U.S. patent number RE35,494 [Application Number 08/236,673] was granted by the patent office on 1997-04-22 for integrated active low-pass filter of the first order.
This patent grant is currently assigned to SGS-Thomson Microelectronics, S.r.l.. Invention is credited to Germano Nicollini.
United States Patent |
RE35,494 |
Nicollini |
April 22, 1997 |
Integrated active low-pass filter of the first order
Abstract
An integrated, low-pass filter of the first order made using the
switched capacitors technique utilizes advantageously a single
switched capacitor and only two switches in contrast to the filters
of the prior art which utilize two switched capacitors and four
switches. The filter of the invention requires a smaller
integration area and moreover exhibits a greater precision of its
DC gain.
Inventors: |
Nicollini; Germano (Piacenza,
IT) |
Assignee: |
SGS-Thomson Microelectronics,
S.r.l. (Agrate Brianza, IT)
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Family
ID: |
27273862 |
Appl.
No.: |
08/236,673 |
Filed: |
May 2, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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832010 |
Feb 6, 1992 |
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Reissue of: |
285204 |
Dec 16, 1988 |
04899069 |
Feb 6, 1990 |
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Foreign Application Priority Data
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Dec 22, 1987 [IT] |
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83685/87 |
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Current U.S.
Class: |
327/554; 327/337;
327/552; 327/558; 327/561; 330/109; 333/173 |
Current CPC
Class: |
H03H
19/004 (20130101) |
Current International
Class: |
H03H
19/00 (20060101); H03B 001/00 (); H03K
005/00 () |
Field of
Search: |
;307/520,521,353,352,354,358 ;328/167,127,26,104,157 ;333/166,173
;330/9,107,109 ;327/552,554,561,311,94,558,337,563,551 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
IEEE J. Solid-State Circuits, vol. SC-15, pp. 301-305, Jun. 1980,
"A Synchronous Switched Capacitor Filter", by Dessoulavy et al.
.
IEEE J. Solid-State Circuits, vol. SC-16, pp. 724-729, Dec. 1981,
"Simplified MOS Switched Capacitor Ladder Filter Structures",
Allstot et al. .
IEEE J. Solid-State Circuits, vol. 71, pp. 967-986, Aug. 1983,
"Technological Design Considerations for Monolithic MOS
Switched-Capacitor Filtering Systems", by Allstot et al. .
IEEE J. Solid-State Circuits, vol. 71, pp. 926-940, Aug. 1983,
"Principles of Operation and Analysis of Switched-Capacitor
Circuits", by Yannis Tsvidis. .
IEEE J. Solid-State Circuits, vol. 67, pp. 61-75, Jan. 1979, "MOS
Switched-Capacitor Filters", by Brodersen et al. .
IEEE J. Solid-State Circuits, vol. SC-7, pp. 302-304, Aug. 1972,
"Analog Sample-Data Filters", by David L. Fried. .
IEEE J. Solid-State Circuits, vol. 26, No. 3, Mar. 1991,
"Switched-Current Circuit Design Issues", by Fiez et al. .
Proceedings of the IEEE, vol. 73, No. 8, Aug. 1985, "A Novel
Two-Amplifier Universal Active Switched-Capacitor Filter", by Mohan
et al..
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Primary Examiner: Callahan; Timothy P.
Assistant Examiner: Le; Dinh T.
Attorney, Agent or Firm: Formby; Betty Groover; Robert
Parent Case Text
.Iadd.This is a continuation of application Ser. No. 07/832,010,
filed Feb. 6, 1992 and now abandoned. .Iaddend.
Claims
What I claim is:
1. An integrated, low-pass, active .Iadd.first order
.Iaddend.filter .[.of the first order operable.]. for producing an
output signal at an output terminal thereof .[.in.]. .Iadd.as a
.Iaddend.function of a signal applied to an input terminal thereof
and comprising an operational amplifier having an inverting input
.[.and.]..Iadd., .Iaddend.a noninverting input.Iadd., .Iaddend.and
.[.a single.]. .Iadd.an .Iaddend.output .[.coinciding.].
.Iadd.conterminous .Iaddend.with said output terminal of the filter
.[.and.]..Iadd., said filter being .Iaddend.provided with a
negative feedback network which comprises a continuous integration
capacitor connected between said output terminal of the filter and
said inverting input of the operational amplifier, the noninverting
input of .[.which.]. .Iadd.said operational amplifier
.Iaddend.being connected to ground, .[.and characterized by.].
comprising.Iadd.: .Iaddend.
a .[.single switched.]. capacitor and two switches operating at a
preset frequency;
a first .[.armature.]. .Iadd.side .Iaddend.of said .[.switched.].
capacitor being switched by a first one of said two switches
between said inverting input of the operational amplifier and
ground;
a second .[.armature.]. .Iadd.side .Iaddend.of said .[.switched.].
capacitor being switched by the other of said two switches between
said input terminal and said output terminal of the filter.
