CMOS image sensor and image signal detecting method
A CMOS image sensor includes a photodiode, a switch configured to transfer a signal sensed by the photodiode to a sensing node, and a comparator electrically and directly connected to the sensing node and configured to compare the sensed signal of the sensing node and a ramp signal. Reset offset of the comparator is maintained at a constant offset voltage level during an initialization mode.
1. A CMOS image sensor comprising:
a photodiode;
a switch configured to transfer a signal sensed by the photodiode to a sensing node; and
a comparator electrically and directly connected to the sensing node and configured to compare the sensed signal of the sensing node and a ramp signal,
wherein reset offset of the comparator is maintained at a constant offset voltage level during an initialization mode,
wherein the switch comprises an RX transistor configured to initialize the sensing node, a TX transistor configured to transfer the signal sensed by the photodiode to the sensing node, an SF1 transistor controlled by the sensed signal; and an SEL1 transistor configured to select the sensing node, and
wherein the comparator comprises an M p transistor configured to be diode-connected during the initialization mode, an M off transistor configured to make diode-connected M p transistor in response to an initialization signal, and an offset capacitor configured to store a voltage level made by the diode-connected M p transistor.
2. The CMOS image sensor of claim 1 , further comprising:
a counter configured to output digital data corresponding to the sensed signal according to a comparison result transferred from the comparator.
3. The CMOS image sensor of claim 2 , further comprising:
a timing controller configured to control the switch, the comparator, and the counter; and
a ramp signal generator configured to output the ramp signal in response to control of the timing controller.
4. The CMOS image sensor of claim 1 , wherein the comparator further comprises:
an SF2 transistor matching in impedance with the SF1 transistor and controlled by the ramp signal; and
an SEL transistor matching in impedance with the SEL1 transistor and controlled by a power supply voltage.
5. The CMOS image sensor of claim 1 , wherein the offset voltage level includes the voltage level stored in the offset capacitor.
6. The CMOS image sensor of claim 1 , wherein the sensing node is initialized by a power supply voltage during the initialization mode.
7. The CMOS image sensor of claim 1 , wherein the reset offset of the comparator is independent of reset offset of the sensing node.
8. The CMOS image sensor of claim 1 , wherein the ramp signal decreases over a defined time period.
9. A method of detecting an image signal using a CMOS image sensor, the method comprising:
initializing a sensing node and setting a reset offset corresponding to the initialized sensing node to a predetermined voltage level; and
converting the predetermined voltage level into a digital signal,
wherein an RX transistor initializes the sensing node, a TX transistor transfers the signal sensed by the photodiode to the sensing node, an SF1 transistor is controlled by the sensed signal, and an SEL1 transistor selects the sensing node, and
wherein an M p transistor of a comparator is diode-connected during the initialization mode, an M off transistor of the comparator makes diode-connected M p transistor in response to an initialization signal, and an offset capacitor stores a voltage level made by the diode-connected M p transistor.
10. The method of claim 9 , wherein initializing a sensing node comprises applying a power supply voltage to the sensing node.
11. The method of claim 9 , wherein the reset offset is independent of a voltage level of the initialized sensing node.
12. The method of claim 9 , wherein converting the predetermined voltage level into a digital signal comprises:
comparing the predetermined voltage level with a ramp signal; and
performing a counting operation while comparing the predetermined voltage level with the ramp signal.
13. The method of claim 12 , wherein the ramp signal decreases over a defined time period during which the counting operation is performed.
14. The method of claim 9 , further comprising:
transferring external image information to the sensing node after receiving the external image information; and
converting the external image information into a digital signal.
15. The method of claim 14 , wherein transferring external image information to the sensing node after receiving the external image information comprises:
receiving the external image information to generate a sensed signal; and
transferring the sensed signal to the sensing node.
16. The method of claim 14 , wherein converting the external image information into a digital signal comprises:
comparing the external image information with a ramp signal; and
performing the counting operation while comparing external image information with the ramp signal.
17. The method of claim 16 , wherein the ramp signal decreases over a defined time period during which the counting operation is performed.