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AN-763
APPLICATION NOTE
Dual Universal Precision Op Amp Evaluation Board
by Giampaolo Marino and Steve Ranta
The EVAL-PRAOPAMP-2R/2RU/2RM is an evaluation board
that accommodates dual op amps in SOIC, TSSOP and
,
MSOP packages. It provides the user with multiple choices
and extensive flexibility for different application 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, com-
posite amplifiers, and external frequency compensation
circuits. Several examples of application circuits are given
in this application note.
TWO STAGE BAND-PASS FILTER
R2
10k
Choosing equal capacitor values minimizes the sensitivity
and also simplifies the expression for f
C
to
(3)
The value of Q determines the peaking of the gain versus
frequency (generally ringing in time domain). Commonly
chosen values for Q are near unity.
Setting Q = 1/÷2 yields minimum gain peaking and minimum
ringing. Use Equation 3 to determine the values for R1 and
R2. For example, set Q = 1/÷2, R1/R2 = 2 in the circuit ex-
ample, and pick R1 = 5 k and R2 = 10 k for simplicity.The
second stage is a low-pass filter whose corner frequency
can be determined in a similar fashion:
C3
680pF
R3
=
R4
=
R
V–
V–
C2
10nF
+
V1
–
C1
10nF
6
5
4
7
R3
33k
R4
33k
C4
330pF
2
3
4
1
V
OUT
8
1/2 OP2177
V+
8
1/2 OP2177
V+
and
Q
=
1/ 2
R1
20k
C3
C4
Figure 1. KRC Filter
The low offset voltage and high CMRR makes the OP2177
a great choice for precision filters such as the KRC filter
shown in Figure 1. This particular filter implementation
offers the flexibility to tune the gain and the cut-off fre-
quency independently. Since the common-mode voltage
into the amplifier varies with the input signal in the KRC
filter circuit, a high CMRR amplifier such as the OP2177
is required to minimize distortion. Furthermore, the low
offset voltage of the OP2177 allows a wider dynamic range
when the circuit gain is chosen to be high.
The circuit in Figure 1 consists of two stages.The first stage
is a simple high-pass filter whose corner frequency f
C
is
1
2π
C1C2R1R2
and whose
(1)
HALF-WAVE, FULL-WAVE RECTIFIER
Rectifying circuits are used in a multitude of applications.
One of the most popular uses is in the design of regulated
power supplies where a rectifier circuit is used to convert
an input sinusoid to a unipolar output voltage. There are
some potential problems for amplifiers used in this manner.
When the input voltage V
IN
is negative, the output is zero.
When the magnitude of V
IN
is doubled at the input of the
op amp, this voltage could exceed the power supply volt-
age which would damage the amplifiers permanently.The
op amp must come out of saturation when V
IN
is negative.
This delays the output signal because the amplifier needs
time to enter its linear region.The AD8510/AD8512/AD8513
have very fast overdrive recovery time, which makes them
a great choice for rectification of transient signals.The sym-
metry of the positive and negative recovery time is also
very important in keeping the output signal undistorted.
Q
=
K
R1
R2
(2)
K
= is the dc gain.
REV. B
AN-763
R2
10k
5V
V
IN
3V p-p
+
–
R1
1k
2
6
3
R3
10k
HIGH GAIN COMPOSITE AMPLIFIER
R1
1k
R2
99k
V
EE
7
1/2
AD8512
8
1
5
2/2
4
AD8512
8
V
CC
OUT B
(HALF WAVE)
V–
V+
V
IN
AD8603
4
U5
V+
AD8541
V–
V
EE
5V
OUT A
(HALF WAVE)
V
CC
R3
1k
R4
99k
Figure 2a. Half-Wave and Full-Wave Rectifier
Figure 3. High Gain Composite Amplifier
A composite amplifier can provide a very high gain in
applications where high closed-loop dc gain is needed.
The high gain achieved by the composite amplifier comes
at the expense of a loss in phase margin.
Placing a small capacitor, C
F
, in the feedback loop and
in parallel with R2 improves the phase margin. For the
circuit of Figure 3, picking a C
F
= 50 pF will yield a phase
margin of about 45.
R2
100k
V
EE
R3
1k
V
CC
V+
V–
V
CC
C2
V
EE
R4
100
VOLTAGE (1V/DIV)
TIME (1ms/DIV)
R1
1k
V
IN
AD8603
V–
V+
Figure 2b. Half-Wave Rectifier Signal (Output A)
AD8541
C3
VOLTAGE (1V/DIV)
Figure 4. Low Power Composite Amplifier
A composite amplifier can be used to optimize the dc
and ac characteristic. Figure 4 shows an example using
the AD8603 and the AD8541 that offers too many circuit
advantages.The bandwidth is increased substantially and
the input offset voltage and noise of the AD8541 becomes
insignificant since they are divided by the high gain of
the AD8603. The circuit offers a high bandwidth, a high
output current, and a very low power consumption of
less than 100
A.
