參數(shù)資料
型號(hào)： LMC6035ITL
廠(chǎng)商： NATIONAL SEMICONDUCTOR CORP
元件分類(lèi)： 運(yùn)動(dòng)控制電子
英文描述： Low Power 2.7V Single Supply CMOS Operational Amplifiers
中文描述： DUAL OP-AMP, 5000 uV OFFSET-MAX, 1.4 MHz BAND WIDTH, PBGA8
封裝： MICRO, SMD-8
文件頁(yè)數(shù)： 13/20頁(yè)
文件大?。?/td> 786K
代理商： LMC6035ITL
1.0 Application Notes
(Continued)
Figure 4
shows the superior distortion performance of
LMC6035/6 over that of the benchmark op amp. The heavy
loading of the circuit causes the A
VOL
of the benchmark part
to drop significantly which causes increased distortion.
1.2 APPLICATION CIRCUITS
1.2.1 Low-Pass Active Filter
A common application for low voltage systems would be
active filters, in cordless and cellular phones for example.
The ultra low input currents (I
) of the LMC6035/6 makes it
well suited for low power active filter applications, because it
allows the use of higher resistor values and lower capacitor
values. This reduces power consumption and space.
Figure 5
shows a low pass, active filter with a Butterworth
(maximally flat) frequency response. Its topology is a Sallen
and Key filter with unity gain. Note the normalized compo-
nent values in parenthesis which are obtainable from stan-
dard filter design handbooks. These values provide a 1Hz
cutoff frequency, but they can be easily scaled for a desired
cutoff frequency (f
). The bold component values of
Figure 5
provide a cutoff frequency of 3kHz. An example of the scal-
ing procedure follows
Figure 5
.
1.2.1.1 Low-Pass Frequency Scaling Procedure
The actual component values represented in bold of
Figure 5
were obtained with the following scaling procedure:
1.
First determine the frequency scaling factor (FSF) for
the desired cutoff frequency. Choosing f
c
at 3kHz, pro-
vides the following FSF computation:
FSF = 2
π
x 3kHz
(desired cutoff freq.)
= 18.84 x 10
3
2.
Then divide all of the normalized capacitor values by the
FSF as follows:
C1’ = C
(6
/FSF
0.707/18.84 x 10
3
= 37.93 x 10
x 10
3
= 75.05 x 10
6
(C1’ and C2’: prior to impedance
scaling)
3.
Last, choose an impedance scaling factor (Z). This Z
factor can be calculated from a standard value for C2.
Then Z can be used to determine the remaining compo-
nent values as follows:
Z = C2’/C2
(chosen)
= 75.05 x 10
6
/
6.8nF
= 8.4k
C1 = C1’/Z = 37.93 x 10
6
/8.4k = 4.52nF
(Standard capacitor value chosen for C1 is
4.7nF
)
R1
x Z = 1
x 8.4k = 8.4k
x Z = 1
x 8.4k = 8.4k
(Standard value chosen for R1 and R2 is
8.45k
)
C1’ =
C2’ = 1.414/18.84
R1 =
R2 = R2
(normalized)
1.2.2 High Pass Active Filter
The previous low-pass filter circuit of
Figure 5
converts to a
high-pass active filter per
Figure 6
.
01283047
FIGURE 4. THD+Noise Performance of LMC6035 and
“Benchmark” per Circuit of
Figure 1
01283048
FIGURE 5. 2-Pole, 3kHz, Active, Sallen and Key,
Lowpass Filter with Butterworth Response
01283049
FIGURE 6. 2 Pole, 300Hz, Sallen and Key,
High-Pass Filter
L
www.national.com
13
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