ADC temperature self-correction method for current-mode image sensors

Abstract

The present invention discloses a method for ADC temperature self-correction in a current-type image sensor, and belongs to the technical field of integrated circuits. The method is applied to a current readout system of a current-type image sensor, comprising: a clamp readout module, an I-V conversion circuit module, and an ADC module connected in sequence; the present invention uses a reference voltage with the same property change to perform temperature correction on the readout current value that has undergone temperature change, and simultaneously uses the pixel readout column inside the chip to correct the input voltage of the ADC affected by temperature drift under normal readout conditions, and accurately compensates for the influence of the imaging unit parameter value due to temperature change without affecting the normal operation of the imaging unit. The present invention greatly reduces the design difficulty, does not change the process parameters of the pixel device itself and the size of the pixel device itself, and does not affect the normal readout function of the image sensor, improves the readout efficiency, and achieves temperature correction more quickly.

Description

Technical Field

[0001] The invention relates to a method for self-correcting ADC temperature in a current-type image sensor, and belongs to the technical field of integrated circuits. Background Art

[0002] Image sensors are tiny imaging devices that have been commonly used by people, such as mobile phones, cameras, and webcams. However, the imaging characteristics of image sensors will change with temperature, affecting normal use. Therefore, in our daily lives, we need good performance regardless of high or low temperatures.

[0003] As the name implies, the current type image sensor is to amplify and read out the current generated by the pixel device through the readout circuit, convert the current into voltage (or directly read out the current) through IV conversion, and then convert the analog voltage (current) signal into a digital signal. Finally, the digital signal is converted into the image value we need after data transmission and other processing. However, due to temperature changes, the carrier mobility μ of the internal device of the pixel unit in the current type image sensor will change. As the temperature rises, the carrier mobility decreases, the output current decreases, and the output voltage after IV conversion will also change, which will affect the input voltage of the ADC and the output effective number of bits of the ADC, affecting the clarity of the image.

[0004] A common method for designing circuits affected by temperature changes is to reduce the geometric size of the device. In order to ensure that the output current of the circuit pixel device does not change with temperature, the channel width of the pixel device unit is changed accordingly because parameters such as mobility change with temperature. This can reduce the impact of temperature to a certain extent, but the change in size will also change the magnitude of the on-voltage of the imaging unit. The chain reaction will make the circuit design more complicated. The team of Professor Ding Ruijun from the University of the Chinese Academy of Sciences proposed a method for extracting process parameters in a paper on the key technology of infrared focal plane digital readout circuit. In order to ensure that the readout circuit can work normally at different temperatures, the various process parameters of the imaging unit are extracted and modified, and simulations are performed at different temperatures to obtain the corresponding values ​​of each process parameter under different temperatures. This greatly increases the difficulty of design, and it is also difficult to achieve corresponding values ​​in actual circuits for different process parameters at different temperatures.

[0005] In summary, the existing solutions for suppressing the influence of temperature drift still have problems such as complex circuits, high process requirements, and great design difficulty. Summary of the invention

[0006] In order to solve the problems of complex circuit design and high requirements on process parameters in the current solution for suppressing the influence of temperature on image sensors, the present invention provides a method for ADC temperature self-correction, which is applied to a current readout system of a current type image sensor, wherein the current readout system comprises: a clamp readout module, an IV conversion circuit module and an ADC module connected in sequence;

[0007] The clamp readout module includes a plurality of clamp readout circuits connected one by one to a plurality of pixel readout columns of the current-type image sensor, and is used to control the voltage on the pixel readout column to be a fixed value and process the pixel current, wherein the processing process includes: reading, voltage stabilization and amplification;

[0008] The IV conversion circuit module includes a plurality of IV conversion circuits, which are connected to the plurality of clamp readout circuits in a one-to-one correspondence, and are used to convert the pixel current output by the readout circuit into a voltage;

[0009] The ADC module includes a plurality of ADCs, which are connected to the plurality of IV conversion circuits in a one-to-one correspondence, and are used to perform analog-to-digital conversion on the voltage output by the IV conversion circuit, and output a digital signal for subsequent data transmission;

[0010] The output signal correction method comprises:

[0011] Step 1: using a pair of pixel readout columns outside the current-type image sensor as a pair of reference readout columns, and converting a pair of pixel currents output by the reference readout columns into voltages after subtraction through the clamp readout circuit as reference voltages for the ADC;

[0012] Step 2: selecting one of the pixel readout columns inside the current-mode image sensor as a basic readout column for calibration;

[0013] Step 3: At any temperature, the digital signals obtained after the pixel current outputted by the basic readout column and any pixel readout column in the sensor passes through the clamp readout circuit, the IV conversion circuit and the ADC are subtracted to obtain a fixed value that does not change with temperature;

[0014] Step 4: Under any temperature, the fixed value and the digital signal output by the ADC corresponding to the basic readout column are used to correct the ADC input voltage affected by temperature corresponding to each pixel readout column.