.Iadd.2. The active filter of claim 1 wherein said preset frequency
and the values of said first and second capacitors determines the
cut-off frequency of the filter. .Iaddend..Iadd.3. The active
filter of claim 1 wherein said active
filter has a D.C. gain of exactly 1. .Iaddend..Iadd.4. The filter
of claim 1, wherein said first and second switches are jointly
connected so that
said first side of said capacitor is connected to ground while said
second side thereof is connected to said input terminal, and
said first side of said capacitor is connected to said inverting
input of said amplifier while said second side of said capacitor is
connected to
said output terminal of the filter. .Iaddend..Iadd.5. An integrated
circuit filter, comprising:
an amplifier having inverting and noninverting inputs and an
output, said noninverting input being connected to a reference
potential;
a capacitor having first and second terminals;
a first switch operable to connect said first terminal of said
capacitor either to said inverting input of said amplifier or to
said reference potential; and
a second switch operable to connect said second terminal of said
capacitor either to said output of said amplifier or to an input
voltage;
whereby, when said first and second switches are switched
synchronously and periodically said output carries a signal which
is filtered with respect
to said input voltage. .Iaddend..Iadd.6. The filter of claim 5,
further comprising an additional capacitor connected from said
inverting input of said amplifier to said output thereof.
.Iaddend..Iadd.7. The filter of claim 5, configured to have a DC
gain of exactly 1. .Iaddend..Iadd.8. The filter of claim 5, wherein
said first and second switches are each connected to switch at a
common preset frequency. .Iaddend..Iadd.9. The filter of claim 5,
wherein said first and second switches are jointly connected so
that
said first terminal of said capacitor is connected to said
reference potential while said second terminal thereof is connected
to the input voltage, and
said first terminal of said capacitor is connected to said
inverting input of said amplifier while said second terminal of
said capacitor is
connected to said output of said amplifier. .Iaddend..Iadd.10. An
integrated circuit filter, comprising:
an amplifier having inverting and noninverting inputs and an
output, said noninverting input being connected to a reference
potential;
a capacitor having first and second terminals;
a first switch operable to connect said first terminal of said
capacitor either to said inverting input of said amplifier or to
said reference potential;
a second switch operable to connect said second terminal of said
capacitor either to said output of said amplifier or to an input
voltage; and
an additional capacitor connected between said inverting input of
said amplifier and said output of said amplifier;
said first and second switches being jointly connected so that
said first terminal of said capacitor is connected to said
reference potential while said second terminal thereof is connected
to the input voltage, and
said first terminal of said capacitor is connected to said
inverting input of said amplifier while said second terminal of
said capacitor is connected to said output of said amplifier;
whereby, when said first and second switches are switched
synchronously and periodically, said output carries a signal which
is filtered with respect
to said input voltage. .Iaddend..Iadd.11. The filter of claim 10,
configured to have a DC gain of exactly 1. .Iaddend..Iadd.12. The
filter of claim 10, wherein said first and second switches are each
connected to switch at a common preset frequency. .Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to integrated circuits and in
particular to integrated active filters.
2. Description of the Prior Art
Filters for selective frequency filtering of signals are most
common circuits within analog electronic systems. Along with the
development of large scale integration techniques, it has become
ever more important to develop techniques for efficiently
implementing these filters. In many applications a large number of
filters, monolithically integrated together with the ancillary
circuitry for implementing certain system's functions, are required
and it is desirable that the filters be wholly integrated, that
they do not require adjustment and that they occupy as little area
as possible on the silicon chip.
Passive filters don't lend themselves to integration for various
reasons along which the inadequate precision of the R and C values
in integrated form, as well as the fact that the output impedance
is tied to R and C values, i.e. to the filtering function, and
therefore in case a resistive and/or capacitive load is driven,
this would modify the transfer function of the filter itself thus
modifying both the DC gain and the cut-off frequency.
In integrated circuits, active filters employing an operational
amplifier with a suitable feedback network are almost exclusively
employed in order to implement the desired transfer function.
Moreover in order to obviate .[.also to.]. the above recalled
problem of precision of the cut-off frequency value, which remains
tied to the values of the integrated R and C, the resistance R is
commonly .[.substitute.]. .Iadd.substituted .Iaddend.by a
capacitance Cx, switched at a frequency f.sub.s. As it is well
known to the expert technician such a switched capacitor behaves
electrically as a resistance having a value given by: ##EQU1##
According to the known technique it is necessary to use at least
two switched capacitors and four switching for realizing a low-pass
filter of the first order.
OBJECTIVES AND SUMMARY OF THE INVENTION
A main objective of the present invention is to provide an active
low-pass filter of the first order which utilizes a lower number of
components than the number of components utilized by a similar
filter made in accordance with the prior art and which requires a
smaller area of integration.
