# Patent

## Title
Method of reducing column fixed pattern noise

## Abstract

A method of reducing column fixed pattern noise including calibrating a readout circuit, wherein the readout circuit is electrically connected to at least one programmable gain amplifier and an analog-to-digital converter. Calibrating the readout circuit includes electrically disconnecting the readout circuit from a pixel output and electrically connecting a pixel reset input of the readout circuit to a pixel output signal input of the readout circuit. Calibrating the readout circuit further includes comparing a measured output of the readout circuit to a predetermined value and storing the comparison result in a non-transitory computer readable medium. The method further includes operating the readout circuit, the operating the readout circuit includes receiving a pixel sample signal and outputting a calibrated output based on an operating output and the stored comparison result.

## Background
BACKGROUND
Image capturing devices such as sensor pixel arrays convert incident light into electrical signals and use the electrical signals to reproduce an image. The quality of the reproduced image is determined in part by the precision of the pixels themselves and in part by the precision of readout circuitry coupled to the sensor pixel array. Column fixed pattern noise (CFPN) is a result of variations, i.e., mismatches, in the readout circuitry due to small variations during manufacturing, temperature of the circuitry or operating time. Some of the parameters which impact CFPN include offset voltage, feedback capacitance value, transistor threshold voltage, overlap capacitance value and bias current variations. As the CFPN increases, the quality of the reproduced image decreases.
CFPN problems are separate from any degradation which occurs based on imprecision within the pixels themselves. Conventional techniques attempt to correct image degradation by using the combined effect of CFPN and pixel imprecision. However, when the intensity of incident light is low, image degradation issues with respect to CFPN are often more significant than image degradation introduced by the pixels. Conventional techniques fail to account for the relative increase in CFPN induced degradation at low light levels.
Readout circuitry is often electrically connected to gain amplifiers in order to boost signals output by the sensor pixel array to provide higher contrast in the reproduced image. As the gain amplifiers increase the strength of the signals output by the sensor pixel array, the impact of CFPN is also increased. Conventional techniques fail to account for the different amounts of CFPN introduced at different gain conditions.



BRIEF DESCRIPTION OF THE DRAWINGS
One or more embodiments are illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout. It is emphasized that, in accordance with standard practice in the industry various features may not be drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features in the drawings may be arbitrarily increased or reduced for clarity of discussion.
FIG. 1A is a schematic diagram of a readout device in a calibration mode according to one or more embodiments;
FIG. 1B is a schematic diagram of a readout device in a normal operation mode according to one or more embodiments; and
FIG. 2 is a flowchart of a method of reducing column fixed pattern noise according to one or more embodiments.



