JP2014143508A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging apparatus whose dynamic range is enlarged.SOLUTION: An imaging apparatus includes: a plurality of photoelectric conversion sections arranged in a plurality of pixels one by one; a plurality of switch elements connected to output terminals of the plurality of photoelectric conversion sections, respectively; a floating diffusion which is connected to an output side of the plurality of switch elements and sequentially adds and accumulates output signals of the plurality of photoelectric conversion sections via the plurality of switch elements; and a signal reading circuit connected to the floating diffusion. When a charge amount accumulated in the floating diffusion reaches a saturation capacity of the floating diffusion, the signal reading circuit multiplies a signal level obtained by adding the output signals of the photoelectric conversion sections before the charge amount reaches the saturation capacity by N/m to output it when the number of pixels to which the output signals of the photoelectric conversion sections are added before the charge amount reaches the saturation capacity is set to be m, and the number of whole pixels of the plurality of pixels to be N.

Description

  The present invention relates to an imaging apparatus.

  Conventionally, in order to expand the dynamic range (ratio of signal level to noise level) of an imaging apparatus, a method of combining a plurality of pixel outputs with different sensitivities (for example, see Patent Document 1), a plurality of pixels with different exposure times There is a method of synthesizing outputs (see, for example, Patent Document 2).

  In addition, in a pixel sharing type image pickup device in which photoelectric conversion elements of a plurality of pixels are joined by floating diffusion (FD), there is a method of reading out signals obtained from each pixel while adding them by FD (for example, Patent Document 3). reference).

JP 2000-125209 A JP 2007-235656 A Japanese Patent No. 4821921

  However, in Patent Document 1, it is necessary to manufacture a plurality of pixels having different sizes. Manufacturing pixels having different sizes has a problem that the manufacturing process becomes complicated and the manufacturing cost increases.

  Moreover, in patent document 2, there exists a subject that a shift | offset | difference arises in a moving image by the difference in the exposure time between pixels.

  Japanese Patent Laid-Open No. 2004-228688 can use a FD to read a signal at a high speed, but has a problem that the dynamic range cannot be expanded.

  SUMMARY An advantage of some aspects of the invention is that it provides an imaging apparatus with an expanded dynamic range.

  An imaging device according to one aspect of the present invention includes a plurality of photoelectric conversion units arranged one by one in a plurality of pixels, a plurality of switch elements respectively connected to output terminals of the plurality of photoelectric conversion units, and a plurality of switch elements Including a floating diffusion that sequentially adds and accumulates output signals of a plurality of photoelectric conversion units via a plurality of switch elements, and a signal readout circuit that is connected to the floating diffusion. When the amount of charge accumulated in the floating diffusion reaches the saturation capacity of the floating diffusion, the circuit calculates the number of pixels to which the output signal of the photoelectric conversion unit is added before the amount of charge reaches the saturation capacity, m Where N is the total number of pixels, the signal obtained by adding the output signals of the photoelectric conversion unit before the charge amount reaches the saturation capacity And it outputs the level N / m times to.

  According to the present invention, it is possible to provide a specific effect that an imaging device with an expanded dynamic range can be provided.

2 is a diagram illustrating a pixel unit 100 included in the imaging apparatus according to Embodiment 1. FIG. 1 is a diagram illustrating an imaging apparatus 500 according to Embodiment 1. FIG. 6 is a diagram illustrating a driving sequence of the imaging apparatus 500 according to Embodiment 1. FIG. 6 is a diagram illustrating a driving sequence of the imaging apparatus 500 according to Embodiment 1. FIG. 6 is a diagram illustrating a pixel unit 200 according to Embodiment 2. FIG. It is a figure which shows an example of a drive sequence. FIG. 6 is a diagram schematically showing the charge amounts of photodiodes PD0 and PD1 of a pixel unit 200 according to the second embodiment. 6 is a diagram illustrating a drive sequence of the imaging apparatus according to Embodiment 2. FIG.

  Hereinafter, embodiments to which the imaging apparatus of the present invention is applied will be described.

