OP1177/OP2177/OP4177
REALIZATION OF ACTIVE FILTERS
BAND-PASS KRC OR SALLEN-KEY FILTER
CHANNEL SEPARATION
The low offset voltage and the high CMRR of the OPx177 make
it an excellent choice for precision filters, such as the band-pass
KRC filter shown in Figure 67. This filter type offers the capability
to tune the gain and the cutoff frequency independently.
Multiple amplifiers on a single die are often required to reject
any signals originating from the inputs or outputs of adjacent
channels. OP2177 input and bias circuitry is designed to prevent
feedthrough of signals from one amplifier channel to the other.
As a result, the OP2177 has an impressive channel separation of
greater than −120 dB for frequencies up to 100 kHz and greater
than −115 dB for signals up to 1 MHz.
Because the common-mode voltage into the amplifier varies with
the input signal in the KRC filter circuit, a high CMRR is required
to minimize distortion. Also, the low offset voltage of the OPx177
allows a wider dynamic range when the circuit gain is chosen to
be high.
C3
680pF
R2
10kΩ
The circuit of Figure 67 consists of two stages. The first stage is
a simple high-pass filter where the corner frequency (fC) is
V+
2
3
1
8
1/2
OP2177
1
6
5
(2)
V
R3
33kΩ
R4
33kΩ
OUT
2π C1C2R1R2
1/2
OP2177
7
C2
10nF
C1
10nF
C4
330pF
and
4
+
V1
–
R1
20kΩ
V–
R1
(3)
Q = K
R2
Figure 67. Two-Stage, Band-Pass KRC Filter
where K is the dc gain.
Choosing equal capacitor values minimizes the sensitivity and
simplifies Equation 2 to
10kΩ
V+
1
8
100Ω
6
5
2
3
1/2
OP2177
2πC R1R2
1/2
OP2177
7
1
4
The value of Q determines the peaking of the gain vs. frequency
(ringing in transient response). Commonly chosen values for Q
are generally near unity.
+
V1
50mV
V–
–
Figure 68. Channel Separation Test Circuit
1
Setting Q =
yields minimum gain peaking and minimum
REFERENCES ON NOISE DYNAMICS
AND FLICKER NOISE
2
ringing. Determine values for R1 and R2 by using Equation 3.
S. Franco, Design with Operational Amplifiers and Analog
Integrated Circuits. McGraw-Hill, 1998.
1
For
R1/R2 = 2 in the circuit example. Select R1 = 5 kꢁ
,
Q =
2
Analog Devices, Inc., The Best of Analog Dialogue, 1967 to
1991. Analog Devices, Inc., 1991.
and R2 = 10 kꢁ for simplicity.
The second stage is a low-pass filter where the corner frequency
can be determined in a similar fashion. For R3 = R4 = R
1
1
2
C3
C4
fC
=
and Q =
C3
C4
2πR
Rev. G | Page 21 of 24
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