Method and system to enhance differential dynamic range and signal/noise in CMOS range finding systems using differential sensors

• US 7,157,685 B2
• Filed: 02/08/2006
• Issued: 01/02/2007
• Est. Priority Date: 04/12/2004
• Status: Active Grant

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Canesta Incorporated (Microsoft Corporation)
• Inventors: Bamji, Cyrus, Salama, Khaled
• Primary Examiner(s): Lee; John R.
• Application Number:
  US11/350,209
• Publication Number:
  US 20060157643A1
• Time in Patent Office: 328 Days
• Field of Search: 250/214.A, 250/214.R, 250/214.1, 348/294, 348/296, 348/301, 348/302, 348/308
• US Class Current: 250/214.00A
• CPC Class Codes: G01C 3/06 Use of electric means to obtain final indication
• CPC Class Codes: G01S 17/08 for measuring distance only indirect measurement G01S17/46; active triangulation systems G01S17/48
• CPC Class Codes: G01S 17/894 3D imaging with simultaneous measurement of time-of-flight at a 2D array of receiver pixels, e.g. time-of-flight cameras or flash lidar
• CPC Class Codes: G01S 7/4863 Detector arrays, e.g. charge-transfer gates
• CPC Class Codes: G01S 7/4868 Controlling received signal intensity or exposure of sensor
• CPC Class Codes: G01S 7/487 Extracting wanted echo signals , e.g. pulse detection
• CPC Class Codes: H04N 25/58 involving two or more exposures
• CPC Class Codes: H04N 25/77 Pixel circuitry, e.g. memories, A/D converters, pixel amplifiers, shared circuits or shared components

Abstract

Dynamic range of a differential pixel is enhanced by injecting, synchronously or asynchronously, a fixed compensating offset (ΔV) into a differential signal capacitor whenever magnitude of the differential signal across the capacitor exceeds a predetermined value. The number (N) of ΔV offsets made is counted. Effective differential signal capacitor voltage V(t)=Vo+N·ΔV, where Vo is capacitor voltage. Differential pixel signal/noise ratio is increased by dynamically maximizing operational amplifier gain A_G for each differential pixel.

Claims

1. A method to increase effective differential dynamic range of a differential pixel responsive to optical energy, a detection response of said differential pixel having a common mode component and a differential component, the method including:

(a) adding a fixed magnitude compensating offset selected from +Δ

V and −

Δ

V to a differential signal V(t) detected by said differential pixel, to avoid a saturation magnitude differential signal level; and

(b) counting number N of times said compensating offset has been added to avoid said saturation magnitude;

wherein effective magnitude of said differential signal V(t) is proportional to V(t)±

N Δ

V.

2. The method of claim 1, wherein step (a) injects an offset of charge to produce said compensating offset.

3. The method of claim 1, wherein step (a) is carried out synchronously by examining magnitude of V(t) at time intervals and compensating at that time if V(t) exceeds a threshold voltage.

4. The method of claim 1, wherein step (a) is carried out asynchronously by compensating V(t) whenever magnitude of V(t) exceeds a threshold voltage.

5. The method of claim 1, wherein said differential pixel includes a differential photodetector, and further including resetting common mode of said differential photodetector;

wherein common mode saturation of said differential photodetector is avoided.

6. The method of claim 5, further including preserving a differential mode component of said differential photodetector in a differential signal capacitor to reduce differential signal loss during common mode resetting of said differential photodetector.

7. The method of claim 6, wherein steps of resetting common mode and preserving a differential mode component are implemented by sharing at least one component selected from a group consisting of a capacitor, an amplifier, and a MOS transistor.

8. The method of claim 1, wherein value of N is processed in a manner selected from a group consisting of (i) N is stored within said differential pixel and is read-out at end of integration, and (ii) N is stored external to said pixel detector and is periodically updated by reading from said pixel detector whether said differential signal capacitor was changed by +Δ

V, by −

Δ

V, or was not changed.

9. For use with a differential pixel responsive to optical energy, a detection response of said differential pixel having a common mode component and a differential component, differential dynamic range enhancing circuitry to enhance effective dynamic range of said differential pixel, said differential dynamic range enhancing circuitry comprising:

means for adding a fixed magnitude compensating offset selected from +Δ

V and −

Δ

V to a differential signal V(t) detected by said differential pixel, to avoid a saturation magnitude differential signal level; and

means for counting number N of times said compensating offset has been added to avoid said saturation magnitude;

wherein effective magnitude of said differential signal V(t) is proportional to V(t)+N Δ

V.

