# Patent

## Title
Photodetector having wide dynamic range and low temperature dependence

## Abstract

A photodetector of a wide dynamic range of incident light amount detection and low temperature dependence is provided. A first signal processing unit 10m,n includes an integrating circuit 11, a first holding circuit 12, a comparing circuit 13, a second holding circuit 14, and a latching circuit 15. The integrating circuit 11 has a variable capacitor unit that is selectively set to a capacitance value among a plurality of capacitance values, accumulates charges, output from the photodiode, into the variable capacitor unit over an accumulating period that is in accordance with the capacitance value set at the variable capacitor unit, and outputs a voltage V1 that is in accordance with the amount of the accumulated charges. The comparing circuit 13 inputs the voltage V1 output from the integrating circuit 11, performs a quantitative comparison of the voltage V1 with a reference voltage Vref, outputs a compared signal S3 expressing the result of comparison, and, when the voltage V1 output from the integrating circuit 11 at the end of an accumulating period is less than the reference voltage Vref, instructs the first holding circuit 12 to hold the voltage.

## Background
TECHNICAL FIELD
The present invention relates to a photodetector.
BACKGROUND ART
Among photodetectors, there are known types in which current signals, generated by photodiodes according to incident light amounts, are logarithmically compressed and output as voltage signals (see, for example, Patent Document 1 and Patent Document 2). This type of photodetector has a merit of being wide in the dynamic range of incident light amount detection.

    Patent Document 1: Japanese Published Unexamined Patent Application No. Hei 11-155105
    Patent Document 2: Japanese Published Unexamined Patent Application No. Hei 5-219443


However, a photodetector that employs such a logarithmic compression method has a problem that the logarithm compression characteristics (that is, the incident light amount detection characteristics) vary largely with temperature.
The present invention has been made to resolve the above problem and an object thereof is to provide a photodetector with a wide dynamic range of incident light amount detection and a small temperature dependence.
DISCLOSURE OF THE INVENTION
A photodetector according to the present invention includes: (1) a photodiode, generating charges of an amount that is in accordance with an incident light amount; (2) an integrating circuit, having a variable capacitor unit that is selectively set to a capacitance value Cn among K capacitance values C1 to CK, accumulating charges, output from the photodiode, into the variable capacitor unit over an accumulating period Tk that is in accordance with a capacitance value Ck set in the variable capacitor unit, and outputting a voltage according to the amount of the accumulated charges; (3) a first holding circuit, holding the voltage output from the integrating circuit at the end of an accumulating period Tk among accumulating periods T1 to TK and outputting the held voltage; and (4) a comparing circuit, inputting the voltage output from the integrating circuit or the first holding circuit, performing a quantitative comparison of the input voltage with a reference voltage, outputting a compared signal expressing the result of comparison, and, when the voltage output from the integrating circuit or the first holding circuit at the end of an accumulating period Tk is less than the reference voltage, instructing the first holding circuit to hold the voltage.
The photodetector is also characterized in that the value of a ratio, (Tk/Ck), differs according to the value of k. Preferably, a ratio of a ratio (Tk1/Ck1) with respect to a ratio (Tk2/Ck2) is a power of 2. Here, K is an integer no less than 2, each of k, k1, and k2 is an arbitrary integer no less than 1 and no more than K, and k1 and k2 differ from each other.
With this photodetector, the variable capacitor unit of the integrating circuit is selectively set to a capacitance value Cn among K capacitance values C1 to CK. Charges output from the photodiode are accumulated into the variable capacitor unit over the accumulating period Tk that is in accordance with the capacitance value Ck set in the variable capacitor unit, and a voltage is output from the integrating circuit according to the amount of the accumulated charge.
The first holding circuit holds the voltage output from the integrating circuit at the end of an accumulating period Tk among accumulating periods T1 to TK and the held voltage is output from the first holding circuit. The comparing circuit performs quantitative comparison of the voltage output from the integrating circuit or the first holding circuit with the reference voltage and outputs the compared signal expressing the result of comparison.
When the voltage output from the integrating circuit or the first holding circuit at the end of an accumulating period Tk is less than the reference voltage, the voltage held by the first holding circuit is held thereafter as well. Thus by performing predetermined computations based on the voltage, output from the comparing circuit at the end of an accumulating period Tk and held by the first holding circuit, and the compared signal, output from the comparing circuit and expressing the accumulating period Tk, the photodetector can perform image pickup over a wide dynamic range.
Preferably with the photodetector according to the present invention, the first holding circuit serves in common as a CDS circuit that inputs the voltage output from the integrating circuit and holds and outputs a voltage that is in accordance with the difference of the voltages that are respectively input at a beginning and an end of the accumulating period Tk. When the first holding circuit thus serves in common as a CDS (Correlated Double Sampling) circuit, error due to noise generated during resetting of an amplifier that is included in the integrating circuit (that is, kTC noise) is reduced and the photodetector can obtain more accurate image pickup data.
Preferably, the photodetector according to the present invention further includes (1) a second holding circuit, inputting a voltage output from the first holding circuit, holding the voltage at a specific point in time, and outputting the held voltage, and (2) a latching circuit, inputting a compared signal output from the comparing circuit, storing the compared signal at the specific point in time, and outputting the stored compared signal.
By the provision of the second holding circuit and the latching circuit that respectively holds and stores the data obtained by image pickup, image pickup data can be obtained in the same period in which image pickup data, obtained by the photodiode, the integrating circuit, the first holding circuit, and the comparing circuit in a previous period, are being processed. This photodetector can thus perform high-speed image pickup.
The photodetector according to the present invention preferably further includes an A/D converting circuit, inputting the voltage output from the first holding circuit, converting the input voltage into a digital value, and outputting the digital value. It is furthermore preferable for a bit shifting circuit, which inputs the digital value output from the A/D converting circuit, inputs the compared signal output from the comparing circuit, performs bit shifting of the digital value based on the compared signal, and outputs the bit-shifted digital value, to be included.
It is furthermore preferable for a switched capacitor circuit, which inputs the voltage output from the first holding circuit, holds the voltage in a capacitor unit, and outputs the held voltage to the A/D converting circuit, to be included. In these cases, the image pickup data, obtained by the photodiode, the integrating circuit, the first holding circuit, and the comparing circuit are converted into digital values by the A/D converting circuit. The digital values output from the A/D converting circuit are bit shifted by just the necessary number of bits based on the compared signal by the bit shifting circuit.
The photodetector according to the present invention preferably includes a plurality of sets of the photodiode, the integrating circuit, the first holding circuit, and the comparing circuit and has a single the A/D converting circuit for the plurality of sets. It is furthermore preferable that a plurality of sets of the photodiode, the integrating circuit, the first holding circuit, the comparing circuit, the second holding circuit, and the latching circuit be equipped, and that one set of the switched capacitor circuit, the A/D converting circuit, and the bit shifting circuit be equipped for the plurality of the abovementioned sets.
The photodetector according to the present invention is wide in the dynamic range of incident light amount detection and small in temperature dependence.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of a photodetector 1 according to an embodiment;
FIG. 2 is a block diagram of a first signal processing unit 10m,n of the photodetector 1 according to the embodiment;
FIG. 3 is a circuit diagram of a comparing circuit 13 of the photodetector 1 according to the embodiment;
FIG. 4 is a block diagram of a second signal processing unit 20 of the photodetector 1 according to the embodiment;
FIG. 5 shows diagrams for describing operations of a bit shifting circuit 28 of the photodetector 1 according to the embodiment;
FIG. 6 is a timing chart for describing operations of the photodetector 1 according to the embodiment;
FIG. 7 is a timing chart for describing operations of the photodetector 1 according to the embodiment; and
FIG. 8 is a timing chart for describing operations of the photodetector 1 according to the embodiment.



