JP2011055535A - Sequential comparison type a/d converter and imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a time required for processing when performing over sampling, and thereby to reduce a conversion time of an A/D conversion, in a sequential comparison type A/D converter. <P>SOLUTION: In over sampling, in the first A/D conversion processing, an N-bit digital value of an analog signal is found in N steps. Then, in the second and subsequent A/D conversion processing, A/D conversion processing is not performed for high-order n bits of the N-bit digital value obtained in the first A/D conversion processing and it is left fixed, and A/D conversion is started from the low-order bits equal to or lower than the (N-n)th bit, that is, from the middle of the bits. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a successive approximation A / D converter, a control method for a successive approximation A / D converter, a solid-state imaging device, and an imaging device.

  One type of A / D converter is a successive approximation A / D converter. The successive approximation A / D converter repeatedly performs a size comparison with a single comparator. More specifically, first, a voltage having a half of the maximum amplitude (full scale; FS) is compared with the input voltage. The comparison result at this time corresponds to the most significant bit (MSB: Most Significant Bit) of the digital value.

  Subsequently, the FS / 4 or 3FS / 4 voltage, which is higher or lower by a half (1/4) FS depending on the value of the most significant bit, is again compared with the input voltage. The comparison result at this time becomes the value of the next bit. Thereafter, the same procedure is repeated to complete the A / D conversion process in N steps.

  In this type of successive approximation type A / D converter, as a technique for shortening the conversion time, the upper bits that were the same i times are detected from the results of the previous i times (i ≧ 2) A / D conversion. However, a technique that is fixed and applied to this successive comparison is known (for example, see Patent Document 1).

JP 2006-108893 A

  By the way, in the successive approximation A / D converter, the noise component included in the input signal can be reduced by taking the averaging by oversampling. Here, oversampling means obtaining a plurality of digital data by executing A / D conversion processing that is completed in N steps a plurality of times. In addition, the averaging means that a plurality of data obtained by oversampling are added and averaged. By adding n pieces of data, the signal component becomes n times and the noise component becomes √n times. And S / N can be improved by taking the average.

  Here, a normal A / D conversion operation in the case of performing N-bit A / D conversion and M oversampling in the successive approximation A / D converter will be considered. As an example, when 10-bit (N = 10) A / D conversion and four times (M = 4) oversampling are performed, as shown in FIG. 13, first, 1 bit per cycle (period T). The A / D conversion process is performed 10 cycles. Subsequently, the same 10-bit A / D conversion processing for 10 bits as the first time is repeated four times.

  As described above, the conventional general successive approximation type A / D converter needs to perform the N-time comparison in the N-bit A / D conversion processing, and therefore, when the input signal is oversampled M times. In addition, since N × (M + 1) comparison operations are required, the conversion time for A / D conversion becomes long. On the other hand, the technique for shortening the conversion time described in Patent Document 1 does not assume addition averaging by oversampling. That is, even if the technique for shortening the conversion time described in Patent Document 1 is applied to addition averaging by oversampling as it is, the time required for processing when oversampling is performed, and hence the conversion time for A / D conversion is reduced. I can't.

  Therefore, the present invention relates to a successive approximation type A / D converter and a successive approximation type A / D converter that can shorten the time required for processing in the case of oversampling, and consequently the conversion time of A / D conversion. It is an object to provide a control method, a solid-state imaging device, and an imaging device.

According to the present invention, a successive approximation A / D converter that performs oversampling processing, sequentially compares an analog signal to be converted and a reference signal bit by bit, and converts the analog signal to be converted and the reference signal. A comparison result signal indicating a logical value corresponding to a magnitude comparison with the comparator, a comparator for AD conversion processing, a storage circuit for holding the comparison result signal output from the comparator, and an AD in the comparator Performs oversampling processing for performing conversion processing and storage processing in the storage circuit a plurality of times, calculates a bit string used for the next reference signal, decodes the bit string output from the control unit, and performs the decoding A reference signal generation unit that generates the analog reference signal according to a result and inputs the reference signal to the comparator;
The arithmetic control unit of the control unit performs the conversion in the first A / D conversion process when performing oversampling in which the A / D conversion process in the comparator is performed a plurality of times on the analog signal to be converted. A / D conversion is performed on the target analog signal to obtain an N-bit digital value. In the second and subsequent A / D conversion processes, the upper n bits of the N-bit digital value obtained in the first A / D conversion process are obtained. A successive approximation A / D converter is provided which controls the reference signal so as to start and start A / D conversion from lower bits of (N−n) bits or less.
The present invention also provides a method for controlling the successive approximation A / D converter.
The present invention also provides a solid-state imaging device using the successive approximation A / D converter.
According to the present invention, an imaging device using the successive approximation A / D converter is provided.

  In the above-described successive approximation A / D converter, when oversampling is performed in which the A / D conversion process is performed a plurality of times, an N-bit digital value is obtained for the analog signal in the first A / D conversion process. The A / D conversion process at this time is a normal A / D conversion process in which A / D conversion is performed in N steps. Then, oversampling is performed by the second and subsequent A / D conversion processes. At this time, if the correlation between the analog signal input in the first A / D conversion process and the analog signal input in the second and subsequent A / D conversion processes is high, the upper n The digital value of the bit will not change.