10. The differential dynamic range enhancing circuitry of claim 9, wherein said means for adding injects an offset of charge to produce said compensating offset.

11. The differential dynamic range enhancing circuitry of claim 9, wherein said means for adding functions in a manner selected from a group consisting of (i) synchronously by examining magnitude of V(t) at time intervals and compensating at that time if V(t) exceeds a threshold voltage, and (ii) asynchronously by compensating V(t) whenever magnitude of V(t) exceeds a threshold voltage.

12. The differential dynamic range enhancing circuitry of claim 9, wherein said means for counting operates in a manner selected from a group consisting of counting N within said differential pixel and reading-out N at end of integration, and storing N external to said pixel detector and periodically updating N by reading from said pixel detector whether said differential signal capacitor was changed by +Δ

V or by −

Δ

V.

13. The differential dynamic range enhancing circuitry of claim 9, wherein said differential pixel includes a differential photodetector, and wherein said circuitry is part of a time-of-flight system that includes at least one of (i) means for resetting common mode of said differential photodetector, and (ii) means for preserving a differential mode component in a differential signal capacitor to reduce differential signal loss,

wherein said differential dynamic range enhancing circuitry shares at least one component selected from a group consisting of a capacitor, an amplifier, and a MOS transistor, with at least one of said means for resetting common mode and said means for preserving.

14. A method to reduce effective noise in the pixel signal readout path of an array of differential pixels, the method including:

(a) individually varying signal gain for each pixel to determine a maximum allowable gain;

(b) for each said pixel, recording said pixel'"'"'s maximum allowable gain determined at step (a) and also recording a signal value for said pixel at said maximum allowable gain; and

(c) operating said pixel at said maximum allowable gain;

wherein effect of noise in components downstream of said pixel is reduced and signal/noise ratio is enhanced.

15. The method of claim 14, wherein:

step (a) includes attempting operation at all possible gain values for each said pixel; and

within a row of said array, step (b) includes row-wise recording maximum-non saturating gain for each said pixel in said row.

16. For use with a differential pixel responsive to optical energy, said differential pixel having a detection response with a common mode component and a differential component, signal/noise enhancement circuitry to reduce effective noise in the pixel signal readout path of an array of differential pixels, the signal/noise enhancement circuitry including:

means for individually varying signal gain for each pixel to determine a maximum allowable gain;

for each said pixel, means for recording said pixel'"'"'s maximum allowable gain determined at step (a) and also recording a signal value for said pixel at said maximum allowable gain; and

operating said pixel at said maximum allowable gain;

wherein effect of noise in components downstream of said pixel is reduced and signal/noise ratio is enhanced.

17. The signal/noise enhancement circuitry of claim 16, wherein:

said means for individually incrementing attempts operation at all possible gain values for each said pixel; and

said means for recording records, within a row of said array, a row-wise record of maximum-non saturating gain for each said pixel in said row.

18. A CMOS-implementable range finding system that emits amplitude modulated optical energy towards a target object and includes an array of differential pixels including pixel photodetectors to differentially detect at least a fraction of said optical energy reflected by said target object even in the presence of ambient optical energy, said differential pixel photodetectors having a detection response with a common mode component, the range finding system including:

means for substantially rejecting said common mode component such that saturation of said differential pixel photodetectors therefrom is reduced;

means for retaining differentially detected reflected said optical energy; and

at least one of;

(i) means for increasing effective differential dynamic range of said differential pixels; and

(ii) means for enhancing signal/noise through a detection signal path of said system.

19. The range finding system of claim 18, wherein at least one of said means for increasing effective differential dynamic range, and said means for enhancing signal/noise share, on a per differential pixel basis, at least one component with at least one of (i) said means for substantially rejecting, (ii) said means for retaining, and (iii) each other.

20. The range finding system of claim 18, wherein said target object includes at least one of (i) a virtual input device, (ii) an object presenting a potential hazard to a motor vehicle, (iii) an object within a motor vehicle.