Time of flight sensing system and image sensor used therein
An image sensor employed in a time-of-flight (TOF) sensing system includes a pixel array including plural pixels, each pixel including at least one photo diode and each pixel generating an amount of charge corresponding to an incident light, comparing circuitry configured to compare voltage levels, each voltage level individually changed based on the amount of charge outputted from each pixel, with a reference voltage to output a comparison result, and calibration circuitry configured to adjust the voltage levels equally based on the comparison result.
1. An image sensor employed in a time-of-flight (TOF) sensing system, comprising:
a pixel array including plural pixels, each pixel including at least one photo diode and each pixel generating an amount of charge corresponding to an incident light;
comparing circuitry configured to compare voltage levels, each voltage level individually changed based on the amount of charge outputted from each pixel, with a reference voltage to output a comparison result;
calibration circuitry configured to adjust the voltage levels equally based on the comparison result; and
current supply circuitry configured to determine an amount of current supplied by the calibration circuitry in response to a current control signal, wherein the current supply circuitry and the calibration circuitry work as a current mirror.
2. The image sensor according to claim 1 , wherein the calibration circuitry is configured to maintain a difference between the voltage levels when adjusting the voltage levels by a predetermined amount.
3. The image sensor according to claim 1 , wherein the calibration circuitry is further configured to supply a preset amount of current into the pixel array.
4. The image sensor according to claim 1 , wherein the pixel comprises:
a reset gate coupled to the at least one photo diode and configured to reset the amount of charge in response to a reset signal;
a transfer gate configured to output a pixel voltage based on the amount of charge in response to a modulation signal;
an access gate configured to be turned on in response to the pixel voltage transferred from the transfer gate; and
a select gate configured to selectively output a voltage outputted from the access gate in response to a selection signal.
5. The image sensor according to claim 4 , wherein the calibration circuitry is further configured to:
check a potential, varied based on the amount of charge, between the transfer gate and the at least one photo diode; and
adjust the pixel voltage between the transfer gate and the access gate.
6. The image sensor according to claim 1 , wherein the calibration circuitry includes a current source configured to supply a current to the pixel array for adjusting the voltage levels.
7. The image sensor according to claim 6 , wherein the current resource comprises:
a switching transistor coupled to each pixel and turned on in response to the comparison result; and
a variable resistor coupled to a power supply and configured to determine an amount of the current.
8. The image sensor according to claim 1 ,
wherein the plural pixels are arranged along a plurality of rows and a plurality of columns, and
wherein the calibration circuitry controls pixels row by row.
9. The image sensor according to claim 1 ,
wherein the calibration circuitry is configured to increase the voltage levels by a predetermined amount, and
wherein the predetermined amount is proportional to the amount of current and inversely proportional to a driving frequency of each pixel.
10. A time-of-flight (TOF) sensing system, comprising:
an emitter configured to output a modulated signal having a preset phase;
a receiver including an image sensor configured to receive a reflected signal which is reflected from a target; and
signal processing circuitry configured to determine a distance from the target based on a phase relationship between the modulated light and the reflected light,
wherein the image sensor comprises:
a pixel array including plural pixels, each pixel including at least one photo diode and each pixel generating an amount of charge corresponding to the reflected light;
comparing circuitry configured to compare voltage levels, each voltage level individually changed based on the amount of charge outputted from each pixel, with a reference voltage to output a comparison result;
calibration circuitry configured to adjust the voltage levels equally based on the comparison result; and
current supply circuitry configured to determine an amount of current supplied by the calibration circuitry in response to a current control signal, wherein the current supply circuitry and the calibration circuitry work as a current mirror.
11. The TOF sensing system according to claim 10 , wherein the calibration circuitry is configured to maintain a difference between the voltage levels when adjusting the voltage levels by a predetermined amount.
12. The TOF sensing system according to claim 10 , wherein the calibration circuitry is further configured to supply a preset amount of current into the pixel array.
13. The TOF sensing system according to claim 10 , wherein the pixel comprises:
a reset gate coupled to the at least one photo diode and configured to reset the amount of charge in response to a reset signal;
a transfer gate configured to output a pixel voltage based on the amount of charge in response to a modulation signal;
an access gate configured to be turned on in response to the pixel voltage transferred from the transfer gate; and
a select gate configured to selectively output a voltage outputted from the access gate in response to a selection signal.
14. The TOF sensing system according to claim 13 , wherein the calibration circuitry is further configured to:
check a potential, varied based on the amount of charge, between the transfer gate and the at least one photo diode; and
adjust the pixel voltage between the transfer gate and the access gate.
15. The TOF sensing system according to claim 10 , wherein the calibration circuitry includes a current source configured to supply a current to the pixel array for adjusting the voltage levels.
16. The TOF sensing system according to claim 15 , wherein the current resource comprises:
a switching transistor coupled to each pixel and turned on in response to the comparison result; and
a variable resistor coupled to a power supply and configured to determine an amount of the current.
17. The TOF sensing system according to claim 10 ,
wherein the plural pixels are arranged along a plurality of rows and a plurality of columns, and
wherein the calibration circuitry controls pixels row by row.
18. The TOF sensing system according to claim 10 ,
wherein the calibration circuitry is configured to increase the voltage levels by a predetermined amount, and
wherein the predetermined amount is proportional to the amount of current and inversely proportional to a driving frequency of each pixel.