Time-of-flight (TOF) system, sensor pixel, and method
View Patent ↗A time-of-flight (TOF) sensor pixel is provided that performs in-pixel subtraction. The TOF sensor pixel includes a photodetector, a capacitor, and circuitry. The photodetector detects light pulses emitted at a clock frequency, after a time of flight, to provide a photocurrent. The capacitor integrates the photocurrent over an integration period, while the circuitry reverses a flow direction of the photocurrent through the capacitor at twice the clock frequency. At the end of the integration period, the capacitor provides a differential photocharge, corresponding to a capacitor voltage. The capacitor voltage is related to the time of flight of the light pulses and may be used to determine a distance to a target.
1. A time-of-flight (TOF) system, comprising:
a light source for emitting light pulses at a clock frequency;
a TOF sensor including one or more TOF sensor pixels, each TOF sensor pixel including:
a photodetector for detecting the light pulses, after a time of flight, to provide a photocurrent;
a capacitor for integrating the photocurrent, over a first integration period, to provide a first-period photocharge, corresponding to a first-period capacitor voltage, at an end of the first integration period; and
circuitry for reversing a flow direction of the photocurrent through the capacitor at twice the clock frequency, during the first integration period, so that the first-period photocharge is a differential photocharge, and the first-period capacitor voltage is related to the time of flight of the light pulses.
2. The TOF system of claim 1 , wherein each TOF sensor pixel includes only one photodetector for detecting the light pulses, and only one capacitor for integrating the photocurrent.
3. The TOF system of claim 1 , wherein the photodetector is for detecting background light, as well as the light pulses; wherein the photocurrent has a contribution from the background light; and wherein the first-period photocharge has substantially no contribution from the background light.
4. The TOF system of claim 1 , wherein the light source is for emitting the light pulses towards a target; wherein the photodetector is for detecting the light pulses reflected by the target; further comprising a processing system, connected to the light source and the TOF sensor, for determining a distance to the target by using the first-period capacitor voltage.
5. The TOF system of claim 1 , wherein the light source is for emitting the light pulses in response to a clock signal; and wherein the circuitry is for reversing the flow direction of the photocurrent through the capacitor in response to the clock signal and an inverted clock signal, during the first integration period.
6. The TOF system of claim 5 , further comprising a processing system, connected to the light source and the TOF sensor, including a clock for providing the clock signal.
7. The TOF system of claim 5 , wherein the circuitry includes:
a first set of transistors, each having a control input for receiving the clock signal during the first integration period; and
a second set of transistors, each having a control input for receiving the inverted clock signal during the first integration period.
8. The TOF system of claim 5 , wherein the capacitor is also for integrating the photocurrent, over a second integration period, to provide a second-period photocharge, corresponding to a second-period capacitor voltage, at an end of the second integration period; and wherein the circuitry is also for reversing the flow direction of the photocurrent through the capacitor at twice the clock frequency in response to a 90°-phase-delayed clock signal and a 270°-phase-delayed clock signal, during the second integration period, so that the second-period photocharge is a differential photocharge, and the second-period capacitor voltage is related to the time of flight of the light pulses.
9. The TOF system of claim 8 , further comprising a processing system, connected to the light source and the TOF sensor, for calculating a ratio of the first-period capacitor voltage to the second-period capacitor voltage.
10. The TOF system of claim 9 , wherein the light source is for emitting the light pulses towards a target; wherein the photodetector is for detecting the light pulses reflected by the target; wherein the processing system is also for determining a distance to the target by comparing the ratio of the first-period capacitor voltage to the second-period capacitor voltage to a look-up table of ratio values and corresponding distance values.
11. A time-of-flight (TOF) sensor pixel, comprising:
a photodetector for detecting light pulses emitted at a clock frequency, after a time of flight, to provide a photocurrent;
a capacitor for integrating the photocurrent, over a first integration period, to provide a first-period photocharge, corresponding to a first-period capacitor voltage, at an end of the first integration period; and
circuitry for reversing a flow direction of the photocurrent through the capacitor at twice the clock frequency, during the first integration period, so that the first-period photocharge is a differential photocharge, and the first-period capacitor voltage is related to the time of flight of the light pulses.
12. The TOF sensor pixel of claim 11 , wherein the TOF sensor pixel includes only one photodetector for detecting the light pulses, and only one capacitor for integrating the photocurrent.
13. The TOF sensor pixel of claim 11 , wherein the photodetector is for detecting background light, as well as the light pulses; wherein the photocurrent has a contribution from the background light; and wherein the first-period photocharge has substantially no contribution from the background light.
14. The TOF sensor pixel of claim 11 , wherein the circuitry is for reversing the flow direction of the photocurrent through the capacitor in response to a clock signal and an inverted clock signal, during the first integration period.
15. The TOF sensor pixel of claim 14 , wherein the circuitry includes:
a first set of transistors, each having a control input for receiving the clock signal during the first integration period; and
a second set of transistors, each having a control input for receiving the inverted clock signal during the first integration period.
16. The TOF sensor pixel of claim 14 , wherein the capacitor is also for integrating the photocurrent, over a second integration period, to provide a second-period photocharge, corresponding to a second-period capacitor voltage, at an end of the second integration period;
and wherein the circuitry is also for reversing the flow direction of the photocurrent through the capacitor at twice the clock frequency in response to a 90°-phase-delayed clock signal and a 270°-phase-delayed clock signal, during the second integration period, so that the second-period photocharge is a differential photocharge, and the second-period capacitor voltage related to the time of flight of the light pulses.
17. A method of determining a distance to a target, comprising:
emitting light pulses towards a target at a clock frequency;
detecting the light pulses reflected from the target, after a time of flight, to provide a photocurrent;
integrating the photocurrent with a capacitor, over a first integration period, to provide a first-period photocharge, corresponding to a first-period capacitor voltage, at an end of the first integration period;
reversing a flow direction of the photocurrent through the capacitor at twice the clock frequency, during the first integration period, so that the first-period photocharge is a differential photocharge, and the first-period capacitor voltage is related to the time of flight of the light pulses; and
determining a distance to the target by using the first-period capacitor voltage.
18. The method of claim 17 , wherein the light pulses are emitted in response to a clock signal; and wherein the flow direction of the photocurrent through the capacitor is reversed in response to the clock signal and an inverted clock signal, during the first integration period;
further comprising:
integrating the photocurrent, over a second integration period, with the capacitor to provide a second-period photocharge, corresponding to a second-period capacitor voltage, at an end of the second integration period; and
reversing the flow direction of the photocurrent through the capacitor at twice the clock frequency in response to a 90°-phase-delayed clock signal and a 270°-phase-delayed clock signal, during the second integration period, so that the second-period photocharge is a differential photocharge, and the second-period capacitor voltage is related to the time of flight of the light pulses.
19. The method of claim 18 , wherein determining the distance to the target includes calculating a ratio of the first-period capacitor voltage to the second-period capacitor voltage.
20. The method of claim 19 , wherein determining the distance to the target further includes comparing the ratio of the first-period capacitor voltage to the second-period capacitor voltage to a look-up table of ratio values and corresponding distance values.