DETAILED DESCRIPTION
The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are of course, merely examples and are not intended to be limiting.
FIG. 1A is a schematic diagram of a readout device 100 in a calibration mode including a readout circuit 102. Outputs of readout circuit 102 are electrically connected to a programmable gain amplifier (PGA) 104. First and second outputs of PGA 104 are electrically connected to an analog-to-digital converter (ADC) 106. ADC 106 is electrically connected to a memory device 108 and to a signal combiner 110. Memory device 108 is also electrically connected to signal combiner 110.
Readout circuit 102 includes a first switch 51 configured to receive a pixel reset signal. Readout circuit 102 further includes a second switch S2 configured to receive a pixel output signal. First switch S1 is electrically connected to a first capacitor C1 and a first side of a third switch S3. Second switch S2 is electrically connected to a second capacitor C2 and a second side of third switch S3. First capacitor C1 is also electrically connected to a fourth switch S4. Second capacitor C2 is also electrically connected to a fifth switch S5. A sixth switch S6 is electrically connected to first capacitor C1. Sixth switch S6 is configured to output a first readout output to a first variable capacitor 103a and then to a first input of PGA 104. A seventh switch S7 is electrically connected to second capacitor C2. Seventh switch S7 is configured to output a second readout output to a second variable capacitor 103b and then to a second input of PGA 104.
In some embodiments, switches S1-S7 comprise transistors such as metal oxide semiconductor (MOS) transistors, MOS field effect transistors (MOSFETs), bipolar junction transistors (BJTs), insulated gate bipolar transistors (IGBTs), or other suitable transistors. In some embodiments, switches S1-S7 comprise thyristors, such as gate turn-off thyristors (GTOs), silicon controlled rectifiers (SCRs), MOS controlled thyristors (MCTs), or other suitable thyristors.
Each of switches S1-S7 introduces mismatches between an expected power transfer and an actual power transfer. The mismatches in turn create column fixed pattern noise, which reduces the quality of the signal passing through readout circuit 102.
First and second variable capacitors 103a and 103b have a tunable capacitance. In some embodiments, first and second variable capacitors 103a and 103b are mechanically controlled. In some embodiments, first and second variable capacitors 103a and 103b are electronically controlled. In some embodiments, first and second variable capacitors 103a and 103b are controlled by a same controller. In some embodiments, first and second variable capacitors 103a and 103b are controlled by different controllers. In some embodiments, first and second variable capacitors 103a and 103b are controlled to have the same capacitance. In some embodiments, first and second variable capacitors 103a and 103b are controlled to have different capacitances.
PGA 104 is an electronic amplifier whose gain is able to be controlled by an external controller. In some embodiments, the ratio between an amplitude of an input signal and an amplitude of an output signal for PGA 104 ranges from 1:1 to 1:8. In some embodiments, a maximum ratio between the amplitude of the input signal and the amplitude of the output signal for PGA 104 is greater than 1:8.
ADC 106 receives an analog input signal and outputs a digital signal proportional to a magnitude of the analog input signal. In some embodiments, ADC 106 is a direct conversion ADC. In some embodiments, ADC 106 is a different type of ADC such as a success approximation ADC, a ramp-compare ADC, a Wilkinson ADC, an integrating ADC or other suitable ADC.
During calibration, memory device 108 stores calibration information related to mismatches associated with each switch S1-S7. In some embodiments, memory device 108 comprises a random access memory (RAM). In some embodiments, memory device 108 comprises a static RAM, a dynamic RAM or other suitable information storage devices.
FIG. 1B is a schematic diagram of readout circuit 100 in a normal operation mode. The components of readout circuit 100 are the same in calibration mode and normal operation mode. During normal operation, signal combiner 110 combines the calibration information stored in memory device 108 with an output of ADC 106 to provide an output signal having reduced noise. When the calibration information indicates readout circuit 102 produces a signal above the expected signal, signal combiner 110 subtracts the calibration information stored in memory device 108 from the pixel output signal. When the calibration information indicates readout circuit 102 produces a signal below the expected signal, signal combiner 110 adds the calibration information stored in memory device 108 to the pixel output signal. Signal combiner 110 comprises a processor capable of performing calculations.
Readout device 100 is part of a sensor pixel array. During normal operation of the sensor pixel array, the output of signal combiner 110 is received by a display viewable by a user. In some embodiments, each column of the sensor pixel array is independently connected to a separate readout circuit 102. In some embodiments, a number of readout circuits 102 electrically connected to the sensor pixel array is greater than 1,000. In some embodiments, PGA 104, ADC 106, memory device 108 and signal combiner 110 are shared by all readout circuits 102 in the sensor pixel array.