This and other objectives and advantages of the active low-pass
filter of the first order of the present invention are achieved by
employing a single switched capacitor and only two switching means,
arranged as defined in the annexed claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The different aspects and advantages of the invention will be more
easily understood through the following description with reference
to the annexed drawing wherein:
FIG. 1 is a diagram of a low-pass active filter of the first
order;
FIG. 2 is a circuit diagram of the low-pass active filter of the
first order of FIG. 1, made with switched capacitors in accordance
with the known technique;
FIG. 3 is the wave shape of the switching signal (clock signal);
and
FIG. 4 is a circuit diagram of a low-pass active filter of the
first order made in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
For better pointing out the characteristics of the invention, in
FIGS. 1 and 2 the diagrams of an active low-pass filter of the
first order made according to the prior art are illustrated. In
relation to the diagram of FIG. 1, the cut-off frequency of the
filter (defined on the dynamic characteristic at the point where
the modulus of the transfer function is reduced by -3 dB in respect
to the DC gain, which is substantially equal to unity) is given by:
##EQU2## and it is hardly controllable because of the difficulty of
obtaining very precise absolute values of resistance in integrated
(diffused) resistors, which furthermore have rather poor linearity
and temperature coefficient characteristics besides requiring a
relatively large area of integration on the silicon chip.
Furthermore, the DC gain is affected by the ratio between the two
equal resistances (R), which is also hardly controllable in terms
of precision (.+-.0.5%).
The known solution depicted in FIG. 2 overcomes these problems by
exploiting the switched capacitor technique for functionally
substituting the two equal resistors (R) of the basic circuit
diagram of FIG. 1.
Each of the two switched capacitances Cx, switched at the f.sub.s
frequency, electrically behaves as a resistor having a value given
by: ##EQU3##
The DC gain of these filters is very precise, because it is
determined by the ratio between the two identical switched
capacitances (Cx), which according to modern fabrication techniques
of integrated circuits has a precision of about .+-.0.1%.
The cut-off frequency of the filter, which is given by: ##EQU4## is
also precisely presettable because it is determined by a ratio
between two integrated capacitors (the precision of which, as
already said, may be 0.1%) and by the value of the switching
frequency f.sub.s, which is normally obtained by means of an
external quartz oscillator and is therefore highly precise
(.+-.0.01%).
In practice all active low-pass filters of the first order having a
high precision, implemented in integrated circuits (especially in
CMOS circuits), have the circuit diagram of FIG. 2, that is an
operational amplifier, a continuous integration capacitance Ci, two
switched capacitances Cx and four switches operating at the
frequency f.sub.s, as shown in the circuit diagram of FIG. 2.
The maximum precision, both on the DC gain and on the cut-off
frequency is of about 0.1%.
The wave shape of the switching signal is shown in FIG. 3, the
switching frequency being f.sub.s =1/T.sub.s, where T.sub.s is the
sampling period.
The active low-pass filter of the present invention has a circuit
diagram as shown in FIG. 4 and, in contrast to the low-pass active
filters of the prior art, utilizes a single switched capacitor Cx
and only two switches driven at the frequency f.sub.s.
Essentially the active low-pass filter of the first order of the
invention comprises an operational amplifier having an inverting
input and a noninverting input and a single output coinciding with
the output terminal of the filter and the operational amplifier is
provided with a negative feedback network which comprises a
continuous integration capacitor Ci, which is connected between the
output terminal of the filter and the inverting input terminal of
the operational amplifier, while the noninverting .[.ting.]. input
of the operational amplifier is connected to ground. A single
switched capacitor Cx and the two switches driven at the frequency
f.sub.s are connected so that a first .[.armature.]. .Iadd.side
.Iaddend.of the switched capacitor is switched by a first one of
said two switches between the inverting input of the operational
amplifier and the circuit's ground node. The second .[.armature.].
.Iadd.side .Iaddend.of the switched capacitor is switched by the
other of said two switches between an input terminal of the filter
and the output terminal thereof.
An analysis of the operation of the circuit of the active low-pass
filter of the first order of the invention is herein shown, by
utilizing the time scale indicated in the diagram of FIG. 3.
##EQU5## by introducing the z- transform: ##EQU6##
As it is well known to the expert technician, the frequency
response of the system is obtained by substituting e.sup.j2.pi.fT s
in place of z; where Ts is the sampling period of the circuit and f
is the current frequency.
It may be immediately observed that the DC gain (i.e. f=0 and
therefore z=1) is 1, i.e. 0 db, the cut-off frequency of the
filter, as already indicated before, is given by: ##EQU7##
In respect to the known solutions, the integrated low-pass, active
filter of the first order of the invention offers the advantage of
requiring a reduced number of components thus permitting to save
integration area. Furthermore, the DC gain of the filter has an
infinite precision because it is no longer dependent from the
precision of a ratio between integrated capacitors, as in the
filters of the prior art.
* * * * *