out DC components. The signals are then input to an input attenuator and an amplifier.

The main applications of the two couplings are as follows.

[AC coupling］

- Accurate measurement of AC components of the signals that contain DC components
- Measurement of overall values (effective values) of AC components only
- Normal acoustic vibration measurement
- When using a CCLD sensor

[DC coupling］

- Measurement including DC components
- Accurate measurement of low frequency (1Hz or lower) AC components