Method of reading an image sensor signal and image sensor
The invention relates to matrix-array image sensors with MOS-technology active pixels, comprising a matrix-array of pixels arranged in rows and columns. To read the signal from a pixel, the reset potential present on a column conductor is sampled in two capacitors. The first capacitor is linked to an input of a comparator; the other input receives, through the second capacitor, a linear voltage ramp varying in a first direction for a first duration, then a linear voltage ramp in the reverse direction; a digital value N of the time between the start of this second ramp and the switching over of the comparator is counted; the useful potential present on the column conductor and representing the lighting of the pixel is sampled again, but in the first capacitor only; two ramps identical to the preceding ones are applied to the second input, through the second capacitor; a digital value N′ of the time between the start of the second reverse ramp and the switching over of the comparator is counted, the difference between the two counts N and N′ representing a measurement of the lighting of the pixel.
1. A method of measuring and converting to a digital value the lighting of the photosensitive pixels of an matrix-array of pixels arranged in rows and columns, all the pixels of one and the same column being able to be selectively linked to one respective column conductor to apply to this conductor in succession two potential values, of which one is a reset potential value and the other is a useful potential value linked to the lighting of a pixel, this method using for each column a voltage ramp generator and a comparator, said method comprising the following steps:
a)—the reset potential present on the column conductor is sampled in a first and a second sampling capacitors, and this potential is applied during the following steps b and c to a first input of the comparator by means of the first sampling capacitor;
b)—a linear voltage ramp starting from a reference value and varying in a first direction during a first duration is applied to a second input of the comparator, through the second capacitor;
c)—still through the second capacitor, a linear voltage ramp in the reverse direction is applied to this second input of the comparator; a digital value of the time that elapses between the start of this second ramp and the switching over of the comparator is counted at a fixed frequency in a counter; and the voltage of the ramp is returned to its reference value;
d)—the useful potential value present on the column conductor and representing the lighting of the pixel is sampled again, but in the first capacitor only, and this potential is applied, during the following steps e and f, to the first input of the comparator by means of the first capacitor;
e)—a linear voltage ramp varying in the first direction, starting from the same reference value, and identical in slope and in duration to that of the step b, is applied to the second input of the comparator, through the second capacitor;
f)—still through the second capacitor, a second linear voltage ramp in the reverse direction, identical in slope to that of the step c, is applied to the second input of the comparator; a second digital value of the time that elapses between the start of this second ramp and the switching over of the comparator is counted at the same frequency in the counter, the difference between the second and the first digital values representing a measurement of the lighting of the pixel.
2. An image sensor comprising an matrix-array of photosensitive pixels arranged in rows and columns, at least one counter, and, for each column of the matrix-array, a column conductor and a reading circuit to supply a digital value representing the lighting of the pixels of the column, the reading circuit comprising:
a first and a second sampling capacitors and switches for sampling and storing on these capacitors voltages representing the potential of the column conductor;
a comparator having a first input connected to a first terminal of the first capacitor and a second input connected to a first terminal of the second capacitor;
a linear voltage ramp generator having an output applied to a second terminal of the second capacitor and that is able to establish on this output either the reference potential, or a linear voltage ramp in one direction, or a linear voltage ramp in the other direction,
a memory for storing and retaining the content of the counter at one or more predetermined instants,
means of controlling the switches to sample simultaneously in the two capacitors a reset potential of the column conductor, and to subsequently sample, only in the first capacitor, a useful potential of the column conductor;
means of controlling the ramp generator for imposing on the output of the latter the reference potential at least during the phases of sampling the potential of the column conductor in the second capacitor, then for initiating the ramp in the first direction from the reference potential for a fixed short duration, after sampling, and finally for then initiating the ramp in the second direction;
means controlled by the comparator for storing the content of the counter at the instant when the ramp in the second direction starts and the content at the instant when the comparator switches over during this ramp, or else the difference between these contents.