lmv358-mda National Semiconductor Corporation, lmv358-mda Datasheet - Page 16

• Manufacturer Part Number: lmv358-mda
• Description: General Purpose, Low Voltage, Rail-to-rail Output Operational Amplifiers
• Manufacturer: National Semiconductor Corporation

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ACTIVE FILTER
Simple Low-Pass Active Filter
The simple low-pass filter is shown in Figure 11. Its low-fre-
quency gain (ω
frequency gains other than unity to be obtained. The filter has
a −20 dB/decade roll-off after its corner frequency fc. R
should be chosen equal to the parallel combination of R
R
sponse of the filter is shown in Figure 12.
Note that the single-op-amp active filters are used in the ap-
plications that require low quality factor, Q(
quency (
the product of gain times Q (
an open loop voltage gain at the highest frequency of interest
at least 50 times larger than the gain of the filter at this fre-
quency. In addition, the selected op amp should have a slew
rate that meets the following requirement:
where ω
output peak-to-peak voltage.
FIGURE 12. Frequency Response of Simple Low-Pass
3
to minimize errors due to bias current. The frequency re-
FIGURE 11. Simple Low-Pass Active Filter
H
Slew Rate
≤
is the highest frequency of interest, and V
5 kHz), and low gain (
→
Active Filter in Figure 11
0) is defined by −R
≥
0.5 × (ω
≤
100). The op amp should have
H
10006037
V
OPP
≤
10), or a small value for
) × 10
3
/R
1
−6
. This allows low-
10006015
V/µsec
≤
10006014
10), low fre-
OPP
is the
1
and
2
16
Sallen-Key 2nd-Order Active Low-Pass Filter
The Sallen-Key 2nd-order active low-pass filter is illustrated
in Figure 13. The DC gain of the filter is expressed as
Its transfer function is
FIGURE 13. Sallen-Key 2nd-Order Active Low-Pass Filter
The following paragraphs explain how to select values for
R
as A
The standard form for a 2nd-order low pass filter is
where
ω
A comparison between Equation 2 and Equation 3 yields
To reduce the required calculations in filter design, it is con-
venient to introduce normalization into the components and
design parameters. To normalize, let ω
C
4 and Equation 5. From Equation 4, we obtain
From Equation 5, we obtain
1
1
, R
= C
LP
Q: Pole Quality Factor
2
, R
C
, Q, and f
2
: Corner Frequency
= C
3
, R
n
4
= 1F, and substitute these values into Equation
, C
c
.
1
, and C
2
for given filter requirements, such
C
= ω
n
= 1 rad/s, and
10006016
(1)
(2)
(3)
(4)
(5)
(6)
(7)