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Электронный компонент: UAF42AU

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1
FIGURE 1. Two-Pole Low-Pass Filter Using UAF42.
NOTE: A UAF42 and two external resistors make a unity-gain, two-pole, 1.25dB ripple
Chebyshev low-pass filter. With the resistor values shown, cutoff frequency is 10kHz.
Although active filters are vital in modern electronics, their
design and verification can be tedious and time consuming.
To aid in the design of active filters, Burr-Brown provides a
series of FilterProTM computer-aided design programs. Us-
ing the FILTER42 program and the UAF42 it is easy to
design and implement all kinds of active filters. The UAF42
is a monolithic IC which contains the op amps, matched
resistors, and precision capacitors needed for a state-variable
filter pole-pair. A fourth, uncommitted precision op amp is
also included on the die.
Filters implemented with the UAF42 are time-continuous,
free from the switching noise and aliasing problems of
switched-capacitor filters. Other advantages of the state-
variable topology include low sensitivity of filter parameters
to external component values and simultaneous low-pass,
high-pass, and band-pass outputs. Simple two-pole filters
can be made with a UAF42 and two external resistors--see
Figure 1.
The DOS-compatible program guides you through the de-
sign process and automatically calculates component values.
Low-pass, high-pass, band-pass, and band-reject (or notch)
filters can be designed.
Active filters are designed to approximate an ideal filter
response. For example, an ideal low-pass filter completely
eliminates signals above the cutoff frequency (in the stop-
band), and perfectly passes signals below it (in the pass-
band). In real filters, various trade-offs are made in an
attempt to approximate the ideal. Some filter types are
optimized for gain flatness in the pass-band, some trade-off
gain variation or ripple in the pass-band for a steeper rate of
attenuation between the pass-band and stop-band (in the
transition-band), still others trade-off both flatness and rate
of roll-off in favor of pulse-response fidelity. FILTER42
supports the three most commonly used all-pole filter types:
Butterworth, Chebyshev, and Bessel. The less familiar In-
verse Chebyshev is also supported. If a two-pole band-pass
or notch filter is selected, the program defaults to a resonant-
circuit response.
Butterworth (maximally flat magnitude). This filter has the
flattest possible pass-band magnitude response. Attenuation
is 3dB at the design cutoff frequency. Attenuation beyond
the cutoff frequency is a moderately steep 20dB/decade/
pole. The pulse response of the Butterworth filter has mod-
erate overshoot and ringing.
Chebyshev (equal ripple magnitude). (Other transliterations
of the Russian
Heby]ov
are Tschebychev, Tschebyscheff
or Tchevysheff). This filter response has steeper initial rate
of attenuation beyond the cutoff frequency than Butterworth.
A
1
R
2
50k
A
2
A
3
R
4
50k
UAF42
11
R
1
50k
R
F1
15.8k
R
F2
15.8k
C
1
1000pF
C
2
1000pF
13
8
7
14
2
V
IN
R
3
50k
V
O
1
FILTER DESIGN PROGRAM FOR
THE UAF42 UNIVERSAL ACTIVE FILTER
By Johnnie Molina and R. Mark Stitt (602) 746-7592
APPLICATION BULLETIN
Mailing Address: PO Box 11400 Tucson, AZ 85734 Street Address: 6730 S. Tucson Blvd. Tucson, AZ 85706
Tel: (602) 746-1111 Twx: 910-952-111 Telex: 066-6491 FAX (602) 889-1510 Immediate Product Info: (800) 548-6132
1991 Burr-Brown Corporation
AB-035C
Printed in U.S.A. July, 1993
2
This advantage comes at the penalty of amplitude variation
(ripple) in the pass-band. Unlike Butterworth and Bessel
responses, which have 3dB attenuation at the cutoff fre-
quency, Chebyshev cutoff frequency is defined as the fre-
quency at which the response falls below the ripple band.
For even-order filters, all ripple is above the dc-normalized
passband gain response, so cutoff is at 0dB (see Figure 2A).
For odd-order filters, all ripple is below the dc-normalized
passband gain response, so cutoff is at (ripple) dB (see
Figure 2B). For a given number of poles, a steeper cutoff can
be achieved by allowing more pass-band ripple. The
Chebyshev has more ringing in its pulse response than the
Butterworth--especially for high-ripple designs.
