High-precision and low-power readout method and circuit for floating gate image sensor

Abstract

The present invention discloses a high-precision, low-power readout method and circuit for a floating gate image sensor, and belongs to the technical field of integrated circuits. The present invention introduces two bias currents, adds one bias current to the pixel current and then copies it, effectively improving the copy accuracy of the pixel current; by controlling the difference between the two bias currents, the influence of the leakage current on the circuit is offset, ensuring that the high-sensitivity positive feedback circuit will not be erroneously flipped due to the leakage current; a high-sensitivity positive feedback circuit is designed to replace the comparator in the traditional SS-ADC design, and the output flip of the high-sensitivity positive feedback circuit is only determined by the sudden change current of the pixel device from the subthreshold region to the linear region, thereby accelerating the flipping speed to improve the readout accuracy and reduce the system power consumption; a correlated double sampling processing circuit is designed to eliminate the readout noise introduced by the inconsistency of the array threshold voltage, further improve the readout accuracy, and at the same time, the on-chip implementation reduces the data transmission amount and further reduces the system power consumption.

Description

Technical Field

[0001] The invention relates to a high-precision, low-power consumption readout method and circuit for a floating gate image sensor, belonging to the technical field of integrated circuits. Background Art

[0002] Image sensors are widely used in scientific research, motion analysis, machine vision, aerospace and other fields. The current mainstream image sensors include CCD and CMOS image sensors. As a new type of image sensor, floating gate image sensors have special floating gate pixel units. Their pixel size is small and the number of full well electrons is high, making it possible to be used in high-end aerospace, microscopy, wearable devices and other fields. During the process of photosensitive photography, the device can sequentially perform photoelectric conversion, store photoelectrons and convert the photoelectron charge into a threshold voltage. The threshold voltage of the pixel device is positively correlated with the light intensity. Therefore, the photoelectron charge can be effectively read out by reading the threshold voltage, thereby reflecting the light intensity. The wide application demand has led to the rapid development of floating gate image sensors. At the same time, the current floating gate image sensors have also encountered bottlenecks in their own technical characteristics.

[0003] As one of the important components of image sensors, the performance of the pixel array determines the quality of the image to a certain extent. The device will be electrically erased before each exposure to expel the electrons accumulated by dark current or residual electrons from the previous exposure, so that the threshold voltage of the device returns to a certain reset value. For the entire array, the threshold voltages of different devices will be different, resulting in noise interference caused by the inconsistency of the array threshold voltage, which reduces the accuracy of the readout circuit.

[0004] As the core module of floating gate image sensors, the readout circuit aims to quantify the threshold voltage converted by the device after being exposed to light. Its performance largely determines the noise level, power consumption, durability and design cost of the image sensor. When floating gate image sensors are used in the aerospace field, we hope that the imaging quality is as high as possible, which puts high demands on the accuracy of the readout circuit; when it is used in embedded and wearable devices, while ensuring high readout accuracy, its power consumption is often limited. Therefore, it is particularly important to design a high-precision and low-power readout circuit to improve the imaging quality of floating gate image sensors and reduce system power consumption.

[0005] Liu Hui proposed that by adding a ramp voltage to the gate end of the floating gate device, the current generated by the pixel device is used to discharge the pre-charged capacitor, and the voltage on the upper plate of the capacitor is compared with the reference voltage of the comparator, thereby controlling the stop of the counter and reading the threshold voltage of the pixel device (Liu Hui. Design of Submicron Floating Gate Image Sensor System [D]. Tsinghua University, 2014.). However, this solution requires that the capacitance of the capacitor be large to solve the problem that the small current in the subthreshold region causes the capacitor to be filled in advance and the comparator to flip in advance. Therefore, the readout circuit proposed in this solution has low readout accuracy and high power consumption.

[0006] In a readout circuit provided by patent CN114025114A, after the switch unit receives the pixel signal, the ramp signal is compared with the pixel signal voltage in the SS-ADC, and the counter is stopped by the flipping of the comparator. This method can realize the readout of pixel information, but the readout will have errors due to the slow flipping speed of the comparator and the circuit power consumption is large, which is not desirable for the integration of ultra-large-scale pixel arrays.