DESCRIPTION OF THE SYMBOLS


    1 photodetector
    10 first signal processing unit
    11 integrating circuit
    12 first holding circuit
    13 comparing circuit
    14 second holding circuit
    15 latching circuit
    20 second signal processing unit
    26 switched capacitor circuit
    27 A/D converting circuit
    28 bit shifting circuit
    A amplifier
    C capacitor
    PD photodiode
    SW switch


BEST MODES FOR CARRYING OUT THE INVENTION
A best mode for carrying out the present invention shall now be described in detail with reference to the drawings. In the description of the drawings, elements that are the same shall be provided with the same symbol and redundant description shall be omitted.
FIG. 1 is a schematic block diagram of a photodetector 1 according to an embodiment.
The photodetector 1, shown in this FIGURE, has M×N photodiodes PD1,1 to PDM,N, M×N first signal processing units 101,1 to 10M,N, and a single second signal processing unit 20. As shown in the FIGURE, the M×N photodiodes PD1,1 to PDM,N are positioned within a predetermined rectangular area. The M×N first signal processing units, 101,1 to 10M,N and the single second signal processing unit 20 are positioned outside one side of the abovementioned rectangular region. Each of M and N is an integer no less than 2. In the following, m is an arbitrary integer no less than 1 and no more than M, and n is an arbitrary integer no less than 1 and no more than N.
The M×N photodiodes, PD1,1 to PDM,N are arrayed in two dimensions in M rows and N columns. A photodiode PDm,n is positioned at the m-th row and n-th column. Each photodiode PDm,n generates charges of an amount that is in accordance with an incident light amount. Each first signal processing unit 10m,n is disposed in one-to-one correspondence to a photodiode PDm,n, inputs the charges output from the corresponding photodiode PDm,n, and outputs a voltage that is in accordance with the amount of the charges input. The second signal processing unit 20 inputs the voltages that are successively output from the respective M×N first signal processing units 101,1 to 10M,N, converts the voltages (analog values) to digital values, and outputs the digital values.
FIG. 2 is a block diagram of a first signal processing unit 10m,n of the photodetector 1 according to the embodiment. The M×N first signal processing units 101,1 to 10M,N respectively have a common arrangement. Each first signal processing unit 10m,n includes an integrating circuit 11, a first holding circuit 12, a comparing circuit 13, a second holding circuit 14, and a latching circuit 15. The photodiode PDm,n is also shown in this FIGURE. An anode terminal of the photodiode PDm,n is grounded.
The integrating circuit 11 has an amplifier A1, capacitors C10 to C12, and switches SW10 to SW12. The amplifier A1 has its non-inverted input terminal grounded and its inverted input terminal connected to a cathode terminal of the photodiode PDm,n. Between the inverted input terminal and an output terminal of the amplifier A1, the switch SW10, the capacitor C10, the serially connected switch SW11 and capacitor C11, and the serially connected switch SW12 and capacitor C12 are connected in parallel.
The capacitors C10 to C12 and the switches SW11 and SW12 make up a variable capacitor unit. This variable capacitor unit is selectively set to a capacitance value among capacitance values C1 to C3 as expressed by formulae (1) below. That is, the capacitance value C1 of the variable capacitor unit when both switches SW11 and SW12 are open is equivalent to the capacitance value of the capacitor C10. The capacitance value C2 of the variable capacitor unit when the switch SW11 is closed and the switch SW12 is open is equivalent to the sum of the respective capacitance values of the capacitors C10 and C11. The capacitance value C3 of the variable capacitor unit when both switches SW11 and SW12 are closed is equivalent to the total of the respective capacitance values of the capacitors C10 to C13.

