OP179/OP279
–12–
REV. G
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
0.1F
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
0.01F
C2
0.01F
C1
0.01F
Z
b
R = R2
Figure 15. 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
network Z
b
, where R is equal to R2, and the capacitor provides
a noise bypass.
Low Pass Configurations
Figure 16 is a LP MFB 2-pole filter using an OP179/OP279
section. For this filter, the gain in the pass band is user con-
figurable over a wide range, and the pass band signal phase is
inverting. Given the design parameters for α, F, and H, a simplified
design process is begun by picking a standard value for C2. Then
C1
and resistors R1-R3
are selected as per the relationships
noted. Optional dc bias current compensation is provided by Z
b
,
where R is equal to the value of R3 plus the parallel 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
0.01F
0.1F
Z
b
C1
0.04F
Figure 16. 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 of polyester capacitors.
Parasitic Effects in Sallen-Key Implementations
In designing these circuits, moderately low (10 kΩ 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 14a, a feedback
compensation resistor with a value equal to R2 is used, shown
optionally as Z
f
. This will minimize bias induced offset, reduc-
ing it to the product of the OP179/OP279’s I
OS
and R2. Similar
compensation is applied to the LP filter, using a Z
f
resistance of
R1 + R2. Using dc compensation and relatively low filter values,
filter output dc errors using the OP179/OP279 will be domi-
nated by V
OS
, which is limited to 4 mV or less. A caveat here is
that the additional resistors increase noise substantially—for
example, an unbypassed 10 kΩ resistor generates ≈ 12 nV/√Hz
of noise. However, the resistance can be ac-bypassed to elimi-
nate 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 one-half 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 kΩ 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 signal. This
dc can be taken from an unbypassed dual 100 kΩ 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 band-pass BP mode. They have the property
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 15 shows an HP MFB 2-pole filter using an OP179/
OP279 section. For this filter, the gain in the pass band 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 resis-
tors 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 reactive,
and limits overall practicality of this filter. The dire effect of C1