  Focusing on this point, in the second and subsequent A / D conversion processes, the upper n bits of the N-bit digital value obtained in the first A / D conversion process are fixed without performing the A / D conversion process. The A / D conversion is started from the (N−n) bits which are the lower bits. Here, since the higher the correlation between the analog signals input in the first and second and subsequent A / D conversion processes, the higher the number n of bits on the higher side where the digital value does not change, the A / D The bit for starting the D conversion becomes the lower bit.

  In this way, in the second and subsequent A / D conversion processes, A / D conversion is started from the most significant bit by starting A / D conversion from the lower bits of (N−n) bits, that is, in the middle of the bits. Processing up to one bit before the start bit is not required, and the processing time corresponding to that can be cut. If the processing time for one cut can be T and the number of oversampling is M, the processing time that can be cut in the entire oversampling process is T × M. That is, the time required for processing when oversampling is performed M times can be shortened by a time of T × M compared to the case where this control method is not employed.

  According to the present invention, when oversampling is performed, in the second and subsequent A / D conversion processes, the upper n bits are fixed, so that the time required for the process is reduced by not performing A / D conversion for the n bits. Since it can be shortened, the conversion time of A / D conversion can be shortened.

It is a block diagram which shows the system configuration | structure of the successive approximation type A / D converter which concerns on one Embodiment of this invention. It is a circuit diagram which shows an example of a structure of a reference signal generation part. It is a figure which shows the MODE example which selects 15 unit capacity | capacitances C randomly from 64 unit capacity | capacitances C. It is a block diagram which shows an example of a structure of a unit capacity | capacitance selection circuit. It is a circuit diagram which shows an example of the internal structure of a cell. It is a circuit diagram which shows an example of a structure of a logic circuit. It is a figure which shows a process sequence in case the return value in a successive approximation type A / D converter which concerns on this embodiment is 2 bits. It is a wave form diagram which shows the operation | movement waveform of an A / D converter with a return value of 3 bits. It is a wave form diagram which shows transition of the reference voltage of the case (A) when it is not a redundant configuration when an input signal makes an erroneous determination near FS / 2, and the transition of the reference voltage when it is a redundant configuration (B). 1 is a system configuration diagram illustrating a configuration example of a CMOS image sensor to which the present invention is applied. It is a circuit diagram which shows an example of the circuit structure of a unit pixel. It is a block diagram which shows an example of a structure of the imaging device which concerns on this invention. It is a figure which shows the process sequence of a normal successive approximation type A / D converter.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[System configuration]
FIG. 1 is a block diagram showing a system configuration of a successive approximation A / D converter according to an embodiment of the present invention. As illustrated in FIG. 1, the successive approximation A / D converter 10 according to the present embodiment includes a comparator 11, a data memory 12, a reference signal generator 13, a controller 14, and a data converter 15. ing.

  The comparator 11 sequentially compares the analog signal that is the input signal and the reference signal provided from the reference signal generation unit 13. The data memory 12 has a function as a latch circuit and stores the comparison result of the comparator 11. The reference signal generation unit 13 is configured by, for example, a D / A converter, and generates a reference signal that is sequentially compared with an analog signal in the comparator 11 under the control of the control unit 14. Details of the reference signal generator 13 will be described later.

  The control unit 14 controls the reference signal generation unit 13 that generates a reference signal based on the comparison result of the comparator 11 stored in the data memory 12, and performs oversampling under an external command. Take control. Details of the control unit 14 will be described later. The data memory 12 finally stores N-bit digital data as a result of the successive approximation by the comparator 11. Before outputting the digital data after A / D conversion stored in the data memory 12, the data converter 15 performs a conversion from redundant to non-redundant and a correction operation based on the number of oversamplings to obtain a final N Output as a bit digital signal.

  Next, details of the reference signal generation unit 13 and the control unit 14 will be described. First, the reference signal generation unit 13 will be described.

(Reference signal generator)
In the reference signal generation unit 13, an error occurs due to variations in characteristics of the D / A converters constituting the reference signal generation unit 13. This error rides the reference signal as noise. The variation in the characteristics of the D / A converter can be suppressed by adopting the following method.

  For example, elements corresponding to 1 [LSB (Least Significant Bit)] constituting a D / A converter are prepared for the resolution, and dynamic element matching that selects (pseudo) random elements each time A / D conversion is performed. Take the technique. By adopting this dynamic element matching method, errors are averaged and suppressed by oversampling.

  As an example of the dynamic element matching method, there is the following reference 1 when the reference signal is a current, and there is the following reference 2 when the reference signal is a voltage, for example.

[Reference 1] T. Miki, et. Al, “An 80-MHz 8bit CMOS D / A Converter,” IEEE Journal of Solid-State Circuits, vol sc-21, No6, Dec. 1986.
[Reference 2] GIRadolv, et. Al, “A Binary-To-Thermometer Decoder with built-in redundancy for improved DAC yield,” Circuits and Systems, ISCAS2006, Proc.

  As shown in FIG. 1, the reference signal generation unit 13 includes a reference signal generator 131, a thermometer code converter 132 as a primary decoder, and a reference signal generation control circuit 133 as a secondary decoder. . Hereinafter, a specific configuration of the reference signal generation unit 13 will be described.

  Hereinafter, a specific example of the reference signal generation unit 13 will be described by taking a case where the reference signal is a voltage as an example.

  When the reference signal is a voltage, the reference signal generator 131 includes a plurality of unit capacitors C. Specifically, as shown in FIG. 2, one end of each of the plurality of unit capacitors C is connected in common, and a positive reference voltage Vrefp and a negative reference voltage Vrefn are connected to each other end by a switch SW. It is applied selectively.

