Temperature correction method and readout circuit for ramp voltage of floating gate image sensor

Abstract

The present invention discloses a temperature correction method and a readout circuit for a ramp voltage of a floating gate image sensor, and belongs to the technical field of integrated circuits. The present invention generates a ramp generation circuit reference voltage having the same temperature characteristics as a device threshold voltage through a temperature coding and voltage generation module, controls the integration time to be consistent with that under normal temperature conditions, and generates a ramp voltage having the same temperature characteristics as the device threshold voltage, thereby realizing on-chip temperature correction of the image sensor readout circuit, and effectively solves the problem of device threshold voltage drift caused by temperature changes, which reduces the accuracy of the readout circuit, without changing the process parameters of the pixel device and increasing the size of the pixel device, thereby improving the imaging quality of the image sensor.

Description

Technical Field

[0001] The invention relates to a temperature correction method and a readout circuit for a slope voltage of a floating gate type image sensor, and belongs 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 wide application demand has led to the rapid development of image sensors. At the same time, current image sensors have also encountered bottlenecks in their own technical characteristics. For example, when image sensors are used in the aerospace field, it is hoped that the image quality is as high as possible, but due to the complexity of the space environment, the image quality of image sensors is greatly affected by temperature. As the core module of the image sensor, the readout circuit plays a decisive role in the image quality. The error of the temperature characteristic has a great influence on the accuracy of the readout circuit. The change in temperature will cause the threshold voltage of the imaging device to drift or even damage the device, thereby reducing the readout accuracy of the pixel and deteriorating the imaging quality. Therefore, it is particularly important to perform temperature correction on the readout circuit of the image sensor to suppress the influence of temperature drift on the accuracy of the readout circuit, thereby improving the imaging quality of the image sensor.

[0003] At present, the accuracy problem of the readout circuit caused by temperature can be roughly divided into indirect compensation and direct compensation. Indirect compensation uses image enhancement algorithms to improve the image quality. In patent CN110850500A, after the image information is collected, the multi-segment correction compensation parameters of different temperature segments are corrected by a single-point correction algorithm outside the chip. It has a good effect on a single image, but for the image data stream, the change in temperature reduces its improvement effect. Direct compensation is divided into physical compensation and circuit and process compensation. Physical compensation increases heat dissipation, but it is not applicable because the image sensor device is small. Circuit and process compensation improves the image sensor circuit or increases the size of the pixel device. The University of Chinese Academy of Sciences proposed to extract and modify the various process parameters of the imaging unit at room temperature to adapt to the changes in the characteristics of the pixel device under different temperatures. This can reduce the impact of temperature drift on the accuracy of the readout circuit to a certain extent, but the increase in size makes it difficult to integrate more pixel devices in a limited area, resulting in a limited pixel array size of the image sensor, which is inconsistent with the demand for high pixels in the aerospace field. Summary of the invention

[0004] In order to solve the problem that the device threshold voltage drifts due to temperature change and reduces the accuracy of the readout circuit, the present invention provides a temperature correction method and a readout circuit for the ramp voltage of a floating gate image sensor. The technical solution is as follows:

[0005] The first object of the present invention is to provide an image sensor readout circuit for reading out a pixel device threshold voltage, the image sensor readout circuit comprising: a ramp generation module, a clamping circuit, a readout capacitor, a comparator, and a counter;

[0006] The ramp generation module includes: a temperature acquisition module, a temperature encoding module, a voltage generation module and a ramp generation circuit connected in sequence, and is used to generate a ramp voltage having the same temperature characteristics as the threshold voltage of the pixel device to control the conduction of the pixel device;

[0007] The clamp circuit is connected to the output terminal of the pixel device and is used to control the drain-source voltage of the pixel device to remain constant;

[0008] The upper plate of the readout capacitor is connected to the output terminal of the pixel device and the input terminal of the comparator, and is connected to the power supply through a switch, and the lower plate of the readout capacitor is grounded;

[0009] The output terminal of the comparator is connected to the input terminal of the counter.

