An apparatus for monitoring a physiological parameter over a plurality of measurement periods is presented. The apparatus includes a circuit coupled to a light emitter and to a light receiver. The circuit is adapted to drive the light emitter with a constant first charge over the plurality of measurement periods. Also presented is a wearable device provided with an apparatus for monitoring a physiological parameter over a plurality of measurement periods. Also presented is a method for monitoring a physiological parameter over a plurality of measurement periods, the steps include providing a light emitter and a light receiver and driving the light emitter with a constant first charge over the plurality of measurement periods.
