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

摘要

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_CONV, of nominal duration d_n and a counting which starts with a ramp of nominal duration d_n and stops upon the toggling of the output S_CMP of the comparator; and a second mode which provides for multiple conversions by comparison with a ramp of reduced duration d_r, 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 S_CMP of the comparator after a predetermined duration after the instant t_i of 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.