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AN-797
APPLICATION NOTE
Quad Precision Op Amp Evaluation Board
by Giampaolo Marino and Steve Ranta
INTRODUCTION
The EVAL-PRAOPAMP- 4R/4RU is an evaluation board
which accommodates quad op amps in SOIC and TSSOP
packages. It is meant to provide the user with multiple
choices and extensive flexibility for different applica-
tions circuits and configurations.
This board is not intended to be used with high frequency
components or high speed amplifiers. However, it provides
the user with many combinations for various circuit
types including active filters, instrumentation amplifiers,
composite amplifier, and external frequency compensa-
tion circuits. For examples of application circuits refer to
ADI amplifier data sheets under the applications section.
This application note will present different implementa-
tions of filter design using quad amplifiers.
STATE VARIABLE FILTER (INVERTING)
The SV filter uses two integrators and a summing amplifier
to provide second-order low-pass, band-pass, and high-
pass responses. A fourth op amp can be used to combine
the existing responses and synthesize the notch or the
all-pass responses.
This filter circuit can be easily constructed using the
quad EVAL_PRAOPAMP board and following the sche-
matic in Figure 1.
Using the superposition principle, we write
V
HP
=
(
–
R6 / R4
)
V
1
–
(
R4
/
R
JUMP
)
V
LP
+
1
+
(
R4 / R6
)
+
(
R4 / R
JUMP
)
/
V
BP
½
1
+
(
R
JUMP1
/ R5
)
Since
A2
and
A3
are integrators, we have:
V
BP
=
(
–1/
R8 C6s
)
V
HP
and
V
LP
=
(
–1
/ R13 C8
)
V
BP
After extended simplifications we have the following
expressions:
ω
o
=
and
(
1
+
R
JUMP1
/
R5
)
R4 R8 C6 /R
JUMP
R13 C8
Q
=
(
1
+
R4 / R6
+
R4 /R
JUMP
)
H
OHP
=
–
R4
/
R6
,
H
OBP
=
(
1
+
R
JUMP1
/
R5
)
/
(
R4
/
R
JUMP
)
/
(
R8 C6 R13 C8
)
(
)
/
½
(
1
+
R6
/
R
JUMP
+
R6
/
R4
)
,
H
OLP
=
–
R
JUMP
/
R6
From the above expressions we observe that
Q
depends
on the resistor ratio
R
JUMP1
/R5.
We therefore expect
Q
to be much less sensitive to resistance tolerances and
drift. Indeed, with proper component selection and
circuit construction, the SV filter can easily yield depend-
able
Qs
in the range of hundreds. For best results, it is
advisable to use metal film resistors and polystyrene or
polycarbonate capacitors, and properly bypass the op
amp supplies.
R
JUMP
*
R6
–V
IN
A
2
V+
3
V–
V
EE
R4
V
CC
A1
6
1
V
HP
= V
OUT
A
V+
5
V–
V
EE
R8
V
CC
A2
9
7
V
BP
= V
OUT
B
V+
10
V–
V
EE
C6
R13
C8
V
CC
A3
8
V
LP
= V
OUT
C
R5
R
JUMP1
*
*R
JUMP
AND
R
JUMP1
IS AN EXTERNAL COMPONENT, NOT PROVIDED ON THE APPLICATION BOARD.
Figure 1. State Variable Filter (Inverting)
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AN-797
The SV filter is usually implemented with
R4 = R6 = R
JUMP
,
R8 = R13 = R and C6 = C8 = C.
So, the above expressions
simplify to
ω
o
=
1/
RC
Q
=
1/ 3
(
1
+
R
JUMP1
/
R5
)
H
OLP
=
–1
H
OHP
=
–1
H
OBP
=
Q
THE BIQUAD FILTER
This particular filter implementation consists of two
integrators. The third op amp is a unity gain inverting
amplifier whose sole purpose is to provide polarity reversal.
Unlike the SV filter, the biquad yields only two significant
responses; however, since all its op amps are operated in
the inverting mode, the circuit is immune from common-
mode limitations.
From the circuit above we have:
H
OBP
=
–
R4 / R6
ω
o
=
1/
H
OLP
=
R
JUMP
/
R6
Q
=
R4 C4
/
The filter is tuned as follows:
1. Adjust
R6
for desired magnitude of the response
of interest.
2. Adjust
R8
or
R13
to tune
o
.
3. Adjust the ratio
R
JUMP1
/R5
to tune
Q.
STATE VARIABLE FILTER (NONINVERTING)
Another popular type of state variable filter can be
formed by just moving the input signal from the
inverting to the noninverting side of A1, resulting in the
noninverting state variable filter. See Figure 2.
By properly choosing the following components:
R4 = R6; R8 = R13 ; R5 = R
JUMP
; C6 = C8
It can be shown that:
ω
o
=
1/
R8 C6
H
OHP
=
1/
Q
Q = 1 + R
JUMP1
/
R3
H
OBP
=
–
1
H
OLP
=
1/
Q
(
R8 R
JUMP
C6 C4
)
(
R8 R
JUMP
C6
)
The biquad filter is usually implemented with
R8 = R
JUMP
= R
C4 = C6 = C,
after which the above expressions simplify as:
H
OBP
=
–
R4 / R6
ω
o
=
1/
RC
H
OLP
=
R
/
R6
Q
=
R4 / R
This filter is tuned as follows:
1. Adjust
R8
or
R
JUMP
to tune
o
.
2. Adjust
R4
to tune
Q.
3. Adjust
R6
for desired values
H
OBP
or of
H
OLP
.
R6
R4
V
CC
2
V+
3
V–
V
EE
R3
A1
R8
JUMP –V
IN
A TO V
OUT
C
C6
V
CC
6
A2
R13
C8
V
CC
9
A3
1
V
HP
= V
OUT
A
V+
5
V–
7
V
BP
= V
OUT
B
V+
10
V–
8
V
LP
= V
OUT
C
R5
+V
IN
A
R
JUMP
*
V
EE
V
EE
*R
JUMP
IS AN EXTERNAL COMPONENT, NOT PROVIDED ON THE APPLICATION BOARD.
Figure 2. State Variable Filter (Noninverting)
R
JUMP
*
R4
C4
V
CC
R6
–V
IN
A
2
V+
3
V–
V
EE
A1
6
1
V
HP
= V
OUT
A
V+
5
V–
V
EE
R8
V
CC
A2
9
7
V
BP
= V
OUT
B
V+
10
V–
V
EE
C6
R13
R16
V
CC
A3
8
V
LP
= V
OUT
C
*R
JUMP
IS AN EXTERNAL COMPONENT, NOT PROVIDED ON THE APPLICATION BOARD.
Figure 3. Biquad Filter
–2–
REV. 0
Microcontroller MCU
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