An inertial sensor includes a substantially planar, rotationally symmetric proof mass, a capacitive pick-off circuit connected to the proof mass, an electrical drive circuit connected to the four pairs of electrodes. The drive circuit is arranged to apply first in-phase and anti-phase pulse width modulation (PWM) drive signals with a first frequency to the first and third electrode pairs, such that one electrode in each pair is provided with in-phase PWM drive signals and the other electrode in each pair is provided with anti-phase PWM drive signals and to apply second in-phase and anti-phase PWM drive signals with a second frequency, different to the first frequency, to the second and fourth electrode pairs, such that one electrode in each pair is provided with in-phase PWM drive signals and the other electrode in each pair is provided with anti-phase PWM drive signals.