Patent Publication Number: US-2023134910-A1

Title: Readout architectures for motion blur reduction in indirect time-of-flight sensors

Description:
BACKGROUND INFORMATION 
     Field of the Disclosure 
     This disclosure relates generally to image sensors, and in particular but not exclusively, relates to time-of-flight sensors. 
     Background 
     Interest in three dimensional (3D) cameras is increasing as the popularity of 3D applications continues to grow in areas such as imaging, movies, games, computers, user interfaces, facial recognition, object recognition, augmented reality, and the like. A typical passive way to create 3D images is to use multiple cameras to capture stereo or multiple images. Using the stereo images, objects in the images can be triangulated to create the 3D image. One disadvantage with this triangulation technique is that it is difficult to create 3D images using small devices because there must be a minimum separation distance between each camera in order to create the 3D images. In addition, this technique is complex and therefore requires significant computer processing power in order to create the 3D images in real time. 
     For applications that require the acquisition of 3D images in real time, active depth imaging systems based on time-of-flight measurements are sometimes utilized. Time-of-flight cameras typically employ a light source that directs light at an object, a sensor that detects the light that is reflected from the object, and a processing unit that calculates the distance to the object based on the round-trip time it takes for the light to travel to and from the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIG.  1    is a block diagram that shows one example of a time-of-flight light sensing system in accordance with the teachings of the present invention. 
         FIG.  2    is a timing diagram that shows an example of light pulses emitted from a light source relative to the receipt of the reflected light pulses and measurements using various phase shifts in an example time-of-flight sensing system accordance with the teachings of the present invention. 
         FIG.  3    is a schematic illustrating one example of a time-of-flight pixel circuit in accordance with the teachings of the present invention. 
         FIG.  4    is a schematic illustrating another example of a time-of-flight pixel circuit in accordance with the teachings of the present invention. 
         FIG.  5    is a schematic illustrating yet another example of a time-of-flight pixel circuit in accordance with the teachings of the present invention. 
         FIG.  6    is an example timing diagram illustrating signals in an example time-of-flight sensing system with pipelined operations in accordance with the teachings of the present invention. 
         FIG.  7    is another example timing diagram illustrating signals in an example time-of-flight sensing system with burst operations in accordance with the teachings of the present invention. 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. In addition, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. 
     DETAILED DESCRIPTION 
     Examples directed to various embodiments of a time-of-flight pixel circuits included in pixel arrays of time-of-flight sensing systems are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of the examples. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail in order to avoid obscuring certain aspects. 
     Reference throughout this specification to “one example” or “one embodiment” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present invention. Thus, the appearances of the phrases “in one example” or “in one embodiment” in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more examples. 
     Spatially relative terms, such as “beneath,” “below,” “over,” “under,” “above,” “upper,” “top,” “bottom,” “left,” “right,” “center,” “middle,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is rotated or turned over, elements described as “below” or “beneath” or “under” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary terms “below” and “under” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated ninety degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. In addition, it will also be understood that when an element is referred to as being “between” two other elements, it can be the only element between the two other elements, or one or more intervening elements may also be present. 
     Throughout this specification, several terms of art are used. These terms are to take on their ordinary meaning in the art from which they come, unless specifically defined herein or the context of their use would clearly suggest otherwise. It should be noted that element names and symbols may be used interchangeably through this document (e.g., Si vs. silicon); however, both have identical meaning. 
     As will be discussed, various examples of indirect time-of-flight (e.g., iTOF) sensing systems are disclosed in which modulated light is emitted from a light source to an object, which is then reflected from the object back to a time-of-flight pixel array included in the time-of-flight sensing system. The object distance is determined in response to the measured phase of the modulation sensed by the time-of-flight pixel circuits, which may be used to yield a 3D frame. 
     In various examples, the photodiode of each time-of-flight pixel circuit is coupled via a plurality of transfer transistors to a plurality of respective floating diffusions. In the various examples, each of the floating diffusions is coupled to a respective reset device as well as to a plurality of memory nodes via a plurality of respective sample and hold transistors. As such, each of the plurality of memory nodes is configured to capture a subframe of a portion of charge information from the photodiode and store the subframe of the portion of charge information during the acquisition of a subsequent subframe of a portion of charge information. 
     In the various examples, the plurality of memory nodes can then be read out in parallel with or subsequent to the acquisition and storage of one or more additional portions of charge information from the photodiode. With the capability of reading out the plurality of memory nodes in parallel with or subsequent to the acquisition and storage of one or more additional portions of charge information from the photodiode, the readout speed requirements from the time-of-flight pixel circuit is significantly relaxed, which enables motion blur performance to be improved significantly with little to no impact on integration time in an example time-of-flight pixel circuit in accordance with the teachings of the present invention. As such, it is appreciated that example time-of-flight pixel circuits in accordance with the teachings of the present invention provide time-of-flight pixel arrays capable of pipelined operation or burst operation with increased image resolution as the example time-of-flight pixel circuits impose less stress on readout speeds. 
     As will be discussed, phase measurements are taken from the various examples of the time-of-flight pixel circuits, which are modulated with 0°/180° and 180°/0° phase modulation signals as well as with 90°/270° and 270°/90° phase modulation signals in multiple subframes when sensing the reflected modulated light from the object. By modulating the time-of-flight pixel circuits with the opposing phases of the 0°/180° and 180°/0° phase modulation signals as well as with the opposing phases of the 90°/270° and 270°/90° phase modulation signals in the multiple subframes, offset errors as well as dark current errors in the time-of-flight pixel circuits are canceled or removed in accordance with the teachings of the present invention. 
     In the various example, the 0°, 90°, 180°, and 270° phase modulation signals are also modulated at multiple different frequencies in the multiple subframes. In the various examples, the 0°, 90°, 180°, and 270° phase modulation signals synchronized to have the same frequency as the modulated light that is emitted to the object from the light source of the time-of-flight sensing system to realize homodyne detection by the indirect time-of-flight sensor. Employing the different phases in the phase modulation signals allows to reconstruction of the encoded distance. Furthermore, measurements at additional frequencies and/or phases improve systematic errors such as harmonic distortions or multipath artifacts. 
     It is noted that phases that are increments of 360° apart cannot be distinguished, which consequently results in ambiguities in the measurements. As a result, the modulation frequency of the phase modulation signals is chosen not to exceed a maximum modulation frequency in order to accommodate a desired depth range. However, a tradeoff is that increasing the modulation frequency improves precision. 
     To illustrate,  FIG.  1    is a block diagram that shows one example of a time-of-flight light sensing system  100  in accordance with the teachings of the present invention. In the depicted example, time-of-flight light sensing system  100  is a 3D camera that calculates image depth information of a scene (e.g., object  106 ) based on indirect time-of-flight (e.g., iToF) measurements with an image sensor that includes a time-of-flight pixel array  110 . In some examples, it is appreciated that although time-of-flight light sensing system  100  is capable of sensing 3D images, time-of-flight light system  100  may also be utilized to capture 2D images. In various examples, time-of-flight light sensing system  100  may also be utilized to capture high dynamic range (HDR) images. 
     As shown in the depicted example, time-of-flight light sensing system  100  includes light source  102  that is synchronized with a time-of-flight sensor that includes the time-of-flight pixel array  110 , which includes a plurality of time-of-flight pixel circuits  112 , and a control circuit  114  that is coupled to control and synchronize the time-of-flight pixel array  110  and light source  102 . 
     As illustrated, light source  102  is configured to emit light  104  to the object  106  over a distance L. The emitted light  104  is then reflected from the object  106  as reflected light  108  (e.g., reflected light waves/pulses), some of which propagates towards the time-of-flight pixel array  110  of time-of-flight light sensing system  100  over the distance L and is incident upon the time-of-flight pixel circuits  112  of time-of-flight pixel array  110  as image light. Each time-of-flight pixel circuit  112  included in the time-of-flight pixel array  110  includes a photodetector (e.g., one or more photodiodes, avalanche photodiodes, or single-photon avalanche diodes, or the like) to detect the reflected light  108  and convert the reflected light  108  into an electric signal (e.g., electrons, image charge, etc.). 
     It is noted that time-of-flight pixel array  110  and control circuit  114  are represented as separate components in  FIG.  1    for explanation purposes. However, it is appreciated that time-of-flight pixel array  110  and components of control circuit  114  may be integrated onto a same integrated circuit chip or wafer in a non-stacked standard planar sensor. In various examples, it as also appreciated that time-of-flight pixel array  110  may implemented in a stacked time-of-flight image sensor. 
     Continuing with the depicted example, each time-of-flight pixel circuit  112  of time-of-flight pixel array  110  determines depth information for a corresponding portion of object  106  such that a 3D image of object  106  can be generated. As will be discussed in greater detail below, depth information is determined by modulating the transfer gates of each time-of-flight pixel circuit  112  with 0°/180° and 180°/0° phase modulation signals as well as with opposing 90°/270° and 270°/90° phase modulation signals in multiple subframes to measure the delay/phase difference between emitted light  104  and the received reflected light  108  to indirectly determine a round-trip time for light to propagate from light source  102  to object  106  and back to the time-of-flight pixel array  110  of time-of-flight light sensing system  100 . In the various example, the 0°, 90°, 180°, and 270° phase modulation signals are also modulated at multiple different frequencies in the multiple subframes. In various examples, each of a plurality of memory nodes included in each time-of-flight pixel circuit  112  is capable of capturing and storing a subframe of a portion of charge information from the photodiode included the time-of-flight pixel circuit  112  during or prior to a readout of another memory node storing another subframe of a portion of charge information acquired in a previous subframe. Thus, the depth information may be based on electric signals generated by the photodiode included in each time-of-flight pixel circuit  112 , which are subsequently transferred and stored in the plurality of memory nodes in each time-of-flight pixel circuit  112 , which are then read out subsequently. 
     As shown in the depicted example, the round-trip time for emitted light  104  to propagate from light source  102  to object  106  and then be reflected back to time-of-flight pixel array  110  can be used to determine the distance L using the following relationships in Equations (1) and (2) below: 
     
       
         
           
             
               
                 
                   
                     T 
                     
                       T 
                       ⁢ 
                       O 
                       ⁢ 
                       F 
                     
                   
                   = 
                   
                     
                       2 
                       ⁢ 
                       L 
                     
                     c 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   L 
                   = 
                   
                     
                       
                         T 
                         
                           T 
                           ⁢ 
                           O 
                           ⁢ 
                           F 
                         