<Embodiment 1>
FIG. 1 is a diagram illustrating a pixel unit 100 included in the imaging apparatus according to the first embodiment.

  The pixel unit 100 includes a plurality of photodiodes PD0 and PD1, switch elements TX0 and TX1, a floating diffusion FD, a reset switch element RT, an output stage 110, and an output terminal 100A.

  The photodiodes PD0 and PD1 are disposed in pixels that are close to each other. The adjacent pixels mean pixels adjacent in a plan view or pixels adjacent in a diagonal direction in a plan view when the pixels are arranged in a matrix. The pixel arrangement may be a Bayer arrangement.

  The photodiodes PD0 and PD1 are examples of photoelectric conversion units. Here, photodiodes PD0 and PD1 are shown as examples of the photoelectric conversion unit. However, the photoelectric conversion unit is not limited to the photodiode, and may be any element that can photoelectrically convert incident light and output an imaging signal.

  The photodiodes PD0 and PD1 have their input terminals grounded and their output terminals connected to the switch elements TX0 and TX1, respectively.

  FIG. 1 shows two photodiodes PD0 and PD1 as an example. However, since there are actually a large number (N) of pixels, the pixel unit 100 includes a large number of photodiodes PD0 to PD (N− 1). Here, N is an arbitrary integer of 2 or more.

  The switch elements TX0 and TX1 are configured by, for example, FET (Field Effect Transistor). Here, as an example, a mode in which the switch elements TX0 and TX1 are n-type FETs will be described. However, the switch elements TX0 and TX1 are not limited to n-type FETs, but may be other types of transistors.

  The switch elements TX0 and TX1 have sources connected to the output terminals of the photodiodes PD0 and PD1, respectively, and drains connected to the floating diffusion FD. The gates of the switch elements TX0 and TX1 are driven by the drive unit of the imaging device including the pixel unit 100 of the first embodiment.

  The number of switch elements TX0 and TX1 is equal to the number of photodiodes PD0 and PD1. That is, the switch elements (TX0, TX1) are connected one by one to the output side of the photodiodes (PD0, PD1).

  The floating diffusion FD is connected to the drains of the switch elements TX0 and TX1. The floating diffusion FD is, for example, an n-type region of a pn junction diode and is electrically floating.

  The floating diffusion FD has a function like a capacitor and accumulates imaging signals (negative charges) output from the photodiodes PD0 and PD1 connected via the switch elements TX0 and TX1.

  Since the floating diffusion FD has a predetermined saturation capacity, it is not possible to accumulate electric charges exceeding the predetermined saturation capacity. The floating diffusion FD is configured to have a predetermined saturation capacity depending on the application of the imaging device 500 and the like. This saturation capacity corresponds to the saturation level of the floating diffusion FD. The saturation level is a level at which no more charge can be accumulated by the imaging signal.

  The reset switch element RT is, for example, an n-type FET, the source is connected to the floating diffusion FD, and the drain is connected to the power supply (VDD). The reset switch element RT is driven by the drive unit of the imaging apparatus including the pixel unit 100 of the first embodiment. When the reset switch element RT is turned on, the floating diffusion is connected to the power source (VDD) and reset. The reset switch element RT is not limited to an n-type FET, but may be another type of transistor.

  The output stage 110 is a circuit for taking out the electric charge accumulated in the floating diffusion FD to the signal readout circuit. In the first embodiment, the output stage 110 includes, for example, a pair of n-type FETs 110A and 110B.

  A pair of n-type FETs included in the output stage 110 are stacked vertically, the drain of the FET 110A is connected to the power supply (VDD), the source is connected to the drain of the FET 110B, and the gate is connected to the floating diffusion FD. The The FET 110A constitutes a so-called source follower circuit by connecting a floating diffusion FD to the gate.

  The drain of the FET 110B is connected to the source of the FET 110A, and the source is connected to the output terminal 120. The select signal (SL) of the imaging device 500 is applied to the gate of the FET 110B. The select signal is a signal for selecting two pixels including the photodiodes PD0 and PD1, and is output from the drive unit of the imaging apparatus 500.