[0010] Optionally, the temperature acquisition module is used to monitor temperature changes in real time;

[0011] The temperature encoding module encodes the reference voltage of the ramp generating circuit according to the relationship between the threshold voltage and the temperature according to the voltage value of the threshold voltage of the pixel device at different temperatures, so as to obtain code values ​​corresponding to different temperatures;

[0012] The voltage generating module includes a decoding circuit and a selection circuit. The decoding circuit performs decoding according to the code values ​​corresponding to the different temperatures and transmits the decoding results to the selection circuit. The selection circuit outputs the slope reference voltage corresponding to the code value condition and controls the integration time of the slope generating circuit to be consistent with that under normal temperature conditions, thereby generating a slope voltage having the same temperature characteristics as the pixel threshold voltage.

[0013] Optionally, the ramp generating circuit comprises: a ramp generating resistor R, a ramp generating capacitor C and an operational amplifier;

[0014] The voltage generation module generates a first ramp reference voltage V ref3 and the second ramp reference voltage V ref4 , the first ramp reference voltage V ref3 The first input terminal of the operational amplifier is input; the second ramp reference voltage V ref4 The second input terminal of the operational amplifier is input through the ramp generating resistor R;

[0015] One end of the ramp generating capacitor C is connected to one end of the ramp generating resistor R and the second input end of the operational amplifier, and the other end is connected to the output end of the operational amplifier.

[0016] Optionally, the ramp voltage Vramp generated by the generating circuit is:

[0017]

[0018] Where t is the integration time.

[0019] Optionally, the temperature acquisition module includes: a temperature sensor.

[0020] A second object of the present invention is to provide an on-chip temperature correction method applied to an image sensor readout circuit, wherein the on-chip temperature correction method is implemented based on the above-mentioned image sensor readout circuit, and comprises:

[0021] Step 1: measuring the voltage values ​​of the pixel threshold voltage of the image sensor at different temperatures to obtain the corresponding relationship between the pixel device threshold voltage and temperature;

[0022] Step 2: The temperature encoding module encodes the corresponding relationship between the threshold voltage and temperature of the pixel device to obtain code values ​​corresponding to different temperatures;

[0023] Step 3: Monitor the temperature change in real time through the temperature acquisition module, transmit the real-time temperature value to the temperature encoding module, and the temperature encoding module outputs the code value corresponding to this temperature and transmits it to the voltage generation module;

[0024] Step 4: The voltage generation module includes a decoding circuit and a selection circuit. The decoding circuit performs decoding according to the code value and transmits the decoding result to the selection circuit. The selection circuit outputs the slope reference voltage corresponding to the code value condition and inputs it into the slope generation circuit.

[0025] Step 5: Control the integration time of the ramp generating circuit to be consistent with that under normal temperature conditions, generate a ramp voltage with the same temperature characteristics as the threshold voltage of the pixel device, so that the pixel current generated by the pixel device is consistent with that under normal temperature conditions, thereby realizing on-chip temperature correction of the image sensor readout circuit.

[0026] A third object of the present invention is to provide a pixel device threshold voltage readout method applied to an image sensor, wherein the pixel device threshold voltage readout method is implemented based on the above-mentioned image sensor readout circuit, and comprises:

[0027] Before the circuit works, the switch is turned on, the read capacitor is precharged to the power supply voltage VDD, and the counter is in standby state;

[0028] After the pre-charging is completed, the switch is turned off, the pixel device is exposed under light conditions, the ramp voltage is added to the input terminal of the pixel device, the circuit starts to work, the comparator outputs a logic level 1, the counter enable terminal is valid, the counter starts counting, and the pixel current generated at the output terminal of the pixel device discharges the readout capacitor to the ground;

[0029] When the voltage on the upper plate of the read capacitor drops to the comparator reference voltage, the comparator flips, the output is logic level 0, the enable terminal of the counter is pulled low, and the counter stops counting;

[0030] The voltage value of the ramp voltage that changes with time is obtained from the output value of the counter, thereby completing the conversion from the threshold voltage of the pixel device to the digital count value.

[0031] Optionally, the formula of the pixel current generated at the output end of the pixel device is equivalent to the MOS tube linear current formula:

[0032]

[0033] 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.

[0034] The third object of the present invention is to provide an image sensor, comprising: a pixel device array and the above-mentioned image sensor readout circuit, wherein the image sensor readout circuit is used to read out the threshold voltage of the pixel device.

