LMV321-N, LMV321-N-Q1, LMV358-N, LMV358-N-Q1
LMV324-N, LMV324-N-Q1
SNOS012I –AUGUST 2000–REVISED FEBRUARY 2013
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(3)
Figure 61. Sallen-Key 2nd-Order Active Low-Pass Filter
The following paragraphs explain how to select values for R1, R2, R3, R4, C1, and C 2 for given filter requirements,
such as ALP, Q, and fc.
The standard form for a 2nd-order low pass filter is
(4)
where
Q: Pole Quality Factor
ωC: Corner Frequency
A comparison between Equation 3 and Equation 4 yields
(5)
(6)
To reduce the required calculations in filter design, it is convenient to introduce normalization into the
components and design parameters. To normalize, let ωC = ωn = 1 rad/s, and C1 = C2 = Cn = 1F, and substitute
these values into Equation 5 and Equation 6. From Equation 5, we obtain
(7)
From Equation 6, we obtain
(8)
For minimum DC offset, V+ = V−, the resistor values at both inverting and non-inverting inputs should be equal,
which means
(9)
20
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