                       
                       · 
                       c 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where c is the speed of light, which is approximately equal to 3×10 8  m/s, and T TOF  Corresponds to the round-trip time, which is the amount of time that it takes for the light to travel to and from the object  106  as shown in  FIG.  1   . Accordingly, once the round-trip time is known, the distance L may be calculated and subsequently used to determine depth information of object  106 . 
     As shown in the depicted example, control circuit  114  is coupled to time-of-flight pixel array  110  and light source  102 , and includes logic and memory that when executed causes time-of-flight light sensing system  100  to perform operations for determining the round-trip time. Determining the round-trip time may be based on, at least in part, timing signals generated by control circuit  114 . For indirect time-of-flight (indirect time-of-flight) measurements, the timing signals are representative of the delay/phase difference between the light waves/pulses of when the light source  102  emits light  104  and when the photodetectors in time-of-flight pixel circuits  112  detect the reflected light  108 . 
     In some examples, time-of-flight light sensing system  100  may be included in a device (e.g., a mobile phone, a tablet, a camera, etc.) that has size and power constraints determined, at least in part, based on the size of the device. Alternatively, or in addition, time-of-flight light sensing system  100  may have specific desired device parameters such as frame rate, depth resolution, lateral resolution, etc. 
       FIG.  2    is a timing diagram that illustrates the timing relationship between example light pulses emitted from a light source relative to the receipt of the reflected light pulses and measurements using various phase shifts in an example time-of-flight imaging system accordance with the teachings of the present invention. Specifically,  FIG.  2    shows emitted light  204 , which represents the modulated light pulses that are emitted from the light source  102  to the object  106 , and corresponding pulses reflected light  208 , which represents the reflected light pulses that are back-reflected from the object  106  and received by the time-of-flight pixel circuits  112  of time-of-flight pixel array  110  of  FIG.  1   . 
     The example depicted in  FIG.  2    also illustrates measurement pulses of the phase modulation signals including a 0° phase modulation signal  214 A (e.g., a first phase modulation signal) and a 180° phase modulation signal  214 B (e.g., a second phase modulation signal), as well as measurement pulses including a 90° phase modulation signal  216 A (e.g., a third phase modulation signal) and a 270° phase modulation signal  216 B (e.g., a fourth phase modulation signal), which as shown are all phase-shifted relative to the phase of the pulses of emitted light  204 . In addition, it is appreciated that in the depicted example the 180° phase modulation signal  214 B is an inverted 0° phase modulation signal  214 A, that the 90° phase modulation signal  216 A is ninety degrees out of phase with the 0° phase modulation signal  214 A, and that the 270° phase modulation signal  216 B is an inverted 90° phase modulation signal  216 A. 
       FIG.  2    also shows that the 0° phase modulation signal  214 A and 180° phase modulation signal  214 B as well as the 90° phase modulation signal  216 A and 270° phase modulation signal  216 B pulses are all modulated at the same frequency as the modulated emitted light  204  and reflected light  208  to realize homodyne detection of the reflected light  208  in accordance with the teachings of the present invention. In various examples, the modulation frequency of the phase modulation signals and the modulated emitted light may be varied across the multiple subframes in which the phase is measured. It is appreciated that the measurements at additional frequencies and/or phases can improve systematic errors, such as for example harmonic distortions and/or multipath artifacts. Utilizing the different phases for the example measurement pulses as shown allows reconstruction of the encoded distance in multiple subframes in accordance with the teachings of the present invention. 
     As will be discussed, the 0° phase modulation signal  214 A and 180° phase modulation signal  214 B as well as the 90° phase modulation signal  216 A and 270° phase modulation signal  216 B pulses correspond to the switching or modulation of transfer transistors that are included in the time-of-flight pixel circuits  112  of time-of-flight pixel array  110 . In operation, the modulation of the transfer transistors in the time-of-flight pixel circuits  112  of time-of-flight pixel array  110  can be used to measure the charge that is photogenerated in the one or more photodiodes that are included the time-of-flight pixel circuits  112  in response to the reflected light  208  to measure the delay or phase difference φ between the pulses of emitted light  204  and the corresponding pulses of reflected light  208 . 
     For instance, the example illustrated in  FIG.  2    shows that a first portion of charge Q1 is photogenerated by the pulses of 0° phase modulation signal  214 A and that a second portion of charge Q2 is photogenerated by the pulses of 180° phase modulation signal  214 B in response to reflected light  208 . Similarly, a third portion of charge Q3 is photogenerated by the pulses of 90° phase modulation signal  216 A and a fourth portion of charge Q4 is photogenerated by the pulses of 270° phase modulation signal  216 B in response to reflected light  208 . As will be discussed in greater detail below, the measurements of four phase portions of charge Q1, Q2, Q3, and Q4 can then be used to determine the delay or phase difference φ between the emitted light  204  and the reflected light  208 , and therefore the time of flight T TOF  of light from the light source  102  to the object  106  and then back to the time-of-flight pixel array  110  in accordance with the teachings of the present invention. 
       FIG.  3    is a schematic illustrating one example of a time-of-flight pixel circuit  312  in accordance with the teachings of the present invention. It is appreciated that the time-of-flight pixel circuit  312  of  FIG.  3    may be an example of one of the time-of-flight pixel circuits  112  included in time-of-flight pixel array  110  shown in  FIG.  1   , and that similarly named and numbered elements described above are coupled and function similarly below. 
     As shown in the example depicted in  FIG.  3   , the time-of-flight pixel circuit  312  includes a photodiode  318  configured to photogenerate charge in response to incident light. In one example, the light that is incident on photodiode  318  is the reflected modulated light  108  that is reflected from an object  106  as described in  FIG.  1   . A first floating diffusion FDA  322 A is configured to store a first portion of charge photogenerated in the photodiode  318 , and second floating diffusion FDB  322 B is configured to store a second portion of charge photogenerated in the photodiode  318 . In one example, a first reset transistor  324 A is coupled between a supply rail and the first floating diffusion FDA  322 A. A second reset transistor  324 B is coupled between the supply rail and the second floating diffusion FDB  322 B. In the various examples, the first reset transistor  334 A is configured to reset the first floating diffusion FDA  322 A in response to a first reset signal RSTA and the second reset transistor  334 B is configured to reset the second floating diffusion FDB  322 B in response to a second reset signal RSTB. In various examples, the first reset signal RSTA and second reset signal RSTB may be the same signal or different signals. In one example, the first reset transistor  334 A and second reset transistor  334 B may also act as overflow transistors. In such examples, the first reset transistor  334 A and second reset transistor  334 B may be operated in such a way that excess carriers generated by photodiode  318  may be guided to the power supply by first reset transistor  334 A and/or the second reset transistor  334 B or in a way such that the photosensitivity of photodiode  318  is disabled. 
     A first transfer transistor  320 A is configured to transfer the first portion of charge from the photodiode  318  to the first floating diffusion FDA  322 A in response to a first modulation signal TXA. In one example, the first modulation signal TXA may be an example of one of the phase modulation signals described in  FIG.  2   . A second transfer transistor  320 B is configured to transfer the second portion of charge from the photodiode  318  to the second floating diffusion FDB  322 B in response to a second modulation signal TXB. In the example, the second modulation signal TXB may also be an example of one of the phase modulation signals described in  FIG.  2   . 
     As shown in the depicted example, the time-of-flight pixel circuit  312  also includes a first plurality of N capacitors  328 A 1 ,  328 A 2 , . . . ,  328 AN, which provide a first plurality of N memory nodes C 1 A  334 A 1 , C 2 A  334 A 2 , . . . , CNA  334 AN, and a second plurality of N capacitors  328 B 1 ,  328 B 2 , . . . ,  328 BN, which provide a second plurality of N memory nodes C 1 B  334 B 1 , CBA  334 B 2 , . . . , CNA  334 BN. In the example, each one of a first plurality of N sample and hold transistors  326 A 1 ,  326 A 2 , . . . ,  326 AN is coupled between the first floating diffusion FDA  322 A and a respective one of the first plurality of N memory nodes C 1 A  334 A 1 , C 2 A  334 A 2 , . . . , CNA  334 AN as shown. Similarly, each one of a second plurality of N sample and hold transistors  326 B 1 ,  326 B 2 , . . . ,  326 BN is coupled between the second floating diffusion FDB  322 B and a respective one of the second plurality of N memory nodes C 1 B  334 B 1 , C 2 A  334 B 2 , . . . , CNA  334 BN as shown. 
     As shown in the depicted example, the time-of-flight pixel circuit  312  further includes a first output source follower transistor  330 A and a second output source follower transistor  330 B. Each one of a first plurality of N readout transistors  336 A 1 ,  336 A 2 , . . . ,  336 AN is coupled between a respective one of the first plurality of N memory nodes C 1 A  334 A 1 , C 2 A  334 A 2 , . . . , CNA  334 AN and a gate of the first output source follower transistor  330 A. Each one of a second plurality of N readout transistors  336 B 1 ,  336 B 2 , . . . ,  336 BN is coupled between a respective one of the second plurality of N memory nodes C 1 B  334 B 1 , CBA  334 B 2 , . . . , CNA  334 BN and a gate of the second output source follower transistor  330 A. A first row select transistor  332 A is coupled to each one of the first plurality of N readout transistors  336 A 1 ,  336 A 2 , . . . ,  336 AN and a second row select transistor  332 B is coupled to each one of the second plurality of N readout transistors  336 B 1 ,  336 B 2 , . . . ,  336 BN. In one example, the output of the first row select transistor  332 A may be considered a first tap of the time-of-flight pixel circuit  312  from which the output VA from first output source follower transistor  330 A may be read out, and the output of the second row select transistors may be considered a second tap of the time-of-flight pixel circuit  312  from which the output VB from second output source follower transistor  330 B may be read out. 
     In one example, the time-of-flight pixel circuit  312  may be configured as a pipelined pixel circuit such that there are two memory nodes (e.g., N=2) per output tap. To illustrate, in a pipelined pixel circuit example with N=2, there are two memory nodes (e.g.,  334 A 1 / 334 A 2 ,  334 B 1 / 334 B 2 ), two sample and hold transistors (e.g.,  326 A 1 / 326 A 2 ,  326 B 1 / 326 B 2 ), and two readout transistors (e.g.,  336 A 1 / 336 A 2 ,  336 B 1 / 336 B 2 ) per output tap. In the example pipelined pixel circuit configuration, one of the memory nodes per output tap can be coupled to the respective floating diffusion through a respective sample and hold transistor to acquire and store (e.g., sample and hold) the portion of charge from the floating diffusion, while the other memory node is read out with the other memory node being coupled to the gate terminal of the respective output source follower transistor through a respective readout transistor. 
     In another example, the time-of-flight pixel circuit  312  may be configured as a burst pixel circuit with two or more memory nodes per output tap (e.g., N=4 or N=8). In a burst pixel circuit configuration, the depth of frame may be formed with, for example, N=4 or N=8 subframes with N=4 or N=8 memory nodes per output tap, which allows the required analog to digital converter (ADC) readout speed, chip-size, and power consumption requirements to be reduced drastically in accordance with the teachings of the present invention. In operation, all of the memory nodes (e.g.,  334 A 1 / 334 B 1 ,  334 A 2 / 334 B 2 , . . . ,  334 AN/ 334 BN) are coupled to sample and hold the first and second portions of the charge from the respective floating diffusions (e.g.,  322 A/ 322 B) through the respective sample and hold transistors (e.g.,  326 A 1 / 326 B 1 ,  326 A 2 / 326 B 2 , . . . ,  326 AN/ 326 BN). 
     In one example, the first and second portions of the charge from the respective first and second floating diffusions (e.g.,  322 A/ 322 B) each include a plurality of subframe portions of the first and second portions of charge. In the various examples, each one of the respective subframe portions of charge is transferred into the respective first and second floating diffusions (e.g.,  322 A/ 322 B) in response to the first and second modulation signals TXA and TXB having a specific phase (e.g., 0°/180°, 90°/270°, 180°/0°, 270°/90°) as well as a specific frequency setting (e.g., f1, f2) combination for each subframe. In the various examples, each one of the plurality of subframe portions of the first and second portions of charge in the respective floating diffusion (e.g.,  322 A/ 322 B) is configured to be sampled and held in sequence (e.g., one after another in time) into a respective one of the memory nodes (e.g.,  334 A 1 / 334 B 1 ,  334 A 2 / 334 B 2 , . . . ,  334 AN/ 334 BN) through a respective one of the sample and hold transistors (e.g.,  326 A 1 / 326 B 1 ,  326 A 2 / 326 B 2 , . . . ,  326 AN/ 326 BN). 
     After all of the memory nodes have received the plurality of subframe portions of the first and second portions of the charge from the respective floating diffusions, each one of the plurality of subframe portions of the first and second portions of the charge may then be read out from the memory nodes through the respective readout transistors (e.g.,  336 A 1 / 336 B 1 ,  336 A 2 / 336 B 2 , . . . ,  336 AN/ 336 BN). In one example, each one of the plurality of subframe portions of the first and second portions of charge that is stored in the respective memory nodes (e.g.,  334 A 1 / 334 B 1 ,  334 A 2 / 334 B 2 , . . . ,  334 AN/ 334 BN) is configured to be read out in sequence (e.g., one after another in time) through a respective one the first plurality of readout transistors (e.g.,  336 A 1 / 336 B 1 ,  336 A 2 / 336 B 2 , . . . ,  336 AN/ 336 BN) after all of the memory nodes have sampled and held the plurality of subframe portions of the first and second portions of the charge from the first and second floating diffusions (e.g.,  332 A/ 332 B). 
     It is appreciated that burst pixel circuit configurations may be utilized in situations where readout speeds are too slow for pipelined pixel circuit configurations. For instance, in an example with N=8, 8 subframes of phase charge information may be sampled and held in 8 memory nodes in sequence. After the 8 subframes of phase charge information have been stored in the respective 8 memory nodes, the 8 memory nodes may then be subsequently read out in sequence with reduced readout speed requirements after the integration period as there is typically a gap between integration exposure periods. 
     In the various examples, the first modulation signal TXA and second modulation signal TXB are configured to modulate the first transfer transistor  320 A and the second transfer transistor  320 B. In the examples, the first modulation signal TXA and second modulation signal TXB are modulation signals that are 180° out of phase or inverted versions of each other during the multiple subframes in which the time-of-flight pixel circuit  312  is modulated. For instance, in one example, in a first subframe, the first modulation signal TXA may be the 0° phase modulation signal  214 A while the second modulation signal TXA is therefore the 180° phase modulation signal  214 B. In that example, in another subframe, the first modulation signal TXA and the second modulation signal TXB are inverted relative to their respective signals in the first subframe. In other words, in that example, in the other subframe, the first modulation signal TXA is the 180° phase modulation signal  216 B while the second modulation signal TXB is the 0° phase modulation signal  214 A. In the various examples, it is appreciated that the first and second transfer transistors  320 A and  320 B may also be modulated with 90°/270° phase modulation signals as well as with the opposing phases of 270°/90° phase modulation signals in multiple subframes. 
     By modulating the first transfer transistor  320 A and the second transfer transistor  320 B with the first modulation signal TXA and second modulation signal TXB with the opposing phases of the 0°/180° and 180°/0° phase modulation signals as well as with the opposing phases of the 90°/270° and 270°/90° phase modulation signals in the multiple subframes as described, offset errors as well as dark current errors in the time-of-flight pixel circuits are canceled or removed in accordance with the teachings of the present invention. 
     In the various examples, the first modulation signal TXA and second modulation signal TXB may also be modulated at different frequencies in different subframes, which improves systematic errors, such as for example harmonic distortions or multipath artifacts in accordance with the teachings of the present invention. For instance, in one example, the first modulation signal TXA and second modulation signal TXB are configured to alternate between two different modulation frequencies f1 and f2 for each subframe. In other words, in one example, the first modulation signal TXA and second modulation signal TXB are configured to be modulated at a modulation frequency f1 during a first subframe, and then a modulation frequency f2 during a second subframe. Then, in a third subframe, the first modulation signal TXA and second modulation signal TXB are configured to be modulated at the modulation frequency f1, and then in a fourth subframe, the first modulation signal TXA and second modulation signal TXB are configured to be modulated at the modulation frequency f2, and so on. 
     To illustrate, a detailed example is described below in which offset errors and dark current errors are canceled or removed by modulating the first transfer transistor  320 A and the second transfer transistor  320 B with opposing 0°/180° and 180°/0° phase modulation signals as well as with 90°/270° and 270°/90° phase modulation signals over multiple subframes in an example time-of-flight pixel circuit  312  configured as pipelined pixel circuit such that there are two memory nodes (e.g., N=2) per output tap. In the example, the following relationships regarding time-of-flight pixel circuit  312  are given. In the example, it is appreciated that systematic errors, such as for example harmonic distortions or multipath artifacts, are also reduced by alternating the modulation frequencies between a first modulation frequency f1 and a second modulation frequency f2 each subframe. It is also noted that a timing diagram illustrating the example described below is provided in  FIG.  6   . 
         V   SF1-A   =V   A   =o   A +CG A1 ·( Q   0-f1 +DC A )  (3)
 
     In Equation (3), V SF1-A  represents the voltage V A  at the output tap on the left side of time-of-flight pixel circuit  312  during a first subframe SF1, o A  represents the offset error associated with the output tap on the left side of time-of-flight pixel circuit  312 , CG A1  represents the conversion gain associated with memory node CIA  334 A 1  of time-of-flight pixel circuit  312 , Q 0-f1  represents the 0° portion of charge (e.g., Q1) measured at a first modulation frequency f1, and DC A  represents the dark current error associated with the output tap on the left side of time-of-flight pixel circuit  312 . 
         V   SF1-B   =V   B   =o   B +CG B1 ·( Q   180-f1 +DC B )  (4)
 
     In Equation (4), V SF1-B  represents the voltage V B  at the output tap on the right side of time-of-flight pixel circuit  312  during the first subframe SF1, o B  represents the offset error associated with the output tap on the right side of time-of-flight pixel circuit  312 , CG B1  represents the conversion gain associated with memory node C 1 B  334 B 1  of time-of-flight pixel circuit  312 , Q 180-f1  represents the 180° portion of charge (e.g., Q2) measured at the first modulation frequency f1, and DC B  represents the dark current error associated with the output tap on the right side of time-of-flight pixel circuit  312 . 
         V   SF2-A   =V   A   =o   A +CG A1 ·( Q   0-f2 +DC A )  (5)
 
     In Equation (5), V SF2-A  represents the voltage V A  at the output tap on the left side of time-of-flight pixel circuit  312  during a second subframe SF1, o A  represents the offset error associated with the output tap on the left side of time-of-flight pixel circuit  312 , CG A1  represents the conversion gain associated with memory node C 1 A  334 A 1  of time-of-flight pixel circuit  312 , Q 0-f2  represents the 0° portion of charge (e.g., Q1) measured at a second modulation frequency f2, and DC A  represents the dark current error associated with the output tap on the left side of time-of-flight pixel circuit  312 . 
         V   SF2-B   =V   B   =o   B +CG B1 ·( Q   180-f2 +DC B )  (6)
 
     In Equation (6), V SF2-B  represents the voltage V B  at the output tap on the right side of time-of-flight pixel circuit  312  during the second subframe SF1, o B  represents the offset error associated with the output tap on the right side of time-of-flight pixel circuit  312 , CG B1  represents the conversion gain associated with memory node C 1 B  334 B 1  of time-of-flight pixel circuit  312 , Q 180-f2  represents the 180° portion of charge (e.g., Q2) measured at the second modulation frequency f2, and DC B  represents the dark current error associated with the output tap on the right side of time-of-flight pixel circuit  312 . 
         V   SF3-A   =V   A   =o   A +CG A2 ·( Q   90-f1 +DC A )  (7)
 
     In Equation (7), V SF3-A  represents the voltage V A  at the output tap on the left side of time-of-flight pixel circuit  312  during a third subframe SF3, o A  represents the offset error associated with the output tap on the left side of time-of-flight pixel circuit  312 , CG A2  represents the conversion gain associated with memory node C 2 A  334 A 2  of time-of-flight pixel circuit  312 , Q 90-f1  represents the 90° portion of charge (e.g., Q3) measured at the first modulation frequency f1, and DC A  represents the dark current error associated with the output tap on the left side of time-of-flight pixel circuit  312 . 
         V   SF3-B   =V   B   =o   B +CG B2 ·( Q   270-f1 +DC B )  (8)
 
     In Equation (8), V SF3-B  represents the voltage V B  at the output tap on the right side of time-of-flight pixel circuit  312  during the third subframe SF3, o B  represents the offset error associated with the output tap on the right side of time-of-flight pixel circuit  312 , CG B2  represents the conversion gain associated with memory node C 2 B  334 B 2  of time-of-flight pixel circuit  312 , Q 270-f1  represents the 270° portion of charge (e.g., Q4) measured at the first modulation frequency f1, and DC B  represents the dark current error associated with the output tap on the right side of time-of-flight pixel circuit  312 . 
         V   SF4-A   =V   A   =o   A +CG A2 ·( Q   90-f2 +DC A )  (9)
 