  The output terminal 100 </ b> A is an output terminal of the pixel unit 100. The pixel unit 100 outputs the imaging signal accumulated in the floating diffusion FD from the output terminal 100A (as PIXEL OUT) via the output stage 110. The output terminal 100A is connected to the input terminal of the signal readout unit of the imaging apparatus including the pixel unit 100 of the first embodiment.

  FIG. 2 is a diagram illustrating the imaging apparatus 500 according to the first embodiment.

  The imaging apparatus 500 includes a pixel unit 100, a CDS circuit 510, a PGA circuit 520, an ADC circuit 530, an ADCLOG circuit 540, and a determination circuit 550.

  The pixel unit 100 has an output terminal 100A shown in FIG. 1 connected to the CDS circuit 510, and outputs an imaging signal based on charges accumulated in the FD to the CDS circuit 510.

The CDS circuit 510 is a circuit that performs correlated double sampling (CDS), and performs signal processing for reducing noise of an imaging signal input from the pixel unit 100. The CDS circuit 510 outputs an imaging signal represented by a potential obtained by subtracting the reset level (S _RST ) of the floating diffusion FD from the imaging signal input from the pixel unit 100. An imaging signal output from the CDS circuit 510 is input to the PGA circuit 520.

  A PGA (Programmable Gain Amplifier) circuit 520 amplifies the imaging signal input from the CDS circuit 510 with a predetermined amplification factor and outputs the amplified signal. The predetermined amplification factor is a fixed value set according to the output level of the pixel unit 100 or the like. The imaging signal amplified by the PGA circuit 520 is input to the ADC circuit 530.

  The ADC circuit 530 is an analog to digital converter that converts the imaging signal (analog value) amplified by the PGA circuit 520 into a digital value. The imaging signal digitally converted by the ADC circuit 530 is input to the ADCLOG circuit 540.

  The ADCLOG circuit 540 includes ADCLOG circuit units 541 and 542. The ADCLOG circuit units 541 and 542 are connected to the output terminal of the ADC circuit 530, and hold the first sample value of the imaging signal and the second sample value of the imaging signal, respectively.

Here, the first sample value of the imaging signal refers to the floating diffusion from the charge level (S _RA ) of the floating diffusion FD when the imaging signal obtained by the photodiode PD0 of the pixel unit 100 is accumulated in the floating diffusion FD. A value obtained by subtracting the reset level (S _RST ) of the FD (S _RA −S _RST ). The ADCLOG circuit unit 541 samples and holds the first sample value of the imaging signal.

Further, the second sample value of the imaging signal means that the floating diffusion FD that accumulates the imaging signal obtained by the photodiode PD0 further accumulates the imaging signal obtained by the photodiode PD1 (of the floating diffusion FD). ) A value (S _RB -S _RST ) obtained by subtracting the reset level (S _RST ) of the floating diffusion FD from the charge level (S _RB ). The ADCLOG circuit unit 542 samples and holds the second sample value of the imaging signal.

  In the first embodiment, the ADCLOG circuit 540 includes two ADCLOG circuit units 541 and 542 in order to describe a mode in which the pixel unit 100 includes two photodiodes PD0 and PD1. The number of ADCLOG circuit portions (541, 542) is equal to the number of photodiodes (PD0, PD1) included in the pixel portion 100.

  Actually, the pixel portion 100 includes a large number (N) of pixels, and the pixel portion 100 includes a large number (N) of photodiodes PD0 to PD (N−1). Therefore, the ADCLOG circuit portions (541, 542) are included. Are also provided in large numbers (N). N is an arbitrary integer of 2 or more.

In the determination circuit 550, the second sample value (S _RB −S _RST ) of the imaging signal held by the ADCLOG circuit unit 542 is obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD. It is determined whether or not the value (S _F −S _RST ) has been reached. Here, the saturation level is a level representing the saturation capacity of the floating diffusion FD, and is a level at which no more charge can be accumulated by the imaging signal.

In the determination circuit 550, the second sample value (S _RB −S _RST ) has reached a value (S _F −S _RST ) obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD. Is determined, the first sample value (S _RA −S _RST ) of the imaging signal held by the ADCLOG circuit unit 541 is output by multiplying it by N / m.

