The present embodiment relates to a method and apparatus for measuring a tissue variation signal (e.g. a photoplethysmographic (PPG) signal) without large direct current (DC) or low frequency (LF) offset which normally limit the sensor accuracy through motion artefacts and/or dynamic range requirements. The proposed solution is based on a separation of the PPG signal from the disturbance. This can be achieved by creation of a modulated PPG signal, or by creation of a differential PPG signal and an optimized sensor configuration which is adapted to remove DC or LF components.