# Patent

## Title
Signal processing circuit and solid-state image pickup device

## Abstract

A VI conversion circuit 40n inputs a voltage output from a hold circuit 30n, converts this input voltage to a current based on the resistance value of a resistor R40, and outputs this current from a drain terminal of a MOS transistor T40. An amp A40 has an adequate open loop gain and MOS transistor T40 operates in the saturated region. Here, if the resistance value of resistor R40 is R, the current value I that is output from VI conversion circuit 40n is in the proportional relationship expressed by the equation, “I=V/R,” with respect to the voltage V input into VI conversion circuit 40n.

## Background
FIELD OF THE ART
This invention concerns a solid-state image pickup device for taking a one-dimensional or two-dimensional optical image.
BACKGROUND ART
A solid-state image pickup device has photosensitive elements aligned in one dimension or two dimensions and can take a one-dimensional or two-dimensional optical image by converting the intensity of light that is incident on each of the photosensitive elements to an electrical signal and outputting this electrical signal. A solid-state image pickup device is used, for example, in a facsimile machine to read an image on a manuscript or to take an image formed by X-rays that have been transmitted through an inspected object in a non-destructive X-ray inspection device. It is demanded that such a solid-state image pickup device takes an image with high speed.
For example, a solid-state image pickup device disclosed in Japanese Unexamined Patent Publication No. Hei-10-126696 comprises: a photoelectric conversion circuit, converting an optical signal to a voltage and outputting this voltage; a VI conversion circuit, converting the voltage output from the photoelectric conversion circuit to a current and outputting this current; and an IV conversion circuit, converting the current output by the VI conversion circuit to a voltage and outputting this voltage. The respective output terminals of a plurality of VI conversion circuits are selectively and successively connected to a single IV conversion circuit.
With a solid-state image pickup device arranged thus, current is transmitted from a VI conversion circuit disposed at a prior stage to the IV conversion circuit disposed at a subsequent stage and the potential at the input terminal of the IV conversion circuit at the subsequent stage is fixed. Thus with this solid-state image pickup device, even if the parasitic capacitance across the respective output terminals of the plurality of VI conversion circuits and the input terminal of the IV conversion circuit is large, the processing of signals among these components can be performed at high speed and image taking can thus be performed at high speed.
DISCLOSURE OF THE INVENTION
With the solid-state image pickup device disclosed in the above-mentioned Publication, each of the plurality of VI conversion circuits and the IV conversion circuit includes a MOS (Metal-Oxide-Semiconductor) type field effect transistor (FET), and it is required that all of these MOS transistors be the same in characteristics. Also with this solid-state image pickup device, each of the plurality of VI conversion circuits and the IV conversion circuit includes a constant current supply, and it is required that all of these constant current supplies be the same in characteristics.
However, even in a case where the MOS transistors that are respectively included in the plurality of VI conversion circuits and the IV conversion circuit are formed on the same substrate, the characteristics (for example, the threshold voltages) of the MOS transistors will not actually be the same. Thus even if light of the same intensity is incident on each of the photosensitive elements aligned in one dimension or two dimensions, the pixel output value (the voltage output from the IV conversion circuit) corresponding to each photosensitive element will not be the same. Also, in a case where the constant current supplies included in the plurality of VI conversion circuits and the IV conversion circuit are not the same in characteristics, the sensitivities of the respective pixels will not be the same and the linearity of the output voltage with respect to the incident light intensity will be poor. Thus with the solid-state image pickup device disclosed in the above-mentioned Publication, the image quality of the taken image is poor due to the fluctuation of the characteristics.
This invention has been made to resolve the above problem and an object thereof is to provide a solid-state image pickup device that can take images of excellent image quality at high speed and a signal processing circuit that is favorably used in such a solid-state image pickup device.
The signal processing circuit of the present invention comprises: (1) a plurality of VI conversion circuits, including each one resistor, converting an input voltage to a current based on the resistance value of the resistor, and outputting this current from an output terminal; (2) an IV conversion circuit, inputting, into an input terminal, the current output from the output terminal of the VI conversion circuit, converting this current to a voltage, and outputting this voltage; and (3) a selection circuit, successively selecting each of the output terminals of a plurality of VI conversion circuits and connecting the selected output terminal to the input terminal of the IV conversion circuit.