     In Equation (9), V SF4-A  represents the voltage V A  at the output tap on the left side of time-of-flight pixel circuit  312  during a fourth subframe SF4, o A  represents the offset error associated with the output tap on the left side of time-of-flight pixel circuit  312 , CG A2  represents the conversion gain associated with memory node C 2 A  334 A 2  of time-of-flight pixel circuit  312 , Q 90-f2  represents the 90° portion of charge (e.g., Q3) measured at the second modulation frequency f2, and DC A  represents the dark current error associated with the output tap on the left side of time-of-flight pixel circuit  312 . 
         V   SF4-B   =V   B   =o   B +CG B2 ·( Q   270-f2 +DC B )  (10)
 
     In Equation (10), V SF4-B  represents the voltage V B  at the output tap on the right side of time-of-flight pixel circuit  312  during the fourth subframe SF4, o B  represents the offset error associated with the output tap on the right side of time-of-flight pixel circuit  312 , CG B2  represents the conversion gain associated with memory node C 2 B  334 B 2  of time-of-flight pixel circuit  312 , Q 270-f2  represents the 270° portion of charge (e.g., Q4) measured at the second modulation frequency f2, and DC B  represents the dark current error associated with the output tap on the right side of time-of-flight pixel circuit  312 . 
         V   SF5-A   =V   A   =o   A +CG A1 ·( Q   180-f1 +DC A )  (11)
 
     In Equation (11), V SF5-A  represents the voltage V A  at the output tap on the left side of time-of-flight pixel circuit  312  during a fifth subframe SF5, o A  represents the offset error associated with the output tap on the left side of time-of-flight pixel circuit  312 , CG A1  represents the conversion gain associated with memory node C 1 A  334 A 1  of time-of-flight pixel circuit  312 , Q 180-f1  represents the 180° portion of charge (e.g., Q3) measured at the first modulation frequency f1, and DC A  represents the dark current error associated with the output tap on the left side of time-of-flight pixel circuit  312 . 
         V   SF5-B   =V   B   =o   B +CG B1 ·( Q   0-f1 +DC B )  (12)
 
     In Equation (12), V SF5-B  represents the voltage V B  at the output tap on the right side of time-of-flight pixel circuit  312  during the fifth subframe SF5, o B  represents the offset error associated with the output tap on the right side of time-of-flight pixel circuit  312 , CG B1  represents the conversion gain associated with memory node C 1 B  334 B 1  of time-of-flight pixel circuit  312 , Q 0-f1  represents the 0° portion of charge (e.g., Q1) measured at the first modulation frequency f1, and DC B  represents the dark current error associated with the output tap on the right side of time-of-flight pixel circuit  312 . 
         V   SF6-A   =V   A   =o   A +CG A1 ·( Q   180-f2 +DC A )  (13)
 
     In Equation (13), V SF6-A  represents the voltage V A  at the output tap on the left side of time-of-flight pixel circuit  312  during a sixth subframe SF6, o A  represents the offset error associated with the output tap on the left side of time-of-flight pixel circuit  312 , CG A1  represents the conversion gain associated with memory node C 1 A  334 A 1  of time-of-flight pixel circuit  312 , Q 180-f2  represents the 180° portion of charge (e.g., Q3) measured at the second modulation frequency f2, and DC A  represents the dark current error associated with the output tap on the left side of time-of-flight pixel circuit  312 . 
         V   SF6-B   =V   B   =o   B +CG B1 ·( Q   0-f2 +DC B )  (14)
 
     In Equation (14), V SF6-B  represents the voltage V B  at the output tap on the right side of time-of-flight pixel circuit  312  during the sixth subframe SF6, o B  represents the offset error associated with the output tap on the right side of time-of-flight pixel circuit  312 , CG B1  represents the conversion gain associated with memory node C 1 B  334 B 1  of time-of-flight pixel circuit  312 , Q 0-f2  represents the 0° portion of charge (e.g., Q1) measured at the second modulation frequency f2, and DC B  represents the dark current error associated with the output tap on the right side of time-of-flight pixel circuit  312 . 
         V   SF7-A   =V   A   =o   A +CG A2 ·( Q   270-f1 +DC A )  (15)
 
     In Equation (15), V SF7-A  represents the voltage V A  at the output tap on the left side of time-of-flight pixel circuit  312  during a seventh subframe SF7, o A  represents the offset error associated with the output tap on the left side of time-of-flight pixel circuit  312 , CG A2  represents the conversion gain associated with memory node C 1 A  334 A 2  of time-of-flight pixel circuit  312 , Q 270-f1  represents the 270° portion of charge (e.g., Q4) measured at the first modulation frequency f1, and DC A  represents the dark current error associated with the output tap on the left side of time-of-flight pixel circuit  312 . 
         V   SF7-B   =V   B   =o   B +CG B2 ·( Q   90-f1 +DC B )  (16)
 
     In Equation (16), V SF7-B  represents the voltage V B  at the output tap on the right side of time-of-flight pixel circuit  312  during the seventh subframe SF7, o B  represents the offset error associated with the output tap on the right side of time-of-flight pixel circuit  312 , CG B2  represents the conversion gain associated with memory node C 2 B  334 B 2  of time-of-flight pixel circuit  312 , Q 90-f1  represents the 90° portion of charge (e.g., Q3) measured at the first modulation frequency f1, and DC B  represents the dark current error associated with the output tap on the right side of time-of-flight pixel circuit  312 . 
         V   SF8-A   =V   A   =o   A +CG A2 ·( Q   270-f2 +DC A )  (17)
 
     In Equation (17), V SF8-A  represents the voltage V A  at the output tap on the left side of time-of-flight pixel circuit  312  during an eighth subframe SF8, o A  represents the offset error associated with the output tap on the left side of time-of-flight pixel circuit  312 , CG A2  represents the conversion gain associated with memory node C 1 A  334 A 2  of time-of-flight pixel circuit  312 , Q 270-f2  represents the 270° portion of charge (e.g., Q4) measured at the second modulation frequency f2, and DC A  represents the dark current error associated with the output tap on the left side of time-of-flight pixel circuit  312 . 
         V   SF8-B   =V   B   =o   B +CG B2 ·( Q   90-f2 +DC B )  (18)
 
     In Equation (18), V SF8-B  represents the voltage V B  at the output tap on the right side of time-of-flight pixel circuit  312  during the eighth subframe SF7, o B  represents the offset error associated with the output tap on the right side of time-of-flight pixel circuit  312 , CG B2  represents the conversion gain associated with memory node C 2 B  334 B 2  of time-of-flight pixel circuit  312 , Q 90-f2  represents the 90° portion of charge (e.g., Q3) measured at the second modulation frequency f2, and DC B  represents the dark current error associated with the output tap on the right side of time-of-flight pixel circuit  312 . 
     With regard to the first modulation frequency f1 relationships given in Equations (3), (4), (7), (8), (11), (12), (15), and (16) above, the phase φ f1  measured at the first modulation frequency f1 from time-of-flight pixel array  312  can be determined according to Equation (19) as follows: 
     
       
         
           
             
               
                 
                   
                     φ 
                     
                       f 
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   5 
                                 
                                 - 
                                 A 
                               
                             
                             - 
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   1 
                                 
                                 - 
                                 A 
                               
                             
                           
                           
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   3 
                                 
                                 - 
                                 A 
                               
                             
                             - 
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   7 
                                 
                                 - 
                                 A 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   1 
                                 
                                 - 
                                 B 
                               
                             
                             - 
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   5 
                                 
                                 - 
                                 B 
                               
                             
                           
                           
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   7 
                                 
                                 - 
                                 B 
                               
                             
                             - 
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   3 
                                 
                                 - 
                                 B 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   19 
                   ) 
                 
               
             
           
         
       
     
     Similarly, with regard to the second modulation frequency f2 relationships given in Equations (5), (6), (9), (10), (13), (14), (17), and (18) above, the phase φ f2  measured at the second modulation frequency f2 from time-of-flight pixel array  312  can be determined according to Equation (20) as follows: 
     
       
         
           
             
               
                 
                   
                     φ 
                     
                       f 
                       ⁢ 
                       2 
                     
                   
                   = 
                   
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   6 
                                 
                                 - 
                                 A 
                               
                             
                             - 
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   2 
                                 
                                 - 
                                 A 
                               
                             
                           
                           
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   4 
                                 
                                 - 
                                 A 
                               
                             
                             - 
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   8 
                                 
                                 - 
                                 A 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   2 
                                 
                                 - 
                                 B 
                               
                             
                             - 
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   6 
                                 
                                 - 
                                 B 
                               
                             
                           
                           
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   8 
                                 
                                 - 
                                 B 
                               
                             
                             - 
                             
                               V 
                               
                                 
                                   SF 
                                   ⁢ 
                                   6 
                                 
                                 - 
                                 B 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   20 
                   ) 
                 
               
             
           
         
       
     
     With regard to the phase measurements at the first modulation frequency f1, substituting Equations (3), (4), (7), (8), (11), (12), (15), and (16) into Equation (19) results in: 
     
       
         
           
             
               
                 
                   
                     φ 
                     
                       f 
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         o 
                                         A 
                                       
                                       + 
                                       
                                         C 
                                         ⁢ 
                                         
                                           
                                             G 
                                             
                                               A 
                                               ⁢ 
                                               1 
                                             
                                           
                                           · 
                                           
                                             ( 
                                             
                                               
                                                 Q 
                                                 
                                                   
                                                     1 
                                                     ⁢ 
                                                     8 
                                                     ⁢ 
                                                     0 
                                                   
                                                   - 
                                                   
                                                     f 
                                                     ⁢ 
                                                     1 
                                                   
                                                 
                                               
                                               + 
                                               
                                                 D 
                                                 ⁢ 
                                                 
                                                   C 
                                                   A 
                                                 
                                               
                                             
                                             ) 
                                           
                                         
                                       
                                     
                                     ) 
                                   
                                   - 
                                 
                               
                             
                             
                               
                                 
                                   ( 
                                   
                                     
                                       o 
                                       A 
                                     
                                     + 
                                     
                                       C 
                                       ⁢ 
                                       
                                         
                                           G 
                                           
                                             A 
                                             ⁢ 
                                             1 
                                           
                                         
                                         · 
                                         
                                           ( 
                                           
                                             
                                               Q 
                                               
                                                 0 
                                                 - 
                                                 
                                                   f 
                                                   ⁢ 
                                                   1 
                                                 
                                               
                                             
                                             + 
                                             
                                               D 
                                               ⁢ 
                                               
                                                 C 
                                                 A 
                                               
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         o 
                                         A 
                                       
                                       + 
                                       
                                         C 
                                         ⁢ 
                                         
                                           
                                             G 
                                             
                                               A 
                                               ⁢ 
                                               2 
                                             
                                           
                                           · 
                                           
                                             ( 
                                             
                                               
                                                 Q 
                                                 
                                                   
                                                     9 
                                                     ⁢ 
                                                     0 
                                                   
                                                   - 
                                                   
                                                     f 
                                                     ⁢ 
                                                     1 
                                                   
                                                 
                                               
                                               + 
                                               
                                                 D 
                                                 ⁢ 
                                                 
                                                   C 
                                                   A 
                                                 
                                               
                                             
                                             ) 
                                           
                                         
                                       
                                     
                                     ) 
                                   
                                   - 
                                 
                               
                             
                             
                               
                                 
                                   ( 
                                   
                                     
                                       o 
                                       A 
                                     
                                     + 
                                     
                                       C 
                                       ⁢ 
                                       
                                         
                                           G 
                                           
                                             A 
                                             ⁢ 
                                             2 
                                           
                                         
                                         · 
                                         
                                           ( 
                                           
                                             
                                               Q 
                                               
                                                 
                                                   2 
                                                   ⁢ 
                                                   7 
                                                   ⁢ 
                                                   0 
                                                 
                                                 - 
                                                 
                                                   f 
                                                   ⁢ 
                                                   1 
                                                 
                                               
                                             
                                             + 
                                             
                                               D 
                                               ⁢ 
                                               
                                                 C 
                                                 A 
                                               
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         o 
                                         B 
                                       
                                       + 
                                       
                                         C 
                                         ⁢ 
                                         
                                           
                                             G 
                                             
                                               B 
                                               ⁢ 
                                               1 
                                             
                                           
                                           · 
                                           
                                             ( 
                                             
                                               
                                                 Q 
                                                 
                                                   
                                                     1 
                                                     ⁢ 
                                                     8 
                                                     ⁢ 
                                                     0 
                                                   
                                                   - 
                                                   
                                                     f 
                                                     ⁢ 
                                                     1 
                                                   
                                                 
                                               
                                               + 
                                               
                                                 D 
                                                 ⁢ 
                                                 
                                                   C 
                                                   B 
                                                 
                                               
                                             
                                             ) 
                                           
                                         
                                       
                                     
                                     ) 
                                   
                                   - 
                                 
                               
                             
                             
                               
                                 
                                   ( 
                                   
                                     
                                       o 
                                       B 
                                     
                                     + 
                                     
                                       C 
                                       ⁢ 
                                       
                                         
                                           G 
                                           
                                             B 
                                             ⁢ 
                                             1 
                                           
                                         
                                         · 
                                         
                                           ( 
                                           
                                             
                                               Q 
                                               
                                                 0 
                                                 - 
                                                 
                                                   f 
                                                   ⁢ 
                                                   1 
                                                 
                                               
                                             
                                             + 
                                             
                                               D 
                                               ⁢ 
                                               
                                                 C 
                                                 B 
                                               
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         o 
                                         B 
                                       
                                       + 
                                       
                                         C 
                                         ⁢ 
                                         
                                           
                                             G 
                                             
                                               B 
                                               ⁢ 
                                               2 
                                             
                                           
                                           · 
                                           
                                             ( 
                                             
                                               
                                                 Q 
                                                 
                                                   
                                                     9 
                                                     ⁢ 
                                                     0 
                                                   
                                                   - 
                                                   
                                                     f 
                                                     ⁢ 
                                                     1 
                                                   
                                                 
                                               
                                               + 
                                               
                                                 D 
                                                 ⁢ 
                                                 
                                                   C 
                                                   B 
                                                 
                                               
                                             
                                             ) 
                                           
                                         
                                       
                                     
                                     ) 
                                   
                                   - 
                                 
                               
                             
                             
                               
                                 
                                   ( 
                                   
                                     
                                       o 
                                       B 
                                     
                                     + 
                                     
                                       C 
                                       ⁢ 
                                       
                                         
                                           G 
                                           
                                             B 
                                             ⁢ 
                                             2 
                                           
                                         
                                         · 
                                         
                                           ( 
                                           
                                             
                                               Q 
                                               
                                                 
                                                   2 
                                                   ⁢ 
                                                   7 
                                                   ⁢ 
                                                   0 
                                                 
                                                 - 
                                                 
                                                   f 
                                                   ⁢ 
                                                   1 
                                                 
                                               
                                             
                                             + 
                                             
                                               D 
                                               ⁢ 
                                               
                                                 C 
                                                 B 
                                               
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   21 
                   ) 
                 
               
             
           
         
       
     
     After expanding Equation (21) above, all of the offset error terms (o A −o A ), (o A −o A ), (o B −o B ), and (o B −o B ) cancel each other out. Similarly, all of the dark current error terms (CG A1 ·DC A )−(CG A1 ·DC A ), (CG A2 ·DC A )−(CG A2 ·DC A ), (CG B1 ·DC B )−(CG B1 ·DC B ), and (CG B2 ·DC B )−(CG B2 ·DC B ) cancel each other out. 
     After removing these canceled offset error terms (o A , o B ) and dark current error terms (CG A1 ·DC A , CG A2 ·DC A , CG B1 ·DC B , CG B2 ·DC B ) shown in Equation (21), the phase φ f1  measured at the first modulation frequency f1 from time-of-flight pixel array  312  can be determined according to Equation (22) as follows: 
     
       
         
           
             
               
                 
                   
                     φ 
                     
                       f 
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               C 
                               ⁢ 
                               
                                 G 
                                 
                                   A 
                                   ⁢ 
                                   1 
                                 
                               
                             
