# Patent

## Title
Image sensor and adjustment method thereof

## Abstract

An image sensor is provided. The image sensor includes a pixel array, a sense amplifier, an analog-to-digital converter, a processor and a voltage generator. The sense amplifier obtains a first signal according to a pixel signal from the pixel array and a reference voltage, wherein the reference voltage has a first voltage level. The analog-to-digital converter converts the first signal into a first digital signal. The processor provides a feedback signal according to the first digital signal. The voltage generator adjusts the reference voltage to a second voltage level corresponding to the feedback signal. The sense amplifier removes a direct current (DC) bias voltage from the pixel signal according to the reference signal having the second voltage level.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This Application claims priority of Taiwan Patent Application No. 102133803, filed on Sep. 18, 2013, the entirety of which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an image sensor, and more particularly to an image sensor capable of signal-range adjustment.
2. Description of the Related Art
A semiconductor image sensor (e.g. a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) sensor), commonly implemented in a camera or a camcorder, is used to convert visible light images into electronic signals, so as to perform subsequent storage, transmission or display.
In some applications, a pixel array of the image sensor has background light. Thus, in addition to the actual sensed signals, the output signals sensed by the image sensor further comprise a background signal caused by the background light. The background signal has a significant signal energy, thus a high-resolution (high bit number) analog-to-digital converter is needed to convert the images traditionally, so as to effectively identify the actual sensing signals.
BRIEF SUMMARY OF THE INVENTION
An image sensor and an adjustment method thereof are provided. An embodiment of an image sensor is provided. The image sensor comprises a pixel array, a sense amplifier, an analog-to-digital converter, a processor and a voltage generator. The sense amplifier obtains a first signal according to a pixel signal from the pixel array and a reference voltage, wherein the reference voltage has a first voltage level. The analog-to-digital converter converts the first signal into a first digital signal. The processor provides a feedback signal according to the first digital signal. The voltage generator adjusts the reference voltage to a second voltage level corresponding to the feedback signal. The sense amplifier removes a direct current (DC) bias voltage from the pixel signal according to the reference signal having the second voltage level.
Furthermore, an embodiment of an adjustment method for an image sensor is provided. A first signal is obtained according to a pixel signal from a pixel array of the image sensor and a reference voltage having a first voltage level. The first signal is converted into a first digital signal, by an analog-to-digital converter of the image sensor. A feedback signal is obtained according to the first digital signal. The reference voltage is adjusted to a second voltage level corresponding to the feedback signal, by a voltage generator of the image sensor. A direct current (DC) bias voltage is removed from the pixel signal according to the reference voltage having the second voltage level.
A detailed description is given in the following embodiments with reference to the accompanying drawings.



BRIEF DESCRIPTION OF DRAWINGS
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
FIG. 1 shows an image sensor according to an embodiment of the invention;
FIG. 2 illustrates a waveform of the pixel signal SPIX and the reference voltage Sref of FIG. 1;
FIG. 3 illustrates a waveform of the signals S2 and S3 of FIG. 1; and
FIG. 4 shows an adjustment method for an image sensor according to an embodiment of the invention.