C1=C10  (1a)

C2=C10+C11  (1b)

C3=C10+C11+C12  (1c)

When the switch SW10 is open, the integrating circuit 11 accumulates charges output from the photodiode PDm,n over an accumulating period Tk that is in accordance with the capacitance value Ck set at the variable capacitor unit and then outputs a voltage V1 that is in accordance with the amount of the accumulated charges. With the integrating circuit 11, by the closing of switches SW10 to SW12, the charges accumulated in the capacitors C10 to C12 are discharged and the output voltage is initialized. Here, k is an arbitrary integer no less than 1 and no more than 3.
The value of a ratio (Tk/Ck) differs according to the value of k. That is, a relationship expressed by a formula (2) indicated below holds among the capacitance values C1 to C3 and the accumulating periods T1 to T3. Preferably, relationships expressed by formulae (3) indicated below also hold among these parameters. Here, each of p and q is an integer no less than 1. More preferably, the accumulating periods T1 to T3 differ mutually and a relationship expressed by a formula (4) indicated below holds among these accumulating periods.




  
    
      
        
          
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The first holding circuit 12 has switches SW21 and SW22, a capacitor C2, and an amplifier A2. One end of the capacitor C2 is grounded via the switch SW21 and is connected to an input terminal of the amplifier A2. The other end of the capacitor C2 is connected via the switch SW22 to the output terminal of the amplifier A1 of the integrating circuit 11.
By the respective opening and closing operations of the switches SW21 and SW22, the first holding circuit 12 holds the voltage V1, output from the integrating circuit 11 in the capacitor C2 at an end of an accumulating period Tk among accumulating periods T1 to T3, inputs the held voltage into the amplifier A2, and outputs a voltage V2 from the amplifier A2. The first holding circuit 12 serves in common as a CDS circuit and can input the voltage V1, output from the integrating circuit 11, and output the output voltage V2, which is in accordance with a difference of the voltages that are respectively input at a start and an end of the accumulating period Tk.
The comparing circuit 13 inputs the voltage V1, output from the integrating circuit 11, performs quantitative comparison of this input voltage V1 and a reference voltage Vref, and outputs a compared signal S3, expressing the result of comparison, to the latching circuit 15. If the voltage V1, output from the integrating circuit 11 at the end of an accumulating period Tk, is less than the reference voltage Vref, the comparing circuit 13 instructs the first holding circuit 12 to hold the voltage V2. The latching circuit 15 stores and outputs the compared signal S3 output from the compared circuit 13. The latching circuit 15 is arranged, for example, from parallel registers of the same number of bits as the number of bits of the compared signal S3.
The second holding circuit 14 has a capacitor C4 and switches SW41 to SW44. One end of the capacitor C4 is connected via the switch SW41 to an output terminal of the amplifier A2 of the first holding circuit 12 and is grounded via the switch SW42. The other end of the capacitor C4 is grounded via the switch SW43 and is connected via the switch SW44 to the exterior. By the respective opening and closing operations of the switches SW41 to SW44, the second holding circuit 14 inputs the voltage V2 output from the first holding circuit 12, holds the voltage at a specific point in time in the capacitor C4, and outputs the held voltage to the exterior. (Actually, because a switched capacitor circuit is connected as shall be described below, charges Q4, the value of which is in accordance with the held voltage, are output to the exterior.)
That is, with the second holding circuit 14, the charges accumulated in the capacitor C4 are discharged by the closing of the switches SW42 and SW43. The voltage that is input when the switches SW41 and SW43 are closed and the switch SW42 is open is held in the capacitor C4, and when the switches SW41 and SW43 open, the voltage is kept held as it is in the capacitor C4. When the switches SW44 and SW42 close, the charges Q4, the value of which is in accordance with the voltage held in the capacitor C4, are output.
FIG. 3 is a circuit diagram of the comparing circuit 13 of the photodetector 1 according to the embodiment. As shown in this FIGURE, the comparing circuit 13 has a comparator 30 and D flip-flops 31 and 32. The comparator 30 inputs the voltage V1, output from the integrating circuit 11, also inputs the reference voltage Vref, and performs a quantitative comparison of the voltage V1 and the reference voltage Vref. An output level V3 of the comparator 30 is set to a high level if the voltage V1 is greater than the reference voltage Vref and is set to a low level if the voltage V1 is not greater than the reference voltage Vref.
With each of the D flip-flops 31 and 32, when a Clr signal that is input into a CLR input terminal is of a high level, an output level from a Q output terminal is of a low level. With each of the D flip-flops 31 and 32, the output voltage V3 from the comparator 30 is input into a clock input terminal, and when this voltage V3 changes from the low level to the high level, the signal level input into a D input terminal up until then is output from the Q output terminal.
The D flip-flops 31 and 32 make up a shift register, and the Q output terminal of the D flip-flop 31 of the preceding stage and the D input terminal of D flip-flop 32 of the subsequent stage are connected to each other. A high level signal is constantly input into the D input terminal of the D flip-flop 31 of the preceding stage. A signal S31, output from the Q output terminal of the D flip-flop 31 of the preceding stage, and a signal S32, output from the Q output terminal of the D flip-flop 32 of the subsequent stage, make up the 2-bit compared signal S3, output from the comparing circuit 13.
