Datasheet

MAX260/MAX261/MAX262
Microprocessor Programmable
Universal Active Filters
______________________________________________________________________________________ 17
Mode 1, along with mode 4, supports the highest clock
frequencies (see Table 1) because the input summing
amplifier is outside the filter’s resonant loop (Figure 7).
The gain of the lowpass and notch outputs is 1, while
the bandpass gain at the center frequency is Q. For
bandpass gains other than Q, the filter input or output
can be scaled by a resistive divider or op amp.
MODE 2 (Figure 8) is used for all-pole lowpass and
bandpass filters. Key advantages compared to mode 1
are higher available Qs (see Table 3) and lower output
noise. Mode 2’s available f
CLK
/f
0
ratios are √2 less than
with mode 1 (see Table 2), so a wider overall range of
f
0
s can be selected from a single clock when both
modes are used together. This is demonstrated in the
Wide Passband Chebyshev Bandpass design example.
MODE 3 (Figure 9) is the only mode that produces
high-pass filters. The maximum clock frequency is
somewhat less than with mode 1 (see Table 1).
MODE 3A (Figure 10) uses a separate op amp to sum
the highpass and lowpass outputs of mode 3, creating
a separate notch output. This output allows the notch to
be set independently of f
0
by adjusting the op amp’s
feedback resistor ratio (R
H
, R
L
). R
H
, R
L
, and R
G
are
external resistors. Because the notch can be indepen-
dently set, mode 3A is also useful when designing
pole-zero filters such as elliptics.
MODE 4 (Figure 11) is the only mode that provides an
allpass output. This is useful when implementing group
delay equalization. In addition to this, mode 4 can also
be used in all pole lowpass and bandpass filters. Along
with mode 1, it is the fastest operating mode for the fil-
ter, although the gains are different than in mode 1.
When the allpass function is used, note that some
amplitude peaking occurs (approximately 0.3dB when
Q = 8) at f
0
. Also note that f
0
and Q sampling errors are
highest in mode 4 (see Figure 20).
SCN
IN
SCN
N
BP
LP
+
-
-
MODE 2
+
-
∫ ∫
SCN
SCN
Σ
Figure 8. Filter Mode 2: Second-Order Bandpass, Lowpass,
and Notch
SCN
IN
SCN
HP
BP
LP
+
-
-
MODE 3
+
-
∫ ∫
SCN
SCN
Σ
SCN = SWITCHED-CAPACITOR NETWORK
Figure 9. Filter Mode 3: Second-Order Bandpass, Lowpass,
and Highpass
SCN
IN
SCN
HP
R
G
R
H
R
L
BP
LP
+
-
-
MODE 3A
N
+
-
+
-
∫ ∫
SCN
SCN
Σ
SCN = SWITCHED-CAPACITOR NETWORK
Figure 10. Filter Mode 3a: Second-Order Bandpass, Lowpass,
Highpass, and Notch. For elliptic LP, BP, HP, and Notch, the N
output is used.