LMC6035 Datasheet, PDF (13/20 Pages) National Semiconductor (TI) – Low Power 2.7V Single Supply CMOS Operational Amplifiers

1.0 Application Notes (Continued)
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FIGURE 4. THD+Noise Performance of LMC6035 and
“Benchmark” per Circuit of Figure 1
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_IN) 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_c). The bold component values of Figure 5
provide a cutoff frequency of 3kHz. An example of the scal-
ing procedure follows Figure 5.
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FIGURE 5. 2-Pole, 3kHz, Active, Sallen and Key,
Lowpass Filter with Butterworth Response
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 = 2π 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_(Normalized)/FSF C1’ =
0.707/18.84 x 10³ = 37.93 x 10^−6 C2’ = 1.414/18.84
x 10³ = 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 =
R1_(normalized) x Z = 1Ω x 8.4k = 8.4kΩ
x Z = 1Ω x 8.4k = 8.4kΩ
R2 = R2_(normalized)
(Standard value chosen for R1 and R2 is 8.45kΩ )
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.
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FIGURE 6. 2 Pole, 300Hz, Sallen and Key,
High-Pass Filter
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