Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
1999-09-17
2001-09-11
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S303000
Reexamination Certificate
active
06288611
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a filter circuit and more particularly to a programmable monolithic high-pass filter.
BACKGROUND OF THE INVENTION
Monolithic differential high-pass filters are used in many mixed signal integrated analog circuits as well as read channels for disk drive circuits in order to eliminate the effect of offset which is generated by internal circuitry. The offsets are often the result of mismatches which are due to various factors such as component mismatches, temperature gradient, etc. The lower cutoff frequency (F
C
) of the high-pass filter is usually within the range of a few kilohertz to a few hundred kilohertz. The low frequency cutoff F
C
is chosen such that it should be sufficiently low in order to prevent any phase distortion on the output signal, and it should be sufficiently high in order to avoid slow transition in startup of the component. In applications where the data rate or the signal frequency shows a large amount of variation in various modes, it is desirable to have a programmable high-pass filter to be able to optimize the operation. Trimming may be necessary since the component tolerances may result in larger deviation than the programming of the high-pass filter. The programmability can only be achieved if the process variation is corrected by trimming and if the trimming accuracy is less than or equal to half of the step sizes in the programming.
FIG. 1
illustrates such a high-pass filter in monolithic form with F
C
as a low-frequency corner frequency. The input to the high-pass filter
100
is voltage V
in
and the output of the high-pass filter
100
is voltage V
outps
. The operational amplifier
102
and the capacitor
104
and resistor
106
, resistor
108
R
1
, and resistor
110
R
2
form an (inverting) Active-RC low-pass filter circuit from the node at the output of summing circuit
116
shown as voltage V
out
to the node at V
L
which is the output of the operational amplifier
102
.
Equation 1 illustrates the Laplace transform.
V
L
(
s
)
V
out
(
s
)
=
-
1
/
RC
s
+
g
0
/
C
(
1
)
where R is the total value of the combination resistances of resistor
106
, resistor
108
and resistor
110
, and depending on input signals B
1
and B
2
where input signal B
1
and input signal B
2
are digital signals as illustrated in Equation 2.
R
=
{
R
0
+
R
1
+
R
2
if
B
1
=
“
LOW
”
and
B
2
=
“
LOW
”
R
0
+
R
1
if
B
2
=
“
HIGH
”
R
0
if
B
1
=
“
HIGH
”
(
2
)
Thus, the input signals B
1
and B
2
control the total resistance by effectively shorting resistor
108
and resistor
102
through switches
112
and
114
.
The parameter g
0
in Equation 1 is the parasitic conductance across capacitor
104
.
The summation circuit
116
sums voltage V
in
and voltage V
L
as illustrated in Equation 3.
V
out
(
S
)=
V
in
(
S
)+
V
L
(
S
). (3)
Substituting Equation 1 in Equation 3 yields Equation 4.
V
out
(
s
)
V
i
n
(
s
)
=
s
+
g
0
/
C
s
+
(
g
0
+
1
/
R
)
/
C
(
4
)
Equation 4 is the first order high-pass filter Laplace domain transfer function, for which the cutoff frequency is illustrated in Equation 5.
F
c
=
1
2
π
g
0
+
1
/
R
C
(
5
)
The zero frequency is derived in Equation 6.
F
z
=
1
2
π
g
0
C
(
6
)
The cutoff frequency and the zero frequency are illustrated as high-pass filter magnitude characteristics as shown in FIG.
2
.
The circuit
100
uses three settings of R which correspond to fast, slow and medium speeds in order to adjust the corner frequency F
C
. The circuit of
FIG. 1
requires an input signal for each switch to control the resistance. Thus, there is no trimming due to process variations.
SUMMARY OF THE INVENTION
The present invention provides a filter which is both programmable and trimmable. The programmable and trimmable features are summed and input to a programmable resistor through a single line. Thus, the programmable feature and trimmable feature are combined.
REFERENCES:
patent: 5208548 (1993-05-01), Van Riezen
patent: 5410729 (1995-04-01), Kumagai et al.
Brady W. James
Mottola Steven J.
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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