The solid-state image pickup device of the present invention is characterized in including the signal processing circuit by the above-described invention. That is, this invention's solid-state image pickup device comprises: (1) a plurality of photoelectric conversion circuits, outputting a voltage of a value corresponding to an incident light intensity; (2) a plurality of VI conversion circuits, including each one resistor, converting the voltage output from the photoelectric conversion circuit to a current based on the resistance value of the resistor, and outputting this current from an output terminal; (3) an IV conversion circuit, inputting, into an input terminal, the current output from the output terminal of the VI conversion circuit, converting this current to a voltage, and outputting this voltage; and (4) a selection circuit, successively selecting each of the output terminals of a plurality of VI conversion circuits and connecting the selected output terminal to the input terminal of the IV conversion circuit.
With this invention, a voltage of a value corresponding to an incident light intensity is output from the photoelectric conversion circuit. This voltage is converted to a current based on the resistance value of the resistor by the VI conversion circuit and this current is output. The current that is output from each of the plurality of VI conversion circuits is then selected successively and input into the IV conversion circuit by the selection circuit, converted into a voltage by the IV conversion circuit, and this voltage is output as a video signal.
Also, in the solid-state image pickup device of the present invention, the VI conversion circuit preferably comprises: (1) an amp, having a first input terminal, a second input terminal, and an output terminal and inputting the voltage output from the photoelectric conversion circuit into the first input terminal and outputting, from the output terminal, a voltage corresponding to the difference in the respective potentials of the first input terminal and the second input terminal; (2) a MOS transistor, having a gate terminal, source terminal, and a drain terminal, the gate terminal being connected to the output terminal of the amp; (3) a resistor, one end of which is connected to the second input terminal of the amp and the source terminal of the MOS transistor and the other end of which is connected to a first reference potential; and (4) a switch, one end of which is connected to the drain terminal of the MOS transistor and the other end of which is set to a second reference potential.
Here, the VI conversion circuit converts the voltage output from the photoelectric conversion circuit to a current and outputs this current from the drain terminal of the MOS transistor. By thus arranging the VI conversion circuit to include a resistor, the current value I output from the VI conversion circuit can be put in the proportional relationship expressed by the equation, “I=V/R,” with respect to the voltage V input into the VI conversion circuit, with R being the resistance value of the resistor.
Also, in the solid-state image pickup device of the present invention, it is preferable for the amp to be put in the ON state in a period from a time that is a fixed amount of time prior to the time at which the VI conversion circuit is connected by the selection circuit to the IV conversion circuit to the time at which the connection with the IV conversion circuit is disengaged by the selection circuit and for the amp to be in the OFF state in other periods. By doing so, since the amp is put in the ON state from a time that is a fixed amount of time (for example, approximately 1 microsecond) prior to the time at which the VI conversion circuit is connected by the selection circuit to the IV conversion circuit, the operation of the VI conversion circuit in the above-mentioned period is started up early. Also since the amp of the VI conversion circuit is put in the OFF state in other periods, the consumption power is reduced.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a solid-state image pickup device 1 of a first embodiment.
FIG. 2 is a specific circuit diagram of solid-state image pickup device 1 of the first embodiment.
FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H, 3I, 3J, and 3K are timing charts for explaining the operations of solid-state image pickup device 1 of the first embodiment.
FIG. 4 is a specific circuit diagram of a solid-state image pickup device of a second embodiment.
FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G, 5H, 5I, 5J, and 5K are timing charts for explaining the operations of the solid-state image pickup device of the second embodiment.
FIG. 6 is a block diagram of a solid-state image pickup device of a third embodiment.



BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of this invention shall now be described with reference to the drawings. In the description of the drawings, the same elements shall be provided with the same symbols and redundant description shall be omitted.
First Embodiment
First, a first embodiment of a solid-state image pickup device by this invention shall be described. FIG. 1 is a block diagram of a solid-state image pickup device 1 of the first embodiment. This solid-state image pickup device 1 has photodiodes PD aligned in one dimension and is equipped with N (where N is an integer of 2 or more) units U1 to UN and an IV conversion circuit 60. Each unit Un (where n is an arbitrary integer of no less than 1 and no more than N) includes a photodiode PD, an integrating circuit 10n, a CDS (Correlated Double Sampling) circuit 20n, a sample-and-hold (referred to hereinafter as “hold”) circuit 30n, a VI conversion circuit 40n and a switch SW5n.