FIG. 4 is a block diagram of the second signal processing unit 20 of the photodetector 1 according to the embodiment. As shown in this FIGURE, the second signal processing unit 20 includes a switched capacitor circuit 26, an A/D converting circuit 27, and a bit shifting circuit 28.
The switched capacitor circuit 26 has an amplifier A6, a capacitor C6, and a switch SW6. The amplifier A6 has its non-inverted input terminal grounded and its inverted input terminal connected to the switch SW44 of the second holding circuit 14. Between the inverted input terminal and an output terminal of the amplifier A6, the switch SW6 and the capacitor C6 are connected in parallel.
When the switch SW6 is open, the switched capacitor circuit 26 inputs the charge value Q4 output from the second holding circuit 14, holds the charge value Q4 in the capacitor C6, and outputs a voltage V6 that is in accordance with the held charge amount to the A/D converting circuit 27. With the switched capacitor circuit 26, by the closing of the switch SW6, the charges accumulated in the capacitor C6 are discharged and the output voltage is initialized.
The A/D converting circuit 27 inputs the voltage V6, output from the second holding circuit 14, held by the switched capacitor circuit 26, and output from the switched capacitor circuit 26, converts this input voltage V6 (analog value) into a digital value, and outputs the digital value D7.
The bit shifting circuit 28 inputs the digital value D7, output from the A/D converting circuit 27, inputs the compared signal S3, output from the comparing circuit 13 and arriving via the latching circuit 15, performs bit shifting of the digital value D7 according to the compared signal S3, and outputs a bit-shifted digital value D8.
FIG. 5 shows diagrams for describing operations of the bit shifting circuit 28 of the photodetector 1 according to the embodiment. Each of FIGS. 5A to 5C shows a relationship between the digital value D7, output from the A/D converting circuit 27 and input into the bit shifting circuit 28, and the digital value D8, output from the bit shifting circuit 28. In the following, the voltage V1, output from the integrating circuit 11 at the end of the accumulating period Tk with the variable capacitor unit of the integrating circuit 11 being set to the capacitance value Ck, shall be expressed as V1,k.
It shall also be deemed that the capacitance values C1 to C3, selectively set in the variable capacitor unit of the integrating circuit 11, and the accumulating periods T1 to T3 satisfy the relationships of the above-described formulae (1) to (3). Here, the number of bits of the output digital value D8 is equal to the result of adding p and q in the formulae (3) to the number of bits of the input digital value D7. Each of p and q is no more than the number of bits of the input digital value D7.
FIG. 5-(a) shows a relationship between the input digital value D7 and the output digital value D8 when the comparing circuit 13 judges that each of voltages V1,1 to V1,3 is less than the reference voltage Vref. In this case, the output digital value D8 is the same in value as the input digital value D7, and the value 0 is placed in each of the upper (p+q) bits.
FIG. 5-(b) shows a relationship between the input digital value D7 and the output digital value D8 when the comparing circuit 13 judges that the voltage V1,1 is equal to or greater than the reference voltage Vref and each of the voltages V1,2 and V1,3 is less than the reference voltage Vref. In this case, the output digital value D8 takes on the value obtained by shifting the input digital value D7 upward by just p bits and the value 0 is placed in each of the lower p bits and the upper q bits.
FIG. 5-(c) shows a relationship between the input digital value D7 and the output digital value D8 when the comparing circuit 13 judges that each of the voltages V1,1 and V1,2 is equal to or greater than the reference voltage Vref and the voltage V1,3 is less than the reference voltage Vref. In this case, the output digital value D8 takes on the value obtained by shifting the input digital value D7 upward by just (p+q) bits and the value 0 is placed in each of the lower (p+q) bits.
For example, let the digital value D7, output from the A/D converting circuit 27 and input into the bit shifting circuit 28, be an 8-bit data and the value of each of p and q be 4. In this case, the digital value D8, output from the bit shifting circuit 28 is a 16 bit data and the dynamic range thereof is 64 k (≅216).
Operations of the photodetector 1 according to the present embodiment shall now be described. The intensity of light incident on each of the M×N photodiodes PD1,1 to PDM,N is not uniform in general and differs according to the position (m, n). In the following, a description shall be provided for each of a case where the amount of light incident on the photodiode PDm,n is comparatively low, a case where the amount of light incident on the photodiode PDm,n is of an intermediate level, and a case where the amount of light incident on the photodiode PDm,n is comparatively high. The following operations are carried out based on control signals output from an unillustrated control circuit.
Each of FIG. 6 to FIG. 8 is a timing chart for describing the operations of the photodetector 1 according to the embodiment. FIG. 6 is for describing the operations of the first signal processing unit 10m,n when the amount of light incident on the photodiode PDm,n is comparatively low. FIG. 7 is for describing the operations of the first signal processing unit 10m,n when the amount of light incident on the photodiode PDm,n is of an intermediate level. FIG. 8 is for describing the operations of the first signal processing unit 10m,n when the amount of light incident on the photodiode PDm,n is comparatively high.
In each FIGURE, the level of the Clr signal that is input into the CLR input terminal of each of the D flip-flops 31 and 32 of the comparing circuit 13, the respective opening and closing operations of the switches SW10 to SW12 of the integrating circuit 11, the respective opening and closing operations of the switches SW21 and SW22 of the first holding circuit 12, the voltage V1 output from the integrating circuit 11, the output level V3 of the comparator 30 of the comparing circuit 13, and the compared signal S3 (S31, S32) output from the respective Q output terminals of the D flip-flops 31 and 32 of the comparing circuit 13 are shown in that order from the upper side.
