Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – By integrating
Reexamination Certificate
1999-11-29
2002-08-06
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
By integrating
C327S345000
Reexamination Certificate
active
06429719
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a signal processing circuit for use in a charge generation type detection device, such as a pyroelectric sensor, an infrared sensor, and a pressure sensor, including a charge-voltage conversion circuit for converting a charge generated in the charge generation type detection device to a voltage.
2. Description of the Related Art
In recent years, in the field of surveillance camera apparatuses, object sorting apparatuses, and the like, there has been a demand for a system capable of performing a high-level information processing operation by obtaining information such as the position, the size, and the traveling speed of an object, as well as the number of objects and whether the object is in contact with another object. For example, in order to realize high level security, a surveillance camera apparatus is required to detect infrared radiation from a human to produce such information.
FIG. 8
illustrates a configuration of a conventional infrared radiation detection circuit
800
as an exemplary signal processing circuit of a charge generation type detection device.
The infrared radiation detection circuit
800
includes a charge-voltage conversion circuit
801
for converting a charge-generated in an infrared sensor
809
to a voltage. The charge-voltage conversion circuit
801
includes an input terminal
812
, an output terminal
813
and a control terminal
814
.
One terminal of the infrared sensor
809
is connected to a signal ground potential V
sg
, and the other terminal thereof is connected to an input terminal
812
of the charge-voltage conversion circuit
801
,
The charge-voltage conversion circuit
801
includes a capacitor
811
for storing a charge generated in the infrared sensor
809
, an operational amplifier
810
which is connected to the capacitor
811
to form a feed-back loop, and a field effect transistor (hereinafter, referred to simply as “FET”)
802
as a reset switch which is connected in parallel with the capacitor
811
.
The FET
802
is controlled according to a control voltage V
c
applied to the control terminal
814
. When V
c
=V
DD
, the FET
802
is turned ON. Herein, V
DD
denotes a power supply voltage and V
SS
denotes a ground voltage. In this way, the FET
802
is repeatedly turned ON/OFF according to the control voltage V
c
.
Thus, the charge-voltage conversion circuit
801
functions as a switched capacitor integration circuit (“SC integration circuit”).
An operation of the charge-voltage conversion circuit
801
will now be described.
When V
c
=V
DD
, the FET
802
is turned ON, thereby initializing the charge-voltage conversion circuit
801
with the amount of charge in the capacitor
811
being reset to zero. In this case, an output voltage V
o
at the output terminal
813
ss expressed by Expression (1) below.
V
o
=V
sg
(1)
Then, when V
o=V
SS
, the FET
802
is turned OFF, thereby starting an operation of integrating the amount of charge being input from the infrared sensor
809
to the charge-voltage conversion circuit
801
. When an amount of charge Q flows from the infrared sensor
809
into the charge-voltage conversion circuit
801
during a period in which the FET
802
is OFF, the output voltage V
o
at the output terminal
813
is expressed by Expression (2) below.
V
o
=V
sg
−Q/C
811
(2)
Herein, C
811
denotes the capacitance value of the capacitor
811
.
Thus, the amount of voltage change &Dgr;V
o
at the output terminal
813
, which corresponds to the amount of charge Q generated in the infrared sensor
809
during d period in which the FET
802
is turned OFF, is expressed by Expression (3) below, based on Expressions (1) and (2).
&Dgr;V
o
=−Q/C
811
(3)
Thus, the charge-voltage conversion circuit
801
can generate a voltage according to the amount of charge Q generated in the infrared sensor
809
.
A parasitic capacitance exists in the FET
802
of the charge-voltage conversion circuit
801
. As illustrated in
FIG. 8
, the parasitic capacitance of the FET
802
includes a parasitic capacitance
803
between the gate and the source of the FET
802
and a parasitic capacitance
804
between the gate and the drain of the FET
802
. Herein, C
gs
denotes the capacitance value of the parasitic capacitance
803
and C
gd
denotes the capacitance value of the parasitic capacitance
804
.
Transition of the FET
802
from ON to OFF (i.e., transition of the control voltage V
c
applied to the control terminal
814
) generates a clock feed-through in the FET
802
. Due to the clock feed-through, a charge q is provided to one end of the capacitor
811
and another charge q
o
is provided to the output of the operational amplifier
810
.
The charge q is expressed by Expression (4) below, and the charge q
o
is expressed by Expression (5) below.
q=−C
gs
·(
V
DD
−V
SS
) (4)
q
o
=−C
gd
·(
V
DD
−V
SS
) (5)
Due to the charge q, an offset voltage &Dgr;V
offset
occurs at the output terminal
813
. The offset voltage &Dgr;V
offset
is expressed by Expression (6) below.
&Dgr;V
offset
=C
gs
/C
811
·(
V
DD
−V
SS
) (6)
As can be seen from Expression (6), the offset voltage &Dgr;V
offset
is a constant voltage. Since the offset voltage &Dgr;V
offset
is added to the output voltage V
o
the offset voltage &Dgr;V
offset
becomes a DC offset voltage error in the output voltage V
o
. The error has been A significant drawback to realization of high-sensitivity infrared radiation detection.
Moreover, the output of the operational amplifier
810
is designed to have the lowest impedance Z
o
among other devices connected to the output terminal
813
. Therefore, all of the charge q
o
as shown in Expression (5) flows into the output of the operational amplifier
810
. As a result, an offset voltage &Dgr;V
offset
(t) occurs at the output terminal
813
due to the charge q
o
. The offset voltage &Dgr;V
offset
(t) is expressed by Expression (7) below.
&Dgr;V
offset
(
t
)=Z
o
·{∂(q
o
)/∂t} (7)
As can be seen from Expression (7), the offset voltage &Dgr;V
offset
(t) Ls a transitional voltage which varies over time. The offset voltage &Dgr;V
offset
(t) is high-frequency noise to the output voltage V
o
. The high-frequency noise has been a significant drawback to realization of high-sensitivity infrared radiation detection.
Moreover, as the intensity of the infrared radiation increases, the amount of charge Q generated in the infrared sensor
809
also increases. In a region where the amount of charge Q is large, the output voltage V
o
does not change according to Expression (2), resulting in a situation where the output voltage V
o
is saturated to the level of the power supply voltage V
DD
or the ground voltage V
SS
. This has been a drawback to realization of an infrared radiation detection circuit having a wide dynamic range.
Furthermore, in order to realize an infrared radiation detection apparatus including a plurality of infrared sensors arranged in a one-dimensional or two-dimensional arrangement, the infrared radiation detection apparatus needs to include a plurality of charge-voltage conversion circuits. This increases the circuit scale of the infrared radiation detection apparatus, thereby making the apparatus high in cost and large in size.
In addition, external noise, particularly a 50 Hz or 60 Hz commercial frequency, may further be superimposed on the output voltage V
o
, thereby reducing the infrared radiation detection capability. In order to reduce such noise, a filter circuit is used. However, it was not possible in the prior art to have such a filter circuit built in the charge-voltage conversion circuit. Therefore, the filter circuit had to be provided in a stage subsequent to the charge-voltage conversion circuit as a separate circuit from the charge-voltage conversion circuit. This has increased the circuit scale of the infra
Kelly Michael K.
Matsushita Electric - Industrial Co., Ltd.
Snell & Wilmer
Wells Kenneth B.
LandOfFree
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