With each unit Un, photodiode PD, integrating circuit 10n, CDS circuit 20n, and hold circuit 30n make up a photoelectric conversion circuit that outputs a voltage of a value corresponding to an incident light intensity. The N switches SW51 to SW5N make up a selection circuit that successively selects the output terminal of VI conversion circuit 40n, of each unit Un and connects the output terminal to the input terminal of IV conversion circuit 60. Integrating circuit 10n, CDS circuit 20n, hold circuit 30n, and VI conversion circuit 40n of each unit Un and IV conversion circuit 60 make up a signal processing circuit of this solid-state image pickup device 1.
FIG. 2 is a specific circuit diagram of solid-state image pickup device 1 of the first embodiment. In this Figure, just one unit Un, among the N units U1 to UN, is shown. With photodiode PD of each unit Un, the anode terminal is grounded and the cathode terminal is connected to the input terminal of integrating circuit 10n. This photodiode PD outputs charges of an amount corresponding to an input light intensity to integrating circuit 10n.
Integrating circuit 10n of each unit Un comprises an amp A10, a switch SW10, and a capacitor C10. The input terminal of amp A10, is connected to the cathode terminal of photodiode PD and the output terminal of amp A10 is connected to CDS circuit 20n. Switch SW10 and capacitor C10 are disposed in parallel between the input terminal and the output terminal of amp A10. Switch SW10 discharges the charges accumulated in capacitor C10 and resets the output level when it is closed, and causes the charges input from photodiode PD to accumulate in capacitor C10 when it is open. Integrating circuit 10n outputs a voltage of a value corresponding to the amount of charges accumulated in capacitor C10. The switching operations of switch SW10 are controlled by a Reset signal.
CDS circuit 20n of each unit Un comprises an amp A20, a switch SW21, a switch SW22, and a capacitor C20. The inverted input terminal of amp A20 is connected to the output terminal of amp A20. The non-inverted input terminal of amp A20 is connected serially via switch SW21 and capacitor C20 to integrating circuit 10 and is connected via switch SW22 to a clamp potential Vclamp. By the opening and closing of switch SW21 and switch SW22 being carried out at appropriate timings, CDS circuit 20n outputs the difference voltage (V(t)−(Vt1)) between a voltage V(t1) at a predetermined time t1 and a voltage V(t) at each time t, which are output from integrating circuit 10n. The switching operations of switch SW21 are controlled by a Sample signal and the switching operations of switch SW22 are controlled by a Clamp signal.
Hold circuit 30n of each unit Un comprises a switch SW30 and a capacitor C30. Switch SW30 has one end connected to CDS circuit 20n and has the other end connected to one end of capacitor C30 and to VI conversion circuit 40n. The other end of capacitor C30 is set to the ground potential. Hold circuit 30n holds the voltage input from CDS circuit 20n in capacitor C30 when switch SW30 is closed and outputs the voltage held in capacitor C30 when switch SW30 is open. The switching operations of switch SW30 are controlled by a Hold signal.
VI conversion circuit 40n of each unit Un comprises an amp A40, a switch SW40, a MOS transistor T40, and a resistor R40. Amp A40 has its non-inverted input terminal connected to hold circuit 30n and has its inverted input terminal connected to the source terminal of MOS transistor T40 and to one end of resistor R40. The other end of resistor R40 is set to the ground potential. MOS transistor T40 has its gate electrode connected to the output terminal of amp A40 and has its drain terminal connected via switch SW40 to a reference potential Vref. VI conversion circuit 40n inputs the voltage output from hold circuit 30n, converts this input voltage to a current based on the resistance value of resistor R40, and outputs this current from the drain terminal of MOS transistor T40. The switching operations of switch SW40 of VI conversion circuit 40n are controlled by a Stdby(n) signal.
By successive closing of switch SW5n of each unit Un, the output terminal of VI conversion circuit 40n of each unit Un is connected successively to the input terminal of IV conversion circuit 60. The switching operations of switch SW5n are controlled by a Shift(n) signal.
IV conversion circuit 60 comprises an amp A60, a resistor R60, and a capacitor C60. Amp A60 has its inverted input terminal connected via switch SW5n to the output terminal of VI conversion circuit 40n and has its non-inverted input terminal set to reference potential Vref. Resistor R60 and capacitor C60 are respectively disposed in parallel between the inverted input terminal and the output terminal of amp A60. IV conversion circuit 60 inputs the current output from the output terminal of VI conversion circuit 40n, converts this current to a voltage, and outputs this voltage (Video signal) from the output terminal of amp A60.
