CN115308756A - Pixel circuit, image sensor and detection device - Google Patents
Pixel circuit, image sensor and detection device Download PDFInfo
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- CN115308756A CN115308756A CN202110493024.5A CN202110493024A CN115308756A CN 115308756 A CN115308756 A CN 115308756A CN 202110493024 A CN202110493024 A CN 202110493024A CN 115308756 A CN115308756 A CN 115308756A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/06—Systems determining position data of a target
- G01S17/08—Systems determining position data of a target for measuring distance only
- G01S17/10—Systems determining position data of a target for measuring distance only using transmission of interrupted, pulse-modulated waves
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/89—Lidar systems specially adapted for specific applications for mapping or imaging
- G01S17/894—3D imaging with simultaneous measurement of time-of-flight at a 2D array of receiver pixels, e.g. time-of-flight cameras or flash lidar
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/483—Details of pulse systems
- G01S7/486—Receivers
- G01S7/4861—Circuits for detection, sampling, integration or read-out
- G01S7/4863—Detector arrays, e.g. charge-transfer gates
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/483—Details of pulse systems
- G01S7/486—Receivers
- G01S7/4865—Time delay measurement, e.g. time-of-flight measurement, time of arrival measurement or determining the exact position of a peak
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- Physics & Mathematics (AREA)
- Computer Networks & Wireless Communication (AREA)
- General Physics & Mathematics (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Electromagnetism (AREA)
- Transforming Light Signals Into Electric Signals (AREA)
Abstract
The invention discloses a pixel circuit, which is characterized by comprising: the photoelectric conversion unit comprises at least two taps and is used for receiving optical signals and converting the optical signals into electric signals; at least two transmission gates respectively connected with the taps and used for controlling the transmission of the electric signals; at least two floating diffusion capacitors respectively connected to the transmission gate for storing the electrical signals; at least two reset transistors respectively connected with the floating diffusion capacitors and used for resetting the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to the coupling capacitance of the transfer gate and the reset transistor.
Description
Technical Field
The present disclosure relates to image sensors, and particularly to a three-dimensional image sensor.
Background
With the technical development of laser radars, time of Flight (TOF) has been receiving increasing attention, and the TOF principle is to obtain a target distance by continuously transmitting light pulses to a target and then receiving light returning from the object with a sensor and detecting the Time of Flight (round trip) of the light pulses.
Direct Time of Flight (DTOF) and Indirect Time of Flight (ITOF) are used as detection methods developed based on TOF, and the two detection methods have advantages in use and are receiving more and more attention.
The indirect time-of-flight time detection mainly acquires a phase difference relationship between a transmitted wave and a reflected echo of a detected object, and obtains distance information of the detected object by using the phase difference relationship.
Therefore, a problem to be solved in the design of a three-dimensional image sensor for reducing the measurement error is developed.
Disclosure of Invention
An object of the present application is to provide a pixel circuit, an image sensor and a detection device for reducing measurement errors and improving the ranging accuracy, which are not enough in the prior art.
In order to achieve the above purpose, the technical solutions adopted in the embodiments of the present application are as follows:
in a first aspect, the present application provides a pixel circuit, comprising: the photoelectric conversion unit comprises at least two taps and is used for receiving optical signals and converting the optical signals into electric signals; at least two transmission gates respectively connected with the taps and used for controlling the transmission of the electric signals; at least two floating diffusion capacitors respectively connected to the transmission gate for storing the electrical signals; at least two reset transistors respectively connected to the floating diffusion capacitors for resetting the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to a coupling capacitance of the transfer gate and the reset transistor.
