OP1177/OP2177/OP4177

Rev. E | Page 21 of 24

REALIZATION OF ACTIVE FILTERS

BAND-PASS KRC OR SALLEN-KEY FILTER

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.

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.

The circuit of Figure 67 consists of two stages. The first stage is

C1C2R1R2

π

2

1

(2)

and

R2

R1

K

Q

=

(3)

where K is the dc gain.

Choosing equal capacitor values minimizes the sensitivity and

simplifies Equation 2 to

R1R2

C

π

2

1

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.

Setting

2

1

Q

=

yields minimum gain peaking and minimum

ringing. Determine values for R1 and R2 by using Equation 3.

For

,

2

1

Q

=

R1/R2 = 2 in the circuit example. Select R1 = 5 kΩ

and R2 = 10 kΩ 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

C4

C3

Q

C4

C3

πR

f

C

2

1

and

2

1

=

=

CHANNEL SEPARATION

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.

C3

680pF

1/2

OP2177

7

8

6

5

4

V+

V–

1/2

OP2177

1

2

3

R2

10kΩ

V1

+

–

R1

20kΩ

C2

10nF

C1

10nF

R3

33kΩ

R4

33kΩ

C4

330pF

Figure 67. Two-Stage, Band-Pass KRC Filter

1

2

3

1/2

OP2177

100Ω

10kΩ

1/2

OP2177

7

8

6

5

4

V+

V–

+

–

V1

50mV

Figure 68. Channel Separation Test Circuit

REFERENCES ON NOISE DYNAMICS

AND FLICKER NOISE

S. Franco, Design with Operational Amplifiers and Analog

Integrated Circuits. McGraw-Hill, 1998.

Analog Devices, Inc., The Best of Analog Dialogue, 1967 to

1991. Analog Devices, Inc., 1991.