[0007] In patent CN110418089A, the pixel bit line voltage is compared with N threshold voltages respectively, and the counter selects the enable signal of the corresponding interval from N+1 counter enable signals according to the interval where the pixel voltage is located. This solution can reduce the power consumption of the counter and solve the problem of image quality degradation due to heating. However, it does not consider the dark current, optical gain, and threshold voltage offset caused by process deviation of pixel devices. This noise will seriously affect the image quality of the image sensor. Summary of the invention

[0008] In order to solve the problems of large error and high power consumption of the current floating gate image sensor readout circuit, the present invention provides a high-precision and low-power readout method and circuit for the floating gate image sensor. The technical solution is as follows:

[0009] The first object of the present invention is to provide a floating gate image sensor readout circuit for reading the threshold voltage of a pixel device, the circuit comprising: a current bias circuit, a clamp circuit, a current copy circuit, a high-sensitivity positive feedback circuit and a correlated double sampling processing circuit;

[0010] The input terminal of the pixel device array is connected to a pixel input voltage, and the pixel input voltage is: a linear voltage related to time;

[0011] The clamp circuit is connected to the pixel device and is used to control the source-drain voltage of the pixel device to maintain a constant value during the readout process;

[0012] The current bias circuit is used to generate a first bias current I bias and the second bias current I bias+ΔI, the first bias current I bias The pixel current I generated by the pixel device pixel Superimpose input to the copied end of the current copy circuit;

[0013] The current copy circuit will superimpose the current I bias +I pixel to the copy terminal and with the second bias current I bias +ΔI subtracted to get the capacitor charging current I ch arg e =(I bias +ΔI)-(I bias +I pixel ), the capacitor charging current I ch arg e Input the high-sensitivity positive feedback circuit;

[0014] The high-sensitivity positive feedback circuit includes a comparison capacitor and an inverter. The capacitor charging current I ch arg e The upper plate of the comparison capacitor is charged. When the voltage of the upper plate of the comparison capacitor is charged to the threshold voltage of the inverter, the output of the inverter is flipped to a low level. At the same time, the positive feedback structure pulls up the voltage of the upper plate of the capacitor, and the output of the high-sensitivity positive feedback circuit is pulled down to complete the level flip;

[0015] The correlated double sampling processing circuit outputs a level signal according to the high-sensitivity positive feedback circuit, counts twice in the reset phase and the exposure phase of the pixel device, completes the correlated double sampling processing, and realizes the reading and digital conversion of the threshold voltage of the pixel device.

[0016] Optionally, the correlated double sampling processing circuit includes: an up-down counter, which starts counting when a pixel input voltage is applied to the pixel device and stops counting when the output level of the high-sensitivity positive feedback circuit flips, thereby realizing the conversion of the pixel device threshold voltage to a digital count value.

[0017] Optionally, the high-sensitivity positive feedback circuit includes: a comparison capacitor C comp , feedback PMOS tube P0 and inverter;

[0018] The comparison capacitor C comp The upper plate is connected to the capacitor charging current I ch arg e , the lower plate is grounded;

[0019] The source of the feedback PMOS tube P0 is connected to the comparison capacitor C comp The upper plate is connected to the upper plate of the inverter, the drain is connected to the power supply VDD, and the gate is connected to the output end of the inverter;

[0020] The input terminal of the inverter is connected to the comparison capacitor C compThe output end is connected to the enable end of the up-down counter.

[0021] A second object of the present invention is to provide a floating gate image sensor readout method, which is implemented based on the above floating gate image sensor readout circuit, and comprises:

[0022] Step 1: The clamping circuit controls the drain-source voltage of the pixel device to remain constant, and the current bias circuit generates a first bias current I bias and the second bias current I bias +ΔI, the up and down counters are in standby state;

[0023] Step 2: Apply the pixel input voltage to the pixel device input terminal, the positive feedback circuit output is VDD, and the up and down counters start counting;

[0024] Step 3: gradually increase the pixel input voltage, the pixel device output terminal generates a pixel current, and the current copy circuit simultaneously copies the pixel current and the first bias current I bias The sum of the second bias current I bias +ΔI is subtracted to generate the charging current I ch arg e Rapidly charging the comparison capacitor;

[0025] Step 4: When the upper plate voltage of the comparison capacitor is gradually charged to the inverter threshold voltage, the inverter output gradually flips to a low level. At the same time, the positive feedback structure quickly pulls up the upper plate voltage of the capacitor, and the positive feedback circuit output quickly pulls down to complete the flip, and the up and down counters stop counting, thereby realizing the conversion of the pixel device threshold voltage to the digital count value.

