Pixel circuit with controlled capacitor discharge time of flight measurement

Patent metadata

Field	Value
Publication number	US-9171985-B2
Application number	US-201314066771-A
Country	US
Kind code	B2
Filing date	Oct 30, 2013
Priority date	Nov 2, 2012
Publication date	Oct 27, 2015
Grant date	Oct 27, 2015

Abstract

A pixel circuit includes a single photon avalanche diode (SPAD) and a measurement circuit including a capacitance. The SPAD detects an incident photon and the measurement circuit discharges the capacitance at a known rate during a discharge time period. The length of the discharge time period is determined by the time of detection of the photon, such that the final amount of charge on the capacitance corresponds to the time of flight of the photon. The pixel circuit may be included in a time resolved imaging apparatus. A method of measuring the time of flight of a photon includes responding to an incident photon detection by discharging a capacitance at a known rate and correlating final capacitance charge to time of flight.

First claim

What is claimed is:

1. A pixel circuit, comprising:
a single photon avalanche diode (SPAD) operable to detect a photon incident on said SPAD; and
a measurement circuit comprising a capacitance; wherein said measurement circuit is operable to discharge said capacitance at a known rate during a discharge time period, the length of said discharge time period being determined by the time of said detection of said photon incident on said SPAD, such that a final amount of charge remaining on said capacitance corresponds to a time of flight of said photon.

2. The pixel circuit as claimed in claim 1 , wherein the charge remaining on said capacitance is a direct or inverse analog representation of the time of flight of said photon.

3. The pixel circuit as claimed in claim 1 , further comprising a memory element operable to disable operation of the pixel after detection of said photon.

4. The pixel circuit as claimed in claim 3 , wherein said memory element comprises a storage element provided by an inherent capacitance of at least one transistor device.

5. The pixel circuit as claimed in claim 3 , wherein said memory element is operable to selectively connect the SPAD output to the measurement circuit depending on its stored content.

6. The pixel circuit as claimed in claim 1 , wherein detection of a photon by said SPAD defines one of the beginning or the end of said discharge time period.

7. The pixel circuit as claimed in claim 1 , the SPAD and measurement circuit operable in a reverse mode wherein detection of a photon by said SPAD during a predetermined time frame begins said discharge time period, said discharge time period ending at the end of said known time frame.

8. The pixel circuit as claimed in claim 7 , the SPAD and measurement circuit operable such that the charge on said capacitance is discharged via a switch controlled by the output of said SPAD.

9. The pixel circuit as claimed in claim 1 , the SPAD and measurement circuit operable in a forward mode where said discharge time period begins at a first known time and ends on detection of said photon by said SPAD.

10. The pixel circuit as claimed in claim 9 , further comprising a signal generator configured to generate a time varying ramp signal and a hold switch, wherein the measurement circuit is operable to apply the time varying ramp signal, via the hold switch, to said capacitance, said capacitance sampling said ramp signal, said hold switch disconnecting said ramp signal from said capacitance on detection of said first photon incident on said SPAD.

11. The pixel circuit as claimed in claim 1 , further comprising a time gating stage operable to begin sensing operation on reception of an enable signal.

12. The pixel circuit as claimed in claim 11 , wherein said time gating stage comprises a switch operable to selectively connect the SPAD output to the measurement circuit on reception of said enable signal.

13. The pixel circuit as claimed in claim 12 , wherein said time gating stage comprises two further switches operable to disable the pixel circuit on reception of a disable signal.

14. A time resolved imaging apparatus, comprising:
an array of pixel circuits, each pixel circuit comprising:
a single photon avalanche diode (SPAD) operable to detect a photon incident on said SPAD; and
a measurement circuit comprising a capacitance; wherein said measurement circuit is operable to discharge said capacitance at a known rate during a discharge time period, the length of said discharge time period being determined by the time of said detection of said photon incident on said SPAD, such that a final amount of charge remaining on said capacitance corresponds to a time of flight of said photon; and

a radiation source for providing photon radiation incident on said SPAD.

15. The time resolved imaging apparatus as claimed in claim 14 , said measurement circuit operable to begin sensing operation of said array of pixels simultaneously with activation of said radiation source.

16. The time resolved imaging apparatus as claimed in claim 14 , being operable to perform three-dimensional imaging.

17. A method of measuring a time of flight of a photon, comprising:
activating a radiation source to emit photons;
receiving one or more of said photons using a single photon avalanche diode (SPAD); and
discharging a capacitance at a known rate;
wherein said discharging is either begun or ended upon detection of a photon received by said SPAD, such that the amount of charge remaining on said capacitance at the end of said time period corresponds to said time of flight of radiation incident on said SPAD.

18. The method as claimed in claim 17 , wherein the charge remaining on said capacitance is an analog representation of the time of flight of radiation incident on said SPAD.

19. The method as claimed in claim 17 , further comprising preventing detection of further photons after detection of the first photon incident on said SPAD.

20. The method as claimed in claim 17 , further comprising:
defining a time frame of known length, the beginning of the time frame coinciding with the activation of the radiation source;
beginning said discharge of said capacitance upon detection of a photon during said time frame; and
preventing further discharge of said capacitance at the end of said known time frame.

21. The method as claimed in claim 20 , wherein the charge on said capacitance is discharged via a switch controlled by the output of said SPAD.

22. The method as claimed in claim 17 , further comprising:
beginning discharge of said capacitance at a first known time; and
preventing further discharge of said capacitance upon detection of a photon.

23. The method as claimed in claim 22 , further comprising:
sampling a time varying ramp signal on said capacitance, and
disconnecting said ramp signal from said capacitance on detection of said first photon incident on said SPAD.

Assignees

• St Microelectronics Res & Dev
• Univ Edinburgh

Inventors

• Dutton Neale
• Henderson Robert K

Classifications

• H04N25/771

  comprising storage means other than floating diffusion · CPC title

• H10F39/80

  Constructional details of image sensors · CPC title

• H10F30/225 Primary

  the potential barrier working in avalanche mode, e.g. avalanche photodiodes · CPC title

• G01S7/4865

  Time delay measurement, e.g. time-of-flight measurement, time of arrival measurement or determining the exact position of a peak (peak detection in noise, signal conditioning G01S7/487) · CPC title

• G01S17/10 Primary

  using transmission of interrupted, pulse-modulated waves (determination of distance by phase measurements G01S17/32) · CPC title