Optionally, the pixel circuit further comprises: at least two readout transistors respectively connected with the floating diffusion capacitors and used for reading the electric signals of the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
Optionally, the pixel circuit further includes: at least two source followers respectively connected to the floating diffusion capacitors for reading out electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
In a second aspect, the present application provides an image sensor, comprising: a pixel array and processing circuitry; the pixel array is composed of a plurality of pixel units, and the pixel units comprise: the photoelectric conversion unit comprises at least two taps and is used for receiving optical signals and converting the optical signals into electric signals; at least two transmission gates respectively connected with the taps and used for controlling the transmission of the electric signals; at least two floating diffusion capacitors respectively connected to the transmission gate for storing the electrical signals; at least two reset transistors respectively connected to the floating diffusion capacitors for resetting the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to a coupling capacitance of the transfer gate and the reset transistor.
Optionally, the pixel circuit further comprises: at least two readout transistors respectively connected with the floating diffusion capacitors and used for reading the electric signals of the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
Optionally, the pixel circuit further includes: at least two source followers respectively connected to the floating diffusion capacitors for reading out electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
In a third aspect, the present application provides a probe apparatus, comprising: the transmitting module is used for transmitting a detection signal; a receiving module, configured to receive the probe signal; the receiving module includes an image sensor, the image sensor including: a pixel array comprised of a plurality of pixel cells, the pixel cells comprising: the photoelectric conversion unit comprises at least two taps and is used for receiving optical signals and converting the optical signals into electric signals; at least two transmission gates respectively connected with the taps and used for controlling the transmission of the electric signals; at least two floating diffusion capacitors respectively connected to the transmission gate for storing the electrical signals; at least two reset transistors respectively connected with the floating diffusion capacitors and used for resetting the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to a coupling capacitance of the transfer gate and the reset transistor.
Optionally, the pixel circuit further comprises: at least two readout transistors respectively connected with the floating diffusion capacitors and used for reading the electric signals of the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
Optionally, the pixel circuit further includes: at least two source followers respectively connected to the floating diffusion capacitors for reading out electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
The beneficial effect of this application is:
the present application provides a pixel circuit, comprising: the photoelectric conversion unit comprises at least two taps and is used for receiving optical signals and converting the optical signals into electric signals; at least two transmission gates respectively connected with the taps and used for controlling the transmission of the electric signals; at least two floating diffusion capacitors respectively connected to the transmission gate for storing the electrical signals; at least two reset transistors respectively connected with the floating diffusion capacitors and used for resetting the floating diffusion capacitors; wherein each floating diffusion capacitance is equal to a coupling capacitance of the transfer gate and the reset transistor.
The coupling capacitors of each floating diffusion capacitor, the transmission gate and the reset transistor are designed to be equal, so that the distance measurement error caused by the two floating diffusion capacitors is reduced, and the distance measurement precision of a pixel unit, an image sensor and a detection device is improved.
Drawings
To more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiments will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present application and therefore should not be considered as limiting the scope, and those skilled in the art can also obtain other related drawings based on the drawings without inventive efforts.
Fig. 1 is a schematic diagram of TOF ranging in the prior art provided in an embodiment of the present application;
fig. 2 is a block diagram of an image sensor provided in an embodiment of the present application;
fig. 3 is a block diagram of a pixel unit according to an embodiment of the present disclosure;
fig. 4 is a circuit diagram of a pixel unit according to an embodiment of the present disclosure;
fig. 5 is a timing diagram of a pixel unit according to an embodiment of the present disclosure;
fig. 6 is a circuit diagram of a pixel unit according to an embodiment of the present disclosure.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are some embodiments of the present application, but not all embodiments. The components of the embodiments of the present application, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations.
Thus, the following detailed description of the embodiments of the present application, presented in the accompanying drawings, is not intended to limit the scope of the claimed application, but is merely representative of selected embodiments of the application. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.
It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, it need not be further defined and explained in subsequent figures.
Fig. 1 is a general TOF ranging diagram, which obtains the flight time between a sensor and an object to be measured by using the phase difference between a transmitted light signal and an echo signal, and then calculates to obtain distance information. As shown, a beam of emitted light signals is emitted from a light source 101. The emitted light signal can be a laser pulse signal modulated by a pseudo-random sequence or a common laser pulse signal. The emitted optical signal is reflected by the object 102 and focused by the lens 103 on the pixels of the image sensor 104. The signals received by the image sensor are echo signals and background light signals or only background light signals, and the background light signals are assumed to be uniform within a certain time. The received signal is subjected to signal cancellation or signal extraction through a pixel circuit in the image sensor 104, a background light part is removed, a pure echo signal is obtained, and finally, the phase shift of the echo signal relative to the emitted light signal in the returned light is detected through each pixel, so that the distance of the surrounding object can be detected.