Integrated circuit 10n, CDS circuit 20n, and hold circuit 30n respectively operate in parallel in each unit Un. The Reset signal that controls the switching operations of switch SW10 of each integrating circuit 10n, the Sample signal that controls the switching operations of switch SW2, of each CDS circuit 20n, the Clamp signal that controls the switching operations of switch SW22 of each CDS circuit 20n, the Hold signal that controls the switching operations of switch SW30 of each hold circuit 30n, the Stdby(n) signal that controls the switching operations of switch SW40 of each VI conversion circuit 40n, and the Shift (n) signal that controls the switching operations of switch SW5n are output respectively at predetermined timings by a control circuit (not shown).
FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H, 3I, 3J, and 3K are timing charts for explaining the operations of solid-state image pickup device 1 of the first embodiment. First, the Reset signal is set to the H level, causing switch SW10 of each integrating circuit 10n to close and the output level of each integrating circuit 10n to be reset. Thereafter, the Reset signal changes to the L level at time to, causing switch SW10 of each integrating circuit 10n to open and the accumulation of charge in capacitor C10 of each integrating circuit 10n to be started.
At time t1, the Sample signal changes from the H level to the L level and switch SW21 of each CDS circuit 20, opens. At time t2, the Clamp signal changes from the H level to the L level and switch SW22 of each CDS circuit 20n opens. At time t3, the Sample signal changes from the L level to the H level and switch SW21 of each CDS circuit 20n closes, and the Hold signal changes from the L level to the H level and switch SW30 of each hold circuit 30n closes. At time t4, the Hold signal changes from the H level to the L level and switch SW30 of each hold circuit 30n opens.
If the voltage output from integrating circuit 10n at a time t is expressed as V(t), the voltage that is output from CDS circuit 20n at a time t prior to the time t4 is (V(t)−V(t1)). From time t4, at which switch SW30 of hold circuit 30n opens, and onwards, the voltage that is output from hold circuit 30n is V(t4)−V(t1). By thus taking the difference, the influence of the fluctuation of the offset of amp A10 is eliminated. At time t5, the Reset signal changes to the H level and switch SW10 of each integrating circuit 10n closes and the Clamp signal changes to the H level and switch SW22 of each CDS circuit 20n closes.
Then at time t6, the Stdby(1) signal changes from the H level to the L level and switch SW40 of VI conversion circuit 401 opens and the Shift(1) signal changes from the L level to the H level and switch SW51 closes. At time t7, the Stdby(1) signal changes to the H level and switch SW40 of VI conversion circuit 401 closes and the Shift(1) signal changes to the L level and switch SW51 opens. In the period from time t6 to time t7, the respective switches SW40 of the other VI conversion circuits 402 to 40N are closed and the other switches SW52 to SW5N are open. The voltage that is output from hold circuit 301 of unit U1 during this period (t6 to t7) is converted to a current by VI conversion circuit 401, and this current is input via switch SW51 into IV conversion circuit 60 and converted to a voltage by IV conversion circuit 60.
Also, at time t7, the Stdby(2) signal changes from the H level to the L level and switch SW40 of VI conversion circuit 402 opens and the Shift (2) signal changes from the L level to the H level and switch SW52 closes. At time t8, the Stdby(2) signal changes to the H level and switch SW40 of VI conversion circuit 402 closes and the Shift (2) signal changes to the L level and switch SW52 opens. In the period from time t7 to time t8, the respective switches SW40 of the other VI conversion circuits 401 and 403 to 40N are closed and the other switches SW51 and SW53 to SW5N are open. The voltage that is output from hold circuit 302 of unit U2 during this period (t7 to t8) is converted to a current by VI conversion circuit 402, and this current is input via switch SW52 into IV conversion circuit 60 and converted to a voltage by IV conversion circuit 60.
Furthermore, at time t8, the Stdby(3) signal changes from the H level to the L level and switch SW40 of VI conversion circuit 403 opens and the Shift (3) signal changes from the L level to the H level and switch SW53 closes. At time t9, the Stdby(3) signal changes to the H level and switch SW40 of VI conversion circuit 403 closes and the Shift (3) signal changes to the L level and switch SW53 opens. In the period from time t8 to time t9, the respective switches SW40 of the other VI conversion circuits 401, 402, and 404 to 40N are closed and the other switches SW51, SW52, and SW54 to SW5N are open. The voltage that is output from hold circuit 303 of unit U3 during this period (t8 to t9) is converted to a current by VI conversion circuit 403, and this current is input via switch SW53 into IV conversion circuit 60 and converted to a voltage by IV conversion circuit 60.