During calibration, readout circuit 102 is disconnected from the sensor pixel array. In some embodiments, readout circuit 102 is disconnected from the sensor pixel array by opening a switch. In some embodiments, one readout circuit 102 is disconnected from the sensor pixel array for calibration, while other readout circuits 102 continue normal operation. In some embodiments where one readout circuit 102 is disconnected from the sensor pixel array during calibration, the readout circuit 102 which is disconnected cycles through each of the readout circuits 102 in turn. In some embodiments, all readout circuits 102 are disconnected from the sensor pixel array during calibration.
Also during calibration, as depicted in FIG. 1A, switches S1, S2, S4 and S5 are closed and the inputs of switches S1 and S2 are connected. In some embodiments, the inputs of switch S1 and switch S2 are electrically connected by closing a switch.
In some embodiments, readout circuit 102 includes a temperature sensing element. In some embodiments, calibration is initiated based a temperature detected by the temperature sensing element. In some embodiments, the temperature sensing element is a thermocouple or other suitable temperature detecting device. In some embodiments, if the temperature voltage exceeds an activation voltage of switches S1-S7, a calibration signal is transmitted to readout circuit 102.
In some embodiments, readout circuit 102 receives a calibration signal to initiate calibration. In some embodiments, the calibration signal is sent by a manual input based on an action by the operator commanding the readout circuit 102 perform calibration. In some embodiments, the manual input is received by an operator pressing a button or other suitable actions. In some embodiments, the calibration signal is sent by a controller based on an operating time of the readout circuit 102. In some embodiments, the operating time between each calibration is selected by the operator. In some embodiments, the operating time between each calibration is predetermined.
FIG. 2 is a flowchart of a method 200 of reducing column fixed pattern noise (CFPN). In operation 202, readout device 100 is electrically disconnected from a pixel output. In some embodiments, readout device 100 is electrically disconnected from the pixel output using at least one switch disposed between the pixel and readout device 100. Disconnecting readout device 100 from the pixel output prevents erroneous data from being sent to memory device 108 during calibration of readout device 100.
The pixel reset signal input is electrically connected to the pixel output signal and an output of readout circuit 102 is measured in operation 204. In some embodiments, a switch between the pixel reset signal and the pixel output signal on an upstream side of switches S1 and S2 is closed to electrically connect the pixel reset signal input and the pixel output signal input. In some embodiments, the upstream side of switches S1 and S2 are tied to a preset voltage.
The output of readout circuit 102 is generated by a charging phase and a readout phase. During the charging phase, switches S1, S2, S4 and S5 are closed to charge first capacitor C1 and second capacitor C2. Switches S3, S6 and S7 are open. By having switches S3, S6 and S7 open, a voltage is held between switches S4 and S5 of readout circuit 102. The voltage held between switches S4 and S5 contacts a second side of first capacitor C1 and a second side of second capacitor C2.
During the readout phase, one switch of S3, S6 and S7 of readout circuit 102 is closed and the output of readout circuit 102 is measured. In some embodiments, the output of readout circuit 102 is measured at the output of PGA 104. In some embodiments, the output of readout circuit 102 is measured at a first output and a second output of PGA 104. In some embodiments, the output of readout circuit 102 is measured at the output of ADC 106.
After measuring the output of readout circuit 102, the charging phase and the readout phase are repeated for each of the open switches, e.g., switches S3, S6 and S7. In some embodiments, readout circuit 102 includes additional open switches and the charging and readout procedures are repeated for each open switch of readout circuit 102.
In optional operation 206, operations 202 and 204 are repeated for each readout circuit in a pixel sensor array. In some embodiments, the pixel sensor array includes more than one readout device. In some embodiments, the pixel sensor array includes one readout device electrically connected to each column of the pixel sensor array. By measuring an output for each readout device of the pixel sensor array individually, the calibration process accounts for mismatches specific to each readout device. Memory device 108 stores output corresponding to each readout device.
In operation 208, the output of readout circuit 102 is compared with the predetermined value based on the gain setting and the comparison result is stored. In some embodiments, the predetermined value is selected based on the number of bits available in readout circuit 102. In some embodiments, the predetermined value is equal to (2n/2)−1, where n is the number of bits of ADC 106. As the number of bits in ADC 106 increases, a resolution of a reproduced image increases.