[0026] Optionally, the threshold voltage is read out twice in step 4, including: a reset readout stage and an exposure readout stage;

[0027] In the reset readout stage, the pixel input voltage is applied to the pixel device input terminal, the pixel input voltage is gradually increased, the readout circuit reads the threshold voltage of the pixel device before exposure, and the up-down counter counts downward;

[0028] In the exposure readout stage, the pixel input voltage is applied to the input end of the pixel device, and the pixel input voltage is gradually increased. The readout circuit reads the threshold voltage of the pixel device before exposure, and the up-down counter counts upward based on the count value in the reset readout stage, thereby completing the correlated double sampling process.

[0029] Optionally, the drain voltage of the pixel device is fixed by a clamping circuit, and the source terminal is grounded, that is, the pixel input voltage V in Equivalent to the MOS tube gate-source voltage V GS, the pixel current formula is equivalent to the MOS tube linear current formula:

[0030]

[0031] Where μ is the electron mobility of the pixel unit, C OX is the pixel unit gate oxide capacitance, is the aspect ratio of the pixel unit, V GS is the pixel input voltage, V th is the pixel unit threshold voltage, V DS is the drain-source voltage of the pixel unit.

[0032] Optionally, the reset readout phase includes:

[0033] The pixel input voltage V in After being added to the pixel device input terminal, the up-down counter starts counting down, gradually increasing the pixel input voltage V in , the current copy circuit will reset the read current I rst With the bias current I bias The sum is copied and the bias current I bias +ΔI is subtracted to generate the charging current I ch arg e For comparison capacitor C comp For fast charging, the comparison capacitor C comp When the upper plate voltage is charged to the inverter threshold voltage, the output of the high-sensitivity positive feedback circuit is reversed, the counter stops counting, and the reading of the pixel device threshold voltage before exposure is completed;

[0034] The reset readout current I generated at the output of the pixel device rst for:

[0035]

[0036] Among them, V th0 is the threshold voltage of the pixel device before exposure.

[0037] Optionally, the exposure readout stage includes:

[0038] After exposure is completed, the pixel input voltage V in After being added to the pixel device input terminal, the up-down counter starts counting upward based on the count value of the reset readout phase counter, gradually increasing the pixel input voltage, and the current copy circuit will read the exposure current I sig With the bias current I bias The sum is copied and the bias current I bias +ΔI is subtracted to generate the charging current I ch arg e The comparison capacitor C compFor fast charging, the comparison capacitor C comp When the upper plate voltage is charged to the inverter threshold voltage, the output of the high-sensitivity positive feedback circuit is reversed, the counter stops counting, and the reading of the pixel device threshold voltage after exposure is completed;

[0039] The exposure readout current I generated by the pixel device BL terminal sig for:

[0040]

[0041] Among them, V th0 +ΔV th is the threshold voltage of the pixel device after exposure.

[0042] The third object of the present invention is to provide a floating gate image sensor, comprising a pixel device array and the above-mentioned floating gate image sensor readout circuit, wherein the threshold voltage of the pixel device is read out by the readout circuit.

[0043] The beneficial effects of the present invention are:

[0044] (1) When reading the threshold voltage of a pixel device, the present invention introduces two bias currents, adds one of the bias currents to the pixel current and then copies it, thereby solving the problem that the pixel current generated by the pixel device in the subthreshold region is small and the copying accuracy of the current copying circuit is too low, and effectively improving the copying accuracy of the pixel current; by controlling the difference between the two bias currents, the influence of the leakage current on the circuit is offset, ensuring that the high-sensitivity positive feedback circuit will not be erroneously flipped due to the leakage current;

[0045] (2) Design a high-sensitivity positive feedback circuit to replace the comparator in the traditional SS-ADC design, ensuring that the output flip of the high-sensitivity positive feedback circuit is only determined by the sudden change of the pixel device from the subthreshold region to the linear region. At the same time, it speeds up the flipping speed, improves the readout accuracy and reduces the system power consumption.