Inverse Chebyshev (equal minima of attenuation in the stop
band). As its name implies, this filter type is cousin to the
Chebyshev. The difference is that the ripple of the Inverse
Chebyshev filter is confined to the stop-band. This filter type
has a steep rate of roll-off and a flat magnitude response in
the pass-band. Cutoff of the Inverse Chebyshev is defined as
the frequency where the response first enters the specified
stop-band--see Figure 3. Step response of the Inverse
Chebyshev is similar to the Butterworth.
Bessel (maximally flat time delay), also called Thomson.
Due to its linear phase response, this filter has excellent
pulse response (minimal overshoot and ringing). For a given
number of poles, its magnitude response is not as flat, nor is
its initial rate of attenuation beyond the 3dB cutoff fre-
quency as steep as the Butterworth. It takes a higher-order
Bessel filter to give a magnitude response similar to a given
Butterworth filter, but the pulse response fidelity of the
Bessel filter may make the added complexity worthwhile.
Tuned Circuit (resonant or tuned-circuit response). If a
two-pole band-pass or band-reject (notch) filter is selected,
the program defaults to a tuned circuit response. When band-
pass response is selected, the filter design approximates the
response of a series-connected LC circuit as shown in Figure
4A. When a two-pole band-reject (notch) response is se-
lected, filter design approximates the response of a parallel-
connected LC circuit as shown in Figure 4B.
CIRCUIT IMPLEMENTATION
In general, filters designed by this program are implemented
with cascaded filter subcircuits. Subcircuits either have a
two-pole (complex pole-pair) response or a single real-pole
response. The program automatically selects the subcircuits
required based on function and performance. A program
option allows you to override the automatic topology selec-
tion routine to specify either an inverting or noninverting
pole-pair configuration.
FIGURE 3. Response vs Frequency for 5-pole, 60dB
Stop-Band, Inverse Chebyshev Low-Pass Filter
Showing Cutoff at 60dB.
FILTER RESPONSE vs FREQUENCY
Normalized Frequency
f /100
C
f /10
C
f
C
10f
C
+10
0
10
20
30
40
50
Filter Response (dB)
5-Pole Chebyshev
3dB Ripple
Ripple
FILTER RESPONSE vs FREQUENCY
Normalized Frequency
f /100
C
f /10
C
f
C
10f
C
+10
0
10
20
30
40
50
Filter Response (dB)
4-Pole Chebyshev
3dB Ripple
Ripple
FIGURE 2A. Response vs Frequency for Even-Order (4-
pole) 3dB Ripple Chebyshev Low-Pass Filter
Showing Cutoff at 0dB.
FIGURE 2B. Response vs Frequency for Odd-Order (5-
pole) 3dB Ripple Chebyshev Low-Pass Filter
Showing Cutoff at 3dB.
FILTER RESPONSE vs FREQUENCY
f
C
/10
Normalized Frequency
f
C
10f
C
100f
C
Normalized Gain (dB)
20
0
20
40
60
80
100
A
MIN
f
STOPBAND
3
FIGURE 6. Multiple-Stage Filter Made with Two or More
Subcircuits.
NOTES:
(1) Subcircuit will be a real-pole high-pass (HP), real-pole low-pass
(LP), or complex pole-pair (PP1 through PP6) subcircuit specified
on the
UAF42 Filter Component Values
and
Filter Block Diagram
program outputs.
(2) If the subcircuit is a pole-pair section, HP Out, BP Out, LP Out, or
Aux Out will be specified on the UAF42
Filter Block Diagram
program output.
V
IN
V
O
Subcircuit N
In
Out
(2)
(1)
Subcircuit 1
In
Out
(2)
(1)
NOTES:
(1) Subcircuit will be a complex pole-pair (PP1 through PP6)
subcircuit specified on the
UAF42 Filter Component Values
and
Filter Block Diagram
program outputs.
(2) HP Out, BP Out, LP Out, or Aux Out will be specified on the
UAF42
Filter Block Diagram
program output.
V
IN
V
O
Subcircuit 1
In
Out
(2)
(1)
FIGURE 5. Simple Filter Made with Single Complex Pole-
Pair Subcircuit.
