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LT1568

Very Low Noise, High Frequency Active RC, Filter Building Block

Linear Technology 

LT1568

APPLICATIO S I FOR ATIO

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, fC, Less Than 1MHz

Start with the resistor values for fC = 1MHz and then scale

them up by the ratio of (1MHz/fC).

Example: Implement a 2nd order lowpass Chebyshev filter

with an fC of 256kHz. From Table 2 the values for fC of

1MHz are R11 = R21 = 976Ω and R31 825Ω.

Scaling for fC = 256kHz:

R11 = R21 = 976Ω • (1MHz/256kHz) ≈ 3.83k

R31 = 825Ω • (1MHz/256kHz) ≈ 3.24k

For a Cutoff Frequency, fC, 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 fC of 3.2MHz. From Table 2 the closest values are

for fC of 3MHz and are R11 = R21 = 316Ω and R31 = 274Ω.

Scaling for fC = 3.2MHz:

R11 = R21 = 316Ω • (3MHz/3.2MHz) ≈ 294Ω

R31 = 274Ω • (3MHz/3.2MHz) ≈ 255Ω

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