  Here, N is the number of pixels (total number of pixels) of the pixel unit 100, and generally N is an arbitrary integer equal to or greater than 2, but the case of N = 2 will be described in the first embodiment.

Since the floating diffusion FD accumulates electrons, the charge level of the floating diffusion FD becomes lower than the reset level (S _RST ) as the imaging signal is accumulated. That is, the saturation level of the floating diffusion FD is lower than the charge level in the state where the floating diffusion FD is reset.

Accordingly, the second sample value (S _RB −S _RST ) of the imaging signal reaches a value (S _F −S _RST ) obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD ( “Achieved” means that (S _RB −S _RST ) = (S _F −S _RST ) holds.

The charge level (S _RB ) in the state where the floating diffusion FD storing the imaging signal obtained by the photodiode PD0 further accumulates the imaging signal obtained by the photodiode PD1 is the saturation level. It cannot occur lower than (S _F ). This is because the floating diffusion FD cannot accumulate charges until the charge level of the floating diffusion FD falls below the saturation level (S _F ) (before the saturation capacity is exceeded).

Therefore, actually, the second sample value (S _RB −S _RST ) of the imaging signal is a value obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD (S _F −S _RST). ), (S _RB −S _RST ) = (S _F −S _RST ). That is, this corresponds to the case where the charge level (S _RB ) reaches the saturation level (S _F ).

  Further, m represents the number of pixels to which the imaging signal is added in the floating diffusion FD before the signal level of the imaging signal accumulated in the floating diffusion FD reaches the saturation level.

  When the signal level of the imaging signal accumulated in the floating diffusion FD reaches the saturation level when the second sample value is obtained, the floating diffusion FD has the first sample value before the first sampling value is obtained. In order to represent the number of pixels to which the imaging signal is added, m = 1. In general, 1 ≦ m ≦ N−1.

For this reason, in the first embodiment, the determination circuit 550 determines that the second sample value (S _RB −S _RST ) of the imaging signal held by the ADCLOG circuit unit 542 is from the saturation level (S _F ) of the floating diffusion FD. If it is determined to have reached the reset level _{(S RST)} obtained by subtracting the value _{(S F -S RST),} the first sample value of the image signal held by the ADCLOG circuit section 541 _{(S RA -S RST)} 2 Double the output. In this case, N = 2, m = 1, and N / m = 2.

Thus, when the second sample value (S _RB −S _RST ) reaches the value obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD, the second sample value Does not correctly reflect the total value of the imaging signals obtained by the photodiodes PD0 and PD1. This is because the signal level reaches a peak due to the saturation level of the floating diffusion FD. In such a case, the dynamic range is reduced.

  For this reason, in the imaging apparatus 500 according to the first embodiment, the dynamic range is expanded by multiplying the imaging signal obtained immediately before the floating diffusion FD is saturated by N / m.

In the first embodiment, since N / m = 2, the second sample value (S _RB −S _RST ) is a value obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD. If it has reached, the first sample value (S _RA −S _RST ) of the imaging signal held by the ADCLOG circuit unit 541 is doubled and output.

On the other hand, in the determination circuit 550, the second sample value (S _RB −S _RST ) of the imaging signal held by the ADCLOG circuit unit 542 changes the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD. If it is determined that the subtracted value has not been reached, the second sample value of the imaging signal held by the ADCLOG circuit unit 542 is output as it is.

When the second sample value (S _RB −S _RST ) does not reach the value obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD, the imaging signals of the photodiodes PD0 and PD1 This is because the correct signal level is obtained and the dynamic range does not deteriorate.

  Here, a drive sequence of the imaging apparatus 500 according to the first embodiment will be described with reference to FIGS. 3 and 4.

  3 and 4 are diagrams illustrating a driving sequence of the imaging apparatus 500 according to the first embodiment. 3 and 4 show the reset switch element RT, the switch elements TX0 and TX1, the select signal SL, and the signal level PIXEL OUT of the output terminal 110A. PIXEL OUT is a signal level of the imaging signal accumulated in the floating diffusion FD.