Generally, in practical applications, the light source 101 generally emits light by a sine wave, and the phase shift between the sine wave received by the image sensor 104 and the sine wave emitted by the light source 101 is proportional to the distance of the object 102. Assuming a transmitted sine wave signal of
After a delay t1, the image sensor 104 receives a signal of
When the image sensor 104 receives four times with equal delay intervals, i.e., with 0 °,90 °,180 °,270 ° phase time delays, the four different signals are expressed as formula (1-3);
when the light signal emitted from the light source 101 is received and integrated by the detector, an electrical signal is obtained, which is shown in formulas 1-4, 1-5, 1-6 and 1-7, wherein B is background light noise.
The background light noise can be eliminated by subtracting Q0 from Q180 in the four formulas, and similarly, the background light noise can also be eliminated by subtracting Q90 from Q270. The subtracted signals are shown in formulas 1-8 and 1-9;
to find the phase w Δ t, it is also necessary to eliminate A R The effect of amplitude on phase. So that the formula 1-9 and the formula 1-10 are divided to obtain the formula1-10；
Thus, the phase can be found and the distance R can be found, see equations 1-11;
from the above theoretical formula, the image sensor needs to obtain the number of charges received in the phases of 0 °,90 °,180 °, and 270 °. Generally, an image sensor needs to obtain the number of charges in a specific phase by sampling voltages in pixel cells.
The structure of the TOF image sensor is explained below with reference to fig. 2. As shown in fig. 2, the TOF image sensor includes a pixel unit 201, a pixel array 202, a row selection circuit 203, a readout circuit 204, and a signal processing block 205. The pixel array 202 is a two-dimensional array in which a plurality of pixel units 201 are arranged. The row selection circuit 203 is used to gate the pixels of a row in the pixel array 202, for example, the row selection circuit 203 gates the pixel cells of a first row, and then the signals of the pixel cells of the row are read out by the readout circuit 204, and the readout circuit 204 may be a sample-and-hold circuit. The readout circuit may be connected to the signal processing module 205, and is configured to process the readout row signal to obtain a TOF ranging result.
Fig. 3 is a block diagram of a general TOF pixel circuit, which includes a photoelectric conversion unit 310, a modulation gate 320, and a charge storage device 330. The photoelectric conversion unit 310 is used for receiving an optical signal and generating photo-generated electrons; a modulation gate 320 for controlling the transport of the photo-generated electrons; the charge storage device 330 is used to store the photo-generated electrons.
Illustratively, as shown in fig. 4, the TOF pixel circuit diagram includes a photodiode 401, a reset transistor 402, a first transfer transistor 403, a second transfer transistor 404, a first Floating Diffusion capacitor 405 (FD 1), a second Floating Diffusion capacitor 406 (FD 2), a first source follower 407, a second source follower 408, a first gate transistor 409 (Select titanium, SEL 1), and a second gate transistor 410 (Select titanium, SEL 2).
The working principle of the TOF pixel circuit is explained in detail below with reference to fig. 5.
When the gate TX1 of the first transfer transistor 403 is at a high level, the gate TX2 of the second transfer transistor 404 is at a low level, the first transfer transistor 403 is turned on, the second transfer transistor 404 is turned off, and photo-generated electrons generated in the photodiode 401 are stored on the capacitor 405 through the first modulation transistor 403;
when the gate TX1 of the first transmission transistor 403 is at a low level, the gate TX2 of the second transmission transistor 404 is at a high level, the first transmission transistor 403 is turned off, the second transmission transistor 404 is turned on, and photo-generated electrons generated in the photodiode 401 are stored on the capacitor 406 through the second modulation transistor 404;
thus, the capacitor 405 and the capacitor 409 store information of the reflected signals at different phases, respectively. For example, capacitor 405 and capacitor 409 now store charge information at 0 ° and 180 ° phases of the reflected signal.