The comparison result is stored in memory device 108. When the output of readout circuit 102 is greater than the predetermined value, the calibration information stored in memory device 108 is the predetermined value subtracted from the output of readout circuit 102. When the output of readout circuit 102 is less than the predetermined value, the calibration information stored in memory device 108 is the output of readout circuit 102 subtracted from the predetermined value. When the output of readout circuit 102 equals the predetermined value, the calibration information stored in memory device 108 is zero.
CFPN changes based on the gain setting value of PGA 104. In some embodiments, during calibration of readout device 100, PGA 104 cycles through every gain setting value of PGA 104. In some embodiments, during calibration of readout device 100, PGA 104 cycles through less than every gain setting value of PGA 104. Memory device 108 stores the comparison result based on the gain setting value of PGA 104, to obtain a comprehensive calibration of readout circuit 102 to reduce CFPN.
In operation 210, during normal operation an output of readout device 100 is calculated by applying a corresponding comparison result to the output of readout circuit 102. During normal operation, as depicted in FIG. 1B, switch S1 is closed to reset the output of readout circuit 102. Once the output is reset, switch S1 is opened and switch S2 is closed to receive the pixel sample signal. Switches S4 and S5 remain closed during normal operation. Switches S3, S6 and S7 are closed one by one and the output of readout circuit is transmitted through PGA 104 and ADC 106 to signal combiner 110. Signal combiner 110 receives signals from ADC 106 as well as corresponding store comparison results from memory device 108 and combines the signals to produce a calibrated output. The calibrated output has an increased precision as a result of the reduced CFPN achieved through calibration of readout circuit 102 for each column of the sensor pixel array.
By using method 200, image resolution at low intensity levels of incident light is increased versus conventional methods. Conventional methods fail to independently account for CFPN in calibrating the sensor pixel array. At low intensity levels of incident light CFPN is the most significant factor in image degradation. Method 200 independently calibrates readout circuits attached to the sensor pixel array and therefore is capable of reproducing a higher quality image at low intensity levels as compared to conventional calibration techniques.
Method 200 also provides calibration information associated with each gain of the programmable gain amplifier. Conventional methods fail to account for the impact of gain on CFPN. By providing calibration information for each switch in readout circuits attached to the sensor pixel array at different gains, method 200 is capable of maintaining a consistent image quality that does not fluctuate as the gain changes.
While the above description of method 200 applies the method to readout circuit 102, readout circuit 102 is used as an example to simplify description. One of ordinary skill will recognize method 200 is applicable to circuit designs other than readout circuit 102.
One aspect of this description relates to a method of reducing column fixed pattern noise including calibrating a readout circuit, where the readout circuit is electrically connected to at least one programmable gain amplifier and to an analog-to-digital converter. Calibrating the readout circuit includes electrically disconnecting the readout circuit from a pixel output and electrically connecting a pixel reset input of the readout circuit to a pixel output signal input of the readout circuit. Calibrating the readout circuit further includes comparing a measured output of the readout circuit to a predetermined value and storing the comparison result in a non-transitory computer readable medium. The method of reducing column fixed pattern noise further includes operating the readout circuit, the operating the readout circuit includes receiving a pixel sample signal and outputting a calibrated output based on an operating output and the stored comparison result.
Another aspect of this description relates to a method of reducing column fixed pattern noise in a sensor pixel array, where the sensor pixel array includes at least one column, at least one programmable gain amplifier and an analog-to-digital converter, and each of the at least one column includes a readout circuit and the method includes calibrating each readout circuit of each of the at least one column. Calibrating the readout circuit includes electrically disconnecting the readout circuit from a pixel output and electrically connecting a pixel reset input of the readout circuit to a pixel output signal of the readout circuit. Calibrating the readout circuit further includes comparing a measured output of the readout circuit to a predetermined value and storing the comparison result in a non-transitory computer readable medium. The method of reducing column fixed pattern noise further includes operating the readout circuit, the operating the readout circuit includes receiving a pixel sample signal and outputting a calibrated output based on an operating output and the stored comparison result.
It will be readily seen by one of ordinary skill in the art that the disclosed embodiments fulfill one or more of the advantages set forth above. After reading the foregoing specification, one of ordinary skill will be able to affect various changes, substitutions of equivalents and various other embodiments as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents thereof.