[0015] Optionally, the clamping circuit in the clamping readout circuit clamps the drain-source voltage of the pixel unit at a fixed value, so that the pixel unit works normally in the linear region.

[0016] Optionally, the readout current of the clamp readout circuit is:

[0017]

[0018] Where, μ is the carrier 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 unit input voltage, V TH is the pixel unit threshold voltage, V DS is the drain-source voltage of the pixel unit.

[0019] Optionally, the reference voltage of the ADC is:

[0020] V REF =I REF R2

[0021]

[0022] Among them, I REF is the readout current difference of the reference readout column, V THA and V THB are the pixel unit threshold voltages of two reference readout columns respectively, and R2 is the load resistor.

[0023] Optionally, the pixel current of the basic readout column is converted into a voltage V via a resistor R1. BASE It is transmitted to ADC and converted into digital signal by ADC. The conversion formula is:

[0024]

[0025] in, The digital signal output by the ADC corresponding to the column is read out based on N, the number of bits of the ADC, and V THBASE The pixel unit threshold voltage corresponding to the basic readout column is obtained.

[0026] Optionally, the current read out by any pixel readout column is converted into a voltage V through a resistor R1. IN It is transmitted to ADC and converted into digital signal by ADC. The conversion formula is:

[0027]

[0028] in, V is the digital signal output by the ADC corresponding to the pixel readout column. THIN A pixel unit threshold voltage corresponding to a column is read out for the pixel.

[0029] Optionally, the fixed value in step 3 is:

[0030]

[0031] Optionally, the carrier mobility of the pixel unit is:

[0032]

[0033] Where T0 is the standard temperature, is the carrier mobility at standard temperature, T is the temperature, and α is a constant.

[0034] The present invention also provides a current type image sensor, which uses the above ADC output signal correction method to perform ADC temperature self-correction;

[0035] The current type image sensor comprises: a pixel unit array, a current readout system, a data transmission module, and an image value acquisition module connected in sequence;

[0036] The pixel readout column reads out the pixel current generated by the pixel unit array and transmits it to the current readout system. The current readout system adopts the above-mentioned current readout system to convert the pixel current into a digital signal, and then transmits it to the image value acquisition module through the data transmission module. The image value acquisition module converts the digital signal into a corresponding image value.

[0037] The beneficial effects of the present invention are:

[0038] The present invention proposes a method for self-correction of ADC temperature in a current-type image sensor, which uses a reference voltage with the same property change to perform temperature correction on a readout current value that has undergone temperature change, and uses a pixel readout column inside the chip to correct the current value that changes due to the influence of temperature. On the other hand, it is only necessary to correct the digital signal affected by temperature drift under normal readout conditions, and the influence of the imaging unit parameter value caused by temperature change can be accurately compensated without affecting the normal operation of the imaging unit.

[0039] The present invention greatly reduces the design difficulty, does not change the process parameters of the pixel device itself and the size of the pixel device itself, and does not affect the normal readout function of the image sensor, thereby improving the readout efficiency and realizing temperature correction more quickly. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] 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.

[0041] Figure 1 It is an overall block diagram of a current-type image sensor readout circuit system corresponding to the second embodiment of the present invention.

[0042] Figure 2 This is the ADC working principle architecture diagram.

[0043] Figure 3 This is a working principle diagram of the reference voltage generated by the chip external pixel readout column in the second embodiment of the present invention.

[0044] Figure 4 This is a working principle diagram of a pixel readout column inside a chip according to the second embodiment of the present invention. DETAILED DESCRIPTION

[0045] 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.