The reset level _SRST is the level of PIXEL OUT in a state where the reset switch element RT is turned on and the floating diffusion FD is reset.

The charge level S _RA is a charge level of the floating diffusion FD in a state where an imaging signal obtained by the photodiode PD0 of the pixel unit 100 is accumulated in the floating diffusion FD.

The charge level _SRB is a charge level (of the floating diffusion FD) in a state in which the floating diffusion FD storing the imaging signal obtained by the photodiode PD0 further accumulates the imaging signal obtained by the photodiode PD1.

S _F is the saturation level of the floating diffusion FD.

FIG. 3 shows a value obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD by the determination circuit 550 for the second sample value (S _RB −S _RST ) of the image signal of the imaging signal. This is a case where it is determined that (S _F −S _RST ) has not been reached.

In this case, at time t0, the reset switch element RT and the select signal SL become H level, and thereby the charge level of the floating diffusion FD is reset to _SRST .

Next, at time t1, the switching element TX0 is turned on, by the image pickup signal of the photodiode PD0 is read, the signal level PIXEL OUT of the output terminal 110A is reduced to _{S RA.}

After that, when the switch element TX1 is turned on at time t2 without turning on the reset switch element RT, the image signal of the photodiode PD1 is read, and the signal level PIXEL OUT of the output terminal 110A is set to S. Decreases to _RB .

Since the charge level S _RB has not reached the saturation level (S _F ), the determination circuit 550 determines that the second sample value (S _RB −S _RST ) of the image signal of the image pickup signal is equal to the saturation level of the floating diffusion FD ( It is determined that the value (S _F −S _RST ) obtained by subtracting the reset level (S _RST ) from S _F ) has not been reached.

Thereafter, at time t3, the reset switch element RT is set to H level, whereby the charge level of the floating diffusion FD is reset to _SRST .

  In this case, the determination circuit 550 outputs the second sample value of the imaging signal held by the ADCLOG circuit unit 542 as it is.

On the other hand, in FIG. 4, the determination circuit 550 causes the second sample value (S _RB −S _RST ) of the image signal of the imaging signal to subtract the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD. This is a case where it is determined that the value (S _F −S _RST ) has been reached.

In this case, at time t0, the reset switch element RT and the select signal SL become H level, and thereby the charge level of the floating diffusion FD is reset to _SRST .

Next, at time t1, the switching element TX0 is turned on, by the image pickup signal of the photodiode PD0 is read, the signal level PIXEL OUT of the output terminal 110A is reduced to _{S RA.} Up to this point, the operation is the same as that of FIG.

After that, when the switch element TX1 is turned on at time t2 without turning on the reset switch element RT, the image signal of the photodiode PD1 is read, and the signal level PIXEL OUT of the output terminal 110A is set to S. Decreases to _RB .

In Figure 4, (for the amount of electrons is large) charge amount representing the imaging signals of the photodiodes PD1 because more than the charge amount representing the imaging signals of the photodiodes PD1 in Figure 3, the charge level S _RB at later time t2 The saturation level (S _F ) is reached, lower than in FIG.

Therefore, the determination circuit 550 determines that the second sample value (S _RB −S _RST ) of the image signal of the imaging signal is a value obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD. S _F −S _RST ) is determined.

Thereafter, at time t3, the reset switch element RT is set to H level, whereby the charge level of the floating diffusion FD is reset to _SRST .

In this case, the determination circuit 550 doubles and outputs the first sample value (S _RA −S _RST ) of the imaging signal held by the ADCLOG circuit unit 541.

If the saturation level is ignored, in FIG. 4, the charge level _SRB is lower than the saturation level (S _F ) as shown by the broken line in the period from time t2 to time t3.

However, as described above, the floating diffusion FD cannot actually store charges until the charge level of the floating diffusion FD falls below the saturation level (S _F ).