                             
                               C 
                               ⁢ 
                               
                                 G 
                                 
                                   A 
                                   ⁢ 
                                   2 
                                 
                               
                             
                           
                           · 
                           
                             
                               
                                 Q 
                                 
                                   
                                     1 
                                     ⁢ 
                                     8 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               - 
                               
                                 Q 
                                 
                                   0 
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     1 
                                   
                                 
                               
                             
                             
                               
                                 Q 
                                 
                                   
                                     9 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               - 
                               
                                 Q 
                                 
                                   
                                     2 
                                     ⁢ 
                                     7 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     1 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               C 
                               ⁢ 
                               
                                 G 
                                 
                                   B 
                                   ⁢ 
                                   1 
                                 
                               
                             
                             
                               C 
                               ⁢ 
                               
                                 G 
                                 
                                   B 
                                   ⁢ 
                                   2 
                                 
                               
                             
                           
                           · 
                           
                             
                               
                                 Q 
                                 
                                   
                                     1 
                                     ⁢ 
                                     8 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               - 
                               
                                 Q 
                                 
                                   0 
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     1 
                                   
                                 
                               
                             
                             
                               
                                 Q 
                                 
                                   
                                     9 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               - 
                               
                                 Q 
                                 
                                   
                                     2 
                                     ⁢ 
                                     7 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     1 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   22 
                   ) 
                 
               
             
           
         
       
     
     It is appreciated that after the offset errors and dark current errors have been removed or compensated for using the modulation signals with opposing phases in multiple subframes as discussed above, the terms (CG A1 /CG A2 ) and (CG B1 /CG B2 ) are remaining conversion gain errors. Assuming the relative conversion gain values CG A1 , CG B1 , CG A2 , and CG B2  are acceptable, the remaining conversion gain error terms (CG A1 /CG A2 ) and (CG B1 /CG B2 ) can be tolerated. 
     Similarly, with regard to the phase measurements at the second modulation frequency f2, substituting Equations (5), (6), (9), (10), (13), (14), (17), and (18) into Equation (20) results in: 
     
       
         
           
             
               
                 
                   
                     φ 
                     
                       f 
                       ⁢ 
                       2 
                     
                   
                   = 
                   
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         o 
                                         A 
                                       
                                       + 
                                       
                                         C 
                                         ⁢ 
                                         
                                           
                                             G 
                                             
                                               A 
                                               ⁢ 
                                               1 
                                             
                                           
                                           · 
                                           
                                             ( 
                                             
                                               
                                                 Q 
                                                 
                                                   
                                                     1 
                                                     ⁢ 
                                                     8 
                                                     ⁢ 
                                                     0 
                                                   
                                                   - 
                                                   
                                                     f 
                                                     ⁢ 
                                                     2 
                                                   
                                                 
                                               
                                               + 
                                               
                                                 D 
                                                 ⁢ 
                                                 
                                                   C 
                                                   A 
                                                 
                                               
                                             
                                             ) 
                                           
                                         
                                       
                                     
                                     ) 
                                   
                                   - 
                                 
                               
                             
                             
                               
                                 
                                   ( 
                                   
                                     
                                       o 
                                       A 
                                     
                                     + 
                                     
                                       C 
                                       ⁢ 
                                       
                                         
                                           G 
                                           
                                             A 
                                             ⁢ 
                                             1 
                                           
                                         
                                         · 
                                         
                                           ( 
                                           
                                             
                                               Q 
                                               
                                                 0 
                                                 - 
                                                 
                                                   f 
                                                   ⁢ 
                                                   2 
                                                 
                                               
                                             
                                             + 
                                             
                                               D 
                                               ⁢ 
                                               
                                                 C 
                                                 A 
                                               
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         o 
                                         A 
                                       
                                       + 
                                       
                                         C 
                                         ⁢ 
                                         
                                           
                                             G 
                                             
                                               A 
                                               ⁢ 
                                               2 
                                             
                                           
                                           · 
                                           
                                             ( 
                                             
                                               
                                                 Q 
                                                 
                                                   
                                                     9 
                                                     ⁢ 
                                                     0 
                                                   
                                                   - 
                                                   
                                                     f 
                                                     ⁢ 
                                                     2 
                                                   
                                                 
                                               
                                               + 
                                               
                                                 D 
                                                 ⁢ 
                                                 
                                                   C 
                                                   A 
                                                 
                                               
                                             
                                             ) 
                                           
                                         
                                       
                                     
                                     ) 
                                   
                                   - 
                                 
                               
                             
                             
                               
                                 
                                   ( 
                                   
                                     
                                       o 
                                       A 
                                     
                                     + 
                                     
                                       C 
                                       ⁢ 
                                       
                                         
                                           G 
                                           
                                             A 
                                             ⁢ 
                                             2 
                                           
                                         
                                         · 
                                         
                                           ( 
                                           
                                             
                                               Q 
                                               
                                                 
                                                   2 
                                                   ⁢ 
                                                   7 
                                                   ⁢ 
                                                   0 
                                                 
                                                 - 
                                                 
                                                   f 
                                                   ⁢ 
                                                   2 
                                                 
                                               
                                             
                                             + 
                                             
                                               D 
                                               ⁢ 
                                               
                                                 C 
                                                 A 
                                               
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         o 
                                         B 
                                       
                                       + 
                                       
                                         C 
                                         ⁢ 
                                         
                                           
                                             G 
                                             
                                               B 
                                               ⁢ 
                                               1 
                                             
                                           
                                           · 
                                           
                                             ( 
                                             
                                               
                                                 Q 
                                                 
                                                   
                                                     1 
                                                     ⁢ 
                                                     8 
                                                     ⁢ 
                                                     0 
                                                   
                                                   - 
                                                   
                                                     f 
                                                     ⁢ 
                                                     2 
                                                   
                                                 
                                               
                                               + 
                                               
                                                 D 
                                                 ⁢ 
                                                 
                                                   C 
                                                   B 
                                                 
                                               
                                             
                                             ) 
                                           
                                         
                                       
                                     
                                     ) 
                                   
                                   - 
                                 
                               
                             
                             
                               
                                 
                                   ( 
                                   
                                     
                                       o 
                                       B 
                                     
                                     + 
                                     
                                       C 
                                       ⁢ 
                                       
                                         
                                           G 
                                           
                                             B 
                                             ⁢ 
                                             1 
                                           
                                         
                                         · 
                                         
                                           ( 
                                           
                                             
                                               Q 
                                               
                                                 0 
                                                 - 
                                                 
                                                   f 
                                                   ⁢ 
                                                   2 
                                                 
                                               
                                             
                                             + 
                                             
                                               D 
                                               ⁢ 
                                               
                                                 C 
                                                 B 
                                               
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         o 
                                         B 
                                       
                                       + 
                                       
                                         C 
                                         ⁢ 
                                         
                                           
                                             G 
                                             
                                               B 
                                               ⁢ 
                                               2 
                                             
                                           
                                           · 
                                           
                                             ( 
                                             
                                               
                                                 Q 
                                                 
                                                   
                                                     9 
                                                     ⁢ 
                                                     0 
                                                   
                                                   - 
                                                   
                                                     f 
                                                     ⁢ 
                                                     2 
                                                   
                                                 
                                               
                                               + 
                                               
                                                 D 
                                                 ⁢ 
                                                 
                                                   C 
                                                   B 
                                                 
                                               
                                             
                                             ) 
                                           
                                         
                                       
                                     
                                     ) 
                                   
                                   - 
                                 
                               
                             
                             
                               
                                 
                                   ( 
                                   
                                     
                                       o 
                                       B 
                                     
                                     + 
                                     
                                       C 
                                       ⁢ 
                                       
                                         
                                           G 
                                           
                                             B 
                                             ⁢ 
                                             2 
                                           
                                         
                                         · 
                                         
                                           ( 
                                           
                                             
                                               Q 
                                               
                                                 
                                                   2 
                                                   ⁢ 
                                                   7 
                                                   ⁢ 
                                                   0 
                                                 
                                                 - 
                                                 
                                                   f 
                                                   ⁢ 
                                                   2 
                                                 
                                               
                                             
                                             + 
                                             
                                               D 
                                               ⁢ 
                                               
                                                 C 
                                                 B 
                                               
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   23 
                   ) 
                 
               
             
           
         
       
     
     After expanding Equation (23) above, all of the offset error terms (o A −o A ), (o A −o A ), (o B −o B ), and (o B −o B ) cancel each other out. Similarly, all of the dark current error terms (CG A1 ·DC A )−(CG A1 ·DC A ), (CG A2 ·DC A )−(CG A2 ·DC A ), (CG B1 ·DC B )−(CG B1 ·DC B ), and (CG B2 ·DC B )−(CG B2 ·DC B ) cancel each other out. 
     After removing these canceled offset error terms (o A , o B ) and dark current error terms (CG A1 ·DC A , CG A2 ·DC A , CG B1 ·DC B , CG B2 ·DC B ) shown in Equation (23), the phase φ f2  measured at the first modulation frequency f1 from time-of-flight pixel array  312  can be determined according to Equation (24) as follows: 
     
       
         
           
             
               
                 
                   
                     φ 
                     
                       f 
                       ⁢ 
                       2 
                     
                   
                   = 
                   
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               C 
                               ⁢ 
                               
                                 G 
                                 
                                   A 
                                   ⁢ 
                                   1 
                                 
                               
                             
                             
                               C 
                               ⁢ 
                               
                                 G 
                                 
                                   A 
                                   ⁢ 
                                   2 
                                 
                               
                             
                           
                           · 
                           
                             
                               
                                 Q 
                                 
                                   
                                     1 
                                     ⁢ 
                                     8 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     2 
                                   
                                 
                               
                               - 
                               
                                 Q 
                                 
                                   0 
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     2 
                                   
                                 
                               
                             
                             
                               
                                 Q 
                                 
                                   
                                     9 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     2 
                                   
                                 
                               
                               - 
                               
                                 Q 
                                 
                                   
                                     2 
                                     ⁢ 
                                     7 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     2 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               C 
                               ⁢ 
                               
                                 G 
                                 
                                   B 
                                   ⁢ 
                                   1 
                                 
                               
                             
                             
                               C 
                               ⁢ 
                               
                                 G 
                                 
                                   B 
                                   ⁢ 
                                   2 
                                 
                               
                             
                           
                           · 
                           
                             
                               
                                 Q 
                                 
                                   
                                     1 
                                     ⁢ 
                                     8 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     2 
                                   
                                 
                               
                               - 
                               
                                 Q 
                                 
                                   0 
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     2 
                                   
                                 
                               
                             
                             
                               
                                 Q 
                                 
                                   
                                     9 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     2 
                                   
                                 
                               
                               - 
                               
                                 Q 
                                 
                                   
                                     2 
                                     ⁢ 
                                     7 
                                     ⁢ 
                                     0 
                                   
                                   - 
                                   
                                     f 
                                     ⁢ 
                                     2 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   24 
                   ) 
                 
               
             
           
         
       
     