It should be noted that the pixel circuit of the image sensor provided in the embodiment of the present application may have a two-tap structure or a four-tap structure, and the present application is not limited in any way.
Taking the example that the phase of the modulation signal is 0 °, in general, the operation of the image processor can be divided into: reset, integrate and read out. At the time of reset, the gate of the reset transistor 402 and the gate of the transfer transistor 404 are turned on, and the charge on the FD is reset; during integration, the gate of the reset transistor 402 is turned off; at the time of reading, the gate of the gate transistor 410 is turned on, and finally, the voltage of the FD1 is output through the read signal line. In this process, each signal transition causes a change in FD1, i.e., a coupling capacitance on FD, which in turn affects the readout FD1 voltage, and since the voltage on FD1 is indicative of the number of electrons in the integration time, these signal transitions also cause an error in the readout voltage. In addition, the reset transistor 402, the gate transistor 410, and the pass transistor 404 have channel charges in the channel region when they are turned on, and at the instant of turning on or off, the charges flow into or out of the MOS switch, thereby changing the voltage of the corresponding node, and these errors can cause the measurement error of the image sensor.
In addition, since the TOF image sensor includes at least two floating diffusion capacitors, such as two-tap or four-tap pixels, in the structure of the two-tap pixel, the difference of the coupling capacitance in the two taps may cause the integrated charge to be inconsistent, thereby affecting the ranging error.
A two-tap pixel is taken as an example, and a specific implementation of the pixel circuit provided in the present application is described with reference to fig. 6.
The pixel circuit shown in fig. 6 includes:
a two-tap pixel including a photoelectric conversion portion 601 for receiving an optical signal and converting the optical signal into an electrical signal, wherein pixel taps, namely an electron well PGA and an electron well PGB, are disposed at two sides of the photoelectric conversion portion, and different taps collect electrons at different phases, for example, PGA collects electrons at a phase of 0 ° and PGB collects electrons at a phase of 180 °;
two transfer gates, as shown in fig. 6, exemplarily, a first transfer gate TX1 on the left and a second transfer gate TX2 on the right, and accordingly, a first floating diffusion capacitance FD1 and a second floating diffusion capacitance FD2 are further included. When the gate of the transfer gate is opened, electrons can be transferred from the electron well PGA to the FD1 through the transfer gate TX1 and stored, and accordingly, electrons can be transferred from the electron well PGB to the FD2 through the transfer gate TX2 and stored. For example, in the phase of 0 °, electrons may be transferred from the electron well PGA to the FD1 through the transfer gate TX1, and in the phase of 180 °, electrons may be transferred from the electron well PGB to the FD2 through the transfer gate TX 2.
In addition, the pixel circuit further includes a first reset transistor 602, a second reset transistor 603, a first source follower 604, and a second source follower 605. When electrons are stored to FD1 or FD2, current will pass through the first source follower 604 and the first gate transistor 606 to output Vout _ a, or pass through the second source follower 605 and the second gate transistor 607 to output Vout _ B.
When the pixel circuit starts to operate, a gate signal of the first reset transistor 602 or the second reset transistor 603 jumps, that is, when the FD1 or the FD2 is reset, the jump on the signal line may generate a coupling capacitance to the FD1 or the FD2, it should be noted that a source of the first reset transistor 602 or the second reset transistor 603 is connected to a power supply, and a disturbance of a signal on the power supply line may also cause the coupling capacitance to the FD1 or the FD2, and similarly, a gate signal of the first gate transistor 606 or the second gate transistor 607 jumps, that is, when an electrical signal on the FD1 or the FD2 is read, the jump on the signal line may also generate a coupling capacitance to the FD1 or the FD2.