[0046] (3) Design a correlated double sampling processing circuit to eliminate the readout noise introduced by the inconsistency of the array threshold voltage, further improving the readout accuracy. At the same time, the on-chip implementation reduces the amount of data transmission and further reduces the system power consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0048] Figure 1It is a framework diagram of the image sensor pixel array readout system of the present invention.

[0049] Figure 2 This is a circuit diagram of a high-speed, low-power readout system provided in Embodiment 1 of the present invention.

[0050] Figure 3 It is a circuit diagram of a high-speed and low-power readout system in a reset readout phase provided by the first embodiment of the present invention.

[0051] Figure 4 It is a circuit diagram of a high-speed and low-power readout system in the exposure readout phase provided by the first embodiment of the present invention. DETAILED DESCRIPTION

[0052] To make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0053] Embodiment 1:

[0054] This embodiment provides a floating gate image sensor readout circuit, such as Figure 1 As shown, it includes: a current bias circuit, a clamp circuit, a current copy circuit, a high-sensitivity positive feedback circuit and an up-down counter.

[0055] The pixel device completes exposure under light conditions, and the pixel input voltage V in is applied to the WL terminal (WL0, WL1…WLN) of the pixel array, and the pixel device is connected to the pixel through the pixel input voltage V in Control conduction, generate current output from the BL terminal (BL0, BL1...BLN) and transmit it to the readout circuit.

[0056] like Figure 2 As shown, the clamp circuit is connected to the pixel device to control the source-drain voltage of the pixel device to maintain a constant value V during the readout process. ref .

[0057] The current bias circuit is designed to change the width-to-length ratio of the left and right transistors N0-N3, so that it generates two bias currents of unequal magnitude, namely the first bias current I bias and the second bias current I bias +ΔI, first bias current I bias The pixel current I generated by the pixel device pixel Superimpose input to the copied end of the current copy circuit;

[0058] The current copy circuit will superimpose the current I bias +I pixel to the copy terminal and with the second bias current I bias +ΔI subtracted to get the capacitor charging current I ch arg e =(Ibias +ΔI)-(I bias +I pixel ), the capacitor charging current I ch arg e Input the high-sensitivity positive feedback circuit;

[0059] The high-sensitivity positive feedback circuit includes a comparison capacitor C comp , feedback PMOS tube P0 and inverter. Comparison capacitor C comp The upper plate is connected to the capacitor charging current I ch arg e , the lower plate is grounded; the source of the feedback PMOS tube P0 is connected to the comparison capacitor C comp The upper plate is connected to the drain of the inverter, the gate is connected to the output of the inverter, and the input of the inverter is connected to the comparison capacitor C comp The output end is connected to the enable end of the up / down counter.

[0060] Capacitor charging current I ch arg e For comparison capacitor C comp When the upper plate voltage of the comparison capacitor is charged to the inverter threshold voltage, the inverter output flips to a low level, and the positive feedback structure will charge the comparison capacitor C comp The voltage on the upper plate is quickly pulled up, and the output of the high-sensitivity positive feedback circuit is quickly pulled down, completing the level flip;

[0061] The up-down counter starts counting when the pixel input voltage is applied to the pixel device, and stops counting when the output level of the high-sensitivity positive feedback circuit is reversed, thereby realizing the conversion of the pixel device threshold voltage to the digital count value.

[0062] Embodiment 2:

[0063] This embodiment provides a floating gate image sensor readout method, which is implemented based on the floating gate image sensor readout circuit described in the first embodiment.

[0064] like Figure 1 As shown, the readout circuit used in this embodiment includes: a clamping circuit, a current biasing circuit, a current copying circuit, a high-sensitivity positive feedback circuit and an up-down counter.

[0065] The pixel device completes exposure under light conditions, and the pixel input voltage V in Control conduction, generate current from BL output, and transmit to the readout circuit. Pixel input voltage V in It is a linear voltage related to time (such as a ramp voltage). Combined with the count value of the up and down counter, the threshold voltage of the pixel device when the counting stops can be read out.

[0066] like Figure 2As shown, the clamp circuit controls the source-drain voltage at both ends of A and B of the pixel device to maintain a constant value V during the readout process. ref .

