Image sensor and image capture apparatus
In a pixel section of an image sensor, pixels are arranged two-dimensionally, each of the pixels including an amplifier transistor that is connected to a power voltage and has a gate into which a voltage of a signal charge generated by a photoelectric conversion area is input and a selection transistor that is connected to the amplifier transistor and a signal line. The image sensor includes an AD conversion circuit having a reference transistor having a gate into which a ramp signal is input and a constant current source that is connected to the reference transistor and the signal line. The voltage of the ramp signal or a driving ability of the reference transistor changes depending on whether one selection transistor or a plurality of selection transistors is/are turned on per signal line to obtain a signal charge to be AD-converted.
1. An image sensor comprising:
a pixel section in which pixels are arranged two-dimensionally, each of the pixels including (i) an amplifier transistor that is connected to a power voltage and has a gate into which a voltage of a signal charge generated by a photoelectric conversion area is input and (ii) a selection transistor that is connected to the amplifier transistor and a signal line; and
an AD conversion circuit including (i) a reference transistor that has a gate into which a ramp signal having a voltage that changes at a constant rate is input and (ii) a constant current source that is connected to the reference transistor and the signal line,
wherein the voltage of the ramp signal or a driving ability of the reference transistor changes between a first mode and a second mode, the first mode being a mode in which a voltage of a signal charge obtained by turning one selection transistor on per signal line is AD-converted by the AD conversion circuit, and the second mode being a mode in which a voltage of a signal charge obtained by turning a plurality of selection transistors on per signal line is AD-converted by the AD conversion circuit.
2. The image sensor according to claim 1 ,
wherein an initial voltage of the ramp signal is set higher in the second mode than in the first mode.
3. The image sensor according to claim 2 ,
wherein the initial voltage is increased more as the number of selection transistors turned on per signal line increases.
4. The image sensor according to claim 2 ,
wherein the amount of increase in the initial voltage is based on the magnitude of the current supplied by the constant current source and a gain coefficient of the selection transistor or the reference transistor.
5. The image sensor according to claim 2 ,
wherein an increasing level of the initial voltage is controlled by reducing the current supplied by the constant current source.
6. The image sensor according to claim 1 ,
wherein the driving ability of the reference transistor is set to be higher in the second mode than in the first mode.
7. The image sensor according to claim 1 ,
wherein the AD conversion circuit includes a plurality of reference transistors, and in the second mode, the driving abilities of the reference transistors are increased by activating more reference transistors than in the first mode.
8. The image sensor according to claim 1 ,
wherein the AD conversion circuit includes a plurality of reference transistors having different driving abilities, and in the second mode, the driving abilities of the reference transistors are increased by activating reference transistors having higher driving abilities than in the first mode.
9. The image sensor according to claim 1 ,
wherein a ramp signal having a lower rate of change than in the first mode is used in the second mode.
10. The image sensor according to claim 1 ,
wherein one of the AD conversion circuits is provided for each vertical signal line.
11. The image sensor according to claim 1 ,
wherein one of the AD conversion circuits is provided for each pixel in a plurality of rows×a plurality of columns.
12. The image sensor according to claim 1 ,
wherein the AD conversion circuit is formed on a different board from the pixel section.
13. An image capture apparatus comprising:
an image sensor that comprises:
a pixel section in which pixels are arranged two-dimensionally, each of the pixels including (i) an amplifier transistor that is connected to a power voltage and has a gate into which a voltage of a signal charge generated by a photoelectric conversion area is input and (ii) a selection transistor that is connected to the amplifier transistor and a signal line; and
an AD conversion circuit including (i) a reference transistor that has a gate into which a ramp signal having a voltage that changes at a constant rate is input and (ii) a constant current source that is connected to the reference transistor and the signal line,
wherein the voltage of the ramp signal or a driving ability of the reference transistor changes between a first mode and a second mode, the first mode being a mode in which a voltage of a signal charge obtained by turning one selection transistor on per signal line is AD-converted by the AD conversion circuit, and the second mode being a mode in which a voltage of a signal charge obtained by turning a plurality of selection transistors on per signal line is AD-converted by the AD conversion circuit,
a ramp signal generation circuit;
a timing generation circuit that generates a control signal for the image sensor; and
a control unit that controls operations of the ramp signal generation circuit and the timing generation circuit.