lmc6035itlx National Semiconductor Corporation, lmc6035itlx Datasheet - Page 13

• Manufacturer Part Number: lmc6035itlx
• Description: Low Power 2.7v Single Supply Cmos Operational Amplifiers
• Manufacturer: National Semiconductor Corporation

1.0 Application Notes
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
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
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
provide a cutoff frequency of 3kHz. An example of the scal-
ing procedure follows Figure 5.
FIGURE 4. THD+Noise Performance of LMC6035 and
“Benchmark” per Circuit of Figure 1
c
). The bold component values of Figure 5
IN
) of the LMC6035/6 makes it
VOL
of the benchmark part
(Continued)
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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
2. Then divide all of the normalized capacitor values by the
3. Last, choose an impedance scaling factor (Z). This Z
Z = C2’/C2
(Standard capacitor value chosen for C1 is 4.7nF )
R1
x Z = 1Ω x 8.4k = 8.4kΩ
(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.
C1 = C1’/Z = 37.93 x 10
(normalized)
FIGURE 5. 2-Pole, 3kHz, Active, Sallen and Key,
the desired cutoff frequency. Choosing f
vides the following FSF computation:
FSF = 2π x 3kHz
FSF as follows:
0.707/18.84 x 10
x 10
scaling)
factor can be calculated from a standard value for C2.
Then Z can be used to determine the remaining compo-
nent values as follows:
Lowpass Filter with Butterworth Response
FIGURE 6. 2 Pole, 300Hz, Sallen and Key,
3
= 75.05 x 10
(chosen)
x Z = 1Ω x 8.4k = 8.4kΩ
= 75.05 x 10
3
High-Pass Filter
(desired cutoff freq.)
−6
= 37.93 x 10
C1’ = C
−6
(C1’ and C2’: prior to impedance
/8.4k = 4.52nF
−6
(Normalized)
/6.8nF = 8.4k
−6
= 18.84 x 10
R2 = R2
C2’ = 1.414/18.84
/FSF
c
at 3kHz, pro-
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(normalized)
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C1’ =
R1 =