[0067] By changing the width-to-length ratio of transistors N0 to N3 on the left and right sides of the current bias circuit, two bias currents I with unequal magnitudes are generated. bias with I bias +ΔI.

[0068] Pixel input voltage V in Before being added to the pixel array WL terminal, the counter is in standby state.

[0069] Pixel input voltage V in When it is added to WL, the high-sensitivity positive feedback circuit outputs VDD, and the up-down counter starts counting, gradually increasing the pixel input voltage V in , the pixel device BL terminal generates a pixel current I pixel , the current copy circuit will simultaneously copy the pixel current I pixel With the bias current I bias The sum of the bias current I bias +ΔI is subtracted to generate the charging current I ch arg e For comparison capacitor C comp For fast charging, the charging current I ch arg e for:

[0070] I ch arg e =(I bias +ΔI)-(I bias +I pixel ) (2)

[0071] When the comparison capacitor C comp When the upper plate voltage is gradually charged to the threshold voltage of the inverter composed of NMOS tube N4 and PMOS tube P1, the inverter output gradually flips to a low level, and the feedback PMOS tube P0 is gradually turned on due to the change of the inverter output, so that the positive feedback structure compares the capacitor C comp The upper plate voltage is quickly pulled up to the power supply voltage VDD, and the output of the high-sensitivity positive feedback circuit is quickly pulled down to complete the flip, and the up and down counters stop counting. At this time, the count value output by the counter is the digital form of the threshold voltage, thereby realizing the conversion of the pixel device threshold voltage to the digital count value.

[0072] In this embodiment, the performance of the current bias circuit and the high-sensitivity positive feedback circuit directly determines the accuracy of the readout circuit. Specifically, considering that the current generated by the BL terminal is small when the pixel device is in the subthreshold region, it is difficult for the current copy circuit to maintain a high copy accuracy; and in order to ensure that the high-sensitivity positive feedback circuit flips quickly, the comparison capacitor C compThe capacitance is very small, and the leakage current of the circuit will cause the high-sensitivity positive feedback circuit to flip prematurely, resulting in a large error in the readout of the threshold voltage of the pixel device.

[0073] Therefore, two bias currents I are introduced bias ,I bias +ΔI, on the one hand, it can improve the current copy circuit's high copy accuracy when the pixel device is in the subthreshold region and low current stage; on the other hand, by controlling the difference ΔI between the two bias currents, the influence of the leakage current on the circuit is offset, and as the pixel current I pixel The gradual increase of I pixel +I bias Gradually approaching I bias +ΔI and I pixel +I bias bias +ΔI,I ch arg e Will not affect the comparison capacitor C comp When the pixel input voltage reaches the pixel device threshold voltage, I pixel Mutation occurs, I pixel +I bias >I bias +ΔI,I ch arg e For comparison capacitor C comp Rapid charging ensures that the output flip of the high-sensitivity positive feedback circuit is only determined by the sudden change of the pixel device from the subthreshold region to the linear region, which increases the flipping speed and reduces the readout error, thereby completing the high-precision readout of the pixel device threshold voltage.

[0074] In this embodiment, in order to further improve the accuracy of the readout circuit, eliminate the readout noise introduced by the inconsistency of the array threshold voltage and reduce the data transmission volume, the threshold voltage is read out twice, which is divided into reset readout and exposure readout. The device drain voltage is fixed by the clamp circuit, and the source terminal is grounded, that is, the pixel input voltage V in Equivalent to the MOS tube gate-source voltage V GS , the formula for reading the current can be equivalent to the MOS tube linear current formula:

[0075]

[0076] like Figure 3 As shown, in the reset readout phase, the pixel input voltage V in After being added to the pixel device WL, the counter starts to count down, gradually increasing the pixel input voltage, and the current copy circuit will reset the read current I rst With the bias current I bias The sum is copied and the bias current I bias +ΔI is subtracted to generate the charging current I ch arg e For comparison capacitor C​comp When the capacitor upper plate voltage is charged to the inverter threshold voltage, the output of the high-sensitivity positive feedback circuit is reversed, the counter stops counting, and the threshold voltage of the pixel device before exposure is read. The reset readout current I generated by the pixel device BL terminal rst for:

[0077]

[0078] Among them, V th0 is the threshold voltage of the pixel device before exposure.

