1. Field of the Invention
The present invention relates to image sensing, and more particularly, to an image sensor using comparators instead of operational amplifiers at output stage to provide low readout noise for passive pixel architecture.
2. Description of the Prior Art
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Due to the demand of high resolution of image sensors, the pixels are designed to be smaller, therefore, the capacitor 114 has to be as small as capacitance Cp to yield enough output swing to overcome readout noise caused by the operational amplifier 112. However, this will raise production cost and the readout noise still persists.
Therefore, there is a need for a readout circuit for a passive pixel sensor providing low readout noise.
In accordance with exemplary embodiments of the present invention, a readout circuit using comparators at output stage to provide low readout noise for passive pixel architecture is proposed to solve the above-mentioned problem.
According to an aspect of the present invention, an exemplary image sensor is disclosed. The exemplary image sensor includes a passive pixel sensor array and a readout circuit. The passive pixel sensor array has a plurality of pixel columns each having at least one column line. The readout circuit includes a ramp signal generating circuit, a ramp signal line and a comparing circuit. The ramp signal generating circuit is arranged for generating a ramp signal during an operating cycle of the readout circuit. The ramp signal line is arranged for receiving the ramp signal, wherein the ramp signal line intersects the column line without electrical connection so as to form a parasitic capacitor between the ramp signal line and the column line. The comparing circuit corresponds to the column lines, wherein during the operating cycle of the readout circuit, a pixel sensor of the passive pixel sensor array outputs a charge signal to the column line, and the comparing circuit is arranged for generating an output signal of the pixel sensor according to the ramp signal and the charge signal.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is electrically connected to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.
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The readout circuit 210 includes, but not limited to, a ramp signal generating circuit 212, a ramp signal line 214, a plurality of comparing circuit 220_1-220_M and a plurality of counters 230_1-230_M. The ramp signal generating circuit 212 is arranged for generating a ramp signal S_RAMP during an operating cycle of the readout circuit 210. The ramp signal line 214 is arranged for receiving the ramp signal S_RAMP and intersects the column line without electrical connection so as to form a parasitic capacitor between the ramp signal line 214 and the column line L1-LM. The ramp signal generating circuit 212 transmits the ramp signal S_RAMP via the ramp signal line 214. Take column line L1 for example, the ramp signal line 214 overlays/crosses but is not directly electrically connected to the column line L1, thereby inducing an inductive capacitor Cr at the intersection between the ramp signal line 214 and the column line L1.
The comparing circuits 220_1-220_M are coupled to the plurality of column lines L1-LM, respectively. The comparing circuits 220_1-220_M receives the ramp signal S_RAMP via the parasitic capacitor Cr from ramp signal generating circuit 212 as well, and during an operating cycle of the readout circuit 210, a pixel sensor of the passive pixel sensor array 240 outputs the charge signal S_CHG to a column line of the plurality of column lines L1-LM, and each of the comparing circuits L1-LM is arranged for generating an output signal S_OUT of the pixel sensor according to the ramp signal S_RAMP and the charge signal of the corresponding column line. For example, the comparing circuits L1 may be responsible for generating an output signal S_OUT of a pixel sensor P11 to the column L1 according to the charge signal S_CHG of the column line L1 and the ramp signal S_RAMP during the operating cycle of the readout circuit 210. Specifically, the state of the output signal S_OUT is altered once the ramp signal S_RAMP exceeds the charge signal S_CHG of the column line L1. In addition, the plurality of counters 230_1-230_M are coupled to the comparing circuits 220_1-220_M, respectively, and each of the plurality of counters is arranged for counting a time period T that the output signal S_OUT takes to alter its state, and accordingly generating a readout value V_OUT.
In short, by way of example, but not limitation, the operations of the readout circuit 210 may be summarized as the following step. First, during the operating cycle of the readout circuit, the comparing circuits 220_1 and the counter 230_1 are reset. Second, the control signal S_TX1 is asserted to make the pixel sensor P11 transfers the charge signal S_CHG of the photodiode to the floating diffusion of the pixel sensor P11. Then after the pixel sensor P11 transfers the charge signal S_CHG, the ramp signal generating circuit 212 generates the ramp signal S_RAMP. Next, the comparing circuit 220_1 compares a voltage level of the ramp signal S_RAMP (manifested by the inductive capacitor Cr) and a voltage level of transferred charges (manifested by the capacitance Cp). When the ramp signal S_RAMP reaches the voltage level of transferred charges, the voltage level of the output signal S_OUT drops. The counter 230_1 outputs the time period T before the ramp signal S_RAMP reaches the voltage level of transferred charges as the readout value V_OUT of the pixel sensor P11.
In addition, since the capacitance Cp is contributed by several of the floating diffusions of the pixel sensors, it is only reasonable to generate the output signal S_OUT of each of the pixel sensors by using the same ramp signal S_RAMP. That is, the ramp signal S_RAMP with a same waveform may be assigned to several rows of the pixel sensor array 240. For example, if the capacitance Cp is contributed by the floating diffusions of the pixel sensors P11-P14, the same ramp signal S_RAMP used to generate the output signal S_OUT of the pixel sensor P11 will also be used to generate the output signal S_OUT of the pixel sensor P12, P13 and P14. The reason why the capacitance Cp needs to be contributed by several donors is that the capacitance Cp should be controlled at the same order of the capacitor Cr for this readout architecture to work optimally. If the capacitance Cp is far smaller than the capacitor Cr, output swing of the readout circuit 210 is very likely to be truncated too much. On the other hand, if the capacitance Cp is far larger than the capacitor Cr, the readout speed of the readout circuit 210 will be compromised. The latter issue may be addressed by partitioning the pixel sensor array 240.
By way of example, but not limitation, please refer to
The structure of the image sensor 300 is substantially the same with that of image sensor 200. The main difference is that each of the comparing circuits 320_1-320_M further includes a switch for selectively coupling the corresponding column line, selected from the column lines of a corresponding pixel column, to a comparator thereof. For example, the comparing circuit 320_1 includes a switch 324_1 to selectively couple the column lines L1 or L2 of the column C1 to the comparator 322_1. The switch 324_1 couples one of the column lines L1 and L2 of the column C1 to the comparator 322_1 before the ramp signal generating circuit 310 generates the ramp signal S_RAMP. As a person skilled in the art can readily understand the operation of the image sensor 300 after reading above paragraphs directed to the image sensor 200, further description is omitted here for brevity.
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In conclusion, the present invention uses comparators instead of operational amplifiers at output stage to provide low readout noise for passive pixel architecture.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.
Number | Date | Country | Kind |
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102139833 | Nov 2013 | TW | national |