  The cells 21 including the unit capacitors C and the switches SW are arranged in a two-dimensional array corresponding to the resolution. Here, as an example, let us consider a case where the number of unit capacitors C is 64, and 15 unit capacitors C are randomly selected from the 64 unit capacitors C. As the selected MODE, as shown in FIG. 3, MODE (A) for selecting the lower 15 units from among 64 unit capacitors C, MODE (B) for selecting the 15 units on the right side, and the like. Conceivable. 3A and 3B, hatched portions indicate selected unit capacities (cells). Hereinafter, the MODE in FIG. 3A is referred to as Mode1, and the MODE in FIG. 3B is referred to as Mode2.

  FIG. 4 is a block diagram showing an example of the configuration of the unit capacitor selection circuit when the number of unit capacitors C is 64.

  In FIG. 4, cells 21 including unit capacitors C and switches SW are arranged in a two-dimensional array to form a capacitor array unit 22. The capacitor array unit 22 corresponds to the reference signal generator 131 in FIG. An X decoder 23 and a Y decoder 24 are arranged around the capacitor array unit 22. The X decoder 23 and the Y decoder 24 are primary decoders that perform primary conversion from binary code to thermometer code. That is, the X decoder 23 and the Y decoder 24 correspond to the thermometer code converter 132 of FIG.

  FIG. 5 shows an example of the configuration of the cell 21 including the unit capacitor C and the switch SW. As shown in FIG. 5, the cell 21 has a configuration including a logic circuit 25 and an inverter 26 in addition to the unit capacitor C and the switch SW. As will be described later, the logic circuit 25 has a function as a secondary decoder. That is, the logic circuit 25 corresponds to the reference signal generation control circuit 133 in FIG.

The logic circuit 25 receives the decoding results X _j and Xa _{j + 1} from the X decoder 23 and the decoding results Y _i and Ya _{i + 1} from the Y decoder 24. The output of the logic circuit 25 is directly supplied to the switch SW, inverted by the inverter 26, and supplied to the switch SW.

FIG. 6 shows an example of the configuration of the logic circuit 25. As illustrated in FIG. 6, the logic circuit 25 includes an OR circuit 251 and an AND circuit 252. In Mode 1 (A), the OR circuit 251 performs a logical sum of the decoding results X _j and Y _{i + 1} . Then, the AND circuit 252 calculates the logical product of the logical sum result of the OR circuit 251 and the decoded result Y _i . As a result, in Mode 1 (A), Ya = Y and Xa = 0 are obtained as the logical product result of the AND circuit 252.

On the other hand, in Mode 2 (B), the OR circuit 251 performs a logical sum of the decoding results X _{j + 1} and Y _i . Then, the AND circuit 252 calculates the logical product of the logical sum result of the OR circuit 251 and the decoded result X _j . As a result, in Mode 1 (B), Ya = 0 and Xa = X are obtained as the logical product result of the AND circuit 252.

  As is apparent from the above description, the unit capacity selection circuit for selecting the unit capacity C is constituted by the thermometer code converter 132 as the primary decoder and the reference signal generation control circuit 133 as the secondary decoder. Yes.

  Next, the operation of the unit capacitor selection circuit having the above configuration will be described. Here, as an example, since the number of cells of the capacity array unit 22 is 64, a 6-bit signal is used as a signal for selecting the cell 21.

  First, in order to select the cell 21, the upper 3 MSB and the lower 3 LSB are input to the X decoder 23 and the Y decoder 24. The input upper 3MSB and lower 3LSB signals are converted from the 3-bit binary code to the 8-bit thermometer code (primary conversion) by the X decoder 23 and the Y decoder 24, respectively, and input to the capacitance array unit 22. The Here, when the LSB is input to the X decoder 23 and the MSB is input to the Y decoder 24, Mode 1 is set, and when the MSB is input to the X decoder 23 and LSB is input to the Y decoder 24, Mode 2 is set (see FIG. 3).

  As an example, when 15 [LSB] is input as an input signal and 15 are selected from 64 unit capacitors C, 001 is input to the upper 3 bits in the Y direction and 111 is input to the lower 3 bits in the X direction. At this time, MODE becomes MODE1. In MODE1, first, the Y decoder 24 converts the upper part into 00000011 (001) and the lower part into 01111111 (111). In this case, as shown in FIG. 3A, the unit capacitors C to be selected are the 15 units below 64 unit capacitors C arranged in a two-dimensional array.

  On the other hand, in the case of MODE2, the upper part is converted to 00000011 and the lower part is converted to 01111111. In the case of MODE 2, the unit capacity C to be selected is 15 on the right side of the 64 unit capacities C arranged in a two-dimensional array as shown in FIG. By averaging the output signals of MODE1 and MODE2 (oversampling), an error due to variation in the capacitance value of the unit capacitance C can be corrected.

  In this example, the case of two types of MODE 1 and MODE 2 has been described as an example. However, the MODE is not limited to two types, and any type of capacitance can be obtained by changing the circuit configuration. It can be designed as a selection pattern, and the number of cells is not limited to 64.

(Control part)
Next, the control unit 14 will be described. As shown in FIG. 1, the control unit 14 includes a computing unit 141, a capacity control memory 142, and a redundant data RAM 143.