[0079] like Figure 4 As shown, in the exposure readout phase, the pixel input voltage V in After being added to the pixel device WL, the counter starts counting up based on the count value of the counter in the reset readout phase, gradually increasing the pixel input voltage, and the current copy circuit will read the exposure current I sig With the bias current I bias The sum is copied and the bias current I bias +ΔI is subtracted to generate the charging current I ch arg e For comparison capacitor C comp When the capacitor upper plate voltage is charged to the inverter threshold voltage, the output of the high-sensitivity positive feedback circuit is reversed, the counter stops counting, and the reading of the pixel device threshold voltage after exposure is completed. The exposure readout current I generated by the pixel device BL terminal sig for:

[0080]

[0081] Where, ΔV th is the threshold voltage applied during exposure, i.e., the threshold voltage to be read out in this embodiment, V th0 +ΔV th is a threshold voltage applied based on the threshold voltage of the pixel device before exposure.

[0082] Subtracting equation (4) from equation (5) yields:

[0083]

[0084] The pixel device threshold voltage to be read in this embodiment is only related to ΔV th Regarding, ΔV th It directly reflects the intensity of the exposure light, thus solving the problem of pixel array threshold voltage inconsistency caused by threshold voltage drift before and after exposure, further improving the imaging quality. At the same time, the on-chip implementation reduces the amount of data transmission and further reduces system power consumption.

[0085] Some steps in the embodiments of the present invention may be implemented using software, and the corresponding software program may be stored in a readable storage medium, such as a CD or a hard disk.

[0086] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A floating gate image sensor readout circuit for reading out the threshold voltage of a pixel device, characterized in that: The floating gate image sensor readout circuit includes: a current bias circuit, a clamp circuit, a current copy circuit, a high-sensitivity positive feedback circuit and a correlated double sampling processing circuit; The input terminal of the pixel device array is connected to a pixel input voltage, and the pixel input voltage is: a linear voltage related to time; The clamp circuit is connected to the pixel device and is used to control the source-drain voltage of the pixel device to maintain a constant value during the readout process, so that the current generated by the pixel device is only determined by the pixel input voltage; The current bias circuit is used to generate two bias currents of unequal magnitude, namely, a first bias current I bias and a second bias current, wherein the first bias current I bias The pixel current I generated by the pixel device pixel Superimpose input to the copied end of the current copy circuit; The current copy circuit will superimpose the current I bias +I pixel to the copy terminal and with the second bias current I bias +ΔI subtracted to get the capacitor charging current I charge =(I bias +ΔI)-(I bias +I pixel ), the capacitor charging current I charge Input the high-sensitivity positive feedback circuit; The high-sensitivity positive feedback circuit includes a comparison capacitor and an inverter. The capacitor charging current I charge The upper plate of the comparison capacitor is charged. When the voltage of the upper plate of the comparison capacitor is charged to the threshold voltage of the inverter, the output of the inverter is flipped to a low level. At the same time, the positive feedback structure will quickly increase the voltage of the upper plate of the capacitor, and the output of the high-sensitivity positive feedback circuit will quickly decrease, thereby accelerating the completion of the level flip; The correlated double sampling processing circuit outputs a level signal according to the high-sensitivity positive feedback circuit, counts twice in the reset phase and the exposure phase of the pixel device, completes the correlated double sampling processing, and realizes the reading and digital conversion of the threshold voltage of the pixel device.

2. The floating gate image sensor readout circuit according to claim 1, characterized in that: The correlated double sampling processing circuit includes an up-down counter, which starts counting when a pixel input voltage is applied to a pixel device and stops counting when an output level of the high-sensitivity positive feedback circuit is flipped, thereby realizing the conversion of a pixel device threshold voltage to a digital count value.

3. The floating gate image sensor readout circuit according to claim 2, characterized in that: The high-sensitivity positive feedback circuit also includes: a feedback PMOS tube P0; The upper plate of the comparison capacitor is connected to the capacitor charging current I charge , the lower plate is grounded; The source of the feedback PMOS tube P0 is connected to the upper plate of the comparison capacitor, the drain is connected to the power supply VDD, and the gate is connected to the output end of the inverter; The input end of the inverter is connected to the upper plate of the comparison capacitor, and the output end is connected to the enable end EN of the up-down counter.