[0035] The beneficial effects of the present invention are:

[0036] The present invention generates a ramp generating circuit reference voltage having the same temperature characteristics as a device threshold voltage through a temperature coding and voltage generating module, controls the integration time to be consistent with that under normal temperature conditions, and generates a ramp voltage having the same temperature characteristics as the device threshold voltage, thereby realizing on-chip temperature correction of an image sensor readout circuit. Under the premise of not changing the process parameters of the pixel device and not increasing the size of the pixel device, the problem of device threshold voltage drift caused by temperature changes, which reduces the accuracy of the readout circuit, is effectively solved, thereby improving the imaging quality of the image sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] 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.

[0038] Figure 1 2 is a diagram of an image sensor pixel array readout architecture according to an embodiment of the present invention.

[0039] Figure 2 is a block diagram of a readout circuit system according to an embodiment of the present invention.

[0040] Figure 3 1 is a graph showing the temperature characteristics of the device threshold voltage and the slope voltage curve according to an embodiment of the present invention.

[0041] Figure 4 4 is a schematic diagram of an on-chip temperature correction circuit for a readout circuit according to an embodiment of the present invention. DETAILED DESCRIPTION

[0042] In order 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.

[0043] Embodiment 1:

[0044] This embodiment provides an image sensor readout circuit for reading out the threshold voltage of a pixel device. Figure 2 ,The image sensor readout circuit includes: a ramp generation module, a clamping circuit, a readout capacitor, a comparator, and a counter;

[0045] The ramp generation module includes: a temperature acquisition module, a temperature encoding module, a voltage generation module and a ramp generation circuit connected in sequence, and is used to generate a ramp voltage having the same temperature characteristics as the threshold voltage of the pixel device to control the conduction of the pixel device;

[0046] The clamping circuit is connected to the output terminal of the pixel device and is used to control the drain-source voltage of the pixel device to remain constant;

[0047] The upper plate of the readout capacitor is connected to the output terminal of the pixel device and the input terminal of the comparator, and is connected to the power supply through a switch, and the lower plate of the readout capacitor is grounded;

[0048] The output terminal of the comparator is connected to the input terminal of the counter.

[0049] Embodiment 2:

[0050] This embodiment provides an on-chip temperature correction method applied to an image sensor readout circuit, which is implemented based on the image sensor readout circuit described in Embodiment 1 and includes the following steps:

[0051] Step 1: measuring the voltage values ​​of the pixel threshold voltage of the image sensor at different temperatures to obtain the corresponding relationship between the pixel device threshold voltage and temperature;

[0052] Step 2: The temperature encoding module encodes the corresponding relationship between the threshold voltage and temperature of the pixel device to obtain code values ​​corresponding to different temperatures;

[0053] Step 3: Monitor the temperature change in real time through the temperature acquisition module, transmit the real-time temperature value to the temperature encoding module, and the temperature encoding module outputs the code value corresponding to this temperature and transmits it to the voltage generation module;

[0054] Step 4: The voltage generation module includes a decoding circuit and a selection circuit. The decoding circuit performs decoding according to the code value and transmits the decoding result to the selection circuit. The selection circuit outputs the slope reference voltage corresponding to the code value condition and inputs it into the slope generation circuit.

[0055] Step 5: Control the integration time of the ramp generating circuit to be consistent with that under normal temperature conditions, generate a ramp voltage with the same temperature characteristics as the threshold voltage of the pixel device, so that the pixel current generated by the pixel device is consistent with that under normal temperature conditions, thereby realizing on-chip temperature correction of the image sensor readout circuit.

[0056] Embodiment three:

[0057] This embodiment provides a readout circuit and a readout method for an image sensor, and uses the temperature correction method described in the second embodiment to correct the readout circuit.

[0058] Figure 1 The image sensor pixel array readout architecture diagram shows that the pixel unit is exposed under light conditions, turned on by ramp voltage control, and generates current output from BL and transmitted to the readout circuit. Figure 2 As shown, the readout circuit includes a clamping circuit, a readout capacitor, a comparator, a counter and other parts. The clamping circuit controls the device drain-source voltage to remain constant, the readout capacitor will be precharged to the power supply voltage, and the counter is in a standby state. After the precharging is completed, the ramp voltage is added to WL, and the counter starts counting. At the same time, the current generated on BL causes the readout capacitor to discharge to the ground. When the upper plate voltage of the capacitor drops to the comparator reference voltage, the comparator flips and the counter stops counting, thereby completing the conversion of the pixel unit threshold voltage to the digital count value.

