AN-763: Dual Universal Precision Op Amp Evaluation Board (Rev. C) PDF

AN-763
APPLICATION NOTE
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Dual Universal Precision Op Amp Evaluation Board
INTRODUCTION
The EVAL-PRAOPAMP-2RZ, EVAL-PRAOPAMP-2RMZ, and
EVAL-PRAOPAMP-2CPZ are universal precision evaluation
boards that accommodate dual op amps in 8-pin SOIC,
MSOP, and LFCSP packages, respectively. For the exposed pad
connection for the LFCSP package, see the appropriate product
data sheet.
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 ADA4077-2, is required to minimize
distortion. Furthermore, the low offset voltage of the ADA4077-2
allows a wider dynamic range when the circuit gain is chosen to
be high.
These PRAOPAMP evaluation boards provide multiple choices
and extensive flexibility for different application circuits and
configurations.
The circuit shown in Figure 1 consists of two stages. The first
stage is a simple high-pass filter with a corner frequency, fC, of
These boards are not intended to be used with high frequency
components or high speed amplifiers. However, they provide
the user with many combinations for various circuit types,
including active filters, instrumentation amplifiers, composite
amplifiers, and external frequency compensation circuits.
Several examples of application circuits are provided in this
application note.
Q=K
C2
10nF
C1
10nF
+
V1
–
R1
20kΩ
6
4
7
R3
33kΩ
R4
33kΩ
1
2πC R1R2
V+
VOUT
C4
330pF
V+
05284-001
8 1/2 ADA4077-2
1
3
(3)
The value of Q determines the peaking of the gain vs. frequency
(generally ringing in the time domain). Commonly chosen
values for Q are near unity.
4
8 1/2 ADA4077-2
5
(2)
Choosing equal capacitor values minimizes the sensitivity and
simplifies the expression for fC to
V–
2
R1
R2
where K is the dc gain.
C3
680pF
V–
(1)
and
TWO STAGE BAND-PASS FILTER
R2
10kΩ
1
2π C1C2R1R2
Figure 1. KRC Filter
The low offset voltage and high CMRR makes the ADA4077-2
a great choice for precision filters, such as the KRC filter shown
in Figure 1.
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 and R1/R2 = 2 in the circuit example,
and pick R1 = 5 kΩ and R2 = 10 kΩ for simplicity. The second
stage is a low-pass filter whose corner frequency can be determined in a similar fashion.
R3 = R4 = R
This particular filter implementation offers the flexibility to
tune the gain and the cut-off frequency independently.
fC =
1
2π × R C3C4
and
Q = 1/ 2
Rev. C | Page 1 of 8
C3
C4
AN-763
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
High Gain Composite Amplifier .....................................................3
Two Stage Band-Pass Filter ............................................................. 1
External Compensation Techniques ...............................................4
Revision History ............................................................................... 2
Snubber Network ...............................................................................5
Half Wave, Full Wave Rectifier ....................................................... 3
REVISION HISTORY
10/13—Rev. B to Rev. C
Updated Format .................................................................. Universal
Replaced All Figures ......................................................................... 1
Changed EVAL-PRAOPAMP-2R/2RU/2RM to EVALPRAOPAMP-2RZ, EVAL-PRAOPAMP-2RMZ, and EVALPRAOPAMP-2CPZ Throughout .................................................... 1
Deleted Authors Names and added Introduction Section
Heading .............................................................................................. 1
Changes to Two Stage Band-Pass Filter Section ........................... 1
Changes to Half Wave, Full Wave Rectifier Section..................... 3
Changes to High Gain Composite Amplifier Sections ................ 3
Rev. C | Page 2 of 8
Application Note
AN-763
HALF WAVE, FULL WAVE RECTIFIER
VOLTAGE (1V/DIV)
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.
05284-004
When the input voltage VIN is negative, the output is zero.
When the magnitude of VIN is doubled at the input of the op
amp, this voltage could exceed the power supply voltage which
would damage the amplifiers permanently. The op amp must
come out of saturation when VIN is negative. This delays the
output signal because the amplifier needs time to enter its
linear region.
TIME (1mS/DIV)
Figure 4. Full Wave Rectifier Signal (Output B)
The ADA4610-2 has a very fast overdrive recovery time, which
makes it a great choice for rectification of transient signals. The
symmetry of the positive and negative recovery time is also very
important in keeping the output signal undistorted.
