A/D conversion circuit and solid imaging device

Details A/D conversion circuit and solid imaging device A/D conversion circuit and solid imaging device

2003-08-26

2004-11-16

Tokar, Michael (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Analog to digital conversion

C341S166000, C341S143000, C341S158000

Reexamination Certificate

active

06819281

ABSTRACT:

TECHNICAL FIELD
This invention relates to an A/D conversion circuit which converts analog values into digital values, and a solid-state image pickup device comprising an A/D conversion circuit.
BACKGROUND ART
A solid-state image pickup device comprises a plurality of photodetector elements, arranged in a one-dimensional or two-dimensional array, and a plurality of integrating circuits which integrate the signal currents output from each photodetector element to convert the signal currents into voltages. In this solid-state image pickup device, signal currents with values corresponding to the incident light intensity are output from the plurality of photodetector elements, voltages corresponding to the integrated signal currents are output from the integrating circuits, and based on the voltages, the distribution of the incident light intensity is obtained, to capture an image. A solid-state image pickup device may further comprise an A/D conversion circuit to convert voltages (analog values) output from the integrating circuits into digital values. In this case, an incident light intensity is obtained as a digital value, and image processing by a computer or similar becomes possible.
One well-known configuration of an A/D conversion circuit comprises N combinations of capacitors and switches; of the N capacitors, the nth capacitor has a capacitance value of 2
n−1
(N≧2, 1≦n≦N) . In this A/D conversion circuit, all capacitance values are set appropriately by appropriately setting the open/close states of each of the N switches according to the input analog value, so that an N-bit digital value corresponding to the open/close states of the N switches is output.
In such a solid-state image pickup device, faster operation and higher precision of the A/D conversion circuit are sought. In order to achieve faster operation, A/D conversion circuits are provided for each integrating circuit, to perform parallel processing. In order to increase precision, the number of bits of the digital value output from A/D conversion circuits is increased. Hence in order to improve both the processing speed and the precision of A/D conversion processing, an A/D conversion circuit may be provided for each integrating circuit to perform parallel processing, and the number of capacitors N in each A/D conversion circuit may be increased.
DISCLOSURE OF THE INVENTION
However, when attempting to fabricate a solid-state image pickup device comprising an A/D conversion circuit as described above on a single semiconductor chip, the following problems are encountered. The area occupied by capacitors on the chip is substantially proportional to the capacitance value. Hence if the number of bits is N, then the area occupied by N capacitors of an A/D conversion circuit is equivalent to the area occupied by one capacitor with capacitance value 2
N
C (≈C+2C+2
2
C+. . . +2
N−1
C) . That is, if the number of bits is increased by one, the area occupied by all the capacitors of the A/D conversion circuit increases by a factor of 2. Also, if the capacitance value of a capacitor is large, the parasitic capacitance also increases, and fast A/D conversion processing becomes impossible. Hence conventional solid-state image pickup devices comprising A/D conversion circuits encounter limits in attempting to improve both speed, by providing an integrating circuit for each A/D conversion circuit, and precision, by increasing the number of bits of the digital values output from A/D conversion circuits.
The present invention was devised in order to resolve the above problems, and it is an object of the invention to provide an A/D conversion circuit which, while occupying a small area, easily achieves both fast operation and high precision, as well as a solid-state image pickup device comprising this A/D conversion circuit.
An A/D conversion circuit of this invention converts an analog value input to an input end into a digital value and outputs this digital value from an output end, and is characterized in comprising: (1) an amplifier, having a first input terminal, a second input terminal and an output terminal, in which the first input terminal is connected to the input end via a coupling capacitor, and a common voltage V
com
is input to the second input terminal; (2) a feedback capacitor, provided between the first input terminal and the output terminal of the amplifier; (3) a switch, provided between the first input terminal and the output terminal of the amplifier; (4) a number M of variable-capacitance portions, each having a number N
m
of capacitors with different capacitance values, one end of each of which is connected to the first input terminal of the amplifier, and voltage switching means to switch the voltage input to each of the other ends of the N
m
capacitors to a common voltage V
com
and to P
m
reference voltages V
ref,m,1
to V
ref,m,Pm
(M≧1, N
m
≧1, P
m
≧1, 1≦m≦M, but excluding the case M=P
1
=1); (5) a comparison portion, which compares the magnitudes of the voltage output from the amplifier output terminal and the common voltage V
com
, and outputs a signal indicating-the comparison result; and, (6) a capacitance control portion, which controls the switching operation in each of the voltage switching means of the M variable-capacitance portions, and which outputs a digital value to the output terminal based on the switched state in the respective voltage switching means of the M variable-capacitance portions as well as the signal output from the comparison portion.
By means of this A/D conversion circuit, when the switch between the first input terminal and the output terminal of the amplifier is closed, the feedback capacitor between the first input terminal and the output terminal of the amplifier is discharged. When the switch is then opened and a voltage (analog value) for A/D conversion is input from the input end, electric charge corresponding to the input voltage integrates across the feedback capacitor. Then, either the common voltage V
com
or one of the P
m
reference voltages V
ref,m,1
to V
ref,m,Pm
is switched by the voltage switching means to each of the other ends of the N
m
capacitors comprised by each of the M variable capacitance portions controlled by the capacitance control portion. One end of the N
m
capacitors comprised by each of the M variable capacitance portions is connected to the first input terminal of the amplifier, either directly or via a switch, so that upon switching, charge moves from the feedback capacitor to the M variable capacitance portions. Then, a voltage corresponding to the amount of charge remaining in the feedback capacitor is output from the output terminal of the amplifier. The magnitude of the voltage output from the amplifier is compared with the common voltage V
com
by the comparison portion, and a signal indicating the comparison result is output from the comparison portion to the capacitance control portion. Based on the switching states of the voltage switching means in each of the M variable capacitance portions and the signal output from the comparison portion, a digital value is output from the capacitance control portion to the output end.
In an A/D conversion circuit of this invention, it is preferable that M be 1, and that N
1
and P
1
be pluralities. In this case, the area occupied on the chip by the N
m
capacitors comprised by each of the M variable capacitance portions is further reduced. In an A/D conversion circuit of this invention, it is preferable that M and N
1
each be 1, and that P
1
be a plurality; in this case, even compared with the above-described case, the area occupied on the chip by the N
m
capacitors comprised by each of the M variable capacitance portions is further reduced.
A solid-state image pickup device of this invention is characterized in comprising (1) a photodetector element which outputs a signal current according to the incident light intensity; (2) an integrating circuit which inputs and integrates the signal current output from the

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