[0046] Embodiment 1:

[0047] This embodiment provides an ADC temperature self-calibration method, which is applied to a current readout system of a current-type image sensor, characterized in that the current readout system comprises: a clamp readout module, an IV conversion circuit module and an ADC module connected in sequence;

[0048] The clamp readout module includes a plurality of clamp readout circuits connected one by one to a plurality of pixel readout columns of the current-type image sensor, and is used to control the voltage on the pixel readout column to be a fixed value and process the pixel current, wherein the processing process includes: reading, voltage stabilization and amplification;

[0049] The IV conversion circuit module includes a plurality of IV conversion circuits, which are connected to the plurality of clamp readout circuits in a one-to-one correspondence, and are used to convert the pixel current output by the readout circuit into a voltage;

[0050] The ADC module includes a plurality of ADCs, which are connected to the plurality of IV conversion circuits in a one-to-one correspondence, and are used to perform analog-to-digital conversion on the voltage output by the IV conversion circuit, and output a digital signal for subsequent data transmission;

[0051] The output signal correction method comprises:

[0052] Step 1: using a pair of pixel readout columns outside the current-type image sensor as a pair of reference readout columns, and converting a pair of pixel currents output by the reference readout columns into voltages after subtraction through the clamp readout circuit as reference voltages for the ADC;

[0053] Step 2: Select one of the pixel readout columns in the current-mode image sensor as a basic readout column for calibration;

[0054] Step 3: At any temperature, the digital signals obtained after the pixel current outputted by the basic readout column and any pixel readout column in the sensor passes through the clamp circuit, the readout circuit, the IV conversion circuit and the ADC are subtracted to obtain a fixed value that does not change with temperature;

[0055] Step 4: Under any temperature, the fixed value and the digital signal output by the ADC corresponding to the basic readout column are used to correct the input voltage value of the ADC corresponding to the pixel readout column affected by temperature.

[0056] The specific working principle of the method in this embodiment is described as follows:

[0057] Figure 1 : is an overall block diagram of the current-type image sensor readout circuit system corresponding to this embodiment, wherein the pixel unit array is composed of each pixel device arranged in a storage array manner. The pixel device is similar to a MOS tube device and has several ports such as a gate, a source, a drain, and a substrate. All of their gates are connected together to form a word line WL (Word Line, WL), and all of their drains are connected together to form a bit line BL (Bit Line, BL). Storage cells in different columns are different BLs. BL transmits the output current to the readout circuit, which is understood as a pixel readout column.

[0058] The pixel unit is turned on by external light and controlled by the on-chip driving voltage, generating current output from the BL, transmitted to the readout circuit, and amplified and controlled by the readout circuit, and converted into voltage by the IV conversion circuit and transmitted to the subsequent ADC.

[0059] The word line WL is controlled by the internal drive voltage, that is, the input voltage V of the MOS device. GS , thereby controlling the conduction of the pixel unit.

[0060] The readout system includes the clamp readout circuit and subsequent IV conversion and ADC analog-to-digital conversion.

[0061] The clamping circuit in the clamp readout circuit controls all voltages on the BL column to a fixed value, so that the output current of the pixel device can be expressed by the linear region output current formula of the MOS tube, thereby facilitating the subsequent current reading and correction, and transmitting the output current to the subsequent IV conversion voltage through operations such as reading, stabilizing, and amplifying.

[0062] ADC converts the voltage converted by IV into a digital signal for subsequent data transmission. Before calibration, the reference voltage of ADC is a reference voltage supplied inside the chip that does not change with temperature.

[0063] like Figure 3As shown, this embodiment uses a pair of pixel readout columns outside the chip to output currents, and then converts them into a voltage after subtraction as a reference voltage for the analog-to-digital converter ADC. The reference voltage is also a voltage value that changes with temperature. On the other hand, one of the pixel readout columns inside the chip is selected as the basic readout column for correction.

[0064] At a certain temperature, the pixel current at that temperature is read out using one of the basic readout columns and the pixel readout column. When the two currents are read out, they are converted into voltages and transmitted to the analog-to-digital converter ADC respectively. They are converted into digital signals by analog-to-digital conversion with the reference voltage. At this time, it can be determined that the subtraction formula of the digital signal obtained after subtracting the two currents is a certain value and has nothing to do with temperature. Further, it can be obtained that at any temperature, the currents generated by the basic readout column and the pixel readout column are converted into voltages and the digital signals generated by the ADC are subtracted to a fixed value. On the other hand, at a random temperature, the digital signal converted by the basic readout column through the ADC can be corrected by the above fixed value for the current value affected by temperature changes generated by the normal pixel readout column at a random temperature.

