  The nitrogen-vacancy (NV) centers in diamond have ability to sense
alternating-current (AC) magnetic fields with high spatial resolution. However,
the frequency range of AC sensing protocols based on dynamical decoupling (DD)
sequences has not been thoroughly explored experimentally. In this work, we
aimed to determine the sensitivity of ac magnetic field as a function of
frequency using sequential readout method. The upper limit at high frequency is
clearly determined by Rabi frequency, in line with the expected effect of
finite DD-pulse width. In contrast, the lower frequency limit is primarily
governed by the duration of optical repolarization rather than the decoherence
time (T$_2$) of NV spins. This becomes particularly crucial when the repetition
(dwell) time of the sequential readout is fixed to maintain the acquisition
bandwidth. The equation we provide successfully describes the tendency in the
frequency dependence. In addition, at the near-optimal frequency of 1 MHz, we
reached a maximum sensitivity of 229 pT/$\sqrt{\mathrm{Hz}}$ by employing the
XY4-(4) DD sequence.