Therefore, it can be seen that the coupling capacitance of FD1 and the coupling capacitance of FD2 are disturbed by signals on different transistors, respectively, which results in asymmetry of the coupling capacitances of FD1 and FD2. Therefore, the pixel circuit provided by the application ensures that the coupling capacitances of the FD1 and the FD2 are equal through circuit and layout design. Therefore, the distance measurement errors of the read-out pixels of the two TAPs can be ensured to be equal left and right, and the precision of the pixels is improved.
It is to be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrases "comprising one of 8230; \8230;" 8230; "does not exclude the presence of additional like elements in a process, method, article, or apparatus that comprises the element.
The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made to the present application by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application. It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, it need not be further defined and explained in subsequent figures. The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.
Claims (9)
1. A pixel circuit, comprising:
the photoelectric conversion unit comprises at least two taps and is used for receiving optical signals and converting the optical signals into electric signals;
at least two transmission gates respectively connected to the taps for controlling the transmission of the electrical signals;
at least two floating diffusion capacitors respectively connected to the transmission gate for storing the electrical signals;
at least two reset transistors respectively connected to the floating diffusion capacitors for resetting the floating diffusion capacitors;
wherein each floating diffusion capacitance is equal to the coupling capacitance of the transfer gate and the reset transistor.
2. The pixel circuit of claim 1, wherein the pixel circuit further comprises:
at least two readout transistors respectively connected to the floating diffusion capacitors for reading electrical signals of the floating diffusion capacitors;
wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
3. The pixel circuit according to claim 1 or 2, wherein the pixel circuit further comprises:
at least two source followers respectively connected to the floating diffusion capacitors for reading out electrical signals of the floating diffusion capacitors;
wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
4. An image sensor, comprising:
a pixel array and processing circuitry;
the pixel array is composed of a plurality of pixel units, and the pixel units comprise:
the photoelectric conversion unit comprises at least two taps and is used for receiving optical signals and converting the optical signals into electric signals;
at least two transmission gates respectively connected to the taps for controlling the transmission of the electrical signals;
at least two floating diffusion capacitors respectively connected to the transmission gate for storing the electrical signals;
at least two reset transistors respectively connected with the floating diffusion capacitors and used for resetting the floating diffusion capacitors;
wherein each floating diffusion capacitance is equal to a coupling capacitance of the transfer gate and the reset transistor.
5. The image sensor of claim 4, wherein the pixel cell further comprises:
at least two readout transistors respectively connected to the floating diffusion capacitors for reading electrical signals of the floating diffusion capacitors;
wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
6. The image sensor of claim 4 or 5, wherein the pixel cell further comprises:
at least two source followers respectively connected to the floating diffusion capacitors for reading out electrical signals of the floating diffusion capacitors;
wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
7. A probe apparatus, comprising:
the transmitting module is used for transmitting a detection signal;
a receiving module, configured to receive the probe signal;
the receiving module includes an image sensor, the image sensor including:
a pixel array comprised of a plurality of pixel cells, the pixel cells comprising:
the photoelectric conversion unit comprises at least two taps and is used for receiving optical signals and converting the optical signals into electric signals;
at least two transmission gates respectively connected to the taps for controlling the transmission of the electrical signals;
at least two floating diffusion capacitors respectively connected to the transmission gate for storing the electrical signals;
at least two reset transistors respectively connected with the floating diffusion capacitors and used for resetting the floating diffusion capacitors;
wherein each floating diffusion capacitance is equal to a coupling capacitance of the transfer gate and the reset transistor.
8. The detection apparatus of claim 7, wherein the pixel cell further comprises:
at least two readout transistors respectively connected with the floating diffusion capacitors and used for reading the electric signals of the floating diffusion capacitors;
wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
9. A detection arrangement according to claim 7 or 8, wherein the pixel cell further comprises:
at least two source followers respectively connected to the floating diffusion capacitors for reading out electrical signals of the floating diffusion capacitors;
wherein each floating diffusion capacitance is equal to a coupling capacitance of the readout transistor.
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