A closed loop method of controlling a capacitive accelerometer comprising a proof mass moveable relative to first and second fixed capacitor electrodes (10, 12), the method comprising: applying in-phase and anti-phase pulse width modulation drive signals (30a, 30b) to the first and second fixed capacitor electrodes (10, 12) with an adjustable mark/space ratio; operating in closed loop so that mechanical inertial forces are balanced by electrostatic forces to maintain the operating point of the proof mass at a null position; detecting an output signal from the accelerometer representing a displacement of the proof mass from the null position to provide an error signal; and using the error signal so as to vary the mark/space ratio of the PWM drive signals (30a, 30b) so that the accelerometer output signal is proportional to acceleration; characterised in that the method further comprises: adding a sine wave modulation at a frequency f to the output signal from the proof mass before providing the error signal; further detecting a phase shift resulting from the sine wave modulation at the frequency f so as to recognise a critical drive signal magnitude (Vcrit) representing the null position; and providing a Vcrit control signal so as to adjust the magnitude of the PWM drive signals (30a, 30b) applied to the first and second fixed capacitor electrodes (10, 12), or applying a separate, adjustable drive signal to at least one further fixed capacitor electrode, to lock to Vcrit and thereby optimise open loop gain. applying in-phase and anti-phase pulse width modulation drive signals (30a, 30b) to the first and second fixed capacitor electrodes (10, 12) with an adjustable mark/space ratio; operating in closed loop so that mechanical inertial forces are balanced by electrostatic forces to maintain the operating point of the proof mass at a null position; detecting an output signal from the accelerometer representing a displacement of the proof mass from the null position to provide an error signal; and using the error signal so as to vary the mark/space ratio of the PWM drive signals (30a, 30b) so that the accelerometer output signal is proportional to acceleration; characterised in that the method further comprises: adding a sine wave modulation at a frequency f to the output signal from the proof mass before providing the error signal; further detecting a phase shift resulting from the sine wave modulation at the frequency f so as to recognise a critical drive signal magnitude (Vcrit) representing the null position; and providing a Vcrit control signal so as to adjust the magnitude of the PWM drive signals (30a, 30b) applied to the first and second fixed capacitor electrodes (10, 12), or applying a separate, adjustable drive signal to at least one further fixed capacitor electrode, to lock to Vcrit and thereby optimise open loop gain. adding a sine wave modulation at a frequency f to the output signal from the proof mass before providing the error signal; further detecting a phase shift resulting from the sine wave modulation at the frequency f so as to recognise a critical drive signal magnitude (Vcrit) representing the null position; and providing a Vcrit control signal so as to adjust the magnitude of the PWM drive signals (30a, 30b) applied to the first and second fixed capacitor electrodes (10, 12), or applying a separate, adjustable drive signal to at least one further fixed capacitor electrode, to lock to Vcrit and thereby optimise open loop gain.