Time-of-flight device and 3D optical detector
A Time-of-flight optical device and a 3D optical detector comprising a CMOS integrated circuit with an array of photosensitive pixels that are, at least in part, interconnected to form macro-pixels ( 180 ). Each macro-pixel ( 180 ) groups a plurality of individual pixels contributing their photocarriers to a common sense node SN through a plurality of transistors in parallel. Preferably the integrated circuit includes switched capacitor circuits arranged to combine the potential of the sense nodes SN of a plurality of macro-pixels, and/or to perform correlated double samplings in an energy efficient way. Each pixel has now an additional sink gate ( 194 ) between the pinned photodetector, PPD, potential well and a positive voltage source. By this additional sink gate ( 194 ), the storage well of the PPDs can be emptied without transferring the charge to the sense node. The value of the transfer gate voltage V_TG may be adapted such that the potential barrier is not lowered all the way down, but decreased to a value V B <V P . In this manner, the potential well of the PPDs is emptied only in part. This amounts to subtracting a constant value from V TRAN and can be used to zero a background illumination value. Importantly, the charge left in the potential well is discharged by the sink transistor ( 194 ) before the next integration and does not affect successive cycles.
1 . A Time-of-flight optical device for the contactless determination of the distance to a target comprising:
a source generating a modulated beam of light impinging on a target;
an optical detector receiving light backscattered from the target; and
a logic circuit, arranged to determine the distance of the target from the time structure of the backscattered light received by the optical detector;
wherein the optical detector comprises an integrated circuit with an array of photosensitive pixels in which at least a subset of the pixels are interconnected to form a macro-pixel, the pixels in the macro-pixel sharing a common sense node, and wherein the pixels in the macro-pixel have each a potential well for collecting photocarriers, a first transistor between the potential well and the common sense node of the macro-pixel, and wherein the gates of the first transistors in a macro-pixel are connected in parallel for transferring photocarriers stored into the potential wells of the macro-pixel to the common sense node at the same time.
2 . The time-of-flight optical device of claim 1 , wherein the pixels in the macro-pixel comprise each a second transistor between the potential well and a positive voltage source held at a given potential, and wherein the gates of the second transistors in a macro-pixel are connected in parallel or operated simultaneously for emptying the potential wells of the pixels in the macro-pixel at the same time.
3 . The time-of-flight optical device of claim 1 , wherein the macro-pixel includes a reset transistor for forcing the sense node to a determined potential, and an amplifier whose input is connected to the sense node.
4 . The time-of-flight optical device of claim 1 , wherein the integrated circuit is a CMOS integrated circuit and comprises one or more switched capacitor circuits arranged to perform a correlated double sampling of the potential of the sense node of a macro-pixel.
5 . The time-of-flight optical device of claim 1 , wherein the integrated circuit is a CMOS integrated circuit and comprises one or more switched capacitor circuits arranged to combine the potential of the sense nodes of a plurality of macro-pixels.
6 . The time-of-flight optical device of claim 1 , wherein the logic circuit is arranged to integrate a received signal of the backscattered light in multiple periodic windows, synchronised with the modulated beam.
7 . The time-of-flight optical device of claim 6 , wherein the logic circuit computes a first value that represents the intensity of received pulses, and second value that integrates only part of the pulses that are received after the end of transmitted pulses by a determined interval.
8 . The time-of-flight optical device of claim 6 , wherein the logic circuit is arranged to integrate the received signal in windows where no signal from the source is expected, to evaluate a background illumination value.
9 . A 3D optical detector comprising the time-of-flight optical device of claim 1 , an imaging optics system arranged to project an image of the target on the array and comprising a plurality of macro-pixels for determining the distance of a corresponding plurality of points on the target.
10 . A method for determining the distance of a target comprising:
generating a modulated beam comprising pulses of light impinging on the target;
receiving light backscattered from the target with an optical detector comprising an integrated circuit with an array of photosensitive pixels in which at least a subset of the pixels are interconnected to form a macro-pixel, the pixels in the macro-pixel sharing a common sense node, and wherein the pixels in the macro-pixel have each a potential well for collecting photocarriers, a first transistor between the potential well and the common sense node of the macro-pixel, and wherein the gates of the first transistors in a macro-pixel are connected in parallel for transferring photocarriers stored into the potential wells of the macro-pixel to the common sense node at the same time;
integrating the received light signal in multiple periodic windows, synchronised with the transmitted signal;
determining a time shift between the generated light and the received signal based on the integrated light signals.
11 . The method of claim 10 , comprising:
computing a first value that represents the intensity of received pulses corresponding to the received backscattered light, and second value that integrates only part of the received pulses that are received after the end of the transmitted pulses by a determined interval;
determining a time shift between the generated light and the received signal based on said first and second value.
12 . The method of claim 10 , comprising measuring a background illumination by integrating the received light signal in windows where no light from the source is expected.
13 . The time-of-flight optical device of claim 4 , wherein the correlated double sampling comprises computing a difference between two consecutive correlated samples.
14 . The time-of-flight optical device of claim 4 , wherein the one or more switched capacitor circuits are configured to perform correlated double sampling by storing a first potential of the common sense node during a reset phase and a second potential of the common sense node after the transferring of the photocarriers stored in the potential wells of the macro-pixel to the common sense node, and wherein the integrated circuit computes a difference between the stored first potential and the stored second potential.
15 . The time-of-flight optical device of claim 14 , wherein the switched capacitor circuit comprises a first capacitor configured to store the first potential and a second capacitor configured to store the second potential, and wherein the first and second capacitors are connected in parallel with an input corresponding to an output of the macro-pixel and with an output corresponding to the difference between the stored first potential and the stored second potential.
16 . The time-of-flight optical device of claim 4 , wherein the switched capacitor circuit is further configured to subtract a background illumination signal by storing an integrated background signal in a capacitor during a time interval when no signal from the modulated light source is expected and subtracting this stored signal from the detected backscattered light signal.
17 . The time-of-flight optical device of claim 5 , wherein the one or more switched capacitor circuits are configured to combine the potentials of sense nodes of a plurality of macro-pixels, such that the integrated circuit averages the charge signals from the plurality of macro-pixels before readout.
18 . The time-of-flight optical device of claim 17 , wherein the integrated circuit reads all of the plurality of macro-pixels simultaneously and delivers an average illumination value.