A method for closed loop operation of a capacitive accelerometer, the capacitive accelerometer comprising: a substantially planar proof mass mounted to a fixed substrate by flexible support legs so as to be linearly moveable in an in-plane sensing direction in response to an applied acceleration; first and second sets of moveable capacitive electrode fingers extending from the proof mass, substantially perpendicular to the sensing direction and spaced apart in the sensing direction; and first and second sets of fixed capacitive electrode fingers (110, 112) extending substantially perpendicular to the sensing direction and spaced apart in the sensing direction; wherein the first set of fixed capacitive electrode fingers is arranged to interdigitate with the first set of moveable capacitive electrode fingers and the second set of fixed capacitive electrode fingers is arranged to interdigitate with the second set of moveable capacitive electrode fingers; the method comprising: applying an in-phase Pulse Width Modulation drive signal (V 1) to the first set of fixed capacitive electrode fingers and a corresponding anti-phase Pulse Width Modulation drive signal (V 2) to the second set of fixed capacitive electrode fingers; and sensing a displacement of the proof mass and increasing or decreasing the mark length of each pulse in the Pulse Width Modulation drive signals, thereby changing the mark:space ratio, to provide a net electrostatic restoring force on the proof mass that balances the inertial force of the applied acceleration and maintains the proof mass at a null position; characterised in that the method further comprises: detecting when the mark:space ratio required to maintain the proof mass at the null position is beyond a predetermined upper or lower threshold; and further modulating the Pulse Width Modulation drive signals by extending or reducing x pulses in every y cycles, where x≥1 and y>1, so as to provide an average mark:space ratio beyond the upper or lower threshold without further increasing or decreasing the mark length of the other pulses. a substantially planar proof mass mounted to a fixed substrate by flexible support legs so as to be linearly moveable in an in-plane sensing direction in response to an applied acceleration; first and second sets of moveable capacitive electrode fingers extending from the proof mass, substantially perpendicular to the sensing direction and spaced apart in the sensing direction; and first and second sets of fixed capacitive electrode fingers (110, 112) extending substantially perpendicular to the sensing direction and spaced apart in the sensing direction; wherein the first set of fixed capacitive electrode fingers is arranged to interdigitate with the first set of moveable capacitive electrode fingers and the second set of fixed capacitive electrode fingers is arranged to interdigitate with the second set of moveable capacitive electrode fingers; the method comprising: applying an in-phase Pulse Width Modulation drive signal (V 1) to the first set of fixed capacitive electrode fingers and a corresponding anti-phase Pulse Width Modulation drive signal (V 2) to the second set of fixed capacitive electrode fingers; and sensing a displacement of the proof mass and increasing or decreasing the mark length of each pulse in the Pulse Width Modulation drive signals, thereby changing the mark:space ratio, to provide a net electrostatic restoring force on the proof mass that balances the inertial force of the applied acceleration and maintains the proof mass at a null position; characterised in that the method further comprises: detecting when the mark:space ratio required to maintain the proof mass at the null position is beyond a predetermined upper or lower threshold; and further modulating the Pulse Width Modulation drive signals by extending or reducing x pulses in every y cycles, where x≥1 and y>1, so as to provide an average mark:space ratio beyond the upper or lower threshold without further increasing or decreasing the mark length of the other pulses. applying an in-phase Pulse Width Modulation drive signal (V 1) to the first set of fixed capacitive electrode fingers and a corresponding anti-phase Pulse Width Modulation drive signal (V 2) to the second set of fixed capacitive electrode fingers; and sensing a displacement of the proof mass and increasing or decreasing the mark length of each pulse in the Pulse Width Modulation drive signals, thereby changing the mark:space ratio, to provide a net electrostatic restoring force on the proof mass that balances the inertial force of the applied acceleration and maintains the proof mass at a null position; characterised in that the method further comprises: detecting when the mark:space ratio required to maintain the proof mass at the null position is beyond a predetermined upper or lower threshold; and further modulating the Pulse Width Modulation drive signals by extending or reducing x pulses in every y cycles, where x≥1 and y>1, so as to provide an average mark:space ratio beyond the upper or lower threshold without further increasing or decreasing the mark length of the other pulses. detecting when the mark:space ratio required to maintain the proof mass at the null position is beyond a predetermined upper or lower threshold; and further modulating the Pulse Width Modulation drive signals by extending or reducing x pulses in every y cycles, where x≥1 and y>1, so as to provide an average mark:space ratio beyond the upper or lower threshold without further increasing or decreasing the mark length of the other pulses.