[0059] WL is used as the pixel input voltage column to control the conduction of the pixel unit, and BL is used as the pixel readout column to transmit the current of the pixel unit to the subsequent readout circuit. The formula of the readout current can be equivalent to the MOS tube linear current formula:

[0060]

[0061] 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.

[0062] Figure 3 The device threshold voltage temperature characteristics and ramp voltage curve are shown in Figure 2. When the temperature changes by ΔT, the device threshold voltage changes by ΔV1. To ensure that the counter count value is constant, the ramp voltage V needs to be increased. ramp value, that is, maintain ΔV2 = ΔV1.

[0063] The working principle of this embodiment is as follows: Figure 4 As shown, the temperature characteristic of the threshold voltage of the device is measured, and the voltage values ​​of the threshold voltage under the conditions of Temp=0 to Temp=100 are obtained respectively, and then the change value ΔV1 of the threshold voltage under normal temperature conditions can be obtained.

[0064] The reference voltage V ref3 With V ref4 Temperature coding is performed so that the voltage generation module generates the reference voltage required by the ramp generation circuit at different temperatures, and the reference voltage V ref3 With V ref4 The temperature variation value ΔV2 remains the same as ΔV1, and the ramp generating circuit is controlled to maintain the same integration time at different temperatures, so that the temperature characteristic of the ramp voltage is consistent with the temperature characteristic of the device threshold voltage, thereby achieving the purpose of on-chip temperature correction of the image sensor readout circuit.

[0065] Specifically, taking the first column of devices as an example, assuming that the electron mobility does not change with temperature, its drain current I BL0 for:

[0066]

[0067] After the temperature changes:

[0068]

[0069] The current change caused by temperature change is:

[0070]

[0071] The formula for charging and discharging a capacitor is:

[0072]

[0073] Therefore, when the temperature changes, the current changes. For the same capacitor and the same ΔU, the charging and discharging time of the capacitor will change, which will affect the count value of the counter and cause errors in the reading of the device threshold. By measuring the voltage value of the threshold voltage at different temperatures, multiple ΔV th , the reference voltage V for the ramp generating circuit ref3 With V ref4 Temperature encoding.

[0074] In this embodiment, 10-bit accuracy is used as an example. 0°C to 100°C is divided into 10 levels, and the code values ​​corresponding to different temperatures are obtained respectively. The voltage generating module generates the required V at the corresponding temperature. ref3 With V ref4 , the ramp voltage Vramp is:

[0075]

[0076] The control integration time is kept consistent with the normal temperature condition, so that the temperature characteristics of the ramp voltage are consistent with the temperature characteristics of the device threshold voltage. GS →ΔV th When ΔI→0, the charging and discharging time Δt of the capacitor does not change with temperature, and the count value of the counter remains constant. The reading of the threshold voltage will not produce an error, thereby achieving the purpose of temperature correction on the image sensor readout circuit chip.

[0077] This embodiment can maintain the same temperature characteristics as the threshold voltage when the temperature changes and causes the threshold voltage of the pixel device to drift without increasing the size of the pixel device, thereby offsetting the influence of the device threshold voltage drift caused by the temperature change on the readout accuracy, realizing on-chip temperature correction of the image sensor readout circuit, solving the problem of the device threshold voltage drift caused by the temperature change and reducing the accuracy of the readout circuit, thereby improving the imaging quality of the image sensor.

[0078] 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.

[0079] 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. An image sensor readout circuit for reading out a pixel device threshold voltage, characterized in that: The image sensor readout circuit includes: a ramp generation module, a clamping circuit, a readout capacitor, a comparator, and a counter; The ramp generation module includes: a temperature acquisition module, a temperature encoding module, a voltage generation module and a ramp generation circuit connected in sequence, and is used to generate a ramp voltage having the same temperature characteristics as the threshold voltage of the pixel device to control the conduction of the pixel device; The clamp circuit is connected to the output terminal of the pixel device and is used to control the drain-source voltage of the pixel device to remain constant; The upper plate of the readout capacitor is connected to the output terminal of the pixel device and the input terminal of the comparator, and is connected to the power supply through a switch, and the lower plate of the readout capacitor is grounded; The output terminal of the comparator is connected to the input terminal of the counter; The temperature acquisition module is used to monitor temperature changes in real time; The temperature encoding module encodes the reference voltage of the ramp generating circuit according to the relationship between the threshold voltage and the temperature according to the voltage value of the threshold voltage of the pixel device at different temperatures, so as to obtain code values ​​corresponding to different temperatures; The voltage generating module includes a decoding circuit and a selection circuit. The decoding circuit performs decoding according to the code values ​​corresponding to the different temperatures and transmits the decoding results to the selection circuit. The selection circuit outputs the slope reference voltage corresponding to the code value condition and controls the integration time of the slope generating circuit to be consistent with that under normal temperature conditions, thereby generating a slope voltage having the same temperature characteristics as the pixel threshold voltage.