Therefore, the signal level of the imaging signal of the photodiode PD1 is large, and the sum of the signal level of the imaging signal of the photodiode PD0 and the signal level of the imaging signal of the photodiode PD1 is equal to the reset level (S _RST ) and the floating diffusion. When the difference is larger than the difference between the saturation level (S _F ) of FD, the signal level PIXEL OUT of the output terminal 110A will bottom out at the saturation level (S _F ).

  The determination circuit 550 as described above can be realized by a combinational circuit, for example.

As described above, in the imaging apparatus 500 according to Embodiment 1, the determination circuit 550 determines that the second sample value (S _RB −S _RST ) of the imaging signal held by the ADCLOG circuit unit 542 is the saturation level (S _F ) is subtracted from the reset level (S _RST ) to determine whether it has reached a value (S _F −S _RST ), and if it is determined that the saturation level has been reached, the imaging held by the ADCLOG circuit unit 541 The first sample value (S _RA −S _RST ) of the signal is multiplied by N / m and output.

  In the first embodiment, N = 2 and m = 1.

For this reason, in the first embodiment, the determination circuit 550 determines that the second sample value (S _RB −S _RST ) of the imaging signal held by the ADCLOG circuit unit 542 is from the saturation level (S _F ) of the floating diffusion FD. If it is determined to have reached the reset level _{(S RST)} obtained by subtracting the value _{(S F -S RST),} the first sample value of the image signal held by the ADCLOG circuit section 541 _{(S RA -S RST)} 2 Double the output.

That is, when (S _RB −S _RST ) = (S _F −S _RST ) holds, the determination circuit 550 outputs an imaging signal having a signal level represented by (S _RA −S _RST ) × 2.

  Therefore, the dynamic range can be doubled even when the imaging signal of the photodiode PD1 cannot be stored in the floating diffusion FD.

  In Embodiment 1, the case where the pixel unit 100 of the imaging device 500 includes two photodiodes PD0 and PD1 will be described. However, the pixel unit 100 actually includes a large number (N) of photodiodes. Correspondingly, the ADCLOG circuit 540 includes N ADCLOG circuit units.

  In addition, when the pixel unit 100 includes a large number (N) of photodiodes, the imaging signal is added in the floating diffusion FD before the signal level of the imaging signal accumulated in the floating diffusion FD reaches the saturation level. The number of pixels is m. m is an integer satisfying 1 ≦ m ≦ N−1.

  For this reason, in the imaging apparatus 500 according to the first embodiment, when the imaging signals of the N photodiodes of the pixel unit 100 are sequentially stored in the floating diffusion FD, the imaging signal of the (m + 1) th photodiode is stored in the floating diffusion. If the charge level of the floating diffusion FD reaches the saturation level when stored in the FD, the determination circuit 550 multiplies the mth sample value held by the mth ADCLOG circuit unit by N / m and outputs the result. To do.

  Thereby, in the case where charges cannot be accumulated in the floating diffusion FD, the dynamic range of the imaging apparatus 500 can be expanded N / m times.

On the other hand, in the determination circuit 550, the N-th sample value of the imaging signal held by the Nth ADCLOG circuit unit reaches a value obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD. If it is determined that it is not, the Nth sample value of the imaging signal held by the Nth ADCLOG circuit unit is output as it is.

If the N-th sample value does not reach the value obtained by subtracting the reset level (S _RST ) from the saturation level (S _F ) of the floating diffusion FD, the signal levels of the imaging signals of the N photodiodes are correct values. This is because the dynamic range is not reduced.

  As described above, according to the imaging apparatus 500 of the first embodiment, when the imaging signal obtained by a plurality of pixels is accumulated in the floating diffusion FD, the amount of charge due to the accumulated imaging signal reaches the saturation capacity of the floating diffusion FD. In this case, the dynamic range of the imaging apparatus 500 can be expanded to N / m times.

  Further, in order to suppress a decrease in dynamic range, a method of taking an average of a plurality of samples has been proposed (see, for example, Patent Document 3). However, an increase in the number of samples causes a decrease in shooting speed. Yes.

  On the other hand, the imaging apparatus 500 according to the first embodiment does not require processing for obtaining the average value as described above, so that the photographing speed does not decrease.