## Claims

1. A method of reducing column fixed pattern noise comprising:
calibrating a readout circuit, wherein the readout circuit is electrically connected to at least one programmable gain amplifier and an analog-to-digital converter, and calibrating the readout circuit comprises:
electrically disconnecting the readout circuit from a pixel output;
electrically connecting a pixel reset signal received by the readout circuit to a pixel output signal received by the readout circuit; and
comparing a measured output of the readout circuit to a predetermined value, the predetermined value being equal to (2n/2)−1, where n is the number of bits of the analog-to-digital converter, and storing a comparison result in a non-transitory computer readable medium.

2. The method of claim 1, wherein comparing an output of the readout circuit comprises:
measuring a first signal at a first output of the at least one programmable gain amplifier; and
measuring a second signal at a second output of the at least one programmable gain amplifier.

3. The method of claim 1, wherein calibrating the readout circuit further comprises:
changing a gain level of the at least one programmable gain amplifier to a different gain level; and
storing the comparison result based on the different gain level.

4. The method of claim 1, wherein electrically connecting a pixel reset signal to a pixel output signal comprises:
connecting the pixel reset signal to the pixel output signal upstream from the readout circuit.

5. The method of claim 1, wherein comparing an output of the readout circuit comprises:
measuring a digital output of the analog-to-digital converter.

6. The method of claim 1, further comprising operating the readout circuit, wherein operating the readout circuit comprises:
combining an operation output of the readout circuit and the comparison result; and
outputting a calibrated output based on the combination.

7. The method of claim 1, further comprising calibrating the readout circuit in response to a calibration signal received as a manual input.

8. The method of claim 1, further comprising calibrating the readout circuit in response to a calibration signal received as a temperature signal from a temperature measuring device.

9. The method of claim 1, further comprising calibrating the readout circuit in response to a calibration signal received after a predetermined time period.

10. A method of reducing column fixed pattern noise in a sensor pixel array, wherein the sensor pixel array comprises at least one column, at least one programmable gain amplifier and an analog-to-digital converter, and each of the at least one column comprises a readout circuit, the method comprising:
calibrating, wherein calibrating the readout circuit comprises:
electrically disconnecting the readout circuit from a pixel output;
electrically connecting a pixel reset signal received by the readout circuit to a pixel output signal received by the readout circuit; and
comparing a measured output of the readout circuit to a predetermined value and storing a comparison result in a non-transitory computer readable medium, the predetermined value being equal to (2n/2)−1, where n is the number of bits of the analog-to-digital converter; and

operating the sensor pixel array, the operating the sensor pixel array comprises:
outputting a calibrated output based on a pixel sample signal at the at least one column and a corresponding stored comparison result.

11. The method of claim 10, wherein comparing an output of the readout circuit comprises:
measuring a first signal at a first output of the at least one programmable gain amplifier; and
measuring a second signal at a second output of the at least one programmable gain amplifier.

12. The method of claim 10, wherein calibrating the readout circuit further comprises:
changing a gain level of the at least one programmable gain amplifier to a different gain level; and
storing the comparison result based on the different gain level.

13. The method of claim 10, wherein electrically connecting a pixel reset signal to a pixel output signal comprises:
connecting the pixel reset signal to the pixel output signal upstream from the readout circuit.

14. The method of claim 10, wherein comparing an output of the readout circuit comprises:
measuring a digital output of the analog-to-digital converter.

15. The method of claim 10, further comprising repeating the calibrating the readout circuit for each column of the at least one column of the sensor pixel array.

16. The method of claim 10, further comprising operating the readout circuit, wherein operating the readout circuit comprises:
combining an operation output of the readout circuit and the comparison result; and
outputting a calibrated output based on the combination.

17. The method of claim 10, further comprising calibrating the readout circuit in response to a calibration signal received as a manual input.

18. The method of claim 10, further comprising calibrating the readout circuit in response to a calibration signal received as a temperature signal from a temperature measuring device.

19. The method of claim 10, further comprising calibrating the readout circuit in response to a calibration signal received after a predetermined time period.

20. The method of claim 10, further comprising:
calibrating one readout circuit of the at least one column while a remaining readout circuits of a remaining at least one column continue operating.

21. A method of reducing column fixed pattern noise, the method comprising:
electrically disconnecting a readout circuit from a pixel output;
electrically connecting a pixel reset signal received by the readout circuit to a pixel output signal received by the readout circuit;
outputting a readout signal from the readout circuit to at least one programmable gain amplifier connected to the readout circuit;
measuring a first signal at a first output of the at least one programmable gain amplifier;
measuring a second signal at a second output of the at least one programmable gain amplifier;
changing a gain level of the at least one programmable gain amplifier to a different gain level;
comparing the first signal to a predetermined value, the predetermined value being equal to (2n/2)−1, where n is the number of bits of an analog-to-digital converter, and the first signal at the first output of the at least one programmable amplifier is measured based on the different gain level;
comparing the second signal to the predetermined value, the second signal at the second output of the at least one programmable gain amplifier being based on the different gain level; and
storing a comparison result between at least one of the first signal and the predetermined value and the second signal and the predetermined valued when the comparison result is greater than or less than the predetermined value.