TIME (1ms/DIV)
Figure 2c. Full-Wave Rectifier Signal (Output B)
Figure 2a is a typical representation of a rectifier circuit.
The first stage of the circuit is a half-wave rectifier. When
the sine wave applied at the input is positive, the output
follows the input response. During the negative cycle of
the input, the output tries to swing negative to follow the
input, but the power supplies restrains it to zero. Similarly,
the second stage is a follower during the positive cycle of
the sine wave and an inverter during the negative cycle.
Figure 2b and Figure 2c represents the signal response of
the circuit at Output A and Output B, respectively.
–2–
REV. B
AN-763
EXTERNAL COMPENSATION TECHNIQUES
Series Resistor Compensation
The use of external compensation networks may be
required to optimize certain applications. Figure 5a is a
typical representation of a series resistor compensation to
stabilize an op amp driving capacitive loads.The stabilizing
effect of the series resistor can be thought of as a means
to isolate the op amp output and the feedback network
from the capacitive load. The required amount of series
resistance depends on the part used, but values of 5
to
50
are usually sufficient to prevent local resonance. The
disadvantage of this technique is a reduction in gain accuracy
and extra distortion when driving nonlinear loads.
R2
C
L
V
IN
R
L
= 10k
C
L
= 500pF
R
L
Snubber Network
Another way to stabilize an op amp driving a capacitive
load is the use of a snubber, as shown in Figure 6a. This
method has the significant advantage of not reducing the
output swing because there is no isolation resistor in the
signal path. Also, the use of the snubber does not degrade
the gain accuracy or cause extra distortion when driving
a nonlinear load. The exact R
S
and C
S
combination can be
determined experimentally.
V
OUT
V
IN
R
S
C
S
C
L
R
L
V
OUT
Figure 6a. Snubber Network
Figure 5a. Series Resistor Compensation
R
L
= 10k
C
L
= 2nF
VOLTAGE (200mV/DIV)
VOLTAGE (200mV/DIV)
GND
TIME (1s/DIV)
Figure 6b. Cap Load Drive Without Snubber
TIME (10s/DIV)
R
L
= 10k
C
L
= 500pF
R
S
= 100
C
S
= 1nF
VOLTAGE (200mV/DIV)
Figure 5b. Cap Load Drive Without Resistor
R
L
= 10k
R
S
= 200
C
L
= 2nF
C
S
= 0.47F
VOLTAGE (200mV/DIV)
GND
TIME (1s/DIV)
Figure 6c. Cap Load Drive with Snubber
TIME (10s/DIV)
Figure 5c. Cap Load Drive with Resistor
REV. B
–3–
AN-763
C4
C4
R4
R4
V
EE
V
EE
1
1
R2
R2
C2
C2
AMPLIFIER A
AMPLIFIER
A
–V1
–V1
–INA
–INA
G1
G1
G1
G1
+V1
+V1
+INA
+INA
G2
G2
G2
G2
1
1
1
1
Rt1
Rt1
R6
R6
2
2
3
3
4
4
DUTA
DUTA
1
1
DUT
DUT
C1
C1
R8
R8
R
S
1
R
S
1
C
S
1
C
S
1
R
L
1
R
L
1
C
L
1
C
L
1
VO1
1
VO1
1
V
OUT
A
V
OUT
A
G5
1
G5
1
G3
G3
8
8
R7
R7
Rt2
Rt2
R5
R5
C3
C3
R3
R3
R1
R1
1
1
V
CC
V
CC
1
1
1
1
C5
C5
Figure 7. Dual Universal Precision Op Amp Evaluation Board Electrical Schematic
C7
C7
R10
R10
AMPLIFIER
AMPLIFIER B
–V2 1
–V2
1
–INB
–INB
G3 1
G3
1
G3
G3
+V2 1
+V2
1
+INB
+INB
G4 1
G4
1
G4
G4
Rt3
Rt3
B
R12
R12
6
6
5
5
DUTB
DUTB
7
7
DUT
DUT
R14
R14
R
R
2
2
S
S
C
C
2
2
S
S
R
R
2
2
L
L
C
C
2
2
L
L
VO2
VO2
1
1
V
V
OUT
B
OUT
B
R13
R13
Rt4
Rt4
R11
R11
C6
C6
G6
G6
1
1
G6
G6
R9
R9
C8
C8
Figure 8. Dual Universal Precision Op Amp Evaluation Board
Figure 9. Layout Patterns
©
2012
Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners.
AN05284-0-9/12(B)
REV.
–4–
B
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