     It is appreciated that after the offset errors and dark current errors have been removed or compensated for using the modulation signals with opposing phases in multiple subframes as discussed above, the terms (CG A1 /CG A2 ) and (CG B1 /CG B2 ) are remaining conversion gain errors. Assuming the relative conversion gain values CG A1 , CG B1 , CG A2 , and CG B2  are acceptable, the remaining conversion gain error terms (CG A1 /CG A2 ) and (CG B1 /CG B2 ) can be tolerated. 
       FIG.  4    is a schematic illustrating another example of a time-of-flight pixel circuit  412  in accordance with the teachings of the present invention. It is appreciated that the example of time-of-flight pixel circuit  412  illustrated in  FIG.  4    shares many similarities with the time-of-flight pixel circuit  312  shown in  FIG.  3   , and/or may be an example of the time-of-flight pixel circuits  112  shown in  FIG.  1   , and that similarly named and numbered elements described above are coupled and function similarly below. 
     It is appreciated that one of the differences between the example of time-of-flight pixel circuit  412  illustrated in  FIG.  4    and the example time-of-flight pixel circuit  312  shown in  FIG.  3    is that example time-of-flight pixel circuit  412  of  FIG.  4    has a single output tap per time-of-flight pixel circuit  412  instead of two output taps per time-of-flight pixel circuit  312  as shown in  FIG.  3   . In the example, the output VA may be read out from the one output tap on the left side of time-of-flight pixel circuit  412 . As such, the example time-of-flight pixel circuit  412  illustrated in  FIG.  4    is a simplified version of time-of-flight pixel circuit  312  of  FIG.  3    and does not include a corresponding second floating diffusion FDB  322 B, second reset transistor  324 B, a second plurality of sample and hold transistors  326 B 1 ,  326 B 2 , . . . ,  326 BN, a second plurality of memory nodes C 1 B  334 B 1 , C 2 B  334 B 2 , . . . , CNB  334 BN, a second plurality of capacitors  328 B 1 ,  328 B 2 , . . . ,  328 BN, a second plurality of readout transistors  336 B 1 ,  336 B 2 , . . . ,  336 BN, a second output source follower transistor  330 B, or a second row select transistor  332 B compared to the example time-of-flight pixel circuit  312  depicted in  FIG.  3   . 
     Therefore, as illustrated in the example depicted in  FIG.  4   , example time-of-flight pixel circuit  412  includes a photodiode  418  configured to photogenerate charge in response to incident light. In one example, the light that is incident on photodiode  418  is the reflected modulated light  108  that is reflected from an object  106  as described in  FIG.  1   . The first floating diffusion FDA  422 A is configured to store a first portion of charge photogenerated in the photodiode  418 . In the example, a first reset transistor  424 A is coupled between a supply rail and the first floating diffusion FDA  422 A. In the various examples, the first reset transistor  424 A is configured to reset the first floating diffusion FDA  422 A in response to a first reset signal RSTA. In one example, the first reset transistor  424 A may also act as an overflow transistor. In such an example, the first reset transistor  424 A may be operated in such a way that excess carriers generated by photodiode  418  may be guided to the power supply by first reset transistor  424 A or in a way such that the photosensitivity of photodiode  418  is disabled. 
     In the example, the first transfer transistor  420 A is configured to transfer a first portion of charge from the photodiode  418  to the first floating diffusion FDA  422 A in response to a first modulation signal TXA. In one example, the first modulation signal TXA may be an example of one of the phase modulation signals described in  FIG.  2   . A second transfer transistor  420 B is configured to transfer a second portion of charge from the photodiode  418  in response to a second modulation signal TXB. In the example, the second modulation signal TXB may also be an example of one of the phase modulation signals described in  FIG.  2   . 
     As shown in the depicted example, time-of-flight pixel circuit  412  also includes a first plurality of N capacitors  428 A 1 ,  428 A 2 , . . . ,  428 AN, which provide a first plurality of N memory nodes C 1 A  434 A 1 , C 2 A  434 A 2 , . . . , CNA  434 AN. In the example, each one of a first plurality of N sample and hold transistors  426 A 1 ,  426 A 2 , . . . ,  426 AN is coupled between the first floating diffusion FDA  422 A and a respective one of the first plurality of N memory nodes CIA  434 A 1 , C 2 A  434 A 2 , . . . , CNA  434 AN as shown. 
     As shown in the depicted example, time-of-flight pixel circuit  412  further includes a first output source follower transistor  430 A. Each one of a first plurality of N readout transistors  436 A 1 ,  436 A 2 , . . . ,  436 AN is coupled between a respective one of the first plurality of N memory nodes C 1 A  434 A 1 , C 2 A  434 A 2 , . . . , CNA  434 AN and a gate of the first output source follower transistor  430 A. A first row select transistor  432 A is coupled to each one of the first plurality of N readout transistors  436 A 1 ,  436 A 2 , . . . ,  436 AN. In the example, the output of the first row select transistor  432 A may be considered to be the single output tap of the time-of-flight pixel circuit  412  from which the output VA from first output source follower transistor  430 A may be read out. 
     Similar to the example time-of-flight pixel circuit  312  of  FIG.  3   , the time-of-flight pixel circuit  412  of  FIG.  4    also may be configured as a pipelined pixel circuit such that there are two memory nodes (e.g., N=2) coupled to the output tap. To illustrate, in a pipelined pixel example with N=2, there are two memory nodes (e.g.,  434 A 1 / 434 A 2 ), two sample and hold transistors (e.g.,  426 A 1 / 426 A 2 ), and two readout transistors (e.g.,  436 A 1 / 436 A 2 ) coupled to the output tap. In the example pipelined pixel circuit configuration, one of the memory nodes can be coupled to the floating diffusion through a respective sample and hold transistor to acquire and store (e.g., sample and hold) the portion of charge from the floating diffusion, while the other memory node is read out with the other memory node being coupled to the gate terminal of the output source follower transistor through a respective readout transistor. 
     In another example, the time-of-flight pixel circuit  412  may be configured as a burst pixel circuit with two or more memory nodes coupled output tap (e.g., N=4 or N=8). In a burst pixel circuit configuration, the depth of frame may be formed with, for example, N=4 or N=8 subframes with N=4 or N=8 memory nodes, which allows the required analog to digital converter (ADC) readout speed, chip-size, and power consumption requirements to be reduced drastically in accordance with the teachings of the present invention. In operation, all of the memory nodes (e.g.,  434 A 1 ,  434 A 2 , . . . ,  434 AN) are coupled to sample and hold the portion of charge from the floating diffusion (e.g.,  422 A) through the respective sample and hold transistors (e.g.,  426 A 1 ,  426 A 2 , . . . ,  426 AN). 
     After all of the memory nodes have received the portions of the charge from the floating diffusion, the portions of the charge may then be read out from the memory nodes through the respective readout transistors (e.g.,  436 A 1 ,  436 A 2 , . . . ,  436 AN). It is appreciated that burst pixel circuit configurations may be utilized in situations where readout speeds are too slow for pipelined pixel circuit configurations. For instance, in an example with N=8, 8 subframes of phase charge information may be sampled and held in 8 memory nodes. After the 8 subframes of phase charge information have been stored in the respective 8 memory nodes, the 8 memory nodes may then be subsequently read out with reduced readout speed requirements after the integration period as there is typically a gap between integration exposure periods. 
     In one example, the portions of the charge from the floating diffusion  422 A include a plurality of subframe portions. In the example, each of the plurality of subframe portions of charge is transferred into the floating diffusion  422 A in response to the first and modulation signal TXA having a specific phase (e.g., 0°, 90°, 180°, 270°) as well as a specific frequency setting (e.g., f1, f2) combination for each subframe. In the various examples, each one of the plurality of subframe portions is configured to be sampled and held in sequence (e.g., one after another in time) into a respective one of the memory nodes (e.g.,  434 A 1 ,  434 A 2 , . . . ,  434 AN) from the floating diffusion  422 A through a respective one of the sample and hold transistors (e.g.,  426 A 1 ,  426 A 2 , . . . ,  426 AN). 
     After all of the memory nodes have received the plurality of subframe portions of the charge from the floating diffusion  422 A, each one of the plurality of subframe portions of charge may then be read out from the memory nodes through the respective readout transistors (e.g.,  436 A 1 ,  436 A 2 , . . . ,  436 AN). In one example, each of the plurality of subframe portions of the phase portions of charge that is stored in the memory nodes (e.g.,  434 A 1 ,  434 A 2 , . . . ,  434 AN) is configured to be read out in sequence (e.g., one after another in time) through a respective one the readout transistors (e.g.,  436 A 1 ,  436 A 2 , . . . ,  436 AN) after all of the memory nodes have sampled and held the plurality of subframe portions of the phase portions of the charge from the floating diffusion (e.g.,  422 A). 
     In operation, the first modulation signal TXA and second modulation signal TXB are configured to modulate the first transfer transistor  420 A and the second transfer transistor  420 B. In the various examples, the first modulation signal TXA and second modulation signal TXB are modulation signals that are 180° out of phase or inverted versions of each other during the multiple subframes in which the time-of-flight pixel circuit  412  is modulated. For instance, in one example, in a first subframe, the first modulation signal TXA may be the 0° phase modulation signal  214 A while the second modulation signal TXA is therefore the 180° phase modulation signal  214 B. In that example, in another subframe, the first modulation signal TXA and the second modulation signal TXB are inverted relative to their respective signals in the first subframe. In other words, in that example, in the other subframe, the first modulation signal TXA is the 180° phase modulation signal  216 B while the second modulation signal TXB is the 0° phase modulation signal  214 A. In the various examples, it is appreciated that the first and second transfer transistors  420 A and  420 B may also be modulated with 90°/270° phase modulation signals as well as with the opposing phases of 270°/90° phase modulation signals in multiple subframes. 
     In the various examples, the first modulation signal TXA and second modulation signal TXB may also be modulated at different frequencies in different subframes, which improves systematic errors, such as for example harmonic distortions or multipath artifacts in accordance with the teachings of the present invention. For instance, in one example, the first modulation signal TXA and second modulation signal TXB are configured to alternate between two different modulation frequencies f1 and f2 for each subframe. In other words, in one example, the first modulation signal TXA and second modulation signal TXB are configured to be modulated at a modulation frequency f1 during a first subframe, and then a modulation frequency f2 during a second subframe. Then, in a third subframe, the first modulation signal TXA and second modulation signal TXB are configured to be modulated at the modulation frequency f1, and then in a fourth subframe, the first modulation signal TXA and second modulation signal TXB are configured to be modulated at the modulation frequency f2, and so on. 
     Similar to the time-of-flight pixel circuit  312  example depicted in  FIG.  3   , by modulating the first transfer transistor  420 A and the second transfer transistor  420 B with the first modulation signal TXA and second modulation signal TXB with the opposing phases of the 0°/180° and 180°/0° phase modulation signals as well as with the opposing phases of the 90°/270° and 270°/90° phase modulation signals in the multiple subframes as described, offset errors as well as dark current errors in the time-of-flight pixel circuits are canceled or removed in accordance with the teachings of the present invention. 
     To illustrate, an example is described below in which N=2 and offset errors and dark current errors are canceled or removed by modulating the first transfer transistor  420 A and the second transfer transistor  420 B with opposing 0°/180° and 180°/0° phase modulation signals as well as with 90°/270° and 270°/90° phase modulation signals over multiple subframes. In the example, the following relationships regarding time-of-flight pixel circuit  412  are given. 
         V   SF1-A   =V   A   =o   A +CG A1 ·( Q   0 +DC A )  (25)
 
     In Equation (25), V SF1-A  represents the voltage V A  at the output tap of time-of-flight pixel circuit  412  during a first subframe SF1, o A  represents the offset error associated with the output tap of time-of-flight pixel circuit  412 , CG A1  represents the conversion gain associated with memory node CIA  434 A 1  of time-of-flight pixel circuit  412 , Q 0  represents the 0° portion of charge (e.g., Q1), and DC A  represents the dark current error associated with the output tap of pixel circuit  12 . 
         V   SF2-A   =V   A   =o   A +CG A2 ·( Q   90 +DC A )  (26)
 
     In Equation (26), V SF2A  represents the voltage V A  at the output tap of time-of-flight pixel circuit  412  during a second subframe SF2, o A  represents the offset error associated with the output tap of time-of-flight pixel circuit  412 , CG A2  represents the conversion gain associated with memory node C 2 A  434 A 2  of time-of-flight pixel circuit  412 , Q 90  represents the 90° portion of charge (e.g., Q2), and DC A  represents the dark current error associated with the output tap of time-of-flight pixel circuit  412 . 
         V   SF3-A   =V   A   =o   A +CG A1 ·( Q   180 +DC A )  (27)
 
     In Equation (27), V SF3-A  represents the voltage V A  at the output tap of time-of-flight pixel circuit  412  during a third subframe SF3, o A  represents the offset error associated with the output tap of time-of-flight pixel circuit  412 , CG A1  represents the conversion gain associated with memory node C 1 A  434 A 1  of time-of-flight pixel circuit  412 , Q 180  represents the 180° portion of charge (e.g., Q3), and DC A  represents the dark current error associated with the output tap of time-of-flight pixel circuit  412 . 
         V   SF4-A   =V   A   =o   A +CG A2 ·( Q   270 +DC A )  (28)
 
     In Equation (28), V SF4-A  represents the voltage V A  at the output tap of time-of-flight pixel circuit  412  during a fourth subframe SF4, o A  represents the offset error associated with the output tap of time-of-flight pixel circuit  412 , CG A2  represents the conversion gain associated with memory node C 2 A  434 A 2  of time-of-flight pixel circuit  412 , Q 270  represents the 270° portion of charge (e.g., Q4), and DC A  represents the dark current error associated with the output tap on the right side of time-of-flight pixel circuit  412 . 
     With the relationships given in Equations (25)-(28), the measured phase P from pixel circuit  412  can be determined as follows: 
     
       
         
           
             
               
                 
                   φ 
                   = 
                   
                     
                       1 
                       2 
                     
                     · 
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                       ( 
                       
                         
                           
                             V 
                             
                               
                                 SF 
                                 ⁢ 
                                 3 
                               
                               - 
                               A 
                             
                           
                           - 
                           
                             V 
                             
                               
                                 SF 
                                 ⁢ 
                                 1 
                               
                               - 
                               A 
                             
                           
                         
                         
                           
                             V 
                             
                               
                                 SF 
                                 ⁢ 
                                 2 
                               
                               - 
                               A 
                             
                           
                           - 
                           
                             V 
                             
                               
                                 SF 
                                 ⁢ 
                                 4 
                               
                               - 
                               A 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   29 
                   ) 
                 
               
             
           
         
       
     
     Substituting Equations (27), (25), (26), and (28) into Equation (29) results in the following equation to determine the measured phase p from pixel circuit  412 : 
     
       
         
           
             
               
                 
                   φ 
                   = 
                   
                     
                       1 
                       2 
                     
                     · 
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                       ( 
                       
                         
                           
                             ( 
                             
                               
                                 o 
                                 A 
                               
                               + 
                               
                                 C 
                                 ⁢ 
                                 
                                   
                                     G 
                                     
                                       A 
                                       ⁢ 
                                       1 
                                     
                                   
                                   · 
                                   
                                     ( 
                                     
                                       
                                         Q 
                                         
                                           1 
                                           ⁢ 
                                           8 
                                           ⁢ 
                                           0 
                                         
                                       
                                       + 
                                       
                                         D 
                                         ⁢ 
                                         
                                           C 
                                           A 
                                         
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                             ) 
                           
                           - 
                           
                             ( 
                             
                               
                                 o 
                                 A 
                               
                               + 
                               
                                 C 
                                 ⁢ 
                                 
                                   
                                     G 
                                     
                                       A 
                                       ⁢ 
                                       1 
                                     
                                   
                                   · 
                                   
                                     ( 
                                     
                                       
                                         Q 
                                         0 
                                       
                                       + 
                                       
                                         D 
                                         ⁢ 
                                         
                                           C 
                                           A 
                                         
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                             ) 
                           
                         
                         
                           
                             ( 
                             
                               
                                 o 
                                 A 
                               
                               + 
                               
                                 C 
                                 ⁢ 
                                 
                                   
                                     G 
                                     
                                       A 
                                       ⁢ 
                                       2 
                                     
                                   
                                   · 
                                   
                                     ( 
                                     
                                       
                                         Q 
                                         
                                           9 
                                           ⁢ 
                                           0 
                                         
                                       
                                       + 
                                       
                                         D 
                                         ⁢ 
                                         
                                           C 
                                           A 
                                         
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                             ) 
                           
                           - 
                           
                             ( 
                             
                               
                                 o 
                                 A 
                               
                               + 
                               
                                 C 
                                 ⁢ 
                                 
                                   
                                     G 
                                     
                                       A 
                                       ⁢ 
                                       2 
                                     
                                   
                                   · 
                                   
                                     ( 
                                     
                                       
                                         Q 
                                         
                                           2 
                                           ⁢ 
                                           7 
                                           ⁢ 
                                           0 
                                         
                                       
                                       + 
                                       
                                         D 
                                         ⁢ 
                                         
                                           C 
                                           A 
                                         
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   30 
                   ) 
                 
               
             
           
         
       
     
     After expanding Equation (30) above, the offset error terms (o A −o A ), (o A −o A ) cancel each other out. Similarly, the dark current error terms (CG A1 ·DC A )−(CG A1 ·DC A ), (CG A2 ·DC A )−(CG A2 ·DC A ) cancel each other out. 
     After removing these canceled offset error terms (o A ) and dark current error terms (CG A1 ·DC A , CG A2 ·DC A ) shown in Equation (30), the measured phase φ from pixel circuit  412  can be determined according to Equation (31) as follows: 
     
       
         
           
             
               
                 
                   φ 
                   = 
                   
                     
                       1 
                       2 
                     
                     · 
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                       ( 
                       
                         
                           
                             C 
                             ⁢ 
                             
                               G 
                               
                                 A 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             C 
                             ⁢ 
                             
                               G 
                               
                                 A 
                                 ⁢ 
                                 2 
                               
                             
                           
                         
                         · 
                         
                           
                             
                               Q 
                               
                                 1 
                                 ⁢ 
                                 8 
                                 ⁢ 
                                 0 
                               
                             
                             - 
                             
                               Q 
                               0 
                             
                           
                           
                             
                               Q 
                               
                                 9 
                                 ⁢ 
                                 0 
                               
                             
                             - 
                             
                               Q 
                               
                                 2 
                                 ⁢ 
                                 7 
                                 ⁢ 
                                 0 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   31 
                   ) 
                 
               
             
           
         
       
     