  The computing unit 141 calculates a bit string for the next reference signal from the comparison result of the comparator 11 stored in the data memory 12 and the redundant data given from the redundant data RAM 143. The capacity control memory 142 stores the calculation result of the calculator 141, that is, the bit string for the reference signal. The thermometer code converter 132 described above converts the calculation result of the calculator 141 stored in the capacity control memory 142 into a thermometer code. The redundant data RAM 143 stores redundant data that can be written from the outside.

(Sample / hold circuit)
As is apparent from FIG. 1, the successive approximation A / D converter 10 according to the present embodiment is characterized in that no sample / hold circuit is provided on the terminal side of the input signal of the comparator 11.

  Normally, the successive approximation A / D converter always has a sample / hold circuit on the terminal side of the input signal. This is because the successive approximation A / D converter cannot perform accurate A / D conversion processing if the signal level of the input signal fluctuates during A / D conversion.

  On the other hand, the successive approximation A / D converter 10 according to the present embodiment does not have a sample / hold circuit on the input signal terminal side of the comparator 11 and sequentially performs the input / output sampling without holding the input signal. Make a comparison. Since sampling / holding is not performed, there is a concern that an error may occur if the input signal changes during the comparison.

  However, when the operation speed of the successive approximation A / D converter 10 is sufficiently high (above a predetermined speed) compared to the input signal change speed, or when the input signal is DC until the end of the A / D conversion process. Therefore, it is possible to perform A / D conversion with high accuracy without performing sample / hold. Examples of the input signal in these cases include a voltage of a DC power source that outputs a DC signal for a long time, a pixel output signal of a solid-state imaging device that can output a DC signal over an arbitrary time by external control, and the like. It is done.

  Thus, by not having the sample / hold circuit, the kT / C noise of the capacitor C is superimposed on the input signal compared to a normal A / D converter having a sample / hold circuit composed of the capacitor C. There is no advantage. In addition, since the capacitor C used in the sample / hold circuit occupies a large area, the sample / hold circuit is not required, thereby reducing the size of the successive approximation A / D converter 10 and simplifying the circuit configuration. Can be planned.

(Oversampling operation)
Subsequently, an oversampling operation executed by the successive approximation A / D converter 10 according to the present embodiment having the above-described configuration will be described. Here, the number of oversamplings will be described as M times.

  First, a general successive approximation A / D converter will be described. In a general successive approximation type A / D converter, the reference signal generation unit 13 outputs a voltage having a half of the maximum amplitude (full scale; FS) as a reference signal, and then a further half (1/4). ) Output FS / 4 or 3FS / 4 higher or lower by FS. Subsequently, a reference signal is generated such that it is higher or lower by half (1/8) FS, higher or lower by half (1/16) FS,. In the oversampling operation, a plurality of digital data is obtained by executing A / D conversion processing that is completed in N steps a plurality of times.

  On the other hand, the successive approximation A / D converter 10 according to the present embodiment performs the following control in order to realize a short-time oversampling operation. That is, in the first (first) A / D conversion process, an N-bit digital value is obtained by successive comparison for an input analog signal. The A / D conversion process at this time is an A / D conversion process based on the normal sequential comparison described above, in which A / D conversion is performed in N steps.

  Then, oversampling is performed by the second and subsequent A / D conversion processes. At this time, if the correlation between the analog signal input in the first A / D conversion process and the analog signal input in the second and subsequent A / D conversion processes is high (that is, the analog signal). If there is no change, the upper n-bit digital value will not change. Focusing on this point, in the second and subsequent A / D conversion processing, that is, oversampling processing, A / D conversion is started in the middle of a bit.

  Specifically, in the second and subsequent A / D conversion processes, the upper n bits of the N-bit digital value obtained in the first A / D conversion process are fixed without performing the A / D conversion process. , A / D conversion is started from lower bits of (N−n) bits or less. That is, the oversampling operation is performed by repeating A / D conversion of lower bits of (N−n) bits or less M times. Here, since the higher the correlation between the analog signals input in the first and second and subsequent A / D conversion processes, the higher the number n of bits on the higher side where the digital value does not change, the A / D The bit for starting the D conversion becomes the lower bit.

  In this way, in the second and subsequent A / D conversion processing, that is, oversampling processing, by starting A / D conversion from the middle of the bit, one bit before the bit at which A / D conversion starts from the most significant bit. Therefore, the processing time can be cut. If the processing time for one cut can be T, the processing time that can be cut in the entire oversampling process is T × M. That is, the time required for processing when oversampling is performed M times can be shortened by a time of T × M compared to the case where this control method is not employed. As a result, the conversion time for A / D conversion can be shortened.

  As an example, FIG. 7 shows a timing chart in the case where the return value to the lower bits is 2 bits (two before). Here, that the return value to the lower bits is 2 bits means that the A / D conversion process is executed for the second and subsequent times after returning by 2LSB. By integrating (adding and averaging) the oversampled signals, noise components such as random noise included in the input signal can be suppressed, and thus the S / N can be improved.

  Taking the case of 10-bit (N = 10) A / D conversion as an example, one A / D conversion process by successive approximation is normally completed in 10 clocks (10 steps / cycle). In the A / D conversion according to the present embodiment, the first A / D conversion process takes a processing time corresponding to 10 clocks, but the second and subsequent A / D conversion processes are about 2 clocks at a time. Corresponding processing time is sufficient. Therefore, the conversion time when taking the average of four times by oversampling is 40T in the conventional A / D conversion, whereas it is 16T in the A / D conversion according to this embodiment, and the time ratio is 0. .4.