Also, in each FIGURE, the period from a time t1 to a time t2 is the accumulating period T1, the period from a time t3 to a time t4 is the accumulating period T2, and the period from a time t5 to a time t6 is the accumulating period T3.
In all of the cases illustrated in FIG. 6 to FIG. 8, for a fixed period before the time t1, the Clr signal is set to the high level and the signals (that is, the compared signal S3 (S31, S32)) output from the respective Q output terminals of the D flip-flops 31 and 32 of the comparing circuit 13 are thereby initialized. Also for fixed periods before the time t1, the respective switches SW10 to SW12 of the integrating circuit 11 are closed to discharge the respective capacitors C10 to C12 and initialize the voltage V1 output from the integrating circuit 11.
During the accumulating period T1, from the time t1 to the time t2, the switches SW10 to SW12 of the integrating circuit 11 are opened and the variable capacitor unit of the integrating circuit 11 is set to the capacitance value C1 (the abovementioned formula (1a)). The charges generated at the photodiode PDm,n are accumulated into the variable capacitor unit of the integrating circuit 11 of the first signal processing unit 10m,n, and the voltage V1 that is in accordance with the amount of the accumulated charges is output from the integrating circuit 11. This voltage V1 increases gradually from the initial value at the time t1.
During the period from the time t2 to the time t3 (that is, the period between the accumulating period T1 and the accumulating period T2), the switches SW10 and SW11 of the integrating circuit 11 are closed to discharge the capacitors C10 and C11 and initialize the voltage V1 output from the integrating circuit 11.
During the accumulating period T2, from the time t3 to the time t4, the switch SW10 of the integrating circuit 11 is opened, the SW12 is kept open, the switch SW11 is closed, and the variable capacitor unit of the integrating circuit 11 is thereby set to the capacitance value C2 (the abovementioned formula (1b)). The charges generated at the photodiode PDm,n are accumulated into the variable capacitor unit of the integrating circuit 11 of the first signal processing unit 10m,n, and the voltage V1 that is in accordance with the amount of the accumulated charges is output from the integrating circuit 11. This voltage V1 increases gradually from the initial value at the time t3.
During the period from the time t4 to the time t5 (that is, the period between the accumulating period T2 and the accumulating period T3), the switches SW10, SW11, and SW12 of the integrating circuit 11 are closed to discharge the capacitors C10, C11, and C12 and initialize the voltage V1 output from the integrating circuit 11.
During the accumulating period T3, from the time t5 to the time t6, the switch SW10 of the integrating circuit 11 is opened, the respective switches SW11 and SW12 are closed, and the variable capacitor unit of the integrating circuit 11 is thereby set to the capacitance value C3 (the abovementioned formula (1c)). The charges generated at the photodiode PDm,n are accumulated into the variable capacitor unit of the integrating circuit 11 of the first signal processing unit 10m,n, and the voltage V1 that is in accordance with the amount of the accumulated charges is output from the integrating circuit 11. This voltage V1 increases gradually from the initial value at the time t5.
In each of an initial fixed period of the accumulating period T1 (that is, a fixed period immediately after the time t1), an initial fixed period of the accumulating period T2 (that is, a fixed period immediately after the time t3), and an initial fixed period of the accumulating period T3 (that is, a fixed period immediately after the time t5), the switch SW21 of the first holding circuit 12 is closed and the voltage V2 output from the first holding circuit 12 is initialized. The switch SW22 of the first holding circuit 12 is also closed before the time t1.
The description of the operations up until now applies in common to each of the cases of FIG. 6 to FIG. 8. However, the first signal processing unit 10m,n operates differently according to the magnitude of the amount of light incident onto the photodiode PDm,n as shall be described below.
The operations of the first signal processing unit 10m,n when the amount of light incident on the photodiode PDm,n is comparatively low are shown in FIG. 6. In this case, each of the voltage V1,1 output from the integrating circuit 11 at the end of the accumulating period T1, the voltage V1,2 output from the integrating circuit 11 at the end of the accumulating period T2, and the voltage V1,3 output from the integrating circuit 11 at the end of the accumulating period T3 is less than the reference voltage Vref. Thus in all of the accumulating periods T1 to T3, the output level V3 of the comparator 30 of the comparing circuit 13 stays at the low level, and the signals S31 and S32 output from the respective Q output terminals of the D flip-flops 31 and 32 stay at the low level.
At the point in time at which the voltage V1,1, output from the integrating circuit 11 at the end of the accumulating period T1 (immediately before the time t2), is judged to be less than the reference voltage Vref, the switch SW22 of the first holding circuit 12, which was closed until then, opens and the switch SW22 stays open thereafter. Consequently, the voltage V2 that is output from the first holding circuit 12 from the time t2 onward is in accordance with the difference between the voltage V1,1 output from the integrating circuit 11 at the point in time at which the switch SW22 became open at the end of the accumulating period T1 and the voltage, output from the integrating circuit 11 at the point in time at which the switch SW21 became open at the beginning of the accumulating period T1. Furthermore, this voltage V2 is held by the second holding circuit 14.