DETAILED DESCRIPTION OF THE INVENTION
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
FIG. 1 shows an image sensor 100 according to an embodiment of the invention. The image sensor 100 comprises a pixel array 110, a sense amplifier 120, a gain amplifier 130, an analog-to-digital converter (ADC) 140, a processor 150 and a voltage generator 160. First, the sense amplifier 120 obtains a pixel signal SPIX from the pixel array 110. Next, according to the pixel signal SPIX and a reference voltage Sref from the voltage generator 160, the sense amplifier 120 provides a signal S1 to the gain amplifier 130, wherein the reference voltage Sref has a first voltage level V1, i.e. Sref=V1. In the embodiment, the sense amplifier 120 comprises an operational amplifier AMP and a capacitor C, wherein the capacitor C is coupled between an output terminal and a non-inverting input terminal (+) of the operational amplifier AMP. Furthermore, the non-inverting input terminal of the operational amplifier AMP is further coupled to the pixel array 110, to receive the pixel signal SPIX. An inverting input terminal of the operational amplifier AMP is coupled to the voltage generator 160 to receive the reference voltage Sref. Next, the gain amplifier 130 amplifies the signal S1 to obtain a signal S2 according to a gain signal Gain corresponding to a unity gain, i.e. S2=S1×1, wherein the gain signal Gain is provided by the processor 150. Next, the ADC 140 converts the signal S2 into a digital signal S3, and provides the digital signal S3 to the processor 150. Next, the processor 150 obtains a background signal according to the digital signal S3. In one embodiment, the background signal is produced by a background light of the pixel array 110. Next, according to the digital signal S3, the processor 150 provides a feedback signal FB corresponding to a signal level of the background signal to the voltage generator 160. Moreover, the processor 150 also provides the gain signal Gain to the gain amplifier 130 according to the digital signal S3. Next, the voltage generator 160 adjusts the reference voltage Sref according to the feedback signal FB, such that the reference voltage Sref has a second voltage level V2, i.e. Sref=V2. In one embodiment, the voltage generator 160 is a digital-to-analog converter (DAC). Next, the sense amplifier 120 removes a direct current (DC) component SDC of the pixel signal SPIX according to the reference voltage Sref having the second voltage level, to generate the signal S1. In the embodiment, the DC component SDC is an offset voltage of the pixel signal SPIX, wherein the offset voltage is produced by the background signal of the pixel array 110. Next, the gain amplifier 130 amplifies the signal S1 to obtain the signal S2 according to the gain signal Gain, wherein the gain signal Gain has a gain that is greater than the unity gain. Therefore, after the DC component SDC is removed, a signal range of the signal S2 is increased. Next, the ADC 140 converts the amplified signal S2 into the digital signal S3, and provides the digital signal S3 to the processor 150. Next, the processor 150 provides the pixel data DPIX according to the digital signal S3. In the embodiment, by removing the DC component SDC caused by the background signal from the pixel signal SPIX, the gain amplifier 130 can amplify the major sensed signal. Thus, the pixel data DPIX having a satisfactory signal level is obtained without using a high-resolution (high-bit number) ADC.
FIG. 2 and FIG. 3 show waveform diagrams of the image sensor 100 of FIG. 1, wherein FIG. 2 illustrates a waveform of the pixel signal SPIX and the reference voltage Sref, and FIG. 3 illustrates a waveform of the signals S2 and S3. Referring to FIG. 1 and FIG. 2 together, in period P1, the pixel signal SPIX provided by the pixel array 110 only comprises the DC component SDC caused by the background signal. Furthermore, in the period P1, the reference voltage Sref is 0V, i.e. the first voltage level V1 is 0. Thus, the sense amplifier 120 obtains the signal S1 having the DC component SDC according to the pixel signal SPIX and the reference voltage Sref, as shown in label 310 of FIG. 3. Therefore, the processor 150 can provide the feedback signal FB corresponding to the DC component SDC and the gain signal Gain to the voltage generator 160 and the gain amplifier 130, respectively. Next, the voltage generator 160 adjusts the reference voltage Sref according to the feedback signal FB, such that the reference voltage Sref is substantially equal to the DC component SDC. Thus, in period P2, the sense amplifier 120 removes the DC component SDC from the pixel signal SPIX to obtain the signal S1 according to the reference voltage Sref, as shown in label 320 of FIG. 3. Next, in period P3, the pixel signal SPIX provided by the pixel array 110 further comprises a major sensed component S. Because the reference voltage Sref is substantially equal or close to the DC component SDC, the sense amplifier 120 can remove the DC component SDC from the pixel signal SPIX to obtain the signal S1 according to the reference voltage Sref, wherein the signal S1 is substantially equal or close to the major sensed component SV, as shown in label 330 of FIG. 3. Next, the gain amplifier 130 amplifies the signal S1 to obtain the signal S2 according to the gain signal Gain, as shown in label 340 of FIG. 3. Thus, the ADC 140 can convert the amplified signal S2 into the signal S3. In FIG. 3, the signal S2 has a larger signal range, thus the image sensor can obtain the pixel data DPIX having satisfactory signal level without using a high-resolution (high-bit number) ADC.
FIG. 4 shows an adjustment method for an image sensor according to an embodiment of the invention. Referring to FIG. 1 and FIG. 4 together, first, in step S410, the sense amplifier 120 obtains the signal S1 according to the pixel signal SPIX and the reference voltage Sref having the first voltage level V1. Next, in step S420, the ADC 140 converts the signal S2 into the digital signal S3, wherein the signal S2 is similar to the signal S1. Next, in step S430, the processor 150 provides the feedback signal FB to the voltage generator 160 according to the digital signal S3. Next, in step S440, the voltage generator 160 adjusts the reference voltage Sref to the second voltage level V2 according to the feedback signal FB, such that the reference voltage Sref has the second voltage level V2. Next, in step S450, the sense amplifier 120 re-obtains the signal 51 according to the pixel signal SPIX and the reference voltage Sref having the second voltage level V2. Next, in step S460, the gain amplifier 130 amplifies the signal S1 to obtain the signal S2 according to the gain signal Gain. Next, in step S470, the ADC 140 converts the amplified signal S2 into the digital signal S3. Next, in step S480, the processor 150 provides the pixel data DPIX having a sufficient signal range to other devices according to the digital signal S3 for subsequent processes. Therefore, by removing the DC component SDC caused by the background signal from the pixel signal SPIX, the image sensor 100 can provide pixel data having a sufficient signal range. Moreover, the image sensor 100 can use the gain amplifier 130 to adjust the signal S2 suitably. Therefore, the pixel data Dm can have the satisfactory signal range without an additional amplifier.
While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

