Datasheet
MAX260/MAX261/MAX262
Microprocessor Programmable
Universal Active Filters
24 ______________________________________________________________________________________
In Figure 23, a series of response curves are shown for
the previous configuration using a MAX261 with clock
frequencies ranging from 750kHz to 4MHz (f
0
from
500Hz to 30kHz). Note that the rightmost curve shows
about 2dB of gain peaking compared to the lower fre-
quency curves, indicating the upper limit of usable filter
accuracy at this Q (see Table 1).
Wide Passband Chebyshev Bandpass
In this example (Figure 24), the desired parameters
are:
Center frequency (f
0
) = 1kHz
Pass bandwidth = 1kHz
Stop bandwidth = 3kHz
Max passband ripple = 1dB
Min stopband attenuation = 20dB
From the previous parameters, we use either lookup
tables, design texts, or Maxim's filter design programs
to generate the order (number of poles), and the f
0
and
Q of each second-order section. The A and B parame-
ters are:
f
0A
= 639Hz f
0B
= 1564Hz
Q
A
= 2.01 Q
B
= 2.01
To implement this filter, section A operates in mode 1
and section B uses mode 2 to provide a wider overall
range of f
CLK
/f
0
ratios. This way, one clock frequency
can drive both sections A and B. See Tables 2 and 3.
CLK
A
= CLK
B
= 120kHz
f
CLK
/f
0A
= 188.49 (Mode 1, N = 56), actual f
0A
= 636.6Hz
f
CLK
/f
0B
= 76.64 (Mode 2, N = 5), actual f
0B
= 156.5Hz
Q
A
= 2.000 (Mode 1, N = 96), Q
B
= 2.01 (Mode 2, N =
83)
The overall passband gain at f
0
is 0.64V/V or
-3.9dB.
High-Frequency Chebyshev Bandpass
The same Chebyshev response shape shown in Figure
24 is implemented at higher frequencies with a
MAX262 in Figure 25. The curves show plots for center
frequencies of 15.6kHz, 31.3kHz, and 47kHz. Not only
is this faster than the MAX260 implementation, but
mode 1 can be used in both halves of the MAX262 for
this filter because the range of available f
CLK
/f
0
ratios is
wider with the MAX262 than the MAX260.
51
11 12
23 21
MAX260
V
IN
IN
A
BP
A
CLK
A
CLK
PROGRAM
CLK
B
IN
B
BP
B
WR, AX, DX
V
OUT
40
-60
200 2K1K500 20K10K5K
-35
FREQUENCY (Hz)
GAIN (dB)
PHASE (DEGREES)
-10
15
-180
180
90
0
-90
GAIN
PHASE
Figure 22. Fourth-Order Chebyshev Bandpass Filter
CLK
A,B
MODE
f
OA
f
OB
Q
A
Q
B
150kHz 1 N = 42 N = 23 N = 119 N = 119
30
-30
1K 10K5K2K 100K50K20K
-15
FREQUENCY (Hz)
GAIN (dB)
0
15
Figure 23. MAX261 Fourth-Order Chebyshev Bandpass Using
Coefficients of Figure 22