  Ensembles of nitrogen-vacancy (NV) centers in diamond are widely used as AC
magnetometers. While such measurements are usually performed using standard
(XY) dynamical decoupling (DD) protocols at room temperature, we study the
sensitivities achieved by utilizing various DD protocols, for measuring
magnetic AC fields at frequencies in the 10-250 kHz range, at room temperature
and 77 K. By performing measurements on an isotopically pure $^{12}$C sample,
we find that the Carr-Purcell-Meiboom-Gill (CPMG) protocol, which is not robust
against pulse imperfections, is less efficient for magnetometry than robust
XY-based sequences. The concatenation of a standard XY-based protocol may
enhance the sensitivities only for measuring high-frequency fields, for which
many ($> 500$) DD pulses are necessary and the robustness against pulse
imperfections is critical. Moreover, we show that cooling is effective only for
measuring low-frequency fields (~10 kHz), for which the experiment time
apporaches $T_1$ at a small number of applied DD pulses.
