AC mode indicates that a high-pass (HP) filter has been applied to the signal that you are recording. An HP filter removes the low-frequency components of a signal. The most common AC mode settings on BIOPAC amplifiers are 0.5 Hz HP and 0.05 Hz HP. In AC mode, the signal is centered around zero.
AC mode is useful in studying changes in the signal. AC mode is not appropriate for recording absolute measures or for detecting the baseline level of the signal.
