Ramp-type analogue-digital conversion, with multiple conversions or single conversion, depending on the light level received by a pixel

Abstract

In a matrix image sensor, a method of reading a pixel of a column allows two modes of analogue-digital conversion of the voltage level provided by the column: a first mode in which are carried out a single analogue-digital conversion in a nominal conversion time window F, of nominal duration dand a counting which starts with a ramp of nominal duration dand stops upon the toggling of the output Sof the comparator; and a second mode which provides for multiple conversions by comparison with a ramp of reduced duration d, in the same nominal conversion time window. The selection of the mode of conversion to be applied is based on the observation of the state of the output Sof the comparator after a predetermined duration after the instant tof ramp start: if the output has toggled, the useful level to be converted represents a low light level to which the second mode with multiple conversions will be applied; if the output has not toggled, the useful level to be converted represents a high light level and the first, conventional, mode with single conversion will be applied. The invention makes it possible to improve the signal-to-noise ratio at the output of the sensor, for low light levels, by decreasing the amount of the Gaussian noise due to the circuits of the conversion chain.