Datasheet
MAX260/MAX261/MAX262
Microprocessor Programmable
Universal Active Filters
______________________________________________________________________________________ 25
Fourth-Order Butterworth Lowpass
Figure 26 shows a fourth-order Butterworth lowpass
with a cutoff frequency of 3kHz. Sections A and B of a
MAX260 are cascaded. The f
0
and Q parameters for
each section are:
f
0A
= 3kHz f
0B
= 3kHz
Q
A
= 1.307 Q
B
= 0.541
Mode 1 and a 400kHz clock are used. Because of low
Q values, the sampling errors of Figure 20 begin to look
significant in this case. From the graphs, using f
CLK
/f
0
ratio near 133, f
0
A is about 4% high, f
0
B is 1.5% high,
Q
A
is -1.2% low, and Q
B
is -0.5% low. If these errors
are not a problem, the corrections can be ignored.
They are included here for best possible accuracy:
CLK
A
= CLK
B
= 400kHz
f
CLK
/f
0A
= 135.08 (N = 22), f
0B
= 2961Hz
(-1.3% correction)
f
CLK
/f
0B
= 139.80 (N = 25), f
0A
= 2861Hz
(-4.6% correction)
Q
A
= 1.306 (N = 79, Q resolution prevents +0.5%
correction)
Q
B
= 0.547 (N = 11 +1.1% correction)
Measured wideband noise for this filter is 123µV RMS.
If mode 2 were used, the noise would be 87µV RMS.
For lower noise with either mode, the first section
should have the highest Q (section A in this example).
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
10
-70
100 1K500200 10K5K2K
-50
FREQUENCY (Hz)
GAIN (dB)
-30
-10
-180
180
90
PHASE (DEGREES)
0
-90
GAIN
PHASE
Figure 24. Wide Passband Chebyshev Bandpass Filter
CLK
A,B
MODE
A
MODE
B
f
OA
f
OB
Q
A
Q
B
120kHz 1 2 N = 56 N = 5 N = 96 N = 83
51
11 12
23 21
MAX262
V
IN
IN
A
BP
A
CLK
A
CLK
PROGRAM
CLK
B
IN
B
BP
B
WR, AX, DX
V
OUT
0
-50
1K 10K5K2K 100K50K20K
-40
-30
FREQUENCY (Hz)
GAIN (dB)
-20
-10
f
O
= 15.6kHz
f
CLK
= 1MHz
f
O
= 31.3kHz
f
CLK
= 2MHz
f
O
= 47kHz
f
CLK
= 3MHz
Figure 25. High-Frequency Chebyshev Bandpass Filter
CLK
A,B
MODE
f
OA
f
OB
Q
A
Q
B
1 to 3MHz 1 N = 38 N = 0 N = 96 N = 96