FIGURE 4B. n = 2 Band-Reject (Notch) Filter Using
UAF42 (approximates the response of a par-
allel-connected tuned L, C, R circuit).
FIGURE 4A. n = 2 Band-Pass Filter Using UAF42 (ap-
proximates the response of a series-connected
tuned L, C, R circuit).
The simplest filter circuit consists of a single pole-pair
subcircuit as shown in Figure 5. More complex filters
consist of two or more cascaded subcircuits as shown in
Figure 6. Even-order filters are implemented entirely with
UAF42 pole-pair sections and normally require no external
capacitors. Odd-order filters additionally require one real
pole section which can be implemented with the fourth
uncommitted op amp in the UAF42, an external resistor, and
an external capacitor. The program can be used to design
filters up to tenth order.
The program guides you through the filter design and gen-
erates component values and a block diagram describing the
filter circuit. The Filter Block Diagram program output
shows the subcircuits needed to implement the filter design
labeled by type and connected in the recommended order.
The Filter Component Values program output shows the
values of all external components needed to implement the
filter.
C
L
V
IN
V
O
R
C
L
V
IN
V
O
R
SUMMARY OF FILTER TYPES
Butterworth
Advantages:
Maximally flat magnitude
response in the pass-band.
Good all-around performance.
Pulse response better than
Chebyshev.
Rate of attenuation better than
Bessel.
Disadvantages:
Some overshoot and ringing in
step response.
Chebyshev
Advantages:
Better rate of attenuation
beyond the pass-band than
Butterworth.
Disadvantages:
Ripple in pass-band.
Considerably more ringing in
step response than Butterworth.
Inverse Chebyshev
Advantages:
Flat magnitude response in
pass-band with steep rate of
attenuation in transition-band.
Disadvantages:
Ripple in stop-band.
Some overshoot and ringing in
step response.
Bessel
Advantages:
Best step response--very little
overshoot or ringing.
Disadvantages:
Slower initial rate of attenua-
tion beyond the pass-band than
Butterworth.
4
At low frequencies, the value required for the frequency-
setting resistors can be excessive. Resistor values above
about 5M
can react with parasitic capacitance causing
poor filter performance. When f
O
is below 10Hz, external
capacitors must be added to keep the value of R
F1
and R
F2
below 5M
. When f
O
is in the range of about 10Hz to 32Hz,
An external 5.49k
resistor, R
2A
, is added in parallel with
the internal resistor, R
2
, to reduce R
F1
and R
F2
by
10 and
eliminate the need for external capacitors. At the other
extreme, when f
O
is above 10kHz, R
2A
, is added in parallel
with R
2
to improve stability.
External filter gain-set resistors, R
G
, are always required
when using an inverting pole-pair configuration or when
using a noninverting configuration with Q < 0.57.
PP1 (Noninverting pole-pair subcircuit using internal gain-
set resistor, R
3
)--See Figure 7. In the automatic topology
selection mode, this configuration is used for all band-pass
filter responses. This configuration allows the combination
of unity pass-band gain and high Q (up to 400). Since no
external gain-set resistor is required, external parts count is
minimized.
PP2 (Noninverting pole-pair subcircuit using an external
gain-set resistor, R
G
)--See Figure 8. This configuration is
used when the pole-pair Q is less than 0.57.
PP3 (Inverting pole-pair subcircuit)--See Figure 9A. In the
automatic topology selection mode, this configuration is
used for the all-pole low-pass and high-pass filter responses.
This configuration requires an external gain-set resistor, R
G
.
With R
G
= 50k
, low-pass and high-pass gain are unity.
PP4 (Noninverting pole-pair/zero subcircuit)--See Figure
10. In addition to a complex pole-pair, this configuration
produces a j
-axis zero (response null) by summing the low-
pass and high-pass outputs using the auxiliary op amp, A
4
,
in the UAF42. In the automatic topology selection mode,
this configuration is used for all band-reject (notch) filter
responses and Inverse Chebyshev filter types when
Q > 0.57. This subcircuit option keeps external parts count
low by using the internal gain-set resistor, R
3
.