Thereafter the respective switching operations of switch SW40 of each VI conversion circuit 40n and each switch SW5n are performed in likewise manner and a Video signal is output from IV conversion circuit 60. With this Video signal, the voltage corresponding to the intensity of incident light on photodiode PD of each unit Un appears in a time sequence.
The operations of each VI conversion circuit 40n shall now be described in detail. VI conversion circuit 40n has the circuit arrangement shown in FIG. 2 and inputs the voltage output from hold circuit 30n, converts this input voltage into a current based on the resistance value of resistor R40, and outputs this current from the drain terminal of MOS transistor T40. Here, amp A40 has an adequate open loop gain and MOS transistor T40 operates in the saturated region. Here, if the resistance value of resistor R40 is R, the current value I that is output from VI conversion circuit 40n is in the proportional relationship expressed by the equation, “I=V/R,” with respect to the voltage V input into VI conversion circuit 40n.
As can be understood from the above, when light of the same intensity is made incident on photodiode PD of each unit Un the fluctuation of the value of the current output from each VI conversion circuit 40n depends on the fluctuation of the resistance value R of resistor R40 of each VI conversion circuit 40n. In general, the fluctuation in manufacture of the resistance values of resistors is small in comparison to the fluctuation in manufacture of the threshold voltages of MOS transistors. Solid-state image pickup device 1 of the present embodiment can thus take images of excellent quality at high speed in comparison to those of the prior art. Also, with solid-state image pickup device 1 of this embodiment, since the influence of the fluctuation of the offset possessed by amp A10 of integrating circuit 10n is eliminated by the provision of CDS circuit 20n, images of excellent quality can be taken for this reason as well.
Also with solid-state image pickup device 1 of this embodiment, since the N photodiodes PD and the other circuits are formed at mutually different areas on a substrate, the photosensitive area and interval of each of the N photodiodes PD may be set as suited to improve the aperture efficiency or the sensitivity. Though the N photodiodes PD and the other circuits may be formed on the same substrate, they may also be formed on mutually different substrates.
Second Embodiment
A second embodiment of a solid-state image pickup device by this invention shall now be described. The general arrangement of the solid-state image pickup device of the second embodiment is the same as that shown in FIG. 1. FIG. 4 is a specific circuit diagram of the solid-state image pickup device of the second embodiment. In comparison to the solid-state image pickup device of the first embodiment (FIG. 2), the solid-state pickup device of the second embodiment differs in that the switching operations of switch SW40 of VI conversion circuit 40n are performed according to a Stdby1(n) signal and the operation of amp A40 of VI conversion circuit 40n is switched ON/OFF by a Stdby2 (n) signal. That is, by amp A40 of VI conversion circuit 40n being put in the ON state only for a predetermined period by the Stdby2 (n) signal and being put in the OFF state in other periods, low power consumption is achieved.
FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G, 5H, 5I, 5J, and 5K are timing charts for explaining the operations of the solid-state image pickup device of the second embodiment. The operations of the solid-state image pickup device of the second embodiment are the same as those of the first embodiment up to time t5.
With the present embodiment, at time t6, each of the Stdby1 (1) signal and the Stdby2 (1) signal changes from the L level to the H level and switch SW40 of VI conversion circuit 401 closes and amp A40 of VI conversion circuit 401 is put in the ON state. At time t7, the Stdby1(1) signal changes to the L level and switch SW40 of VI conversion circuit 401 opens and the Shift (1) signal changes from the L level to the H level and switch SW51 closes. At time t8, each of the Stdby2(1) signal and the Shift(1) signal changes to the L level and amp A40 of VI conversion circuit 401 is put in the OFF state and switch SW51 opens.
Thus in the period from time t7 to time t8, switch SW40 of VI conversion circuit 401 is open, amp A40 of VI conversion circuit 401 is in the ON state, and switch SW51 is closed. Thus in this period from time t7 to time t8, the voltage that is output from hold circuit 301 of unit U1 is converted into a current by VI conversion circuit 401, and this current is input via switch SW51 into IV conversion circuit 60 and converted into a voltage by IV conversion circuit 60. During this period from t7 to t8, the other switches SW52 to SW5N are respectively open.