  Incidentally, in the case of 12-bit A / D conversion, the conversion time for taking the four averages is 48T in the conventional A / D conversion, whereas the A / D according to the present embodiment is A / D conversion. In the conversion, 18T is obtained, and the time ratio can be reduced to 0.375. In the case of 14-bit A / D conversion, the conversion time for taking the average of four times is 56T in the conventional A / D conversion, whereas in the A / D conversion according to this embodiment, the conversion time is 56T. 20T, and the time ratio can be reduced to 0.357.

  Here, the magnitude of the return value to the lower bits is defined by the noise level superimposed on the input signal and the magnitude of 1 [LSB]. Assuming that the random noise has a normal distribution and the standard deviation is σ_r, the return value may be a return of an LSB value of 6σ_r or more. For example, if 1LSB = σ_r, a return value of 6LSB or more is sufficient, so a return of 3 bits (8LSB) is sufficient.

  As an example, FIG. 8 shows an operation waveform of an A / D converter having a 3-bit return value. Of course, this noise level σ depends on the performance of the mounted semiconductor. In FIG. 8, the determination threshold corresponds to a reference signal that is generated by the reference signal generation unit 13 and serves as a comparison reference for the comparator 11.

  By the way, the successive approximation type A / D converter causes an erroneous determination due to the influence of disturbance such as noise when the input signal is present in the vicinity of the determination threshold (determination level) of the successive comparison. In order to suppress such erroneous determination, the successive approximation A / D converter 10 according to the present embodiment employs a technique of providing redundancy to the reference signal generator 131 formed of a capacitor array.

  Specifically, under the control of the control unit 14, the reference signal generator 131 is made redundant by widening the determination threshold value after the erroneous determination has occurred by a predetermined value (for example, by increasing the predetermined value). . As described above, by providing the reference signal generator 131 with redundancy, it is possible to increase resistance to erroneous determination.

  FIG. 9 shows the transition of the reference voltage when the input signal is not in a redundant configuration when an erroneous determination is caused in the vicinity of FS / 2 (A) and the transition of the reference voltage when the input signal is in a redundant configuration (B). Here, a case where the maximum amplitude FS is 64 LSB and the redundancy is 12.7% is taken as an example.

  When the redundant configuration is not adopted, the comparison threshold value at the time of A / D conversion in (A) changes with the size of FS / 2, FS / 4, FS / 8,. In this case, for example, if the first input signal is affected by disturbance or the like and the comparator makes an erroneous determination, error data is generated in the A / D conversion after the erroneous determination occurs, and the signal is not recovered.

  On the other hand, when the redundant configuration is adopted, the comparison threshold for A / D conversion in (B) is FS / 2, FS / 4 + Red1, (FS / 4 + Red1) / 2 + Red2,. Here, Red1, Red2,... Are values (predetermined values) when the determination threshold is raised. Compared to the case of (A) when the redundant configuration is not taken, even if the comparator makes a false determination due to the influence of disturbance or the like at the beginning or midway of the A / D conversion, the redundant configuration is taken. Data recovery is possible from the output of the comparator after the next determination.

  In the above embodiment, when oversampling is performed, the upper n bits are fixed in all of the A / D conversion processes after the second time. However, this is a preferable example and is not limited thereto. That is, even if the upper n bits are fixed in at least one A / D conversion process after the second time, the conversion time for A / D conversion can be shortened as compared with the case where this control method is not adopted. .

  In the above embodiment, it is assumed that the input / output terminal side of the comparator 11 does not have a sample / hold circuit. However, the present invention provides a successive approximation type A / D having a configuration without a sample / hold circuit. The application to the converter is not limited. That is, the present invention can also be applied to a successive approximation A / D converter having a configuration having a sample / hold circuit. However, as described above, by adopting the configuration without the sample / hold circuit, there is an advantage that the kT / C noise of the capacitor C is not superimposed on the input signal and the circuit configuration can be simplified.

[Solid-state imaging device]
FIG. 10 is a system configuration diagram showing a configuration example of a solid-state imaging device, for example, a CMOS image sensor to which the present invention is applied.

  As shown in FIG. 10, the CMOS image sensor 50 according to this application example includes unit pixels (hereinafter sometimes simply referred to as “pixels”) 60 including photoelectric conversion elements arranged two-dimensionally in a matrix. The pixel array unit 51 and its peripheral circuit are included.

  The peripheral circuit of the pixel array unit 51 includes, for example, a vertical scanning circuit 52, a column circuit 53, a horizontal scanning circuit 54, and an output circuit 55, and is integrated on the same chip (semiconductor substrate) as the pixel array unit 51, for example. .

  A vertical signal line 511 is wired for each pixel column with respect to the matrix-like pixel arrangement of the pixel array unit 51, and a drive control line such as a transfer control line 512, a reset control line 513, and a selection control line 514 is provided for each pixel row. Wired (see FIG. 11).

  The vertical scanning circuit 52 includes a shift register, an address decoder, and the like. Here, although the illustration of a specific configuration is omitted, the vertical scanning circuit 52 has a configuration including a readout scanning system and a sweep scanning system. The readout scanning system sequentially performs selective scanning in units of rows for unit pixels from which signals are read out.

