  Ensembles of solid state defects in diamond make promising quantum sensors
with high sensitivity and spatiotemporal resolution. The inhomogeneous
broadening and drive amplitude variations across such ensembles have differing
impacts on the sensitivity depending on the sensing scheme used, adding to the
challenge of choosing the optimal sensing scheme for a particular sensing
regime. In this work, we numerically investigate and compare the predicted
sensitivity of schemes based on continuous-wave (CW) optically detected
magnetic resonance (ODMR) spectroscopy, pi-pulse ODMR and Ramsey interferometry
for sensing using nitrogen-vacancy centers in the low-frequency (<10 kHz) range
typical for signals from biological sources. We show that inhomogeneous
broadening has the strongest impact on the sensitivity of Ramsey
interferometry, and drive amplitude variations least impact the sensitivity of
CW ODMR, with all methods constrained by the Rabi frequency. Based on our
results, we can identify three different regions of interest. For inhomogeneous
broadening less than 0.3 MHz, typical of diamonds used in state of the art
sensing experiments, Ramsey interferometry yields the highest sensitivity. In
the regime where inhomogeneous broadening is greater than 0.3 MHz, such as for
standard optical grade diamonds or in miniaturized integrated devices, drive
amplitude variations determine the optimal protocol to use. For low to medium
drive amplitude variations, the highest sensitivity is reached using pi-pulse
ODMR. For high drive amplitude variations, relevant for widefield microscopic
imaging, CW ODMR can yield the best sensing performance.