[0065] The pixel unit is illuminated by external light and driven by the internal driving voltage of the chip, namely V GS , controls the conduction of the pixel device, and the clamping circuit controls the drain voltage of the pixel device to a fixed value, namely V DS , the current is generated and output from the BL, transmitted to the readout circuit, and converted into voltage through amplification, control and other operations of the readout circuit and transmitted to the subsequent IV conversion circuit. The converted voltage is converted into a digital signal through the analog-to-digital conversion of the ADC and transmitted to the subsequent data transmission.

[0066] Figure 3 It provides a reference voltage for the analog-to-digital converter ADC. The currents IA and IB generated by the two pixel readout columns BLA and BLB outside the chip are subtracted through the readout circuit to obtain I REF Converted into voltage V by resistor R2 REF , the formula is:

[0067] I REF R2=V REF (2)

[0068] The formula for the readout current is equivalent to the MOS tube linear current formula:

[0069]

[0070] Where μ is the carrier 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 THis the pixel unit threshold voltage, V DS is the drain-source voltage of the pixel unit. The clamping circuit clamps the drain-source voltage at a fixed value so that the pixel device can work normally in the linear region.

[0071] From the above formula, we can get:

[0072]

[0073] When the temperature changes, the carrier mobility μ of the pixel unit will change, and its change formula is as follows:

[0074]

[0075] Where μ is the carrier mobility after the pixel unit changes, is the carrier mobility at standard temperature, T is the temperature after the change, and α is a constant. As the temperature increases, the carrier mobility will decrease, which will affect the output current and the conversion accuracy of the ADC.

[0076] The analog-to-digital converter (ADC) converts the input analog signal into a digital signal. First, the ADC divides the reference voltage into the minimum resolution voltage (LSB) according to its bit number, that is:

[0077]

[0078] For example, when the number of bits is 8,

[0079] The pixel readout column generates a voltage V IN The value obtained by multiplying the current I1 of the pixel readout column by the resistance R1, R1 is the same as the resistance R1 of the base readout column because the same symbol is used. The formula is:

[0080] V IN =I1·R1 (7)

[0081] When the input voltage V IN When transmitted to the ADC, the input voltage and the reference voltage are converted into a digital signal D by the ADC. OUT , at this time the input voltage V IN , output digital signal D OUT The ADC conversion formula between the minimum resolution voltage LSB is:

[0082]

[0083] The image sensor works at any temperature. The current of the basic readout column inside the chip is converted into a voltage V through a resistor. BASEIt is transmitted to ADC and converted into digital signal by ADC. The conversion formula is:

[0084]

[0085] in It is the output digital signal converted by the ADC in the basic readout column.

[0086] The current read out by the pixel readout column inside the chip is converted into a voltage V through a resistor. IN It is transmitted to ADC and converted into digital signal by ADC. The conversion formula is:

[0087]

[0088] in It is the output digital signal converted by ADC under the pixel readout column.

[0089] Subtracting (10) from (9) yields:

[0090]

[0091] It can be seen from the above formula that in each image sensor readout state, the digital signal generated by the chip internal pixel readout column and the chip internal basic readout column after ADC conversion is subtracted to a fixed value.

[0092] At any temperature, the current read out by the basic readout column inside the chip is converted into a voltage and then converted into a digital signal by ADC. The formula is:

[0093]

[0094] From the above formula, we can get:

[0095]

[0096] From the above formula, we can see that on the same pixel readout column, no matter how much the temperature changes, the ADC readout digital signal is a certain value. Therefore, combining formulas (11) and (13), we can get the formula for the ADC output digital signal corresponding to the pixel readout column under normal working conditions:

[0097]

[0098] Right now:

[0099] DOUT IN0 =DOUT BASE -DOUT IN +DOUT BASE0 (15)

[0100] In summary, on the same pixel readout column, no matter how much the temperature changes, the output value of the ADC read out by the pixel readout column is a certain value, which is also the output value of the ADC we need at the standard temperature and is unique.

