A sigma-delta converter suitable for measuring a photocurrent comprises an input node adapted to receive a current to be measured (Imeas), a capacitor connected to the input node, a clocked comparator coupled to the input node and to a reference voltage Vref at respective inputs, and a switchable current source connected to the input node which conducts a reference current Iref when switched on. The converter is arranged in a sigma-delta configuration, with the current source switched on to pull down the voltage (VCMP) at the input node when the comparator output toggles due to VCMP increasing above Vref, and to be switched off when the comparator output toggles due to VCMP falling below Vref, such that the comparator output comprises a digital bitstream which varies with Imeas.