  Here, the noise level in the case of low illuminance (when the charge amount does not reach the saturation capacity) is the noise level when reading one pixel at a time. On the other hand, the noise level at high illuminance (when the charge amount reaches the saturation capacity) is N / m times, but the light level is ignored on the high illuminance side, so the noise level is ignored. it can.

  For this reason, while the noise level on the low illuminance side is the same, the maximum signal level is N / m times. As a result, the dynamic range can be expanded to N / m times. Here, the saturation capacity (saturation level) of the floating diffusion FD can be determined arbitrarily.

  Since each pixel has a saturation level variation due to a manufacturing error, it is possible to eliminate an erroneous determination by setting the saturation level variation or less for each pixel when the addition processing is performed.

<Embodiment 2>
In the second embodiment, a case where there is a relatively large difference in the amount of incident light of the photodiodes PD0 and PD1 will be described.

  The imaging device of the second embodiment is different from the imaging device 500 of the first embodiment in the configuration of the pixel unit. The configuration other than the pixel portion is the same as that of the imaging apparatus 500 of the first embodiment. For this reason, the pixel unit 200 and the driving sequence of the second embodiment will be described below.

  FIG. 5 is a diagram illustrating the pixel unit 200 according to the second embodiment. FIG. 6 is a diagram illustrating an example of a drive sequence.

  The pixel unit 200 according to the second embodiment is obtained by adding a switching element SP that connects the output terminals of the photodiodes PD0 and PD1 to the pixel unit 100 (see FIG. 1) according to the first embodiment. The switch element SP is an example of a combining unit that combines (equally divides) the output signals of the photodiodes PD0 and PD1.

  The switch element SP is composed of, for example, an n-type FET, the drain is connected to the output terminal of the photodiode PD0, the source is connected to the output terminal of the photodiode PD1, and the gate is the drive of the imaging device of the second embodiment. Connected to the part. The drain and source connection may be reversed.

  For example, when the positions of the photodiodes PD0 and PD1 are far apart from the positional relationship between the photodiodes PD0 and PD1 of the first embodiment or when the subject has a high contrast when the pixel pitch is relatively large. Therefore, a difference occurs in the signal amount between the photodiodes PD0 and PD1.

  This is because the photodiodes PD0 and PD1 exist at different positions on the imaging surface, and a difference occurs in the amount of incident light.

  For example, consider a case where the incident light amount of the photodiode PD0 is smaller than the incident light amount of the photodiode PD1, and when the photodiode PD0 and the PD1 imaging signal are accumulated in the floating diffusion FD, the total charge amount reaches a saturation level. .

In such a case, as shown in FIG. 6, the first sample value _{(S RA -S RST)} twice the amount of signal _{2 × (S RA -S RST)} , considered ignoring saturation level In this case, there is a great difference from the second sample value (S _RB −S _RST ). These values should be substantially the same when the incident light amounts of the photodiodes PD0 and PD1 are substantially equal. The imaging apparatus 500 according to Embodiment 1 is based on such a premise.

  The imaging apparatus according to the second embodiment corresponds to such a state.

  FIG. 7 is a diagram schematically showing the charge amounts of the photodiodes PD0 and PD1. 7A to 7C, the vertical axes indicate the charge amounts of the photodiodes PD0 and PD1. In FIG. 7A, the switch elements TX0, TX1, and SP are all off. In FIG. 7B, the switch elements TX0 and TX1 are off, and the switch element SP is on. In FIG. 7C, the switch elements TX0, TX1, and SP are all off.

  For example, as shown in FIG. 7A, it is assumed that the amount of charge obtained by the photodiode PD0 is small and the amount of charge obtained by the photodiode PD1 is larger when the switch element SP is off.

  In such a case, as shown in FIG. 7B, the charge amounts of the photodiodes PD0 and PD1 can be made equal by turning on the switch element SP.

  After the switch element SP is turned on as shown in FIG. 7B and the charge amounts of the photodiodes PD0 and PD1 are made equal, even if the switch element SP is turned off, as shown in FIG. The charge amounts of PD0 and PD1 remain equal.

