OP179/OP279

–12–

REV. F

reactive, and limits overall practicality of this filter. The dire

effect of C1 loading can be tempered somewhat by using a small

series input resistance of about 100

Ω, but can still be an issue.

7

6

5

0.1 F

GIVEN:

ALPHA, F AND H (PASSBAND GAIN)

ALPHA = 1/Q

PICK A STD C1 VALUE, THEN:

C3 = C1, C2 = C1/H

R1 = ALPHA / ((2*PI*F*C1)*(2+(1/H)))

R2 = (H*(2+(1/H))) / (ALPHA*(2*PI*F*C1))

1kHz BW EXAMPLE SHOWN

(NOTE: SEE TEXT ON C1 LOADING

CONSIDERATIONS)

IN

R1

7.5k

OUT

U1B

OP279

R2

33.6k

C3

0.01 F

C2

0.01 F

C1

0.01 F

R = R2

Figure 36. Two-Pole, High Pass Multiple Feedback Filters

In this example, the filter gain is set to unity, the corner fre-

quency is 1 kHz, and the response is a Butterworth type. For

applications where dc output offset is critical, bias current com-

pensation can be used for the amplifier. This is provided by

a noise bypass.

Low Pass Configurations

Figure 37 is a LP MFB 2-pole filter using an OP179/OP279

section. For this filter, the gain in the pass band is user config-

urable over a wide range, and the pass band signal phase is

inverting. Given the design parameters for

α, F, and H, a sim-

plified design process is begun by picking a standard value for

C2. Then C1 and resistors R1-R3 are selected as per the rela-

tionships noted. Optional dc bias current compensation is pro-

equivalent value of R1 and R2.

7

5

6

(R1

R2)+R3

GIVEN:

ALPHA, F AND H (PASSBAND GAIN)

ALPHA = 1/Q

PICK A STD C2 VALUE, THEN:

C1 = C2 • (4 • (H +1))/ALPHA^2

R1 = ALPHA/(4 • H • PI • F • C2)

R2 = H • R1

R3 = ALPHA/(4 • (H + 1) • PI • F • C2)

1kHz BW EXAMPLE SHOWN

(NOTE: SEE TEXT ON C1 LOADING

CONSIDERATIONS)

IN

OUT

U1B

OP279

R1

11.3k

R2

11.3k

R3

5.62k

C2

0.01 F

0.1 F

C1

0.04 F

Figure 37. Two-Pole, Low-Pass Multiple Feedback Filters

Gain of this filter, H, is set here by resistors R2 and R1 (as in a

standard op amp inverter), and can be just as precise as these

resistors allow at low frequencies. Because of this flexible and

accurate gain characteristic, plus a low range of component

value spread, this filter is perhaps the most practical of all the

MFB types. Capacitor ratios are best satisfied by paralleling

two or more common types, as in the example, which is a 1 kHz

unity gain Butterworth filter.

Capacitors should be 1% or 2% film types preferably, such as

polypropylene or polystyrene, or NPO (COG) ceramic for

smaller values. Somewhat lesser performance is available with

the use polyester capacitors.

Parasitic Effects in Sallen-Key Implementations

In designing these circuits, moderately low (10 k

Ω or less) val-

ues for R1-R2 can be used to minimize the effects of Johnson

noise when critical, with of course practical tradeoffs of capaci-

tor size and expense. DC errors will result for larger values of

resistance, unless bias current compensation is used. To add

bias compensation in the HP filter of Figure 35a, a feedback

compensation resistor with a value equal to R2 is used, shown

R1 + R2. Using dc compensation and relatively low filter val-

ues, filter output dc errors using the OP179/OP279 will be

here is that the additional resistors increase noise substantially—

for example, an unbypassed 10 k

Ω resistor generates ≈ 12 nV/

√Hz of noise. However, the resistance can be ac-bypassed to

eliminate noise with a simple shunt capacitor, such as 0.1

µF.

Sallen-Key Implementations in Single Supply Applications

The hookups shown illustrate a classical dual supply op amp

application, which for the OP179/OP279 would use supplies up

to

±5 V. However, these filters can also use the op amp in a

single-supply mode, with little if any alteration to the filter itself.

To operate single-supply, the OP179/OP279 is powered from

+5 V at Pin 8 with Pin 4 grounded. The input dc bias for the

op amp must be supplied from a dc source equal to 1/2 supply,

or 2.5 V in this case.

For the HP section, dc bias is applied to the common end of R2.

R2 is simply returned to an ac ground that is a well-bypassed

2:1 divider across the 5 V source. This can be as simple as a

pair of 100 k

Ω resistors with a 10 µF bypass cap. The output

from the stage is then ac coupled, using an appropriate coupling

cap from U1A to the next stage. For the LP section dc bias is

applied to the input end of R1, in common with the input sig-

nal. This dc can be taken from an unbypassed dual 100 k

Ω

divider across the supply, with the input signal ac coupled to the

divider and R1.

Multiple Feedback Filters

MFB filters, like their SK relatives, can be used as building

blocks as well. They feature LP and HP operation as well, but

can also be used in a bandpass BP mode. They have the prop-

erty of inverting operation in the pass band, since they are based

on an inverting amplifier structure. Another useful asset is their

ability to be easily configured for gain.

High Pass Configurations

Figure 36 shows an HP MFB 2-pole filter using an OP179/

OP279 section. For this filter, the gain in the passband is user

configurable, and the signal phase is inverting. The circuit uses

one more tuning component than the SK types. For simplicity,

capacitors C1 and C3 are set to equal standard values, and

resistors R1-R2 are selected as per the relationships noted. Gain

of this filter, H, is set by capacitors C1 and C2, and this factor

limits both gain selectability and precision. Also, input capaci-

tance C1 makes the load seen by the driving stage highly