LT1568
11
1568f
The following filter examples are provided to make it easy
to design a variety of filter stages. Both 2nd and 4th order
filters are shown. For each filer, a table of external resistor
values (standard 1% tolerance) is provided. These resistor
values have been adjusted to compensate for the finite
gain bandwidth product of the LT1568 amplifiers.
To implement a filter, simply connect the resistor values
shown in the table for the cutoff frequency desired. If the
desired cutoff frequency is not shown in the table of
values, use interpolation as recommended in the next
section.
DESIGNING FOR ANY CUTOFF FREQUENCY
To implement a lowpass filter with a cutoff frequency not
included in the design table, resistor values can be inter-
polated in the following manner:
For a Cutoff Frequency, f
C
, Less Than 1MHz
Start with the resistor values for f
C
= 1MHz and then scale
them up by the ratio of (1MHz/f
C
).
APPLICATIO S I FOR ATIO
WUUU
Example: Implement a 2nd order lowpass Chebyshev filter
with an f
C
of 256kHz. From Table 2 the values for f
C
of
1MHz are R11 = R21 = 976Ω and R31 825Ω.
Scaling for f
C
= 256kHz:
R11 = R21 = 976Ω • (1MHz/256kHz) ≈ 3.83k
R31 = 825Ω • (1MHz/256kHz) ≈ 3.24k
For a Cutoff Frequency, f
C
, Between Values Given in a
Design Table
Start with the resistor values for the cutoff frequency
closest to the desired one and scale the values up or down
accordingly.
Example: Implement a 2nd order lowpass Chebyshev filter
with an f
C
of 3.2MHz. From Table 2 the closest values are
for f
C
of 3MHz and are R11 = R21 = 316Ω and R31 = 274Ω.
Scaling for f
C
= 3.2MHz:
R11 = R21 = 316Ω • (3MHz/3.2MHz) ≈ 294Ω
R31 = 274Ω • (3MHz/3.2MHz) ≈ 255Ω
