Patent ID: 8035538

Claim:
A sigma-delta converter suitable for measuring a photocurrent, comprising: an input node adapted to receive a current to be measured I meas ; an integrator, consisting of: a capacitor connected to said input node such that said capacitor is charged by I meas ; a periodic clock signal; and a clocked comparator which receives said clock signal and is coupled to said input node at one input and to a reference voltage V ref at a second input, said comparator arranged to toggle its output synchronously with said clock signal when the voltage at said input node increases above or falls below said reference voltage; and a switchable current source, consisting of: a fixed current source which conducts a reference current I ref ; and a single switch, connected between said input node and said current source; said switchable current source arranged such that said single switch is switched on such that reference current I ref pulls down the voltage (V CMP ) at said input node when said comparator output toggles due to V CMP increasing above said reference voltage, and such that said single switch is switched off when said comparator output toggles due to V CMP falling below said reference voltage, such that the output of said comparator comprises a digital bitstream which varies with I meas .