[0101] The present embodiment proposes a method for correcting the ADC input voltage affected by temperature in a current-type image sensor. The temperature correction is performed on the read current value that has undergone temperature change using a reference voltage that has changed in the same nature. Without changing the process parameters of the pixel device itself and the size of the pixel device itself, the pixel readout column inside the chip is used to correct the current value that changes due to the influence of temperature. On the other hand, it is only necessary to correct the ADC input voltage affected by temperature drift under normal readout conditions. Without affecting the normal operation of the imaging unit, the influence of the imaging unit parameter value caused by temperature change is accurately compensated. The present invention greatly reduces the design difficulty, does not affect the normal readout function of the image sensor, improves the readout efficiency, and realizes temperature correction more quickly.

[0102] 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.

[0103] 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 method for ADC temperature self-calibration, applied to a current readout system of a current-type image sensor, characterized in that: The current reading system comprises: a clamp reading module, an IV conversion circuit module and an ADC module connected in sequence; The clamp readout module includes a plurality of clamp readout circuits connected one by one to a plurality of pixel readout columns of the current-type image sensor, and is used to control the voltage on the pixel readout column to be a fixed value and process the pixel current, wherein the processing process includes: reading, voltage stabilization and amplification; The IV conversion circuit module includes a plurality of IV conversion circuits, which are connected to the plurality of clamp readout circuits in a one-to-one correspondence, and are used to convert the pixel current output by the readout circuit into a voltage; The ADC module includes a plurality of ADCs, which are connected to the plurality of IV conversion circuits in a one-to-one correspondence, and are used to perform analog-to-digital conversion on the voltage output by the IV conversion circuit, and output a digital signal for subsequent data transmission; The output signal correction method comprises: Step 1: using a pair of pixel readout columns outside the current-type image sensor as a pair of reference readout columns, and converting a pair of pixel currents output by the reference readout columns into voltages after subtraction through the clamp readout circuit as reference voltages for the ADC; Step 2: Select one of the pixel readout columns in the current-mode image sensor as a basic readout column for calibration; Step 3: At any temperature, the digital signals obtained after the pixel current outputted by the basic readout column and any pixel readout column in the sensor passes through the clamp readout circuit, the IV conversion circuit and the ADC are subtracted to obtain a fixed value that does not change with temperature; Step 4: at any temperature, using the fixed value and the digital signal output by the ADC corresponding to the basic readout column to correct the ADC input voltage affected by temperature corresponding to each pixel readout column; The clamping circuit in the clamping readout circuit clamps the drain-source voltage of the pixel unit at a fixed value, so that the pixel unit works normally in the linear region; The readout current of the clamp readout circuit is: in, is the carrier mobility of the pixel unit, is the pixel unit gate oxide capacitance, is the aspect ratio of the pixel unit, is the pixel unit input voltage, is the pixel unit threshold voltage, is the drain-source voltage of the pixel unit; The reference voltage of the ADC is: in, is the read current difference of the reference read column, and are the pixel cell threshold voltages of the two reference readout columns, is the load resistance; The pixel current of the basic readout column is converted into a voltage V through the resistor R1. BASE It is transmitted to ADC and converted into digital signal by ADC. The conversion formula is: in, Based on the readout column, the digital signal output by the ADC corresponding to the column, N is the number of bits of the ADC, A pixel unit threshold voltage corresponding to the basic readout column; The current read out of any pixel readout column passes through the resistor Converted into voltage V IN It is transmitted to ADC and converted into digital signal by ADC. The conversion formula is: in, The digital signal output by the ADC corresponding to the pixel readout column is A pixel unit threshold voltage corresponding to a column is read out for the pixel.

2. The method according to claim 1, characterized in that The fixed value in step 3 is: 。 3. The method according to claim 1, characterized in that The carrier mobility of the pixel unit is: in, is the standard temperature, is the carrier mobility at standard temperature, T is the temperature, and α is a constant.

4. A current-type image sensor, characterized in that: The ADC input voltage is corrected by using the ADC output signal correction method according to any one of claims 1 to 3; The current type image sensor comprises: a pixel unit array, a current readout system, a data transmission module, and an image value acquisition module connected in sequence; The pixel readout column reads out the pixel current generated by the pixel unit array and transmits it to the current readout system. The current readout system adopts the current readout system described in any one of claims 1 to 3. After converting the pixel current into a digital signal, it is transmitted to the image value acquisition module through the data transmission module. The image value acquisition module converts the digital signal into a corresponding image value.

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