PP5 (Noninverting pole-pair/zero subcircuit)--See Figure
11. In addition to a complex pole-pair, this configuration
produces a j
-axis zero (response null) by summing the low-
pass and high-pass outputs using the auxiliary op amp, A
4
,
in the UAF42. In the automatic topology selection mode,
this configuration is used for all band-reject (notch) filter
responses and Inverse Chebyshev filter types when
Q < 0.57. This subcircuit option requires an external gain-set
resistor, R
G
.
PP6 (Inverting pole-pair/zero subcircuit)--See Figure 12. In
addition to a complex pole-pair, this configuration produces
a j
-axis zero (response null) by summing the low-pass and
high-pass outputs using the auxiliary op amp, A
4
, in the
UAF42. This subcircuit is only used when you override the
automatic topology selection algorithm and specify the in-
verting pole-pair topology. Then it is used for all band-reject
(notch) filter responses and Inverse Chebyshev filter types.
The program automatically places lower Q stages ahead of
higher Q stages to prevent op amp output saturation due to
gain peaking. Even so, peaking may limit input voltage to
less than
10V (V
S
=
15V). The maximum input voltage
for each filter design is shown on the filter block diagram.
If the UAF42 is to be operated on reduced supplies, the
maximum input voltage must be derated commensurately.
To use the filter with higher input voltages, you can add an
input attenuator.
The program designs the simplest filter that provides the
desired AC transfer function with a pass-band gain of
1.0V/V. In some cases the program cannot make a unity-
gain filter and the pass-band gain will be less than 1.0V/V.
In any case, overall filter gain is shown on the filter block
diagram.
If you want a different gain, you can add an
additional stage for gain or attenuation as required.
To build the filter, print-out the block diagram and compo-
nent values. Consider one subcircuit at a time. Match the
subcircuit type referenced on the component print-out to its
corresponding circuit diagram--see the Filter Subcircuits
section of this bulletin.
The UAF42 Filter Component Values print-out has places to
display every possible external component needed for any
subcircuit. Not all of these components will be required for
any specific filter design. When no value is shown for a
component, omit the component. For example, the detailed
schematic diagrams for complex pole-pair subcircuits show
external capacitors in parallel with the 1000pF capacitors in
the UAF42. No external capacitors are required for filters
above approximately 10Hz.
After the subcircuits have been implemented, connect them
in series in the order shown on the filter block diagram.
FILTER SUBCIRCUITS
Filter designs consist of cascaded complex pole-pair and
real-pole subcircuits. Complex pole pair subcircuits are
based on the UAF42 state-variable filter topology. Six varia-
tions of this circuit can be used, PP1 through PP6. Real pole
sections can be implemented with the auxiliary op amp in
the UAF42. High-pass (HP) and low-pass (LP) real-pole
sections can be used. The subcircuits are referenced with a
two or three letter abbreviation on the UAF42 Filter Compo-
nent Values
and Filter Block Diagram program outputs.
Descriptions of each subcircuit follow:
POLE-PAIR (PP) SUBCIRCUITS
In general, all complex pole-pair subcircuits use the UAF42
in the state-variable configuration. The two filter parameters
that must be set for the pole-pair are the filter Q and the
natural frequency, f
O
. External resistors are used to set these
parameters. Two resistors, R
F1
and R
F2
, must be used to set
the pole-pair f
O
. A third external resistor, R
Q
, is usually
needed to set Q.
5
FIGURE 7. PP1 Noninverting Pole-Pair Subcircuit Using Internal Gain-Set Resistor R
3
.
FIGURE 8. PP2 Noninverting Pole-Pair Subcircuit Using External Gain-Set Resistor R
G
.
PP1
PP2
A
1
R
2
50k
A
2
A
3
R
3
50k
R
4
50k
UAF42
11
R
1
50k
R
F1
R
F2
C
1
1000pF
C
2
1000pF
V
IN
2
13
8
7
14
3
R
Q
R
2A
C
1A
C
2A
LP Out
BP Out
HP Out
1
12
A
1
R
2
50k
A
2
A
3
R
4
50k
UAF42
11
R
1
50k
R
F1
R
F2
C
1
1000pF
C
2
1000pF
V
IN
3
13
8
7
14
R
Q
R
2A
C
1A
C
2A
LP Out
BP Out
HP Out
1
12
R
G