An apparatus comprising: a drive signal circuit for a micro-electromechanical system MEMS sensor, the drive signal circuit comprising: an input configured to receive a voltage signal representative of charge generated by the MEMS sensor; a phase-shift circuit (340; 540) electrically coupled to the input and configured to phase shift an input signal by substantially ninety degrees; a comparator circuit with hysteresis, wherein an input of the comparator circuit is electrically coupled to an output of the phase-shift circuit (340; 540) and an output of the comparator circuit is electrically coupled to an output of the drive signal circuit; and a feedback loop (360, 365, 370; 560, 565) extending from the output of the drive signal circuit to the input of the phase-shift circuit (340; 540) and configured to generate a self-oscillating signal at an output of the drive signal circuit, and wherein an output signal generated by the drive signal circuit is applied to a drive input of the MEMS sensor, characterized in that the feedback loop (360, 365, 370; 560, 565) includes a first amplifier circuit (360; 560) having an input electrically coupled to the output of the comparator circuit, a second amplifier circuit (365; 565) having an input electrically coupled to an output of the first amplifier circuit and an output electrically coupled to an input of the phase-shift circuit (340; 540), and a capacitor coupled to the output of the first amplifier circuit (360; 560) and circuit ground. a drive signal circuit for a micro-electromechanical system MEMS sensor, the drive signal circuit comprising: an input configured to receive a voltage signal representative of charge generated by the MEMS sensor; a phase-shift circuit (340; 540) electrically coupled to the input and configured to phase shift an input signal by substantially ninety degrees; a comparator circuit with hysteresis, wherein an input of the comparator circuit is electrically coupled to an output of the phase-shift circuit (340; 540) and an output of the comparator circuit is electrically coupled to an output of the drive signal circuit; and a feedback loop (360, 365, 370; 560, 565) extending from the output of the drive signal circuit to the input of the phase-shift circuit (340; 540) and configured to generate a self-oscillating signal at an output of the drive signal circuit, and an input configured to receive a voltage signal representative of charge generated by the MEMS sensor; a phase-shift circuit (340; 540) electrically coupled to the input and configured to phase shift an input signal by substantially ninety degrees; a comparator circuit with hysteresis, wherein an input of the comparator circuit is electrically coupled to an output of the phase-shift circuit (340; 540) and an output of the comparator circuit is electrically coupled to an output of the drive signal circuit; and a feedback loop (360, 365, 370; 560, 565) extending from the output of the drive signal circuit to the input of the phase-shift circuit (340; 540) and configured to generate a self-oscillating signal at an output of the drive signal circuit, and wherein an output signal generated by the drive signal circuit is applied to a drive input of the MEMS sensor, characterized in that the feedback loop (360, 365, 370; 560, 565) includes a first amplifier circuit (360; 560) having an input electrically coupled to the output of the comparator circuit, a second amplifier circuit (365; 565) having an input electrically coupled to an output of the first amplifier circuit and an output electrically coupled to an input of the phase-shift circuit (340; 540), and a capacitor coupled to the output of the first amplifier circuit (360; 560) and circuit ground.