4. A floating gate image sensor readout method, characterized in that: The floating gate image sensor readout method is implemented based on the floating gate image sensor readout circuit according to claim 2, comprising: Step 1: The clamping circuit controls the drain-source voltage of the pixel device to remain constant, and the current bias circuit generates a first bias current I bias and the second bias current I bias +ΔI, the up and down counters are in standby state; Step 2: Apply the pixel input voltage to the pixel device input terminal, the high-sensitivity positive feedback circuit outputs VDD, and the up and down counters start counting; Step 3: gradually increase the pixel input voltage, the pixel device output terminal generates a pixel current, and the current copy circuit simultaneously copies the pixel current and the first bias current I bias The sum of the second bias current I bias +ΔI is subtracted to generate the charging current I charge Rapidly charging the comparison capacitor; Step 4: When the upper plate voltage of the comparison capacitor is gradually charged to the inverter threshold voltage, the inverter output gradually flips to a low level. At the same time, the positive feedback structure quickly pulls up the upper plate voltage of the capacitor, and the output of the high-sensitivity positive feedback circuit is quickly pulled down to complete the flip, and the up and down counters stop counting, thereby realizing the conversion of the pixel device threshold voltage to the digital count value.

5. The floating gate image sensor readout method according to claim 4, characterized in that: The step 4 performs two threshold voltage readouts, including a reset readout phase and an exposure readout phase; In the reset readout stage, the pixel input voltage is applied to the pixel device input terminal, the pixel input voltage is gradually increased, the readout circuit reads the threshold voltage of the pixel device before exposure, and the up-down counter counts downward; In the exposure readout stage, the pixel input voltage is applied to the input end of the pixel device, and the pixel input voltage is gradually increased. The readout circuit reads the threshold voltage of the pixel device before exposure, and the up-down counter counts upward based on the count value in the reset readout stage, thereby completing the correlated double sampling process.

6. The floating gate image sensor readout method according to claim 5, characterized in that: The drain voltage of the pixel device is fixed by the clamping circuit, and the source terminal is grounded, that is, the pixel input voltage V in Equivalent to the MOS tube gate-source voltage V GS , the pixel current formula is equivalent to the MOS tube linear current formula: Where μ is the electron mobility of the pixel unit, C OX is the pixel unit gate oxide capacitance, is the aspect ratio of the pixel unit, V GS is the pixel input voltage, V th is the pixel unit threshold voltage, V DS is the drain-source voltage of the pixel unit.

7. The floating gate image sensor readout method according to claim 6, characterized in that: The reset readout phase includes: Before exposure, the pixel input voltage V in After being added to the pixel device input terminal, the up-down counter starts counting down, gradually increasing the pixel input voltage V in , the current copy circuit will reset the read current I rst With the bias current I bias The sum is copied and the bias current I bias +ΔI is subtracted to generate the charging current I charge The comparison capacitor is quickly charged. When the upper plate voltage of the comparison capacitor is charged to the inverter threshold voltage, the output of the high-sensitivity positive feedback circuit is reversed, the counter stops counting, and the reading of the pixel device threshold voltage before exposure is completed; The reset readout current I generated at the output of the pixel device rst for: Among them, V th0 is the threshold voltage of the pixel device before exposure.

8. The floating gate image sensor readout method according to claim 7, characterized in that: The exposure readout phase includes: After exposure is completed, the pixel input voltage V in After being added to the pixel device input terminal, the up-down counter starts counting upward based on the count value of the reset readout phase counter, gradually increasing the pixel input voltage, and the current copy circuit will read the exposure current I sig With the bias current I bias The sum is copied and the bias current I bias +ΔI is subtracted to generate the charging current I charge The comparison capacitor is quickly charged. When the upper plate voltage of the comparison capacitor is charged to the inverter threshold voltage, the output of the high-sensitivity positive feedback circuit is reversed, the counter stops counting, and the reading of the threshold voltage of the pixel device after exposure is completed; The exposure readout current I generated by the pixel device BL terminal sig for: Among them, V th0 +ΔV th is the threshold voltage of the pixel device after exposure.

9. A floating gate image sensor, comprising a pixel device array, characterized in that: It also includes the floating gate image sensor readout circuit according to any one of claims 1 to 3, and the threshold voltage of the pixel device is read out by using the readout circuit.

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