2. The image sensor readout circuit according to claim 1, characterized in that: The ramp generating circuit comprises: a ramp generating resistor R, a ramp generating capacitor C and an operational amplifier; The voltage generation module generates a first ramp reference voltage V ref3 and the second ramp reference voltage V ref4 , the first ramp reference voltage V ref3 The first input terminal of the operational amplifier is input; the second ramp reference voltage V ref4 The second input terminal of the operational amplifier is input through the ramp generating resistor R; One end of the ramp generating capacitor C is connected to one end of the ramp generating resistor R and the second input end of the operational amplifier, and the other end is connected to the output end of the operational amplifier.

3. The image sensor readout circuit according to claim 2, characterized in that: The ramp voltage Vramp generated by the generating circuit is: Where t is the integration time.

4. The image sensor readout circuit according to claim 1, characterized in that: The temperature acquisition module includes: a temperature sensor.

5. An on-chip temperature correction method for an image sensor readout circuit, characterized in that: The on-chip temperature correction method is implemented based on the image sensor readout circuit according to any one of claims 1 to 4, comprising: Step 1: measuring the voltage values ​​of the pixel device threshold voltage of the image sensor at different temperatures to obtain the corresponding relationship between the pixel device threshold voltage and temperature; Step 2: The temperature encoding module encodes the corresponding relationship between the threshold voltage and temperature of the pixel device to obtain code values ​​corresponding to different temperatures; Step 3: Monitor the temperature change in real time through the temperature acquisition module, transmit the real-time temperature value to the temperature encoding module, and the temperature encoding module outputs the code value corresponding to this temperature and transmits it to the voltage generation module; Step 4: The voltage generation module includes a decoding circuit and a selection circuit. The decoding circuit performs decoding according to the code value and transmits the decoding result to the selection circuit. The selection circuit outputs the slope reference voltage corresponding to the code value condition and inputs it into the slope generation circuit. Step 5: Control the integration time of the ramp generating circuit to be consistent with that under normal temperature conditions, generate a ramp voltage with the same temperature characteristics as the threshold voltage of the pixel device, so that the pixel current generated by the pixel device is consistent with that under normal temperature conditions, thereby realizing on-chip temperature correction of the image sensor readout circuit.

6. A pixel device threshold voltage readout method applied to an image sensor, characterized in that: The pixel device threshold voltage readout method is implemented based on the image sensor readout circuit according to any one of claims 1 to 4, comprising: Before the circuit works, the switch is turned on, the read capacitor is precharged to the power supply voltage VDD, and the counter is in standby state; After the pre-charging is completed, the switch is turned off, the pixel device is exposed under light conditions, the ramp voltage is added to the input terminal of the pixel device, the circuit starts to work, the comparator outputs a logic level 1, the counter enable terminal is valid, the counter starts counting, and the pixel current generated at the output terminal of the pixel device discharges the readout capacitor to the ground; When the voltage on the upper plate of the read capacitor drops to the comparator reference voltage, the comparator flips, the output is logic level 0, the enable terminal of the counter is pulled low, and the counter stops counting; The voltage value of the ramp voltage that changes with time is obtained from the output value of the counter, thereby completing the conversion from the threshold voltage of the pixel device to the digital count value.

7. The pixel device threshold voltage reading method according to claim 6, characterized in that: The formula of the pixel current generated at the output end of the pixel device 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.

8. An image sensor, characterized in that: The image sensor comprises: a pixel device array and an image sensor readout circuit according to any one of claims 1 to 4, wherein the image sensor readout circuit is used to read out a threshold voltage of the pixel device.

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