## Claims

1. An image sensor, comprising:
a pixel array;
a sense amplifier, obtaining a first signal according to a pixel signal from the pixel array and a reference voltage, wherein the reference voltage has a first voltage level;
an analog-to-digital converter, converting the first signal into a first digital signal;
a processor, providing a feedback signal according to the first digital signal; and
a voltage generator, adjusting the reference voltage to a second voltage level corresponding to the feedback signal,
wherein the sense amplifier removes a direct current (DC) bias voltage from the pixel signal according to the reference signal having the second voltage level,
wherein the sense amplifier comprises:
an operational amplifier, having a non-inverting input terminal for receiving the pixel signal, an inverting input terminal for receiving the reference voltage, and an output terminal coupled to the analog-to-digital converter; and
a capacitor coupled between the non-inverting input terminal and the output terminal of the operational amplifier.

2. The image sensor as claimed in claim 1, wherein the voltage generator is a digital-to-analog converter.

3. The image sensor as claimed in claim 1, wherein the sense amplifier obtains a second signal according to the pixel signal and the reference voltage having the second voltage level.

4. The image sensor as claimed in claim 3, wherein the DC bias voltage corresponds to a background signal of the pixel array, and the sense amplifier removes the DC bias voltage from the pixel signal to obtain the second signal.

5. The image sensor as claimed in claim 3, further comprising:
a gain amplifier coupled between the sense amplifier and the analog-to-digital converter, amplifying the second signal.

6. The image sensor as claimed in claim 5, wherein the analog-to-digital converter converts the amplified second signal into a second digital signal, and the processor provides a pixel data according to the second digital signal.

7. An adjustment method for an image sensor, comprising:
obtaining a first signal according to a pixel signal from a pixel array of the image sensor and a reference voltage having a first voltage level;
converting the first signal into a first digital signal, by an analog-to-digital converter of the image sensor;
obtaining a feedback signal according to the first digital signal;
adjusting the reference voltage to a second voltage level corresponding to the feedback signal, by a voltage generator of the image sensor; and
removing a direct current (DC) bias voltage from the pixel signal according to the reference voltage having the second voltage level,
wherein the first signal is provided by a sense amplifier of the image sensor, and the sense amplifier comprises:
an operational amplifier, having a non-inverting input terminal for receiving the pixel signal, an inverting input terminal for receiving the reference voltage, and an output terminal coupled to the analog-to-digital converter; and
a capacitor coupled between the non-inverting input terminal and the output terminal of the operational amplifier.

8. The adjustment method as claimed in claim 7, further comprising:
obtaining a second signal according to the pixel signal and the reference voltage having the second voltage level.

9. The adjustment method as claimed in claim 7, wherein the voltage generator is a digital-to-analog converter.

10. The adjustment method as claimed in claim 8, wherein the second signal is provided by the sense amplifier of the sensor.

11. The adjustment method as claimed in claim 8, wherein the step of obtaining the second signal further comprises:
removing the DC bias voltage from the pixel signal, to obtain the second signal,
wherein the DC bias voltage corresponds to a background signal of the pixel array.

12. The adjustment method as claimed in claim 8, further comprising: amplifying the second signal.

13. The adjustment method as claimed in claim 12, further comprising:
converting the second signal into a second digital signal; and obtaining a pixel data according to the second digital signal.