OP179 Datasheet, PDF (12/16 Pages) Analog Devices – Rail-to-Rail High Output Current Operational Amplifiers

OP179/OP279
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 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 35a, 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 val-
ues, filter output dc errors using the OP179/OP279 will be
dominated 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
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 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 sig-
nal. 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 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
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.
C1
0.01␮F
IN
C2
0.01␮F
OUT
C3
0.01␮F
R2
33.6k⍀
6
GIVEN:
ALPHA, F AND H (PASSBAND GAIN)
ALPHA = 1/Q
R1
7.5k⍀
7
5
PICK A STD C1 VALUE, THEN:
C3 = C1, C2 = C1/H
U1B
OP279
R1 = ALPHA / ((2*PI*F*C1)*(2+(1/H)))
R2 = (H*(2+(1/H))) / (ALPHA*(2*PI*F*C1))
R = R2
1kHz BW EXAMPLE SHOWN
0.1␮F
Z_b
(NOTE: SEE TEXT ON C1 LOADING
CONSIDERATIONS)
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
network Z_b, where R is equal to R2, and the capacitor provides
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-
vided by Z_b, where R is equal to the value of R3 plus the parallel
equivalent value of R1 and R2.
R1
11.3k⍀
IN
R2
11.3k⍀
OUT
R3
5.62k⍀
C2
0.01␮F
GIVEN:
ALPHA, F AND H (PASSBAND GAIN)
ALPHA = 1/Q
C1
0.04␮F
5
7
PICK A STD C2 VALUE, THEN:
6
C1 = C2 • (4 • (H +1))/ALPHA^2
U1B
R1 = ALPHA/(4 • H • PI • F • C2)
R2 = H • R1
OP279 _{R3 = ALPHA/(4 • (H + 1) • PI • F • C2)}
(R1 R2)+R3
1kHz BW EXAMPLE SHOWN
0.1␮F
Z_b
(NOTE: SEE TEXT ON C1 LOADING
CONSIDERATIONS)
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.
–12–
REV. F