     It is appreciated that after the offset errors and dark current errors have been removed or compensated for using the modulation signals with opposing phases in multiple subframes as discussed above, the term (CG A1 /CG A2 ) is the remaining conversion gain error. Assuming the relative conversion gain values CG A1  and CG A2  are acceptable, the remaining conversion gain error term (CG A1 /CG A2 ) can be tolerated. 
       FIG.  5    is a schematic illustrating yet another example of a time-of-flight pixel circuit  512  in accordance with the teachings of the present invention. It is appreciated that the example of time-of-flight pixel circuit  512  illustrated in  FIG.  5    shares many similarities with the time-of-flight pixel circuit  312  shown in  FIG.  3   , and/or may be an example of the time-of-flight pixel circuits  112  shown in  FIG.  1   , and that similarly named and numbered elements described above are coupled and function similarly below. 
     As will be shown, some of the differences between the example of time-of-flight pixel circuit  512  illustrated in  FIG.  5    and the example time-of-flight pixel circuit  312  shown in  FIG.  3    is that example time-of-flight pixel circuit  512  of  FIG.  5    also includes optional additional amplifying stages that may be included between the floating diffusion nodes and the sample and hold transistors. In the example, additional capacitors may also be coupled to the floating diffusion nodes. Furthermore, optional output reset transistors may also be coupled to the gate terminals of the output source follower transistors that are coupled to the outputs of the readout transistors. 
     To illustrate,  FIG.  5    shows that time-of-flight pixel circuit  512  includes a photodiode  518  configured to photogenerate charge in response to incident light. In one example, the light that is incident on photodiode  518  is the reflected modulated light  108  that is reflected from an object  106  as described in  FIG.  1   . In the example depicted in  FIG.  5   , a first floating diffusion capacitor  542 A is coupled to the first floating diffusion FDA  522 A and a second floating diffusion capacitor  542 B coupled to second floating diffusion  522 B as shown. The first floating diffusion FDA  522 A and first floating diffusion capacitor  542 A are configured to store a first portion of charge photogenerated in the photodiode  518 , and the second floating diffusion FDB  522 B and second floating diffusion capacitor  542 B are configured to store a second portion of charge photogenerated in the photodiode  518 . In one example, a first reset transistor  524 A is coupled between a supply rail and the first floating diffusion FDA  522 A and first floating diffusion capacitor  542 A. A second reset transistor  524 B is coupled between the supply rail and the second floating diffusion FDB  522 B and second floating diffusion capacitor  542 B. In the example depicted in  FIG.  5   , both the first and second reset transistors  524 A and  524 B are coupled to receive an overflow signal OF to function as overflow transistors. As such, the first reset transistor  534 A and second reset transistor  534 B may be operated in such a way that excess carriers generated by photodiode  518  may be guided to the power supply by first reset transistor  534 A and/or the second reset transistor  534 B or in a way such that the photosensitivity of photodiode  518  is disabled. Thus, in various examples, the overflow signal OF may be activated during periods in which integration does not occur. 
     A first transfer transistor  520 A is configured to transfer the first portion of charge from the photodiode  518  to the first floating diffusion FDA  522 A in response to a first modulation signal TXA. In one example, the first modulation signal TXA may be an example of one of the phase modulation signals described in  FIG.  2   . A second transfer transistor  520 B is configured to transfer the second portion of charge from the photodiode  518  to the second floating diffusion FDB  522 B in response to a second modulation signal TXB. In the example, the second modulation signal TXB may also be an example of one of the phase modulation signals described in  FIG.  2   . 
     As shown in the depicted example, the time-of-flight pixel circuit  512  also includes a first plurality of N capacitors  528 A 1 ,  528 A 2 , . . . ,  528 AN, which provide a first plurality of N memory nodes CIA  534 A 1 , C 2 A  534 A 2 , . . . , CNA  534 AN, and a second plurality of N capacitors  528 B 1 ,  528 B 2 , . . . ,  528 BN, which provide a second plurality of N memory nodes C 1 B  534 B 1 , CBA  534 B 2 , . . . , CNA  534 BN. 
     In the example depicted in  FIG.  5   , a first amplifying stage, including for example a first amplifying source follower transistor  544 A, is coupled to the first floating diffusion FDA  522 A and first floating diffusion capacitor  542 A. As such, the gate of the first amplifying source follower transistor  544 A is coupled to the first floating diffusion FDA  522 A and first floating diffusion capacitor  542 A while the source of first amplifying source follower transistor  544 A is coupled to a first plurality of N sample and hold transistors  526 A 1 ,  526 A 2 , . . . ,  526 AN. Similarly, a second amplifying stage, including for example a second amplifying source follower transistor  544 B, is coupled to the second floating diffusion FDB  522 B and second floating diffusion capacitor  542 B. As such, the gate of the second amplifying source follower transistor  544 B is coupled to the second floating diffusion FDB  522 A and second floating diffusion capacitor  542 B while the source of second amplifying source follower transistor  544 B is coupled to a second plurality of N sample and hold transistors  526 B 1 ,  526 B 2 , . . . ,  526 BN. 
     In operation, it is appreciated that the first and second amplifying stages provided with first and second amplifying source follower transistors  544 A and  544 B function as decoupling stages between the respective floating diffusions FDA  522 A and FDB  522 B and the respective sample and hold transistors  526 A 1 ,  526 A 2 , . . . ,  526 AN and  526 B 1 ,  526 B 2 , . . . ,  526 BN. For instance, a pixel requires a large full-well capacity or charge handling capacity, which can be described with the product of C×V=Q. A conventional pipelined or burst mode pixel would require that each memory node C 1 A, C 2 A, . . . , CNA, C 1 B, C 2 B, . . . , CNB has the required C×V full-well capacity. The first and second amplifying source follower transistors  544 A and  544 B provide a voltage buffer/decoupling stage, which only requires the sense-node capacitances (e.g., capacitors  542 A/ 542 B) to be large. With the first and second amplifying source follower transistors  544 A and  544 B, the memory node capacitances C 1 A, C 2 A, . . . , CNA, C 1 B, C 2 B, . . . , CNB can now be reduced in size, which can help reducing pixel pitch (e.g., 500ke- to 50ke-). One drawback associated with this is additive kTC and flicker noise. However, for semiconductor materials such as for example Germanium or III/V technology, dark current noise dominates and the additional kTC noise is negligible. Furthermore, even for Silicon-based technology where dark current is negligible, often read-noise is not of importance for time-of-flight applications. Especially for outdoor applications, mostly photon shot noise is the limiting factor which could require a large FWC. Hence, also for Silicon, this decoupling technique provided with first and second amplifying source follower transistors  544 A and  544 B by can be helpful. This technique helps if one is either photon-shot noise limited or dark current shot noise limited. 
     In one example, an enable transistor  546 A and a bias transistor  548 A are coupled between the source of second amplifying source follower transistor  544 A and ground, and an enable transistor  546 B and a bias transistor  548 B are coupled between the source of second amplifying source follower transistor  544 B and ground. In the example, the gates of the enable transistors  546 A and  546 B are coupled to receive an enable signal EN, and the gates of the bias transistors  548 A and  548 B are coupled to receive a bias signal BIAS. 
     In the example, each one of the first plurality of N sample and hold transistors  526 A 1 ,  526 A 2 , . . . ,  526 AN is coupled between first amplifying source follower transistor  544 A, which is coupled to the first floating diffusion FDA  522 A and first floating diffusion capacitor  542 A, and a respective one of the first plurality of N memory nodes C 1 A  534 A 1 , C 2 A  534 A 2 , . . . , CNA  534 AN as shown. Similarly, each one of a second plurality of N sample and hold transistors  526 B 1 ,  526 B 2 , . . . ,  526 BN is coupled between the second amplifying source follower transistor  544 B, which is coupled to the second floating diffusion FDB  522 B and second floating diffusion capacitor  542 B, and a respective one of the second plurality of N memory nodes C 1 B  534 B 1 , C 2 A  534 B 2 , . . . , CNA  534 BN as shown. 
     As shown in the example depicted in  FIG.  5   , the time-of-flight pixel circuit  512  further includes a first output source follower transistor  530 A and a second output source follower transistor  530 B. In the depicted example, a first output reset transistor  550 A is coupled between the gate of first output source follower transistor  530 A and the voltage supply, and a second output reset transistor  550 B is coupled between the gate of second output source follower transistor  530 B and the voltage supply. In the example, the gate of the first output reset transistor  550 A is coupled to receive a reset signal RSTA and the gate of the second output reset transistor  550 B is coupled to receive a reset signal RSTB. In operation, the gates of the first and second output source follower transistors  530 A and  530 B can be reset or precharged by the first and second output reset transistors  550 A and  550 B between readouts to reduce noise by enabling for example correlated double sampling. In various examples, the reset signal RSTA and reset signal RSTB may be the same signal. 
     As shown in the depicted example, each one of a first plurality of N readout transistors  536 A 1 ,  536 A 2 , . . . ,  536 AN is coupled between a respective one of the first plurality of N memory nodes C 1 A  534 A 1 , C 2 A  534 A 2 , . . . , CNA  534 AN and the gate of the first output source follower transistor  530 A. Each one of a second plurality of N readout transistors  536 B 1 ,  536 B 2 , . . . ,  536 BN is coupled between a respective one of the second plurality of N memory nodes C 1 B  534 B 1 , CBA  534 B 2 , . . . , CNA  534 BN and the gate of the second output source follower transistor  530 A. A first row select transistor  532 A is coupled to each one of the first plurality of N readout transistors  536 A 1 ,  536 A 2 , . . . ,  536 AN and a second row select transistor  532 B is coupled to each one of the second plurality of N readout transistors  536 B 1 ,  536 B 2 , . . . ,  536 BN. In one example, the output of the first row select transistor  532 A may be considered a first tap of the time-of-flight pixel circuit  512  from which the output VA from first output source follower transistor  530 A may be read out, and the output of the second row select transistors may be considered a second tap of the time-of-flight pixel circuit  512  from which the output VB from second output source follower transistor  530 B may be read out. 
     In one example, the time-of-flight pixel circuit  512  may be configured as a pipelined pixel circuit such that there are two memory nodes (e.g., N=2) per output tap. To illustrate, in a pipelined pixel circuit example with N=2, there are two memory nodes (e.g.,  534 A 1 / 534 A 2 ,  534 B 1 / 534 B 2 ), two sample and hold transistors (e.g.,  526 A 1 / 526 A 2 ,  526 B 1 / 526 B 2 ), and two readout transistors (e.g.,  536 A 1 / 536 A 2 ,  536 B 1 / 536 B 2 ) per output tap. In the example pipelined pixel circuit configuration, one of the memory nodes per output tap can be coupled to the respective floating diffusion through a respective sample and hold transistor and through a respective amplifying source follower transistor to acquire and store (e.g., sample and hold) charge in response to the portion of charge from the floating diffusion, while the other memory node is read out with the other memory node being coupled to the gate terminal of the respective output source follower transistor through a respective readout transistor. 
     In another example, the time-of-flight pixel circuit  512  may be configured as a burst pixel circuit with two or more memory nodes per output tap (e.g., N=4 or N=8). In a burst pixel circuit configuration, the depth of frame may be formed with, for example, N=4 or N=8 subframes with N=4 or N=8 memory nodes per output tap, which allows the required analog to digital converter (ADC) readout speed, chip-size, and power consumption requirements to be reduced drastically in accordance with the teachings of the present invention. In operation, all of the memory nodes (e.g.,  534 A 1 / 534 B 1 ,  534 A 2 / 534 B 2 , . . . ,  534 AN/ 534 BN) are coupled to sample and hold the first and second portions of the charge responsive to the charge from the respective floating diffusions (e.g.,  522 A/ 522 B) from the respective amplifying stage through the respective sample and hold transistors (e.g.,  526 A 1 / 526 B 1 ,  526 A 2 / 526 B 2 , . . . ,  526 AN/ 526 BN). 
     In one example, the first and second portions of the charge responsive to the charge in the respective first and second floating diffusions (e.g.,  522 A/ 522 B) each include a plurality of subframe portions. In the example, each of the plurality of subframe portions of the first and second portions of charge is responsive to the first and second modulation signals TXA and TXB, which may be varied in phase and/or modulation frequency for each subframe. In the various examples, each of the plurality of subframe portions of the first and second portions of charge is configured to be sampled and held in sequence (e.g., one after another in time) into a respective one of the memory nodes (e.g.,  534 A 1 / 534 B 1 ,  534 A 2 / 534 B 2 , . . . ,  534 AN/ 534 BN) responsive to the charge from the respective floating diffusion (e.g.,  522 A/ 522 B) through a respective one of the sample and hold transistors (e.g.,  526 A 1 / 526 B 1 ,  526 A 2 / 526 B 2 , . . . ,  526 AN/ 526 BN) and a respective one of the first and second amplifying source follower transistors  544 A/ 544 B. 
     After all of the memory nodes have received the respective subframe portion of the first and second portions of the charge, each one of the plurality of subframe portions of the first and second portions of the charge may then be read out from the memory nodes through the respective readout transistors (e.g.,  536 A 1 / 536 B 1 ,  536 A 2 / 536 B 2 , . . . ,  536 AN/ 536 BN). In one example, each of the plurality of subframe portions of the first and second portions of charge that is stored in the respective memory nodes (e.g.,  534 A 1 / 534 B 1 ,  534 A 2 / 534 B 2 , . . . ,  534 AN/ 534 BN) is configured to be read out in sequence (e.g., one after another in time) through a respective one the first plurality of readout transistors (e.g.,  536 A 1 / 536 B 1 ,  536 A 2 / 536 B 2 , . . . ,  536 AN/ 536 BN) after all of the memory nodes have sampled and held the plurality of subframe portions of the first and second portions of the charge responsive to the charge from the first and second floating diffusions (e.g.,  532 A/ 532 B). 
     It is appreciated that burst pixel circuit configurations may be utilized in situations where readout speeds are too slow for pipelined pixel circuit configurations. For instance, in an example with N=8, 8 subframes of phase charge information may be sampled and held in 8 memory nodes in sequence. After the 8 subframes of phase charge information have been stored in the respective 8 memory nodes, the 8 memory nodes may then be subsequently read out in sequence with reduced readout speed requirements after the integration period as there is typically a gap between integration exposure periods. 
     In the various examples, the first modulation signal TXA and second modulation signal TXB are configured to modulate the first transfer transistor  520 A and the second transfer transistor  520 B. In the examples, the first modulation signal TXA and second modulation signal TXB are modulation signals that are 180° out of phase or inverted versions of each other during the multiple subframes in which the time-of-flight pixel circuit  512  is modulated. For instance, in one example, in a first subframe, the first modulation signal TXA may be the 0° phase modulation signal  214 A while the second modulation signal TXA is therefore the 180° phase modulation signal  214 B. In that example, in another subframe, the first modulation signal TXA and the second modulation signal TXB are inverted relative to their respective signals in the first subframe. In other words, in that example, in the other subframe, the first modulation signal TXA is the 180° phase modulation signal  216 B while the second modulation signal TXB is the 0° phase modulation signal  214 A. In the various examples, it is appreciated that the first and second transfer transistors  520 A and  520 B may also be modulated with 90°/270° phase modulation signals as well as with the opposing phases of 270°/90° phase modulation signals in multiple subframes. 
     By modulating the first transfer transistor  520 A and the second transfer transistor  520 B with the first modulation signal TXA and second modulation signal TXB with the opposing phases of the 0°/180° and 180°/0° phase modulation signal as well as with the opposing phases of the 90°/270° and 270°/90° phase modulation signals in the multiple subframes as described, offset errors as well as dark current errors in the time-of-flight pixel circuits are canceled or removed in accordance with the teachings of the present invention. 