  In the second embodiment, by turning on the switching element SP, the charge amounts of the photodiodes PD0 and PD1 can be made equal even when the incident light amounts of the photodiodes PD0 and PD1 are different.

  FIG. 8 is a diagram illustrating a driving sequence of the imaging apparatus according to the second embodiment.

  As shown in FIG. 8, the switch element SP is turned on immediately before time t1. Thereby, the charge amounts of the photodiodes PD0 and PD1 can be made equal.

Here, in FIG. 8, if the saturation level is ignored, as in the case shown in FIG. 4, the charge level _SRB is indicated by a broken line in the period from time t2 to before time t3. It becomes lower than the saturation level (S _F ). However, in practice, the charge level S _RB cannot be lower than the saturation level (S _F ).

However, in Embodiment 2, since the charge amounts of the photodiodes PD0 and PD1 are equalized by turning on the switch element SP immediately before time t1, the charge level _SRB is saturated after time t2. When (S _F ) is reached, the signal amount 2 × (S _RA −S _RST ) obtained by doubling the first sample value (S _RA −S _RST ), and the second time when the saturation level is considered and ignored Can be made equal to the sample value of (S _RB −S _RST ).

  For this reason, in the imaging apparatus according to the second embodiment, the dynamic range can be expanded as in the first embodiment even when there is a relatively large difference in the amount of incident light between the photodiodes PD0 and PD1.

  The imaging apparatus of Embodiment 2 can expand the dynamic range even when the contrast is intense and the spatial frequency is high, such as a natural image.

  In the above description, the pixel unit 200 includes two photodiodes PD0 and PD1. However, when the pixel unit 200 generally includes N photodiodes, the output terminals of the N photodiodes are denoted by N−1. What is necessary is just to connect by one switch element SP.

  The imaging device according to the exemplary embodiment of the present invention has been described above, but the present invention is not limited to the specifically disclosed embodiment, and does not depart from the scope of the claims. Various modifications and changes are possible.

DESCRIPTION OF SYMBOLS 100 Pixel part 100A Output terminal 110 Output stage PD0, PD1 Photodiode TX0, TX1 Switch element FD Floating diffusion RT Reset switch element 500 Imaging device 510 CDS circuit 520 PGA circuit 530 ADC circuit 540 ADCLOG circuit 541, 542 ADCLOG circuit part 50 Circuit 200 Pixel part SP Switch element

Claims (4)

A plurality of photoelectric conversion units disposed one by one in a plurality of pixels;
A plurality of switch elements respectively connected to output terminals of the plurality of photoelectric conversion units;
Floating diffusion that is connected to the output side of the plurality of switch elements and accumulates the output signals of the plurality of photoelectric conversion units in order through the plurality of switch elements;
A signal readout circuit connected to the floating diffusion,
When the amount of charge accumulated in the floating diffusion reaches the saturation capacity of the floating diffusion, the signal readout circuit sets the number of pixels to which the output signal of the photoelectric conversion unit is added before the amount of charge reaches the saturation capacity, m, An imaging apparatus that outputs a signal obtained by adding N / m times the level of a signal obtained by adding output signals of a photoelectric conversion unit before the amount of charge reaches a saturation capacity, where N is the total number of pixels.   The imaging device according to claim 1, wherein the signal readout circuit reads out the level of the signal accumulated in the floating diffusion when the charge amount does not reach the saturation capacity. The signal readout circuit
An AD converter for digitally converting the output signal of the floating diffusion;
A plurality of holding circuits that are connected to the output side of the AD converter and hold the signals that the floating diffusion sequentially adds and stores in order for each addition, and the same number of photoelectric conversion units;
Determine whether the signal held by any of the multiple holding circuits has reached the saturation capacity, and if any of the signals has reached the saturation capacity, add it before the signal reaches the saturation capacity. The imaging apparatus according to claim 1, further comprising: a determination circuit that outputs a signal held by a corresponding holding circuit by N / m times.   The imaging device according to claim 1, further comprising a combining unit connected to the output side of the plurality of photoelectric conversion units and configured to combine the output signals of the plurality of photoelectric conversion units.

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