Pixel architecture with high dynamic range
A given pixel of a pixel array includes various operation modes with each of the operation modes having a different conversion gain for the charge received from the photodetector of the pixel. When the modes are used in conjunction with one another, the dynamic range of the pixel can be increased. A readout circuit coupled to a photodetector within a given pixel includes a transfer gate between the photodetector and a gain mode select block that includes capacitors of different sizes and one or more switches to control which capacitors are to receive the charge from the photodetector. Depending on the state(s) of the one or more switches, different operation modes with different conversion gains can be selected to increase the dynamic range of the pixel. The adaptability of the readout circuit can allow for a high dynamic range even in extreme temperature environments by lowering the dark current.
1. A charge coupled device (CCD), comprising:
a plurality of pixels, wherein at least one pixel of the plurality of pixels comprises
a photodetector;
a transfer gate coupled to an output of the photodetector; and
a gain mode select block coupled to an output of the transfer gate, wherein the gain mode select block comprises
a first switch coupled between a first node and a second node, the first node coupled to a first capacitor and the second node coupled to a second capacitor, and
a second switch coupled between the second node and a third node, the third node coupled to a third capacitor, wherein the third capacitor has a higher capacitance than the second capacitor, and the second capacitor has a higher capacitance than the first capacitor.
2. The CCD of claim 1 , wherein the transfer gate is coupled to the first node of the gain mode select block.
3. The CCD of claim 1 , wherein the first switch and the second switch are each field effect transistors (FETs).
4. The CCD of claim 3 , wherein the first switch has substantially the same width/length (W/L) ratio as the second switch.
5. The CCD of claim 1 , further comprising a resistive element coupled between the output of the photodetector and the third node.
6. The CCD of claim 5 , wherein the resistive element is a FET.
7. The CCD of claim 1 , further comprising a reset switch coupled to any node of the gain mode select block.
8. An image sensor, comprising:
a pixel array having at least one column of addressable pixels;
a column amplifier coupled to the at least one column of addressable pixels;
an analog-to-digital converter (ADC) coupled to the column amplifier; and
a processor coupled to the ADC;
wherein the at least one column of addressable pixels includes at least one pixel that comprises
a photodetector;
a transfer gate coupled to an output of the photodetector; and
a gain mode select block coupled to an output of the transfer gate, wherein the gain mode select block comprises
a first switch coupled between a first node and a second node, the first node coupled to a first capacitor and the second node coupled to a second capacitor, and
a second switch coupled between the second node and a third node, the third node coupled to a third capacitor, wherein the third capacitor has a higher capacitance than the second capacitor, and the second capacitor has a higher capacitance than the first capacitor.
9. The image sensor of claim 8 , wherein the transfer gate is coupled to the first node of the gain mode select block.
10. The image sensor of claim 8 , wherein the first switch and the second switch are each field effect transistors (FETs).
11. The image sensor of claim 8 , wherein the at least one pixel further comprises a resistive element coupled between the output of the photodetector and the third node.
12. The image sensor of claim 11 , wherein the resistive element is a FET.
13. The image sensor of claim 8 , wherein the at least one pixel further comprises a reset switch coupled to any node of the gain mode select block.
14. A pixel of a pixel array within a CCD device, the pixel comprising:
a photodetector;
a transfer gate having a first terminal coupled to an output of the photodetector;
a first capacitor coupled to a second terminal of the transfer gate;
a first switch having a first terminal coupled to the second terminal of the transfer gate;
a second capacitor coupled to a second terminal of the first switch;
a second switch having a first terminal coupled to the second terminal of the first switch; and
a third capacitor coupled to a second terminal of the second switch, wherein the third capacitor has a higher capacitance than the second capacitor, and the second capacitor has a higher capacitance than the first capacitor.
15. The pixel of claim 14 , further comprising a resistive element coupled between the output of the photodetector and the second terminal of the second switch.
16. The pixel of claim 15 , wherein the resistive element is a FET.
17. The pixel of claim 14 , wherein the second terminal of the transfer gate is further coupled to a gate of an amplifier FET.