CMOS light sensing cell
A light sensing cell comprising output circuitry for generating an output voltage depending on the voltage of a sensing node, the voltage of the sensing node varying as a function of a received light; a reset transistor operable to force the voltage of the sensing node to a reset voltage; a feedback loop including an operational amplifier operable to add through a capacitive voltage divider a correction voltage to the voltage of the sensing node, said correction voltage depending on the output voltage; and preset mean circuitry for, during the operation of the reset transistor and until the amplifier is operated, setting the input of the capacitive voltage divider to a predetermined voltage.
1. A light sensing cell comprising:
output means for generating an output voltage depending on the voltage of a sensing node, the voltage of the sensing node varying as a function of a received light;
a reset transistor operable to force the voltage of the sensing node to a reset voltage;
a feedback loop comprising an operational amplifier operable to add, through a capacitive voltage divider, a correction voltage to the voltage of the sensing node, said correction voltage depending on the output voltage; and
preset means for, during the operation of the reset transistor and until the amplifier is operated, setting an input of the capacitive voltage divider to a predetermined voltage.
2. A light sensing cell according to claim 1 , comprising:
a sensor for converting the received light into a sensing current, the sensing node receiving said sensing current;
a first capacitor connected between the sensing node and an output node, said output node also receiving a supply current; and
a second capacitor connecting the sensing node to ground;
wherein the output means comprises an output transistor controlled by the voltage of the sensing node and coupled between the output node and the ground;
wherein the operational amplifier has a first input receiving the voltage of the output node and a second input receiving a reference voltage, the output of the amplifier being connected to the input of the capacitive voltage divider through a first switch;
and wherein the capacitive voltage divider comprises a third capacitor connected between the input of the capacitive voltage divider and the sensing node and a fourth capacitor connected between the input of the capacitive voltage divider and the ground.
3. A light sensing cell according to claim 2 , wherein the output transistor is coupled to the output node through a cell selection switch controlled by a cell selection signal.
4. A light sensing array comprising a plurality of light sensing cells according to claim 3 .
5. A light sensing array according to claim 4 , wherein the light sensing cells form a plurality of groups, the feedback means of the light sensing cells of a same group sharing a same operational amplifier.
6. A light sensing array according to claim 5 , wherein the output nodes of the light sensing cells of a same group are coupled to a same supply current source.
7. A light sensing array according to claim 5 , wherein the light sensing cells are arranged in rows and columns, each column forming one of said groups.
8. A light sensing array according to claim 6 , wherein the light sensing cells are arranged in rows and columns, each column forming one of said groups.
9. The light sensing array of claim 4 , wherein the light sensing array is incorporated into an integrated circuit.
10. The light sensing cell according to claim 1 , wherein the preset means for setting the input of the capacitive voltage divider to a predetermined voltage comprises a preset transistor having a source connected to a fixed voltage, a drain connected to the input of the capacitive voltage divider, and a gate.
11. The light sensing cell according to claim 10 , wherein the drain of the preset transistor is connected to a drain of the first switch.