廬
DESIGN A 60Hz NOTCH FILTER WITH THE UAF42
by Johnnie Molina, (602) 746-7592
DESIGN A 60HZ NOTCH FILTER WITH THE UAF42
The UAF42 is a monolithic, time-continuous, 2nd-order
active filter building block for complex and simple filter
designs. It uses the classical state-variable analog architec-
ture with a summing amplifier plus two integrators. This
topology offers low sensitivity of filter design parameters f
O
(natural frequency) and Q to external component variations
along with simultaneous high-pass, low-pass and band-pass
outputs. An auxiliary high performance operational ampli-
fier is also provided which can be used for buffering, gain,
real pole circuits, or for summing the high-pass and low-pass
outputs to create a band reject (notch) filter (see Figure 1).
A notch filter is easily realized with the UAF42 and six
external resistors. Figure 2 shows the UAF42 configured
into a 60Hz notch filter. The auxiliary operational amplifier
is used to sum both the high-pass and low-pass outputs. At
f = f
NOTCH
, both of these outputs times their respective gain
at the summing circuit are equal in magnitude but 180擄 out
of phase. Hence, the output goes to zero. Figure 3 shows the
response plot for the circuit shown in Figure 2 where
f
O
= 60Hz and Q = 6.
The notch frequency for the notch filter is set by the
following calculations:
f
NOTCH
=
where,
A
LP
= gain from input to low-pass out at f = 0Hz.
A
HP
= gain from input to high-pass out of f >> f
O
.
Typically, A
LP
/A
HP
鈥?R
Z2
/R
Z1
is equal to one. This simplifies
f
NOTCH
to be,
f
NOTCH
= f
O
f
O
is given by, f
O
=
1
R
F
鈥?C 鈥?2蟺
where, R
F
= R
F1
= R
F2
and C = C
1
= C
2
Note that the notch frequency can be modified by simply
changing the R
F
resistors and/or adding external capacitors.
NPO ceramic, mica or a good film capacitor with low
dissipation factor characteristics is recommended.
The 鈥?dB bandwidth, as shown in Figure 3, can be set by the
following calculations.
BW
鈥?dB
= f
NOTCH
/Q
where, BW
鈥?dB
= f
H
鈥?f
L
The filter Q can be determined by setting R
Q
to a value given
by,
25k鈩?/div>
R
Q
=
Q 鈥?
Low-pass
Out
14
1
5
6
(
A
LP
/ A
HP
鈥?R
Z2
/ R
Z1
)
鈥?f
O
High-pass
Out
13
8
R
1
50k鈩?/div>
R
2
50k鈩?/div>
C
2
1000pF
Band-pass
Out
7
In
1
12
C
2
1000pF
In
3
3
In
2
2
R
3
50k鈩?/div>
R
4
50k鈩?/div>
UAF42
11
4
FIGURE 1. UAF42 Universal Active Filter with High-pass, Band-pass and Low-pass Outputs.
漏
1994 Burr-Brown Corporation
AB-071
1
Printed in U.S.A. January, 1994
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