On the other hand, the sweep-out scanning system removes unnecessary charges from the photoelectric conversion elements of the unit pixels of the readout row preceding the readout row by the time of the shutter speed with respect to the readout row where the readout scanning is performed by the readout scanning system. Sweep out (reset) sweep scanning. A so-called electronic shutter operation is performed by sweeping (reset) unnecessary charges by the sweep scanning system. Here, the electronic shutter operation refers to an operation in which the photoelectric charge of the photoelectric conversion element is discarded and a new exposure is started (photocharge accumulation is started).

The signal read by the reading operation by the reading scanning system corresponds to the amount of light incident after the immediately preceding reading operation or electronic shutter operation. The period from the read timing by the previous read operation or the sweep timing by the electronic shutter operation to the read timing by the current read operation is the photocharge accumulation time (exposure time) in the unit pixel.

  A signal output from each unit pixel in the pixel row selectively scanned by the vertical scanning circuit 52 is supplied to the column circuit 53 through each vertical signal line 511. The column circuit 53 is a signal readout circuit unit having an A / D converter that converts an analog signal output from each pixel 60 of the selected row into a digital signal for each pixel column of the pixel array unit 51. As the A / D converter, the successive approximation A / D converter according to the above-described embodiment can be used.

  The column circuit 53 may have various signal processing functions such as CDS (Correlated Double Sampling) processing in addition to the A / D conversion function. Here, the CDS process is a noise reduction process for reducing fixed pattern noise unique to a pixel such as reset noise and threshold transistor variation of the amplification transistor.

  Here, the column circuit 53 is provided for each unit column of the pixel array of the pixel array unit 51. However, one column circuit 53 is provided for each of the plurality of columns, and one column circuit 53 is divided between the plurality of columns in a time-sharing manner. It is also possible to adopt a configuration to be used.

  The horizontal scanning circuit 54 includes a shift register, an address decoder, and the like, and sequentially selects the column circuit 53 provided for each pixel column of the pixel array unit 51 by sequentially outputting horizontal selection pulses φH1 to φHn. By the selective scanning by the horizontal scanning circuit 54, the pixel signals digitized by the column circuit 53 are sequentially read out to the horizontal signal line 56.

  The output circuit 55 performs various signal processing on the pixel signal transmitted through the horizontal signal line 56. As an example, the output circuit 55 performs signal processing such as black level adjustment, column variation correction, and color-related processing. In some cases, only buffering processing is performed.

  Timing signals and control signals that serve as references for operations of the vertical scanning circuit 52, the column circuit 53, the horizontal scanning circuit 54, the output circuit 55, and the like are generated by a timing control circuit (timing generator) (not shown).

(Pixel circuit)
FIG. 11 is a circuit diagram illustrating an example of a circuit configuration of the unit pixel 60. As shown in FIG. 11, the unit pixel 60 according to this circuit example includes a photoelectric conversion element, for example, a photodiode 61, and four transistors, for example, a transfer transistor 62, a reset transistor 63, an amplification transistor 64, and a selection transistor 65. It has a configuration.

  Here, as the four transistors 62 to 65, for example, N-channel MOS transistors are used. However, the combination of conductivity types of the transfer transistor 62, the reset transistor 63, the amplification transistor 64, and the selection transistor 65 illustrated here is merely an example, and is not limited to these combinations.

  The photodiode 61 has an anode electrode connected to a negative power source (for example, ground), and photoelectrically converts received light into photocharge (here, photoelectrons) having a charge amount corresponding to the light amount. The cathode electrode of the photodiode 61 is electrically connected to the gate electrode of the amplification transistor 64 through the transfer transistor 62. A node 66 electrically connected to the gate electrode of the amplification transistor 64 is called an FD (floating diffusion) portion.

  The transfer transistor 62 is connected between the cathode electrode of the photodiode 61 and the FD unit 66. A transfer pulse TRG whose high level (for example, VDD level) is active (hereinafter referred to as “High active”) is applied to the gate electrode of the transfer transistor 62 via the transfer control line 512. As a result, the transfer transistor 62 is turned on, and the photoelectric charges photoelectrically converted by the photodiode 61 are transferred to the FD unit 66.

  The reset transistor 63 has a drain electrode connected to the pixel power supply VDD and a source electrode connected to the FD unit 66. A high active reset pulse RST is applied to the gate electrode of the reset transistor 63 via the reset control line 513. Accordingly, the reset transistor 63 is turned on, and the FD unit 66 is reset by discarding the charge of the FD unit 66 to the pixel power supply VDD prior to the transfer of the signal charge from the photodiode 61 to the FD unit 66.

  The amplification transistor 64 has a gate electrode connected to the FD unit 66 and a drain electrode connected to the pixel power supply VDD. The amplification transistor 64 outputs the potential of the FD unit 66 after being reset by the reset transistor 63 as a reset signal (reset level) Vreset. Further, the amplification transistor 64 outputs the potential of the FD unit 66 after the signal charge is transferred by the transfer transistor 62 as a light accumulation signal (signal level) Vsig.

  For example, the selection transistor 65 has a drain electrode connected to the source electrode of the amplification transistor 64 and a source electrode connected to the vertical signal line 511. A high active selection pulse SEL is applied to the gate electrode of the selection transistor 65 via a selection control line 514. As a result, the selection transistor 65 is turned on, and the unit pixel 60 is selected and the signal output from the amplification transistor 64 is relayed to the vertical signal line 511.

