An amplifier apparatus and method includes two charge amplifiers 210, 220, configured to receive input from a charge sensor such as an accelerometer 110 or piezoelectric device, and a differential amplifier 230 providing two outputs. The inputs of the differential amplifier are provided by the outputs of the two charge amplifiers. A switched capacitor amplifier may be formed by providing switches 330, 340, and optionally capacitors, between the input signal path of the differential amplifier and its output. When the switches are closed the output state of the differential amplifier may be reset, and when open, the off state resistance may form resistive feedback and provide high pass filtering. The differential outputs may be supplied to a differential analogue to digital converter. A processor, or computer program, may be configured to trigger a measurement state of the apparatus when unwanted motion disturbances are below a threshold level and maintain a motion recognition state when they are not. The apparatus is suitable for use with seismocardiographic signals which are typically noisy and have a desired signal with relatively low amplitude.