R3
10kΩ
5V
VIN
3V p-p
3
R1
1kΩ
+
2
–
6
4
2/2
ADA4610-2
5
8
8
1/2
ADA4610-2
1
7
OUT B
(FULL WAVE)
HIGH GAIN COMPOSITE AMPLIFIER
4
R2
99kΩ
R1
1kΩ
05284-002
5V
OUT A
(HALF WAVE)
VEE
VCC
Figure 2. Half Wave and Full Wave Rectifier
V–
V+
VIN
1/2
ADA4661-2
VEE
VCC
R3
1kΩ
VOLTAGE (1V/DIV)
U5 1/2
V+ ADA4661-2
V–
R4
99kΩ
05284-005
R2
10kΩ
Figure 2 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 3 and Figure 4 represents the signal
response of the circuit at Output A and Output B, respectively.
Figure 5. High Gain Composite Amplifier
05284-003
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.
TIME (1mS/DIV)
Figure 3. Half Wave Rectifier Signal (Output A)
Placing a small capacitor, CF, in the feedback loop and in
parallel with R2 improves the phase margin. For the circuit
of Figure 5, picking a CF = 50 pF yields a phase margin of
about 45°.
Rev. C | Page 3 of 8
AN-763
Application Note
R2
100kΩ
R2
VOUT
V–
V+
CL
R3
1kΩ
VCC
V+
V–
VCC
C2
RL
VIN
05284-007
VIN
VEE
R4
100Ω
Figure 7. Series Resistor Compensation
1/2
AD8657
VEE
C3
05284-006
R1
1kΩ
1/2
AD8657
RL = 10kΩ
CL = 2nF
GND
05284-008
A composite amplifier can be used to optimize the dc and ac
characteristics. Figure 6 shows an example using the AD8657,
which offers many circuit advantages. The bandwidth is
increased substantially and the input offset voltage and noise
of the AD8657 becomes insignificant because they are divided
by the high gain of the amplifier. The circuit offers a high
bandwidth, a high output current, and a very low power
consumption of less than 100 μA.
VOLTAGE (200mV/DIV)
Figure 6. Low Power Composite Amplifier
EXTERNAL COMPENSATION TECHNIQUES
TIME (10µs/DIV)
Series Resistor Compensation
Figure 8. Capacitor Load Drive Without Resistor
RL = 10kΩ
RS = 200Ω
CL = 2nF
CS = 0.47µF
GND
05284-009
VOLTAGE (200mV/DIV)
The use of external compensation networks may be required
to optimize certain applications. Figure 7 shows 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 Ω to 50 Ω 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.
TIME (10µs/DIV)
Figure 9. Capacitor Load Drive with Resistor
Rev. C | Page 4 of 8
Application Note
AN-763
SNUBBER NETWORK
Another way to stabilize an op amp driving a capacitive load
is through the use of a snubber as shown in Figure 10.
05284-011
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 RS and CS combination can be determined
experimentally.
VOLTAGE (200mV/DIV)
RL = 10kΩ
CL = 500pF
VOUT
CL
RL
VIN
CS
TIME (1µs/DIV)
Figure 11. Capacitor Load Drive Without Snubber
05284-010
RS
RL = 10kΩ
CL = 500pF
RS = 100Ω
CS = 1nF
05284-012
VOLTAGE (200mV/DIV)
Figure 10. Snubber Network
TIME (1µs/DIV)
Figure 12. Capacitor Load Drive with Snubber
Rev. C | Page 5 of 8
AN-763
Application Note
C4
R4
VEE
1
AMPLIFIER A
R2
–V1 1
–INA
G1 1
R6
2
Rt1
C2
4
DUTA
1
3
R8
DUT
8
RS1
C1
G1
+V1 1
RL1
CL1
1
CS1
R5
R3
G2
G5
G3
R1
C3
Rt2
VO1
VOUTA
1
VCC
05284-013
+INA
G2 1
R7
1
C5
Figure 13. Dual Universal Precision Op Amp Evaluation Board Electrical Schematic
C7
R10
AMPLIFIER B
R12
6
Rt3
DUTB
7
R14
1
5
RS2
DUT
RL2
CL2
G3
+INB
G4 1
G4
R13
Rt4
CS2
R11
1
G6
G6
C6
R9
C8
Figure 14. Dual Universal Precision Op Amp Evaluation Board
05284-015
+V2 1
VO2
VOUTB
05284-014
–V2 1
–INB
G3 1
Figure 15. Dual SOIC Layout Patterns
Rev. C | Page 6 of 8
Application Note
AN-763
NOTES
Rev. C | Page 7 of 8
AN-763
Application Note
NOTES
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registered trademarks are the property of their respective owners.
AN05284-0-10/13(C)
Rev. C | Page 8 of 8
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