  Note that the selection transistor 65 may have a circuit configuration connected between the pixel power supply VDD and the drain of the amplification transistor 64.

  The unit pixel 60 is not limited to the pixel configuration including the four transistors having the above configuration. For example, a pixel configuration including three transistors that serve as both the amplification transistor 64 and the selection transistor 65 may be used, and the configuration of the pixel circuit is not limited.

  As described above, in the CMOS image sensor 50 having the A / D converter in the column circuit 53, a successive approximation A / D converter is used as the A / D converter, and a plurality of pieces of image data are superimposed by oversampling. The S / N can be improved by taking the averaging. That is, since a plurality of images taken continuously have a high correlation, the noise component included in the pixel signal can be reduced by taking the average of the plurality of image data, so that the S / N can be improved. . In addition, by using the successive approximation A / D converter according to the above-described embodiment, the conversion time of A / D conversion can be shortened, so that the speed of signal processing can be increased.

In the application example described above, the CMOS image sensor 50 having the configuration in which the column circuit 53 includes the A / D converter is taken as an example. However, the CMOS image sensor in which the output circuit 55 includes the A / D converter is described. Also good. It is also possible to adopt a configuration in which an A / D converter is provided outside the chip of the CMOS image sensor.

  In the application example, the case where the present invention is applied to a CMOS image sensor in which unit pixels that detect signal charges corresponding to the amount of visible light as physical quantities are arranged in a matrix has been described as an example. The present invention is not limited to application to a CMOS image sensor. That is, the present invention can be applied to all solid-state imaging devices having an A / D converter.

  Furthermore, the present invention is not limited to a solid-state imaging device that sequentially scans each pixel in the pixel array unit in units of rows and reads signals from each pixel in a selected row, and selects any pixel in units of pixels, and selects the selected pixel. The present invention is also applicable to an XY address type solid-state imaging device that reads out signals in units of pixels.

  Note that the solid-state imaging device may be formed as a single chip, or may be in a modular form having an imaging function in which an imaging unit and a signal processing unit or an optical system are packaged together. Good.

  In addition, the present invention is not limited to application to a solid-state imaging device, but can also be applied to an imaging device. Here, the imaging apparatus refers to a camera system such as a digital still camera or a video camera, or an electronic device having an imaging function such as a mobile phone. Note that the above-described module form mounted on an electronic device, that is, a camera module may be used as an imaging device.

[Imaging device]
FIG. 12 is a block diagram illustrating an example of the configuration of the imaging apparatus according to the present invention. As shown in FIG. 12, the imaging apparatus 100 according to the present invention includes an optical system including a lens group 101 and the like, an imaging element 102, a DSP circuit 103 which is a camera signal processing circuit, a frame memory 104, a display apparatus 105, and a recording apparatus 106. The operation system 107 and the power supply system 108 are included. The DSP circuit 103, the frame memory 104, the display device 105, the recording device 106, the operation system 107, and the power supply system 108 are connected to each other via a bus line 109.

  The lens group 101 captures incident light (image light) from a subject and forms an image on the imaging surface of the imaging element 102. The imaging element 102 converts the amount of incident light imaged on the imaging surface by the lens group 101 into an electrical signal in units of pixels and outputs the electrical signal. As the image sensor 102, a CMOS image sensor having the successive approximation A / D converter according to the above-described embodiment is used.

The display device 105 includes a panel type display device such as a liquid crystal display device or an organic EL (electroluminescence) display device, and displays a moving image or a still image captured by the image sensor 102. The recording device 106 converts a moving image or still image captured by the image sensor 102 into a video tape or a DVD.
Recording on a recording medium such as (Digital Versatile Disk).

  The operation system 107 issues operation commands for various functions of the imaging apparatus under operation by the user. The power supply system 108 appropriately supplies various power supplies serving as operation power supplies for the DSP circuit 103, the frame memory 104, the display device 105, the recording device 106, and the operation system 107 to these supply targets.

  DESCRIPTION OF SYMBOLS 10 ... Successive comparison type A / D converter, 11 ... Comparator, 12 ... Data memory, 13 ... Reference signal production | generation part, 14 ... Control part, 15 ... Data converter, 21 ... Cell, 22 ... Capacitance array part, 23 ... X decoder, 24 ... Y decoder, 50 ... CMOS image sensor, 51 ... pixel array unit, 52 ... vertical scanning circuit, 53 ... column circuit, 54 ... horizontal scanning circuit, 55 ... output circuit, 60 ... unit pixel, 61 ... Photodiode 62 ... Transfer transistor 63 ... Reset transistor 64 ... Amplification transistor 65 ... Select transistor 131 ... Reference signal generator 132 ... Thermometer code converter (primary decoder) 133 ... Reference signal generation control circuit (Secondary decoder), 141 ... arithmetic unit, 142 ... capacity control memory, 143 ... redundant data RAM

Claims (9)

