Dark current calibration for a photosensitive device

Patent metadata

Field	Value
Publication number	US-9310247-B2
Application number	US-201414287445-A
Country	US
Kind code	B2
Filing date	May 27, 2014
Priority date	May 29, 2013
Publication date	Apr 12, 2016
Grant date	Apr 12, 2016

Abstract

An imaging circuit includes at least one photosensitive device that provides an output in response to at least one photon and a compensation circuit configured to provide dark current compensation for the output of said photosensitive device. The applied compensation uses temperature information and temperature dependent calibration information.

First claim

What is claimed is:

1. An apparatus, comprising:
at least one photosensitive device configured to provide an output in response to at least one photon;
at least one shielded compensation photosensitive device configured in use to provide a dark current value; and
a compensation circuit configured to modify said dark current value of said at least one shielded photosensitive device to provide a temperature compensated dark current value; and
wherein said compensation circuit is further configured to provide dark current compensation for the output of said photosensitive device by processing temperature information and temperature dependent calibration information to compensate for dark current;
wherein said temperature dependent calibration information is dependent on a calibration measurement of dark currents output from at least one of the photosensitive device and the shielded compensation photosensitive device at a first and a second, different, calibration temperatures.

2. The apparatus as claimed in claim 1 , wherein said compensation circuit is further configured to use said temperature compensated dark current value as a measure of dark current in said output of said at least one photosensitive device.

3. The apparatus as claimed in claim 1 , wherein said compensation circuit is further configured to subtract said temperature compensated dark current value from said output of said at least one photosensitive device.

4. The apparatus as claimed in claim 1 , wherein said compensation circuit is configured to apply a function to a dark current value of said shielded photosensitive device, said function being dependent on said temperature information and said dark current of said at least one shielded photosensitive device and dark current of said at least one photosensitive device at said first and second, different, calibration temperatures.

5. The apparatus as claimed in claim 4 , wherein said function has a first factor dependent on a first coefficient multiplied by an exponential function of the temperature information divided by a second coefficient.

6. The apparatus as claimed in claim 5 , wherein at least one of said first and second coefficients is dependent on a dark current at the first and second calibration temperatures for said shielded photosensitive device and a dark current at the first and second calibration temperatures for said photosensitive device.

7. The apparatus as claimed in claim 6 , wherein said compensation circuit is configured to store said first and second coefficients.

8. The apparatus as claimed in claim 4 , wherein said compensation circuit comprises a data store configured to store said function.

9. The apparatus as claimed in claim 4 , wherein said compensation circuit comprises a processor configured to execute said function.

10. The apparatus as claimed in claim 1 , wherein at least one calibration temperature is in a range of 10 degrees to 30 degrees.

11. The apparatus as claimed in claim 1 , wherein at least one calibration temperature is in a range of 70 degrees to 90 degrees.

12. The apparatus as claimed in claim 1 , wherein said compensation circuit comprises a temperature sensing device.

13. The apparatus as claimed in claim 12 , wherein said temperature sensing device comprises a proportional to absolute temperature sensor.

14. The apparatus as claimed in claim 1 , wherein said photosensitive device comprises a photodiode.

15. The apparatus of claim 1 , implemented as an integrated circuit.

16. The apparatus of claim 1 , wherein the photosensitive device is a part of an image sensor.

17. The apparatus of claim 16 , wherein the image sensor is an x-ray sensor.

18. A method comprising:
obtaining a dark current value for at least one shielded compensation photosensitive device;
modifying said dark current value of said at least one shielded photosensitive device to provide a temperature compensated dark current value;
providing dark current compensation for an output of a photosensitive device by using said dark current value, temperature information and temperature dependent calibration information;
using said temperature compensated dark current value as a measure of dark current in said output of said at least one photosensitive device; and
calculating said temperature dependent calibration information dependent on a calibration measurement of dark currents output from at least one of the photosensitive device and the shielded compensation photosensitive device at a first and a second, different, calibration temperatures.

19. The method as claimed in claim 18 , further comprising subtracting said temperature compensated dark current value from said output of said at least one photosensitive device.

20. The method as claimed in claim 18 , further comprising applying a function to a dark current value of said shielded photosensitive device, said function being dependent on said temperature information and said dark current of said at least one shielded photosensitive device and dark current of said at least one photosensitive device for the first and second calibration temperatures.

21. The method as claimed in claim 20 , wherein said function has a first factor dependent on a first coefficient multiplied by an exponential function of the temperature information divided by a second coefficient.

22. The method as claimed in claim 21 , wherein at least one of said first and second coefficients is dependent on a dark current at said first and second, different, calibration temperatures for said shielded photosensitive device and a dark current at said first and second, different, calibration temperatures for said photosensitive device.

23. The method as claimed in claim 22 , further comprising storing said first and second coefficients.

24. The method as claimed in claim 18 , wherein at least one calibration temperature is in a range of 10 degrees to 30 degrees.

25. The method as claimed in claim 18 , wherein at least one calibration temperature is in a range of 70 degrees to 90 degrees.

Assignees

• St Microelectronics Res & Dev

Inventors

• Raynor Jeffrey M
• Price Andy

Classifications

• G01J2001/444

  Compensating; Calibrating, e.g. dark current, temperature drift, noise reduction or baseline correction; Adjusting · CPC title

• G01J1/44

  Electric circuits {(for command of an exposure part G03B7/02)} · CPC title

• H04N25/63 Primary

  applied to dark current · CPC title

• G01J1/1626 Primary

  Arrangements with two photodetectors, the signals of which are compared · CPC title

• G01J2001/446

  Photodiode · CPC title