Datasheet
MAX260/MAX261/MAX262
Microprocessor Programmable
Universal Active Filters
18 ______________________________________________________________________________________
Description of Filter Functions
BANDPASS (Figure 12)
For all pole bandpass and lowpass filters (Butterworth,
Bessel, Chebyshev) use mode 1 if possible. If appropri-
ate f
CLK
/f
0
or Q values are not available in mode 1,
mode 2 provides a selection that is closer to the
required values. Mode 1, however, has the highest
bandwidth (see Table 1). For pole-zero filters, such as
elliptics, see mode 3A.
H
OBP
= Bandpass output gain at ω = ω
o
f
0
=
ω
0
/ 2π
= The center frequency of the complex
pole pair. Input-output phase shift is -180° at f
0
.
Q = The quality factor of the complex pole pair.
Also the ratio of f
0
to -3dB bandwidth of the
second-order bandpass response.
LOWPASS See bandpass text. (Figure 13)
H
OLP
= Lowpass output gain at DC
f
0
= ω
0
/ 2π
HIGHPASS (Figure 14)
Mode 3 is the only mode with a highpass output. It
works for all pole filter types such as Butterworth,
Bessel and Chebyshev. Use mode 3A for filters
employing both poles and zeros, such as elliptics.
H
OHP
= Highpass output gain as f approaches f
CLK
/4
f
0
= ω
0
/ 2π
NOTCH (Figure 15)
Mode 3A is recommended for multi-pole notch filters. In
second-order filters, mode 1 can also be used. The
advantages of mode 1 are higher bandwidth, com-
pared to mode 3 (higher f
N
can be implemented), and
no need for external components as required in mode
3A.
H
ON2
= Notch output gain as f approaches f
CLK
/4
H
ON1
= Notch output gain as f approaches DC
f
n
= ω
n
/ 2π
ALLPASS
Mode 4 is the only configuration in which an allpass
function can be realized.
Gs H
s
ss Q
ON
n
oo
()
( / )
=
+
++
2
2
2
2
2
ω
ωω
Gs H
s
ss Q
OHP
oo
()
( / )
=
++
2
2
2ωω
Gs H
ss Q
OLP
o
oo
()
( / )
=
++
ω
ωω
2
2
2
Gs H
sQ
ss Q
OBP
o
oo
()
( / )
( / )
=
++
ω
ωω
2
2
SCN
IN
SCN
AP
LP
+
-
-
MODE 4
BP
+
-
∫ ∫
SCN
Σ
SCN = SWITCHED-CAPACITOR NETWORK
Figure 11. Filter Mode 4: Second-Order Bandpass, Lowpass,
and Allpass
f
L
f
O
f
H
H
OBP
0.707 H
OBP
BANDPASS OUTPUT
f(LOG SCALE)
GAIN (V/V)
Figure 12. Second-Order Bandpass Characteristics
Q
f
ff
f
ff
QQ
ff
QQ
O
HL
Off
LO
HO
LH
,
=
−
=
−
+
+
=+
+
=
1
2
1
2
1
1
2
1
2
1
2
2