     In the various examples, the first modulation signal TXA and second modulation signal TXB may also be modulated at different frequencies in different subframes, which improves systematic errors, such as for example harmonic distortions or multipath artifacts in accordance with the teachings of the present invention. For instance, in one example, the first modulation signal TXA and second modulation signal TXB are configured to alternate between two different modulation frequencies f1 and f2 for each subframe. In other words, in one example, the first modulation signal TXA and second modulation signal TXB are configured to be modulated at a modulation frequency f1 during a first subframe, and then a modulation frequency f2 during a second subframe. Then, in a third subframe, the first modulation signal TXA and second modulation signal TXB are configured to be modulated at the modulation frequency f1, and then in a fourth subframe, the first modulation signal TXA and second modulation signal TXB are configured to be modulated at the modulation frequency f2, and so on. 
       FIG.  6    is an example timing diagram  600  illustrating signals in an example time-of-flight sensing system with pipelined operations in accordance with the teachings of the present invention. It is appreciated that the signals illustrated in the timing diagram  600  of  FIG.  6    may be examples of the signals found during operation of the time-of-flight sensing system  100  as illustrated in  FIG.  1    including a pixel array of time-of-flight pixels  312  as illustrated in  FIG.  3   , and that similarly named and numbered elements described above are coupled and function similarly below. 
     In the example depicted  FIG.  6   , it is assumed for illustration purposes that the time-of-flight pixel circuits  312  are configured as pipelined pixel circuits such that there are two memory nodes (e.g., N=2) per output tap. It is appreciated that the timing diagram  600  of  FIG.  6    illustrates the example signals that occur with respect the example used to describe Equations (3)-(24) above. 
     As shown in the depicted example, at time T0, a laser enable (Laser-EN) signal  602  is activated, which enables the light source  102 . As such, the laser low voltage differential signal (Laser-LVDS)  603  as well as the first and second modulation signals (TXA/B)  620  are synchronized and oscillate as shown. At time T0, it is noted that the reset signal  624  as well as the sample and hold signals SH 1   626 - 1 , SH 2   626 - 2  and the readout signals RD 1   636 - 1 , RD 2   636 - 2  are also activated, which resets or initializes the first and second floating diffusions (FDA, FDB) as well as the memory nodes (C 1 A, C 2 A, C 1 B, C 2 B) in time-of-flight pixel circuit  312 . 
     Between time T1 and T2, the integration of subframe 1 (SF1) occurs, which is indicated with Integration/Readout  640 . During the integration that occurs during subframe SF1, the reset signal  624  is deactivated and the first modulation signal TXA is modulated with a 0° phase modulation signal, which is indicated with Phase TXA  614 A, and the second modulation signal TXB is modulated with a 180° phase modulation signal, which is indicated with Phase TXB  614 B. In addition, it is noted that the modulation signals are modulated at a first modulation frequency f1, as indicated with f mod    638 . During this subframe SF1 integration time, the 0° portion of charge (e.g., Q 0 ) is sampled and held in a first memory node of a first output tap (e.g., C 1 A) and the 180° portion of charge is (e.g., Q 180 ) is sampled and held in a first memory of a second output tap (e.g., C 1 B). 
     After the subframe SF1 integration between time T1 and T2, the reset signal RST  624  is activated, the first sample and hold signal SH 1   626 - 1  is deactivated, and the second sample and hold signal SH 2   626 - 2  is activated between time T2 and T3. As such, the first and second floating diffusions (FDA, FDB) the second memory nodes (e.g., C 2 A, C 2 B) in time-of-flight pixel circuit  312  are reset. However, the subframe portions of charge that are sampled in the first memory nodes (e.g., C 1 A, C 1 B) remain held since the first sample and hold signal SH 1   626 - 1  is deactivated. 
     Between time T3 and T4, the subframe SF2 integration occurs, and a readout occurs of the previous integration that occurred during subframe SF1, which is indicated with the Integration/Readout  640  between time T3 and T4. It is noted that during the subframe SF2 integration that occurs between time T3 and T4, the modulation signals are modulated at a second modulation frequency f2, as indicated with f mod    638 . During the subframe SF1 readout that occurs between time T3 and T4, the first readout signal RD 1  is activated, the second readout signal RD 2  is deactivated, and the row select signals RSA  632 A and RSB  632 B are pulsed to read out the VA and VB signals from the first memory nodes (e.g., C 1 A, C 1 B) at the two output taps of time-of-flight pixel circuit  312 . In addition, during the subframe SF2 integration that occurs between time T3 and T4, the first sample and hold signal SH 1   626 - 1  is deactivated while the second sample and hold signal SH 2   626 - 2  is activated. During this integration time, the 0° portion of charge (e.g., Q 0 ) is sampled and held in a second memory node of a first output tap (e.g., C 2 A) and the 180° portion of charge is (e.g., Q 180 ) is sampled and held in a second memory of a second output tap (e.g., C 2 B). 
     After the subframe SF2 integration and the subframe SF1 readout between time T3 and T4, the reset signal RST  624  is activated, the first sample and hold signal SH 1   626 - 1  is activated, and the second sample and hold signal SH 2   626 - 2  is deactivated between time T4 and T5. As such, the first and second floating diffusions (FDA, FDB) and the first memory nodes (e.g., C 1 A, C 1 B) in time-of-flight pixel circuit  312  are reset. However, the subframe portions of charge that are sampled in the second memory nodes (e.g., C 2 A, C 2 B) remain held since the second sample and hold signal SH 2   626 - 2  is deactivated. 
     Between time T5 and T6, the subframe SF3 integration occurs and the subframe SF2 readout occurs as indicated with the Integration/Readout  640  between time T5 and T6. During the integration that occurs during subframe SF3, the first modulation signal TXA is modulated with a 90° phase modulation signal, as indicated with Phase TXA  614 A and the second modulation signal TXB is modulated with a 270° phase modulation signal, as indicated with Phase TXB  614 B. In addition, it is noted that during the subframe SF3 integration that occurs between time T5 and T6, the modulation signals are modulated at a first modulation frequency f1, as indicated with f mod    638 . During this SF3 integration time, the 90° portion of charge (e.g., Q 90 ) is sampled and held in a first memory node of a first output tap (e.g., C 1 A) and the 270° portion of charge is (e.g., Q 270 ) is sampled and held in a first memory of a second output tap (e.g., C 1 B). During the subframe SF2 readout that occurs between time T5 and T6, the first readout signal RD 1  is deactivated, the second readout signal RD 2  is activated, and the row select signals RSA  632 A and RSB  632 B are pulsed to read out the VA and VB signals from the second memory nodes (e.g., C 2 A, C 2 B) at the two output taps of time-of-flight pixel circuit  312 . 
     After the integration of subframe SF3 and readout of subframe SF2 between time T5 and T6, the reset signal RST  624  is activated, the first sample and hold signal SH 1   626 - 1  is deactivated, and the second sample and hold signal SH 2   626 - 2  is activated between time T6 and T7. As such, the first and second floating diffusions (FDA, FDB) and the second memory nodes (e.g., C 2 A, C 2 B) in time-of-flight pixel circuit  312  are reset. However, the subframe portions of charge that are sampled in the first memory nodes (e.g., CIA, C 1 B) remain held since the first sample and hold signal SH 1   626 - 1  is deactivated. 
     Between time T7 and T8, the subframe SF4 integration occurs and the subframe SF3 readout occurs as indicated with the Integration/Readout  640  between time T7 and T8. It is noted that during the subframe SF4 integration that occurs between time T7 and T8, the modulation signals are modulated at the second modulation frequency f2, as indicated with f mod    638 . During the subframe SF3 readout that occurs between time T7 and T8, the first readout signal RD 1  is activated, the second readout signal RD 2  is deactivated, and the row select signals RSA  632 A and RSB  632 B are pulsed to read out the VA and VB signals from the first memory nodes (e.g., CIA, C 1 B) at the two output taps of time-of-flight pixel circuit  312 . In addition, during the subframe SF4 integration that occurs between time T7 and T8, the first sample and hold signal SH 1   626 - 1  is deactivated while the second sample and hold signal SH 2   626 - 2  is activated. During this integration time, the 90° portion of charge (e.g., Q 90 ) is sampled and held in a second memory node of a first output tap (e.g., C 2 A) and the 270° portion of charge (e.g., Q 270 ) is sampled and held in a second memory of a second output tap (e.g., C 2 B). 
     After the subframe SF4 integration and the subframe SF3 readout between time T7 and T8, the reset signal RST  624  is activated, the first sample and hold signal SH 1   626 - 1  is activated, and the second sample and hold signal SH 2   626 - 2  is deactivated between time T8 and T9. As such, the first and second floating diffusions (FDA, FDB) and the first memory nodes (e.g., C 1 A, C 1 B) in time-of-flight pixel circuit  312  are reset. However, the subframe portions of charge that are sampled in the second memory nodes (e.g., C 2 A, C 2 B) remain held since the second sample and hold signal SH 2   626 - 2  is deactivated. 
     Between time T9 and T10, the subframe SF5 integration occurs and the subframe SF4 readout occurs as indicated with the Integration/Readout  640  between time T9 and T10. During the integration that occurs during subframe SF5, the first modulation signal TXA is modulated with a 180° phase modulation signal, as indicated with Phase TXA  614 A and the second modulation signal TXB is modulated with a 0° phase modulation signal, as indicated with Phase TXB  614 B. In addition, it is noted that during the subframe SF5 integration that occurs between time T9 and T10, the modulation signals are modulated at a first modulation frequency f1, as indicated with f mod    638 . During this SF5 integration time, the 180° portion of charge (e.g., Q 180 ) is sampled and held in a first memory node of a first output tap (e.g., C 1 A) and the 0° portion of charge is (e.g., Q 0 ) is sampled and held in a first memory of a second output tap (e.g., C 1 B). During the subframe SF4 readout that occurs between time T9 and T10, the first readout signal RD 1  is deactivated, the second readout signal RD 2  is activated, and the row select signals RSA  632 A and RSB  632 B are pulsed to read out the VA and VB signals from the second memory nodes (e.g., C 2 A, C 2 B) at the two output taps of time-of-flight pixel circuit  312 . 
     After the subframe SF5 integration and the subframe S4 readout between time T9 and T10, the reset signal RST  624  is activated, the first sample and hold signal SH 1   626 - 1  is deactivated, and the second sample and hold signal SH 2   626 - 2  is activated between time T10 and T11. As such, the first and second floating diffusions (FDA, FDB) and the second memory nodes (e.g., C 2 A, C 2 B) in time-of-flight pixel circuit  312  are reset. However, the subframe portions of charge that are sampled in the first memory nodes (e.g., C 1 A, C 1 B) remain held since the first sample and hold signal SH 1   626 - 1  is deactivated. 
     Between time T11 and T12, the subframe SF6 integration occurs and the S5 readout occurs as indicated with the Integration/Readout  640  between time T11 and T12. It is noted that during the subframe SF6 integration that occurs between time T11 and T12, the modulation signals are modulated at the second modulation frequency f2, as indicated with f mod    638 . During the subframe SF5 readout that occurs between time T11 and T12, the first readout signal RD 1  is activated, the second readout signal RD 2  is deactivated, and the row select signals RSA  632 A and RSB  632 B are pulsed to read out the VA and VB signals from the first memory nodes (e.g., C 1 A, C 1 B) at the two output taps of time-of-flight pixel circuit  312 . In addition, during the subframe SF6 integration that occurs between time T11 and T12, the first sample and hold signal SH 1   626 - 1  is deactivated while the second sample and hold signal SH 2   626 - 2  is activated. During this integration time, the 180° portion of charge (e.g., Q 180 ) is sampled and held in a second memory node of a first output tap (e.g., C 2 A) and the 0° portion of charge (e.g., Q 0 ) is sampled and held in a second memory of a second output tap (e.g., C 2 B). 
     After the subframe SF6 integration and the subframe SF5 readout between time T11 and T12, the reset signal RST  624  is activated, the first sample and hold signal SH 1   626 - 1  is activated, and the second sample and hold signal SH 2   626 - 2  is deactivated between time T12 and T13. As such, the first and second floating diffusions (FDA, FDB) and the first memory nodes (e.g., C 1 A, C 1 B) in time-of-flight pixel circuit  312  are reset. However, the subframe portions of charge that are sampled in the second memory nodes (e.g., C 2 A, C 2 B) remain held since the second sample and hold signal SH 2   626 - 2  is deactivated. 
     Between time T13 and T14, the subframe SF7 integration occurs and the subframe SF6 readout occurs as indicated with the Integration/Readout  640  between time T13 and T14. During the integration that occurs during subframe SF7, the first modulation signal TXA is modulated with a 270° phase modulation signal, as indicated with Phase TXA  614 A and the second modulation signal TXB is modulated with a 90° phase modulation signal, as indicated with Phase TXB  614 B. In addition, it is noted that during the subframe SF7 integration that occurs between time T13 and T14, the modulation signals are modulated at a first modulation frequency f1, as indicated with f mod    638 . During this SF7 integration time, the 270° portion of charge (e.g., Q 270 ) is sampled and held in a first memory node of a first output tap (e.g., C 1 A) and the 90° portion of charge is (e.g., Q 90 ) is sampled and held in a first memory of a second output tap (e.g., C 1 B). During the subframe SF6 readout that occurs between time T13 and T14, the first readout signal RD 1  is deactivated, the second readout signal RD 2  is activated, and the row select signals RSA  632 A and RSB  632 B are pulsed to read out the VA and VB signals from the second memory nodes (e.g., C 2 A, C 2 B) at the two output taps of time-of-flight pixel circuit  312 . 
     After the subframe SF7 integration and the subframe S6 readout between time T13 and T14, the reset signal RST  624  is activated, the first sample and hold signal SH 1   626 - 1  is deactivated, and the second sample and hold signal SH 2   626 - 2  is activated between time T14 and T15. As such, the first and second floating diffusions (FDA, FDB) and the second memory nodes (e.g., C 2 A, C 2 B) in time-of-flight pixel circuit  312  are reset. However, the subframe portions of charge that are sampled in the first memory nodes (e.g., C 1 A, C 1 B) remain held since the first sample and hold signal SH 1   626 - 1  is deactivated. 
     Between time T15 and T16, the subframe SF8 integration occurs and the S7 readout occurs as indicated with the Integration/Readout  640  between time T15 and T16. It is noted that during the subframe SF8 integration that occurs between time T15 and T16, the modulation signals are modulated at the second modulation frequency f2, as indicated with f mod    638 . During the subframe SF7 readout that occurs between time T15 and T16, the first readout signal RD 1  is activated, the second readout signal RD 2  is deactivated, and the row select signals RSA  632 A and RSB  632 B are pulsed to read out the VA and VB signals from the first memory nodes (e.g., C 1 A, C 1 B) at the two output taps of time-of-flight pixel circuit  312 . In addition, during the subframe SF8 integration that occurs between time T15 and T16, the first sample and hold signal SH 1   626 - 1  is deactivated while the second sample and hold signal SH 2   626 - 2  is activated. During this integration time, the 270° portion of charge (e.g., Q 270 ) is sampled and held in a second memory node of a first output tap (e.g., C 2 A) and the 90° portion of charge (e.g., Q 90 ) is sampled and held in a second memory of a second output tap (e.g., C 2 B). 
     After the subframe SF8 integration and the subframe SF7 readout between time T15 and T16, the reset signal RST  624  is activated, the first sample and hold signal SH 1   626 - 1  is activated, and the second sample and hold signal SH 2   626 - 2  is deactivated between time T16 and T17. As such, the first and second floating diffusions (FDA, FDB) and the first memory nodes (e.g., C 1 A, C 1 B) in time-of-flight pixel circuit  312  are reset. However, the subframe portions of charge that are sampled in the second memory nodes (e.g., C 2 A, C 2 B) remain held since the second sample and hold signal SH 2   626 - 2  is deactivated. 
     Between time T17 and T18, there is no integration and the subframe SF8 readout occurs as indicated with the Integration/Readout  640  between time T17 and T18. During the subframe SF8 readout that occurs between time T17 and T17, the first readout signal RD 1  is deactivated, the second readout signal RD 2  is activated, and the row select signals RSA  632 A and RSB  632 B are pulsed to read out the VA and VB signals from the second memory nodes (e.g., C 2 A, C 2 B) at the two output taps of time-of-flight pixel circuit  312 . 
       FIG.  7    is another example timing diagram  700  illustrating signals in an example time-of-flight sensing system with burst operations in accordance with the teachings of the present invention. It is appreciated that the signals illustrated in the timing diagram  700  of  FIG.  7    may be examples of the signals found during operation of the time-of-flight sensing system  100  as illustrated in  FIG.  1    including a pixel array of time-of-flight pixels  512  as illustrated in  FIG.  5   , and that similarly named and numbered elements described above are coupled and function similarly below. 