A successive approximation A / D converter that performs oversampling processing,
An AD conversion process is performed in which an analog signal to be converted and a reference signal are sequentially compared bit by bit, and a comparison result signal indicating a logical value corresponding to a magnitude comparison between the analog signal to be converted and the reference signal is output. A comparator;
A storage circuit for holding the comparison result signal output from the comparator;
Performs oversampling processing for performing AD conversion processing in the comparator and storage processing in the storage circuit a plurality of times, calculates a bit string used for the next reference signal, and decodes the bit string output from the control unit A reference signal generation unit that generates the analog reference signal according to the decoded result and inputs the reference signal to the comparator;
The calculation control unit of the control unit is
When performing oversampling in which the A / D conversion process in the comparator is performed a plurality of times on the analog signal to be converted, the analog signal to be converted is A / D converted in the first A / D conversion process. N-bit digital value is obtained, and in the second and subsequent A / D conversion processes, the upper n bits of the N-bit digital value obtained in the first A / D conversion process are fixed, and (N−n) bits or less Controlling the reference signal to start A / D conversion from the lower bits of
Successive comparison type A / D converter. The magnitude of the return value to the lower bits is defined by the noise level superimposed on the analog signal to be converted and the magnitude of 1 [LSB].
The successive approximation A / D converter according to claim 1. The A / D conversion speed in the comparator is higher than a predetermined speed compared with the change speed of the analog signal to be converted, or the analog signal to be converted is DC until the end of the A / D conversion process.
The successive approximation A / D converter according to claim 1. The comparator performs successive comparisons with the reference signal without sampling / holding the analog signal to be converted;
The successive approximation A / D converter according to claim 3. The reference signal generation unit includes a capacitance array unit in which unit capacitance corresponding to the size of 1 [LSB] is arranged in a two-dimensional array corresponding to the resolution, and the unit every time A / D conversion is performed. A selection circuit that randomly selects a capacitance;
The successive approximation A / D converter according to claim 1. The determination threshold has a redundant configuration wider than a predetermined value so that A / D conversion is possible even if an erroneous determination occurs in the comparator.
The successive approximation A / D converter according to claim 1. A control method of a successive approximation A / D converter that performs oversampling processing,
The successive approximation A / D converter sequentially compares an analog signal to be converted and a reference signal bit by bit, and indicates a logical value corresponding to a magnitude comparison between the analog signal to be converted and the reference signal. A comparator that performs AD conversion processing that outputs a result signal, a storage circuit that holds the comparison result signal output from the comparator, AD conversion processing in the comparator, and storage processing in the storage circuit a plurality of times Performs oversampling processing to be performed, calculates a bit string used for the next reference signal, decodes the bit string output from the control unit, and generates the reference signal in an analog format according to the decoded result And a reference signal generation unit that is input to the comparator.
A control method of a successive approximation A / D converter,
When performing oversampling in which the A / D conversion process in the comparator is performed a plurality of times on the analog signal to be converted, the analog signal to be converted is A / D converted in the first A / D conversion process. N-bit digital value is obtained, and in the second and subsequent A / D conversion processes, the upper n bits of the N-bit digital value obtained in the first A / D conversion process are fixed, and (N−n) bits or less Controlling the reference signal to start A / D conversion from the lower bits of
Control method of successive approximation type A / D converter. A successive approximation A / D converter that converts an analog signal output from a pixel including a photoelectric conversion element into a digital signal;
The successive approximation A / D converter is a successive approximation A / D converter that performs oversampling processing,
A comparison of an analog signal to be converted output from a pixel including the photoelectric conversion element and a reference signal sequentially for each bit, and a comparison indicating a logical value according to a magnitude comparison between the analog signal to be converted and the reference signal A comparator that outputs a result signal and performs AD conversion processing;
A storage circuit for holding the comparison result signal output from the comparator;
Performs oversampling processing for performing AD conversion processing in the comparator and storage processing in the storage circuit a plurality of times, calculates a bit string used for the next reference signal, and decodes the bit string output from the control unit A reference signal generation unit that generates the analog reference signal according to the decoded result and inputs the reference signal to the comparator;
The calculation control unit of the control unit is
When performing oversampling in which the A / D conversion process in the comparator is performed a plurality of times on the analog signal to be converted, the analog signal to be converted is A / D converted in the first A / D conversion process. N-bit digital value is obtained, and in the second and subsequent A / D conversion processes, the upper n bits of the N-bit digital value obtained in the first A / D conversion process are fixed, and (N−n) bits or less Controlling the reference signal to start A / D conversion from the lower bits of
Solid-state imaging device. A solid-state imaging device having a successive approximation A / D converter that converts an analog signal output from a pixel including a photoelectric conversion element into a digital signal;
An optical system that forms an image of incident light on the imaging surface of the solid-state imaging device,
The successive approximation A / D converter is a successive approximation A / D converter that performs oversampling processing,
A comparison of an analog signal to be converted output from a pixel including the photoelectric conversion element and a reference signal sequentially for each bit, and a comparison indicating a logical value according to a magnitude comparison between the analog signal to be converted and the reference signal A comparator that outputs a result signal and performs AD conversion processing;
A storage circuit for holding the comparison result signal output from the comparator;
Performs oversampling processing for performing AD conversion processing in the comparator and storage processing in the storage circuit a plurality of times, calculates a bit string used for the next reference signal, and decodes the bit string output from the control unit A reference signal generation unit that generates the analog reference signal according to the decoded result and inputs the reference signal to the comparator;
The calculation control unit of the control unit is
When performing oversampling in which the A / D conversion process in the comparator is performed a plurality of times on the analog signal to be converted, the analog signal to be converted is A / D converted in the first A / D conversion process. N-bit digital value is obtained, and in the second and subsequent A / D conversion processes, the upper n bits of the N-bit digital value obtained in the first A / D conversion process are fixed, and (N−n) bits or less Controlling the reference signal to start A / D conversion from the lower bits of
Imaging device.

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