In the prior period from time t6 to time t7, since switch SW40 of VI conversion circuit 401 is closed and amp A40 of VI conversion circuit 401 is in the ON state, the operation of VI conversion circuit 401 in the period from t7 to t8 is started up early. Also, since prior to time t6 and from time t8 onwards, amp A40 of VI conversion circuit 401 is put in the OFF state, the consumption power is reduced. Furthermore, since prior to time t6 and from time t7 onwards, the Stdby1(1) signal is at the L level and switch SW40 of VI conversion circuit 401 is open, the consumption power is reduced from this point as well.
Also, at time t7, each of the Stdby1(2) signal and the Stdby2 (2) signal changes from the L level to the H level and switch SW40 of VI conversion circuit 402 closes and amp A40 of VI conversion circuit 402 is put in the ON state. At time t8, the Stdby1(2) signal changes to the L level and switch SW40 of VI conversion circuit 402 opens and the Shift (2) signal changes from the L level to the H level and switch SW52 closes. At time t9, each of the Stdby2(2) signal and the Shift(2) signal changes to the L level and amp A40 of VI conversion circuit 402 is put in the OFF state and switch SW52 opens.
Thus in the period from time t8 to time t9, switch SW40 of VI conversion circuit 402 is open, amp A40 of VI conversion circuit 402 is in the ON state, and switch SW52 is closed. Thus in this period from time t8 to time t9, the voltage that is output from hold circuit 302 of unit U1 is converted into a current by VI conversion circuit 402, and this current is input via switch SW52 into IV conversion circuit 60 and converted into a voltage by IV conversion circuit 60. During this period from t8 to t9, the other switches SW51 and SW53 to SW5N are respectively open.
In the prior period from time t7 to time t8, since switch SW40 of VI conversion circuit 402 is closed and amp A40 of VI conversion circuit 402 is in the ON state, the operation of VI conversion circuit 402 in the period from t8 to t9 is started up early. Also, since prior to time t7 and from time t9 onwards, amp A40 of VI conversion circuit 402 is put in the OFF state, the consumption power is reduced. Furthermore, since prior to time t7 and from time t8 onwards, the Stdby1(2) signal is at the L level and switch SW40 of VI conversion circuit 402 is open, the consumption power is reduced from this point as well.
Thereafter the respective switching operations of switch SW40 of each VI conversion circuit 40n and each switch SW5n are performed in likewise manner and a Video signal is output from IV conversion circuit 60. With this Video signal, the voltage corresponding to the intensity of incident light on photodiode PD of each unit Un appears in a time sequence.
When light of the same intensity is made incident on photodiode PD of each unit Un, the fluctuation of the value of the current output from each VI conversion circuit 40n depends on the fluctuation of the resistance value R of resistor R40 of each VI conversion circuit 40n with this embodiment as well. The solid-state image pickup device of the present embodiment can thus also take images of excellent quality at high speed in comparison to those of the prior art. Also, since the influence of the fluctuation of the offset possessed by amp A10 of integrating circuit 10n is eliminated by the provision of CDS circuit 20n with the solid-state image pickup device of this embodiment as well, images of excellent quality can be taken for this reason as well.
Also, since the N photodiodes PD and the other circuits are formed at mutually different areas on a substrate, the photosensitive area and interval of each of the N photodiodes PD may be set as suited to improve the aperture efficiency or the sensitivity with the solid-state image pickup device of this embodiment as well. Though the N photodiodes PD and the other circuits may be formed on the same substrate, they may also be formed on mutually different substrates.
Third Embodiment
A third embodiment of a solid-state image pickup device by this invention shall now be described. FIG. 6 is a block diagram of solid-state image pickup device 3 of the third embodiment. This solid-state image pickup device 3 has photodiodes PD aligned in two dimensions and is equipped with N (where is an integer of 2 or more) units U1 to UN and an IV conversion circuit 60. Each unit Un (where n is an arbitrary integer of no less than 1 and no more than N) includes M sets (where M is an integer of 2 or more) of photodiodes PD and switches, an integrating circuit 10n, a CDS circuit 20n, a hold circuit 30n, a VI conversion circuit 40n and a switch SW5n.
With each unit Un, the M sets of photodiodes PD and switches SW, integrating circuit 10, CDS circuit 20n, and hold circuit 30n make up a photoelectric conversion circuit that outputs a voltage of a value corresponding to an incident light intensity. The N switches SW51 to SW5N make up a selection circuit that successively selects the output terminal of VI conversion circuit 40n of each unit Un and connects the output terminal to the input terminal of IV conversion circuit 60. Integrating circuit 10n, CDS circuit 20n, hold circuit 30n, and VI conversion circuit 40n of each unit Un and IV conversion circuit 60 make up a signal processing circuit of this solid-state image pickup device 1.
