AN-763
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com
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
MSOPpackages. Itprovidestheuserwithmultiplechoices
and extensive flexibility for different application circuits
and configurations.
Choosing equal capacitor values minimizes the sensitivity
and also simplifies the expression for fC 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.
This board is not intended to be used with high frequency
componentsorhighspeedamplifiers.However,itprovides
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.
SettingQ=1/÷2yieldsminimumgainpeakingandminimum
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:
TWO STAGE BAND-PASS FILTER
C3
680pF
R3 = R4 = R
R2
10k
V–
V–
2
4
R3
R4
6
5
1
4
8
V
C2
C1
33k
33k
OUT
3
7
10nF 10nF
C3
C4
8
1/2 OP2177
and
Q = 1/ 2
1/2 OP2177
C4
330pF
+
V1
–
V+
R1
20k
V+
HALF-WAVE, FULL-WAVE RECTIFIER
Figure 1. KRC Filter
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
somepotentialproblemsforamplifiersusedinthismanner.
When the input voltageVIN 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 volt-
age which would damage the amplifiers permanently.The
op amp must come out of saturation whenVIN is negative.
This delays the output signal because the amplifier needs
timetoenteritslinearregion.TheAD8510/AD8512/AD8513
haveveryfastoverdriverecoverytime, whichmakesthem
agreatchoiceforrectificationoftransientsignals.Thesym-
metry of the positive and negative recovery time is also
very important in keeping the output signal undistorted.
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
offsetvoltageoftheOP2177allowsawiderdynamicrange
when the circuit gain is chosen to be high.
ThecircuitinFigure1consistsoftwostages.Thefirststage
is a simple high-pass filter whose corner frequency fC is
1
2π C1C2R1R2
(1)
(2)
and whose
R1
R2
Q = K
K = is the dc gain.
REV. B
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