The operations of the first signal processing unit 10m,n when the amount of light incident on the photodiode PDm,n is of an intermediate level are shown in FIG. 7. In this case, whereas the voltage V1,1 output from the integrating circuit 11 at the end of the accumulating period T1 is equal to or greater than the reference voltage Vref, each of the voltage V1,2 output from the integrating circuit 11 at the end of the accumulating period T2 and the voltage V1,3 output from the integrating circuit 11 at the end of the accumulating period T3 is less than the reference voltage Vref.
Thus at some point in time within the accumulating period T1 (the point in time at which the output voltage V1 from the integrating circuit 11 becomes equal to or greater than the reference voltage Vref), the output level V3 of the comparator 30 of the comparing circuit 13 switches to the high level, and the signal S31 output from the Q output terminal of the D flip-flop 31 changes to the high level. From the time t2 onward, the signal S31 output from the Q output terminal of the D flip-flop 31 stays at the high level and the signal S32 output from the Q output terminal of the D flip-flop 32 stays at the low level.
At the point in time at which the voltage V1,2, output from the integrating circuit 11 at the end of the accumulating period T2 (immediately before the time t4), is judged to be less than the reference voltage Vref, the switch SW22 of the first holding circuit 12, which was closed until then, opens and the switch SW22 stays open thereafter. Consequently, the voltage V2 that is output from the first holding circuit 12 from the time t4 onward is in accordance with the difference between the voltage V1,2, output from the integrating circuit 11 at the point in time at which the switch SW22 became open at the end of the accumulating period T2, and the voltage, output from the integrating circuit 11 at the point in time at which the switch SW21 became open at the beginning of the accumulating period T2. Furthermore, this voltage V2 is held by the second holding circuit 14.
The operations of the first signal processing unit 10m,n when the amount of light incident on the photodiode PDm,n is comparatively high are shown in FIG. 8. In this case, whereas each of the voltage V1,1 output from the integrating circuit 11 at the end of the accumulating period T1 and the voltage V1,2 output from the integrating circuit 11 at the end of the accumulating period T2 is equal to or greater than the reference voltage Vref, the voltage V1,3 output from the integrating circuit 11 at the end of the accumulating period T3 is less than the reference voltage Vref.
Thus at some point in time within the accumulating period T1 (the time at which the output voltage V1 from the integrating circuit 11 becomes equal to or greater than the reference voltage Vref), the output level V3 of the comparator 30 of the comparing circuit 13 switches to the high level, and the signal S31 output from the Q output terminal of the D flip-flop 31 changes to the high level. Furthermore, at some time within the accumulating period T2 (the time at which the output voltage V1 from the integrating circuit 11 becomes equal to or greater than the reference voltage Vref), the output level V3 of the comparator 30 of the comparing circuit 13 switches to the high level, and the signal S32 output from the Q output terminal of the D flip-flop 32 also changes to the high level. From the time t4 onwards, the signals S31 and S32 output from the respective Q output terminals of the D flip-flops 31 and 32 stay at the high level.
At the point in time at which the voltage V1,3, output from the integrating circuit 11 at the end of the accumulating period T3 (immediately before the time t6), is judged to be less than the reference voltage Vref, the switch SW22 of the first holding circuit 12, which was closed until then, opens and the switch SW22 stays open thereafter. Consequently, the voltage V2 that is output from the first holding circuit 12 from the time t6 onward is in accordance with the difference between the voltage V1,3, output from the integrating circuit 11 at the point in time at which the switch SW22 became open at the end of the accumulating period T3, and the voltage, output from the integrating circuit 11 at the point in time at which the switch SW21 became open at the beginning of the accumulating period T3. Furthermore, this voltage V2 is held by the second holding circuit 14.
The compared signal S3 (S31, S32), output from the comparing circuit 13 at time t6 onward, expresses the level of the amount of light incident on the photodiode PDm,n in three stages.
That the compared signal S3 (S31, S32) is (0, 0) thus signifies that even if charges generated at the photodiode PDm,n are accumulated into the variable capacitor unit over the accumulating period T1 with the variable capacitor unit of the integrating circuit 11 being set to the capacitance value C1, the output voltage V1,1 at the end of the accumulating period T1 is less than the reference voltage Vref, in other words, the amount of light incident on the photodiode PDm,n is comparatively low.
That the compared signal S3 (S31, S32) is (1, 0) signifies that though the output voltage V1,1 at the end of the accumulating period T1 is equal to or greater than the reference voltage Vref, even if charges generated at the photodiode PDm,n are accumulated into the variable capacitor unit over the accumulating period T2 with the variable capacitor unit of the integrating circuit 11 being set to the capacitance value C2, the output voltage V1,2 at the end of the accumulating period T2 is less than the reference voltage Vref, in other words, the amount of light incident on the photodiode PDm,n is of an intermediate level.
That the compared signal S3 (S31, S32) is (1, 1) signifies that though the output voltage V1,2 at the end of the accumulating period T2 is equal to or greater than the reference voltage Vref, even if charges generated at the photodiode PDm,n are accumulated into the variable capacitor unit over the accumulating period T3 with the variable capacitor unit of the integrating circuit 11 being set to the capacitance value C3, the output voltage V1,3 at the end of the accumulating period T3 is less than the reference voltage Vref, in other words, the amount of light incident on the photodiode PDm,n is comparatively high.