The specific circuit arrangement examples of each integrating circuit 10n, each CDS circuit 20n, each hold circuit 30n, each VI conversion circuit 40n, each switch SW5n, and IV conversion circuit 60 are the same as those shown in FIG. 2 or FIG. 4. In comparison to solid-state image pickup device 1 of the first embodiment, solid-state image pickup device 3 of the present embodiment differs in that M sets of photodiodes PD and switches SW are provided in each unit Un and that the M switches SW of each unit Un close successively so that the corresponding photodiode PD becomes connected with integrating circuit 10n. The switching operations of the M switches SW of each unit Un are also output at predetermined timings by a control circuit (not shown).
The operations of solid-state image pickup device of this embodiment are substantially the same as the operations described with the timing charts shown in FIG. 3 or FIG. 5. However, with this embodiment, photodiodes PD are aligned in the two dimensions of M rows and N columns and the M switches SW of each unit Un close successively. Thus with the Video signal output from IV conversion circuit 60, voltages corresponding to the intensities of incident light on N photodiodes PD of the first row appear first in a time sequence, then voltages corresponding to the intensities of incident light on N photodiodes PD of the second row appear in the time sequence, then voltages corresponding to the intensities of incident light on N photodiodes PD of the third row appear in the time sequence, and so for thin likewise manner, voltages corresponding to the intensities of incident light on the respective photodiodes PD appear in the time sequence.
When light of the same intensity is made incident on photodiode PD of each unit Un, the fluctuation of the value of the current output from each VI conversion circuit 40n depends on the fluctuation of the resistance value R of resistor R40 of each VI conversion circuit 40n with this embodiment as well. The solid-state image pickup device of the present embodiment can thus also take images of excellent quality at high speed in comparison to those of the prior art. Also, since the influence of the fluctuation of the offset possessed by amp A10 of integrating circuit 10n is eliminated by the provision of CDS circuit 20n with the solid-state image pickup device of this embodiment as well, images of excellent quality can be taken for this reason as well.
INDUSTRIAL APPLICABILITY
This invention can be applied to a solid-state image pickup device that takes a one-dimensional or two-dimensional optical image.

## Claims

1. A signal processing circuit comprising:
a plurality of VI conversion circuits, including each one resistor, converting an input voltage to a current based on the resistance value of said resistor, and outputting this current from an output terminal;
an IV conversion circuit, inputting, into an input terminal, the current output from said output terminal of said VI conversion circuit, converting this current to a voltage, and outputting this voltage; and
a selection circuit, successively selecting each of said output terminals of a plurality of said VI conversion circuits and connecting the selected output terminal to said input terminal of said IV conversion circuit,
wherein said VI conversion circuit comprises:
an amp, having a first input terminal, a second input terminal, and an output terminal, and inputting said input voltage into said first input terminal and outputting, from said output terminal, a voltage corresponding to the difference in the respective potentials of said first input terminal and said second input terminal;
a MOS transistor, having a gate terminal, source terminal, and a drain terminal, said gate terminal being connected to said output terminal of said amp;
a resistor, one end of which is connected to said second input terminal of said amp and to said source terminal of said MOS transistor and the other end of which is connected to a first reference potential; and
a switch, one end of which is connected to said drain terminal of said MOS transistor and the other end of which is set to a second reference potential;
and converts said input voltage to a current and outputs this current from said drain terminal of said MOS transistor.

2. A solid-state image pickup device comprising:
said signal processing circuit according to claim 1; and
a plurality of photoelectric conversion circuits, outputting a voltage of a value corresponding to an incident light intensity,
wherein said
plurality of VI conversion circuits, including each one resistor, converting the voltage output from one of said plurality of photoelectric conversion circuits to a current based on the resistance value of said resistor, and outputting this current from an output terminal.

3. A signal processing circuit as set forth in claim 1, wherein said amp is put in the ON state in a period from a time that is a fixed amount of time prior to the time at which said VI conversion circuit is connected by said selection circuit to said IV conversion circuit to the time at which the connection with said IV conversion circuit is disengaged by said selection circuit and said amp is put in the OFF state in other periods.