Patent ID: 7659840

Claim:
A sigma-delta converter suitable for measuring a photocurrent, comprising: an input node adapted to receive a current to be measured I meas , a capacitor connected to said input node such that said capacitor is charged by I meas ; a periodic clock signal; 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; a switchable current source which conducts a reference current I ref when switched on and is connected to said input node, said current source arranged to be switched on and pull down the voltage (V CMP ) at said input node when said comparator output toggles due to V CMP increasing above said reference voltage, and to be 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 ; and a digital processor which receives said digital bitstream, said converter and processor arranged to: initialize V CMP by switching on I ref to make V CMP ≈V ref ; and begin a conversion interval that comprises a plurality of sequentially-occurring conversion cycles, each of said cycles comprising: switching off I ref to begin said conversion cycle; allowing V CMP to increase due to I meas until it exceeds V ref ; and switching on I ref to pull V CMP below V ref ; such that I meas is given by: I meas =[x 1s /( x 1s +x 0s )]* I ref , where x 0s is the number of ‘zeroes’ that occur in said bitstream while V CMP is increasing but is less than V ref , and x 1s is the number of ‘ones’ that occur in said bitstream while V CMP is decreasing but is greater than V ref .