The voltages respectively held in the first holding circuit 12 and the second holding circuit 14 at time t6 and onward correspond to being the voltage, among the voltages V1,1 to V1,3 output from the integrating circuit 11 at the respective ends of the accumulating periods T1 to T3, that first became less than the reference voltage Vref.
Onward from time t6, the charges Q4 are output successively from the respective second holding circuits 14 of the M×N first signal processing units 101,1 to 10M,N to the switched capacitor circuit 26, and the compared signal S3 is output from the latching circuit 15 to the bit shifting circuit 28.
At the switched capacitor circuit 26, the charges Q4 that have been successively output from the second holding circuit 14 of each first signal processing unit 10m,n are input, these charges Q4 are held in the capacitor C6, and the voltage V6 that is in accordance with the amount of charges held is output to the A/D converting circuit 27. At the A/D converting circuit 27, the voltage V6 output from the switched capacitor circuit 26 is input, this voltage V6 (analog value) is converted into a digital value, and this digital value D7 is output.
The digital value D7 output from the A/D converting circuit 27 and the compared signal S3 output from the comparing circuit 13 are input into the bit shifting circuit 28. The digital value D7 is then bit shifted based on the compared signal S3, and the bit-shifted digital value D8 is output. Here, if the compared signal S3 (S31, S32) is (0,0), the output digital value D8 is the same in value as the input digital value D7, and the value 0 is placed in each of the upper (p+q) bits (FIG. 5-(a)).
If the compared signal S3 (S31, S32) is (1,0), the output digital value D8 takes on the value obtained by shifting the input digital value D7 upward by just p bits and the value 0 is placed in each of the lower p bits and the upper q bits (FIG. 5-(a)). If the compared signal S3 (S31, S32) is (1,1), the output digital value D8 takes on the value obtained by shifting the input digital value D7 upward by just (p+q) bits and the value 0 is placed in each of the lower (p+q) bits (FIG. 5-(c)).
Data (V2, S3), concerning the amount of light incident on the photodiode PDm,n in the period from the time t1 to the time t6, are thus held in the second holding circuit 14 and the latching circuit 15 of the corresponding first signal processing unit 10m,n. In this state, the relationships of the above-described formula (2) or (3) hold for the capacitance value Ck of the variable capacitor unit of the integrating circuit 11 and the accumulating period Tk, the voltage V2 is held by the second holding circuit 14 when the voltage V1, which is in accordance with the amount of charges accumulated in the variable capacitor unit of the integrating circuit 11 in one of the accumulating periods, is less than the reference voltage Vref, and the compared signal S3, expressing the accumulating period at which the held voltage V2 is obtained, is stored by the loatching circuit 15.
Then based on the data (Q4, S3), output from the second holding circuit 14 and the latching circuit 15 of each first signal processing unit 10m,n from the subsequent time t6 onward, the voltage, which is in accordance with the charges Q4 and results from the conversion by the switched capacitor circuit 26, is A/D converted by the A/D converting circuit 27 of the second signal processing unit 20, and the digital value is bit shifted as necessary by the bit shifting circuit 28.
This photodetector 1 is thus wide in the dynamic range of incident light amount detection. Also because the charges generated at the photodiode PDm,n is accumulated in the capacitor of the integrating circuit 11 and the voltage V1 that is in accordance with the amount of the accumulated charges is output from the integrating circuit 11, the temperature dependence of the incident light amount detection is small.
Also, by the provision of the second holding circuit 14 and the latching circuit 15, in the same period, from the time t6 onward, in which the image pickup data (V2, S3), obtained by the respective photodiodes PDm,n and the respective first signal processing units 10m,n in the period from the time t1 to the time t6, are processed at the second signal processing unit 20, the subsequent set of image pickup data (V2, S3) are obtained by the respective photodiodes PDm,n and the respective first signal processing units 10m,n. The photodetector 1 can thus perform high-speed image pickup.
Also because the first holding circuit 12 serves in common as a CDS circuit, the error of the output voltage V1 due to noise generated during resetting of the amplifier A1 of the integrating circuit 11 is reduced, and the photodetector 1 can thus obtain more accurate image pickup data.
The present invention is not restricted to the above-described embodiment and may be modified in various ways. For example, though with the above-described embodiment, the comparing circuit 13 performs quantitative comparison of the voltage V1 output from the integrating circuit 11 with the reference voltage Vref, quantitative comparison of the voltage V2 output from the first holding circuit 12 with the reference voltage Vref may be performed instead.
Also, the circuit arrangement of the photodetector is not restricted to that described with the embodiment above and may take on other forms. For example, the photodiodes are not restricted to a two-dimensional array and may be arrayed one-dimensionally or may be a single photodiode. The capacitance value of the variable capacitor unit of the integrating circuit may be switchable in two stages or four or more stages. Other circuit arrangements are possible for the first holding circuit, the comparing circuit, the second holding circuit, etc., as well. Also, a single first signal processing unit may be provided for a plurality of photodiodes.
INDUSTRIAL APPLICABILITY
The present invention can be used in a photodetector.

