Circuit for an active pixel sensor
A pixel circuit includes a silicon substrate having a photodiode that converts light intensity into a voltage signal and two metal layers disposed on the substrate having a pixel control circuit. The first metal layer includes a row trace and a reset trace and the second metal layer includes a column trace and a voltage supply trace. The row trace carries a signal that activates a switch for coupling the photodiode to the column trace during a readout phase and clears the voltage at the photodiode during a reset phase. The column trace interfaces with a signal capture circuit in a CMOS array of pixels for capturing a digital image that corresponds to each voltage level at each photodiode.
1. A pixel-capture circuit, comprising:
a pixel-capture device having a pixel node and operable to convert light intensity into a pixel signal at the pixel node, the pixel signal representing a captured pixel;
a row node directly connected to a gate of a row selection transistor for carrying a row signal that is operable to couple the pixel node to a column trace during a read period of the captured pixel and operable to set the pixel node to a predetermined signal level during a reset period; and
a reset node carrying a reset signal that is operable to directly couple the row node to the pixel node during the reset period,
wherein the row signal changes between predetermined voltage levels during at least one portion of the reset period and sets the pixel node to the predetermined signal level such that the row node is coupled to the pixel node during the reset period.
2. The circuit of claim 1 , further comprising
a reset trace carrying the reset signal that is operable to uncouple the pixel node from a row trace during the reading of the captured pixel.
3. The circuit of claim 2 wherein the pixel-capture device is disposed on a silicon substrate.
4. The circuit of claim 3 wherein the row trace, the column trace, and the reset trace are disposed within no more than two conductive layers disposed on the silicon substrate.
5. The circuit of claim 1 wherein the pixel-capture device comprises a photodiode.
6. The circuit of claim 1 wherein the pixel signal comprises a voltage.
7. The pixel-capture circuit of claim 1 , further comprising:
a substrate;
two conductive layers disposed on the substrate; and
one or more conductive paths respectively operable to carry the row signal, each of the conductive paths disposed in a respective one of the two conductive layers.
8. The pixel-capture circuit of claim 7 wherein the pixel capture circuit comprises no conductive layers disposed on the substrate other than the two conductive layers.
9. The circuit of claim 1 , further comprising:
a reset transistor for controlling reset of the pixel signal at the pixel node of the pixel-capture device, the reset transistor being controlled by the reset signal from the reset node, the reset transistor connecting the row signal to the pixel node during the reset period and disconnecting the row signal from the pixel node during an image-capture period;
wherein the row node is coupled to the reset transistor to selectively couple the row node to the pixel node of the pixel-capture device for reset and the row node is further coupled to the row selection transistor to control the row selection transistor to selectively couple the pixel node of the pixel-capture device to the column trace for readout.
10. The circuit of claim 9 , wherein:
the row selection transistor couples the pixel node of the pixel-capture device to the column trace during the readout period and uncouples the pixel node from the column trace during the image-capture period; and
a further transistor connected between the pixel-capture device and the row selection transistor, the row selection transistor being disposed between the further transistor and the column trace.
11. A pixel-capture circuit, comprising:
a pixel-capture device having a first node and a second node, the first node coupled to a first supply node;
a first transistor having a control node, a first drive node, and a second drive node, the control node directly connected to the second node of the pixel-capture device and the first drive node coupled to a second supply node;
a second transistor having a control node, a first drive node, and a second drive node, the control node of the second transistor connected to a row node, the first drive node of the second transistor connected to the second drive node of the first transistor, the second drive node of the second transistor directly connected to a column node; and
a third transistor having a control node, a first drive node, and a second drive node, the control node of the third transistor coupled to a reset node, the first drive node of the third transistor directly connected to the row node, the second drive node of the third transistor directly connected to the second node of the of the pixel-capture device,
wherein the row node carries a row signal, the row signal changes between predetermined levels during at least one portion of a reset period.
12. The circuit of claim 11 wherein the first, second, and third transistors comprise MOSFET transistors.
13. A CMOS array comprising:
a plurality of pixel-capture circuits arranged in rows and columns, each pixel-capture circuit comprising:
a pixel-capture device having a pixel node and operable to convert light intensity into a pixel signal at the node, the pixel signal representing a captured pixel;
a row node directly connected to a gate of a row selection transistor for carrying a row signal that is operable to couple the pixel node to a column trace during a read period of the captured pixel and operable to set the node to a predetermined signal level during a reset period; and
a reset node carrying a reset signal that is operable to directly couple the row node to the pixel node during the reset period,
wherein the row signal changes between predetermined levels during at least one portion of the reset period and sets the pixel node to the predetermined signal level during the reset period such that the row node is coupled to the pixel node during the reset period.
14. The CMOS array of claim 13 , further comprising a reset trace carrying the reset signal that is operable to uncouple the pixel node from the row trace during the reading of the captured pixel.
15. The CMOS array of claim 14 , further comprising a first conductive layer having the row trace and the reset trace disposed therein and a second conductive layer having the column trace disposed therein.
16. A system comprising:
a CMOS array having:
a plurality of pixel-capture circuits arranged in rows and columns, each pixel-capture circuit comprising:
a pixel-capture device having a pixel node and operable to convert light intensity into a pixel signal at the pixel node, the pixel signal representing a captured pixel; and
a row node directly connected to a gate of a row selection transistor for carrying a row signal that is operable to couple the pixel node to a column trace during a read period of the captured pixel and operable to set the pixel node to a predetermined signal level during a reset period; and
a reset node carrying a reset signal that is operable to directly couple the row node to the pixel node during the reset period,
a processor coupled with the CMOS array and operable to facilitate detection of a voltage signal at each column trace in each pixel in the CMOS array,
wherein the row signal changes between predetermined levels during at least one portion of the reset period and sets the pixel node to the predetermined signal level such that the row node is coupled to the pixel node during the reset period.
17. The system of claim 16 , further comprising a memory coupled to the processor and operable to store the pixel signal.
18. A method comprising the steps of:
integrating an amount of light;
generating a light level signal on a pixel node, the pixel node signal having a level related to the integrated amount of light;
generating a first control signal on a first control node which is directly connected to a gate of a row selection transistor;
generating a second control signal on a second control node to control resetting of the light level signal;
reading the light level signal in response to the control signal on the first control node; and
resetting a level of the light level signal at the pixel node in response to the second control signal, wherein the resetting of the level of the light level signal occurs during a reset period and includes connecting the first control node directly to the pixel node via a switching device in response to the second control signal and driving the resetting of the level of the light level signal using the first control signal, the first control signal changing between predetermined levels during at least one portion of the reset period.
19. The method of claim 18 , wherein the reading of the light level signal comprises detecting a level at the second control node.
20. The method of claim 18 wherein the driving of the resetting of the level of the light level signal comprises:
setting a level at the another control node to a predetermined high level; and
pulsing a level at the first control node to a predetermined low level from the predetermined high level, the predetermined high level being higher than the predetermined low level.