PP6
HP Out
LP Out
R
Z2
R
G
V
IN
12
R
2A
R
F1
C
1A
R
F2
C
2A
R
Z1
R
Z3
13
8
R
1
50kΩ
7
14
1
5
R
2
50kΩ
C
1
1000pF
C
2
1000pF
3
A
1
R
4
50kΩ
A
2
A
3
A
4
6
Aux Out
R
Q
UAF42
11
4
FIGURE 12. PP6 Inverting Pole-Pair/Zero Subcircuit.
This subcircuit option requires an external gain-set resistor,
R
G
.
LP
(Real-pole low-pass subcircuit). The basic low-pass
subcircuit (LP) is shown in Figure 13A. A single pole is
formed by R
P
and C
P
. A
2
buffers the output to prevent
loading from subsequent stages. If high input impedance is
needed, an optional buffer, A
1
, can be added to the input.
For an LP subcircuit with gain, use the optional circuit
shown in Figure 13B.
For an LP subcircuit with inverting gain or attenuation, use
the optional circuit shown in Figure 13C.
HP
(Real-pole high-pass subcircuit). The basic high-pass
subcircuit (HP) is shown in Figure 14A. A single pole is
formed by R
P
and C
P
. A
2
buffers the output to prevent
loading from subsequent stages. If high input impedance is
needed, an optional buffer, A
1
, can be added to the input.
For an HP subcircuit with gain, use the optional circuit
shown in Figure 14B.
For an HP subcircuit with inverting gain or attenuation, use
the optional circuit shown in Figure 14C.
IF THE AUXILIARY OP AMP
IN A UAF42 IS NOT USED
If the auxiliary op amp in a UAF42 is not used, connect it as
a grounded unity-gain follower as shown in Figure 15. This
will keep its inputs and output in the linear region of
operation to prevent biasing anomalies which may affect the
other op amps in the UAF42.
ELIMINATING THE LP SUBCIRCUIT
IN ODD-ORDER INVERSE CHEBYSHEV
LOW-PASS FILTERS
Odd-order Inverse Chebyshev low-pass filters can be simpli-
fied by eliminating the LP input section and forming the real
pole in the first pole-pair/zero subcircuit. To form the real
pole in the pole-pair/zero subcircuit, place a capacitor, C
1
, in
parallel with the summing amplifier feedback resistor, R
Z3
.
The real pole must be at the same frequency as in the LP
subcircuit. One way to achieve this is to set C
1
= C
P
and R
Z3
= R
P
, where C
P
and R
P
are the values that were specified for
the LP section. Then, to keep the summing amplifier gains
the same, multiply R
Z1
and R
Z2
by R
P
/R
Z3
.
Figures 16A and 16B show an example of the modification
of a 3-pole circuit. It is a 347Hz-cutoff inverse Chebyshev
low-pass filter. This example is from an application which
required a low-pass filter with a notch for 400Hz system
power-supply noise. Setting the cutoff at 347Hz produced
the 400Hz notch. The standard filter (Figure 16A) consists
of two subcircuits, an LP section followed by a PP4 section.
In the simplified configuration (Figure 16B), the summing
amplifier feedback resistor, R
Z3
is changed from 10kΩ to
130kΩ and paralleled with a 0.01µF capacitor. Notice that
these are the same values used for R
P
and C
P
in the LP
section of Figure 16A. To set correct the summing amplifier
gain, resistors, R
Z1
and R
Z2
are multiplied by R
P
/R
Z3
(130kΩ/
10kΩ). R
Z1
and R
Z2
must be greater than 2kΩ to prevent op
amp output overloading. If necessary, increase R
Z1
, R
Z2
, and
R
Z3
by decreasing C
P
.
8
BURR-BROWN [ BURR-BROWN CORPORATION ]