## Claims

1. A photodetector comprising:
a photodiode, generating charges of an amount that is in accordance with an incident light amount;
an integrating circuit, having a variable capacitor unit that is selectively set to a capacitance value Cn among K capacitance values C1 to CK, accumulating charges, output from the photodiode, into the variable capacitor unit over an accumulating period Tk that is in accordance with a capacitance value Ck set in the variable capacitor unit, and outputting a voltage according to the amount of the accumulated charges;
a first holding circuit, holding the voltage output from the integrating circuit at the end of an accumulating period Tk among accumulating periods T1 to TK and outputting the held voltage; and
a comparing circuit, inputting the voltage output from the integrating circuit or the first holding circuit, performing a quantitative comparison of the input voltage with a reference voltage, outputting a compared signal expressing the comparing result, and, when the voltage output from the integrating circuit or the first holding circuit at the end of an accumulating period Tk is less than the reference voltage, instructing the first holding circuit to hold the voltage; wherein
the value of a ratio, (Tk/Ck), differs according to the value of k (where K is an integer no less than 2 and k is an arbitrary integer no less than 1 and no more than K).

2. The photodetector according to claim 1, wherein a ratio of a ratio (Tk1/Ck1) with respect to a ratio (Tk2/Ck2) is a power of 2 (where each of k1 and k2 is an arbitrary integer no less than 1 and no more than K, and k1≠k2).

3. The photodetector according to claim 1, wherein the first holding circuit serves in common as a CDS circuit that inputs the voltage output from the integrating circuit and holds and outputs a voltage that is in accordance with the difference of the voltages that are input at a beginning and an end of the accumulating period Tk.

4. The photodetector according to claim 1, further comprising:
a second holding circuit, inputting a voltage output from the first holding circuit, holding the voltage at a specific point in time, and outputting the held voltage; and
a latching circuit, inputting a compared signal output from the comparing circuit, storing the compared signal at the specific point in time, and outputting the stored compared signal.

5. The photodetector according to claim 1, further comprising: an A/D converting circuit, inputting the voltage output from the first holding circuit, converting the input voltage into a digital value, and outputting the digital value.

6. The photodetector according to claim 5, further comprising: a bit shifting circuit, inputting the digital value output from the A/D converting circuit, inputting the compared signal output from the comparing circuit, bit shifting the digital value based on the compared signal, and outputting the bit-shifted digital value.

7. The photodetector according to claim 5, further comprising: a switched capacitor circuit, inputting the voltage output from the first holding circuit, holding the voltage in a capacitor unit, and outputting the held voltage to the A/D converting circuit.

8. The photodetector according to claim 5, comprising a plurality of sets of the photodiode, the integrating circuit, the first holding circuit, and the comparing circuit and has a single the A/D converting circuit for the plurality of sets.