     In the example depicted  FIG.  7   , it is assumed for illustration purposes that the time-of-flight pixel circuits  512  are configured as burst pixel circuits such that there are eight memory nodes (e.g., N=8) per output tap. It is appreciated that the timing diagram  700  of  FIG.  7    also illustrates the example signals that occur with respect the example used to describe Equations (3)-(24) above. 
     As shown in the depicted example, at time T0, a laser enable (Laser-EN) signal  702  is activated, which enables the light source  102 . As such, the laser low voltage differential signal (Laser-LVDS)  703  as well as the first and second modulation signals (TXA/B)  720  are synchronized and oscillate as shown. At time T0, it is noted that the overflow signal OF  724 , the sample and hold signals SH 1   726 - 1  to SH 8   726 - 8 , the readout signals RD 1   636 - 1  to RD 8   636 - 8 , and the reset signal RST  750 , which resets or initializes the first and second floating diffusions (FDA, FDB) as well as the memory nodes (C 1 A to C 8 A and C 1 B to C 8 B) in time-of-flight pixel circuit  512 . 
     As will be shown in  FIG.  7   , between time T1 and time T16, an integration period occurs and the TXA and TXB portions of charge from the photodiode  518  are sampled and held in sequence (e.g., one after another in time) into respective memory nodes (e.g.,  534 A 1 / 534 B 1 ,  534 A 2 / 534 B 2 ,  534 A 3 / 534 B 3 ,  534 A 4 / 534 B 4 ,  534 A 5 / 534 B 5 ,  534 A 6 / 534 B 6 ,  534 A 7 / 534 B 7 ,  534 A 8 / 534 B 8 ) from the first and second amplifying source follower transistors (e.g.,  544 A/ 544 B) in response to the charge in respective floating diffusions (e.g.,  522 A/ 522 B) through respective sample and hold transistors (e.g.,  526 A 1 / 526 B 1 ,  526 A 2 / 526 B 2 ,  526 A 3 / 526 B 3 ,  526 A 4 / 526 B 4 ,  526 A 5 / 526 B 5 ,  526 A 6 / 526 B 6 ,  526 A 7 / 526 B 7 ,  526 A 8 / 526 B 8 ). 
     In particular, at time T1, the integration period begins as the overflow signal OF  724 , the sample and hold signals SH 2   726 - 2  to SH 8   726 - 8 , the readout signals RD 1   736 - 1  to RD 8   736 - 8 , and the reset signal RST  750  are all deactivated. Between time T1 and T2, the integration of subframe 1 (SF1) occurs, which is indicated with Integration  740  and the sample and hold signal SH 1   726 - 1  remaining active. During the integration that occurs during subframe SF1, the first modulation signal TXA is modulated with a 0° phase modulation signal, which is indicated with Phase TXA  714 A, and the second modulation signal TXB is modulated with a 180° phase modulation signal, which is indicated with Phase TXB  714 B. In addition, it is noted that the modulation signals are modulated at a first modulation frequency f1, as indicated with f mod    738 . During this subframe SF1 integration time, the 0° portion of charge (e.g., Q 0 ) is sampled and held in a first memory node of a first output tap (e.g., C 1 A) and the 180° portion of charge is (e.g., Q 180 ) is sampled and held in a first memory node of a second output tap (e.g., C 1 B). 
     After the subframe SF1 integration between time T1 and T2, the overflow signal OF  724  may be activated, the first sample and hold signal SH 1   726 - 1  is deactivated, and the second sample and hold signal SH 2   726 - 2  is activated between time T2 and T3. As such, the first and second floating diffusions (FDA, FDB) and the second memory nodes (e.g., C 2 A, C 2 B) in time-of-flight pixel circuit  512  are reset. However, the subframe portions of charge that are sampled in the first memory nodes (e.g., CIA, C 1 B) remain held since the first sample and hold signal SH 1   726 - 1  is deactivated. 
     Between time T3 and T4, the subframe SF2 integration occurs, which is indicated with the Integration  740  between time T3 and T4. It is noted that during the subframe SF2 integration that occurs between time T3 and T4, the modulation signals are modulated at a second modulation frequency f2, as indicated with f mod    738 . During the subframe SF2 integration that occurs between time T3 and T4, the second sample and hold signal SH 2   726 - 2  is activated and the overflow signal OF  724  is deactivated. During this integration time, the 0° portion of charge (e.g., Q 0 ) is sampled and held in a second memory node of a first output tap (e.g., C 2 A) and the 180° portion of charge is (e.g., Q 180 ) is sampled and held in a second memory node of a second output tap (e.g., C 2 B). 
     After the subframe S2 integration between time T3 and T4, the overflow signal OF  724  may be activated, the second sample and hold signal SH 2   726 - 2  is deactivated, and the third sample and hold signal SH 3   726 - 3  is activated between time T4 and T5. As such, the first and second floating diffusions (FDA, FDB) and the third memory nodes (e.g., C 3 A, C 3 B) in time-of-flight pixel circuit  512  are reset. However, the subframe portions of charge that are sampled in the first and second memory nodes (e.g., C 1 A/C 1 B, C 2 A/C 2 B) remain held since the first and second sample and hold signals SH 1   726 - 1 , SH 2   726 - 2  are deactivated. 
     Between time T5 and T6, the subframe SF3 integration occurs, which is indicated with the Integration  740  between time T5 and T6. It is noted that during the subframe SF3 integration that occurs between time T5 and T6, the modulation signals are modulated at the first modulation frequency f1, as indicated with f mod    738 . During the subframe SF3 integration that occurs between time T5 and T6, the third sample and hold signal SH 3   726 - 3  is activated and the overflow signal OF  724  is deactivated. During this integration time, the 90° portion of charge (e.g., Q 90 ) is sampled and held in a third memory node of a first output tap (e.g., C 3 A) and the 270° portion of charge is (e.g., Q 270 ) is sampled and held in a third memory node of a second output tap (e.g., C 3 B). 
     After the subframe S3 integration between time T5 and T6, the overflow signal OF  724  may be activated, the third sample and hold signal SH 3   726 - 3  is deactivated, and the fourth sample and hold signal SH 4   726 - 4  is activated between time T6 and T7. As such, the first and second floating diffusions (FDA, FDB) and the fourth memory nodes (e.g., C 4 A, C 4 B) in time-of-flight pixel circuit  512  are reset. However, the subframe portions of charge that are sampled in the first to third memory nodes (e.g., C 1 A/C 1 B to C 3 A/C 3 B) remain held since the first to third sample and hold signals SH 1   726 - 1  to SH 3   726 - 3  are deactivated. 
     Between time T7 and T8, the subframe SF4 integration occurs, which is indicated with the Integration  740  between time T7 and T8. It is noted that during the subframe SF4 integration that occurs between time T7 and T8, the modulation signals are modulated at the second modulation frequency f2, as indicated with f mod    738 . During the subframe SF4 integration that occurs between time T7 and T8, the fourth sample and hold signal SH 4   726 - 4  is activated and the overflow signal OF  724  is deactivated. During this integration time, the 90° portion of charge (e.g., Q 90 ) is sampled and held in a fourth memory node of a first output tap (e.g., C 4 A) and the 270° portion of charge is (e.g., Q 270 ) is sampled and held in a fourth memory node of a second output tap (e.g., C 4 B). 
     After the subframe S4 integration between time T7 and T8, the overflow signal OF  724  may be activated, the fourth sample and hold signal SH 4   726 - 4  is deactivated, and the fifth sample and hold signal SH 5   726 - 5  is activated between time T8 and T9. As such, the first and second floating diffusions (FDA, FDB) and the fifth memory nodes (e.g., C 5 A, C 5 B) in time-of-flight pixel circuit  512  are reset. However, the subframe portions of charge that are sampled in the first to fourth memory nodes (e.g., C 1 A/C 1 B to C 4 A/C 4 B) remain held since the first to fourth sample and hold signals SH 1   726 - 1  to SH 4   726 - 4  are deactivated. 
     Between time T9 and T10, the subframe SF5 integration occurs, which is indicated with the Integration  740  between time T9 and T10. It is noted that during the subframe SF5 integration that occurs between time T9 and T10, the modulation signals are modulated at the first modulation frequency f1, as indicated with f mod    738 . During the subframe SF5 integration that occurs between time T9 and T10, the fifth sample and hold signal SH 5   726 - 5  is activated and the overflow signal OF  724  is deactivated. During this integration time, the 180° portion of charge (e.g., Q 180 ) is sampled and held in a fifth memory node of a first output tap (e.g., C 5 A) and the 0° portion of charge is (e.g., Q 0 ) is sampled and held in a fifth memory node of a second output tap (e.g., C 5 B). 
     After the subframe S5 integration between time T9 and T10, the overflow signal OF  724  may be activated, the fifth sample and hold signal SH 5   726 - 5  is deactivated, and the sixth sample and hold signal SH 6   726 - 6  is activated between time T10 and T11. As such, the first and second floating diffusions (FDA, FDB) and the sixth memory nodes (e.g., C 6 A, C 6 B) in time-of-flight pixel circuit  512  are reset. However, the subframe portions of charge that are sampled in the first to fifth memory nodes (e.g., C 1 A/C 1 B to C 5 A/C 5 B) remain held since the first to fifth sample and hold signals SH 1   726 - 1  to SH 5   726 - 5  are deactivated. 
     Between time T11 and T12, the subframe SF6 integration occurs, which is indicated with the Integration  740  between time T11 and T12. It is noted that during the subframe SF6 integration that occurs between time T11 and T12, the modulation signals are modulated at the second modulation frequency f2, as indicated with f mod    738 . During the subframe SF6 integration that occurs between time T11 and T12, the sixth sample and hold signal SH 6   726 - 6  is activated and the overflow signal OF  724  is deactivated. During this integration time, the 180° portion of charge (e.g., Q 180 ) is sampled and held in a sixth memory node of a first output tap (e.g., C 6 A) and the 0° portion of charge is (e.g., Q 0 ) is sampled and held in a sixth memory node of a second output tap (e.g., C 6 B). 
     After the subframe S6 integration between time T11 and T12, the overflow signal OF  724  may be activated, the sixth sample and hold signal SH 6   726 - 6  is deactivated, and the seventh sample and hold signal SH 7   726 - 7  is activated between time T12 and T13. As such, the first and second floating diffusions (FDA, FDB) and the seventh memory nodes (e.g., C 7 A, C 7 B) in time-of-flight pixel circuit  512  are reset. However, the subframe portions of charge that are sampled in the first to sixth memory nodes (e.g., C 1 A/C 1 B to C 6 A/C 6 B) remain held since the first to sixth sample and hold signals SH 1   726 - 1  to SH 6   726 - 6  are deactivated. 
     Between time T13 and T14, the subframe SF7 integration occurs, which is indicated with the Integration  740  between time T13 and T14. It is noted that during the subframe SF7 integration that occurs between time T13 and T14, the modulation signals are modulated at the first modulation frequency f1, as indicated with f mod    738 . During the subframe SF7 integration that occurs between time T13 and T14, the seventh sample and hold signal SH 7   726 - 7  is activated and the overflow signal OF  724  is deactivated. During this integration time, the 270° portion of charge (e.g., Q 270 ) is sampled and held in a seventh memory node of a first output tap (e.g., C 7 A) and the 90° portion of charge is (e.g., Q 90 ) is sampled and held in a seventh memory node of a second output tap (e.g., C 7 B). 
     After the subframe S7 integration between time T13 and T14, the overflow signal OF  724  may be activated, the seventh sample and hold signal SH 7   726 - 7  is deactivated, and the eighth sample and hold signal SH 8   726 - 8  is activated between time T14 and T15. As such, the first and second floating diffusions (FDA, FDB) and the eighth memory nodes (e.g., C 8 A, C 8 B) in time-of-flight pixel circuit  512  are reset. However, the subframe portions of charge that are sampled in the first to seventh memory nodes (e.g., C 1 A/C 1 B to C 7 A/C 7 B) remain held since the first to sixth sample and hold signals SH 1   726 - 1  to SH 6   726 - 6  are deactivated. 
     Between time T15 and T16, the subframe SF8 integration occurs, which is indicated with the Integration  740  between time T15 and T16. It is noted that during the subframe SF8 integration that occurs between time T15 and T16, the modulation signals are modulated at the second modulation frequency f2, as indicated with f mod    738 . During the subframe SF8 integration that occurs between time T15 and T16, the eighth sample and hold signal SH 8   726 - 8  is activated and the overflow signal OF  724  is deactivated. During this integration time, the 270° portion of charge (e.g., Q 270 ) is sampled and held in an eighth memory node of a first output tap (e.g., C 8 A) and the 90° portion of charge is (e.g., Q 90 ) is sampled and held in an eighth memory node of a second output tap (e.g., C 8 B). 
     As shown in the depicted example, after the integration period that occurs between time T1 and time T16, all of the memory nodes C 1 A to C 8 A and C 1 B to C 8 B have sampled and hold their respective subframe portions of the charge in sequence as discussed above. As such, the overflow signal OF  724  is activated after time T16. As will be shown, the respective subframe portions of the charge may then be read out in sequence (e.g., one after another in time) through respective readout transistors during a readout period, which occurs during the time period from time T17 to T33. As such, the row select signal RS  732  is activated during time T17 to T33. 
     At time T17, the reset signal RST  750  is activated, which resets or precharges that gate of source followers  530 A/ 530 B. At time T18, the reset signal RST  750  is deactivated and the first readout signal RD 1   736 - 1  is activated, which enables the SF1 portions sampled and held in the first memory nodes (e.g., C 1 A/C 1 B) to be read out through source followers  530 A/ 530 B. 
     At time T19, the first readout signal RD 1   736 - 1  is deactivated and the reset signal RST  750  is activated, which resets or precharges that gate of source followers  530 A/ 530 B. At time T20, the reset signal RST  750  is deactivated and the second readout signal RD 2   736 - 2  is activated, which enables the SF2 portions sampled and held in the second memory nodes (e.g., C 2 A/C 2 B) to be read out through source followers  530 A/ 530 B. 
     At time T21, the second readout signal RD 2   736 - 2  is deactivated and the reset signal RST  750  is activated, which resets or precharges that gate of source followers  530 A/ 530 B. At time T22, the reset signal RST  750  is deactivated and the third readout signal RD 3   736 - 3  is activated, which enables the SF3 portions sampled and held in the third memory nodes (e.g., C 3 A/C 3 B) to be read out through source followers  530 A/ 530 B. 
     At time T23, the third readout signal RD 3   736 - 3  is deactivated and the reset signal RST  750  is activated, which resets or precharges that gate of source followers  530 A/ 530 B. At time T24, the reset signal RST  750  is deactivated and the fourth readout signal RD 4   736 - 4  is activated, which enables the SF4 portions sampled and held in the fourth memory nodes (e.g., C 4 A/C 4 B) to be read out through source followers  530 A/ 530 B. 
     At time T25, the fourth readout signal RD 4   736 - 4  is deactivated and the reset signal RST  750  is activated, which resets or precharges that gate of source followers  530 A/ 530 B. At time T26, the reset signal RST  750  is deactivated and the fifth readout signal RD 5   736 - 5  is activated, which enables the SF5 portions sampled and held in the fifth memory nodes (e.g., C 5 A/C 5 B) to be read out through source followers  530 A/ 530 B. 
     At time T27, the fifth readout signal RD 5   736 - 5  is deactivated and the reset signal RST  750  is activated, which resets or precharges that gate of source followers  530 A/ 530 B. At time T28, the reset signal RST  750  is deactivated and the sixth readout signal RD 6   736 - 6  is activated, which enables the SF6 portions sampled and held in the sixth memory nodes (e.g., C 6 A/C 6 B) to be read out through source followers  530 A/ 530 B. 
     At time T29, the sixth readout signal RD 6   736 - 6  is deactivated and the reset signal RST  750  is activated, which resets or precharges that gate of source followers  530 A/ 530 B. At time T29, the reset signal RST  750  is deactivated and the seventh readout signal RD 7   736 - 7  is activated, which enables the SF7 portions sampled and held in the seventh memory nodes (e.g., C 7 A/C 7 B) to be read out through source followers  530 A/ 530 B. 
     At time T31, the seventh readout signal RD 7   736 - 6  is deactivated and the reset signal RST  750  is activated, which resets or precharges that gate of source followers  530 A/ 530 B. At time T32, the reset signal RST  750  is deactivated and the eighth readout signal RD 8   736 - 8  is activated, which enables the SF8 portions sampled and held in the eighth memory nodes (e.g., C 8 A/C 8 B) to be read out through source followers  530 A/ 530 B. 
     As shown in the depicted example, after the readout period that occurs between time T17 and time T22, the subframe portions of the charge that have been sampled and held in all of the memory nodes C 1 A to C 8 A and C 1 B to C 8 B have now been read out in sequence as described above. 
     As such, the overflow signal OF  724 , the sample and hold signals SH 1   726 - 1  to SH 8   726 - 8 , the readout signals RD 1   636 - 1  to RD 8   636 - 8 , and the reset signal RST  750 , may all be reactivated after time T34 to reset or reinitialize the first and second floating diffusions (FDA, FDB) as well as the memory nodes (C 1 A to C 8 A and C 1 B to C 8 B) in time-of-flight pixel circuit  512 . 
     The above description of illustrated examples of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific examples of the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific examples disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.