LMC6035 Datasheet, PDF (14/29 Pages) National Semiconductor (TI) – Low Power 2.7V Single Supply CMOS Operational Amplifiers

LMC6035, LMC6035-Q1, LMC6036
SNOS875G – JANUARY 2000 – REVISED APRIL 2013
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Not only does the LMC6035/6 provide excellent output swing capability at low supply voltages, it also maintains
high open loop gain (A _VOL) with heavy loads. To illustrate this, the LMC6035 and the benchmark op amp were
compared for their distortion performance in the circuit of Figure 43. The graph of Figure 46 shows this
comparison. The y-axis represents percent Total Harmonic Distortion (THD plus noise) across the loaded
secondary of T1. The x-axis represents the input amplitude of a 1 kHz sine wave. (Note that T1 loses about 20%
of the voltage to the voltage divider of R_L (600Ω) and T1's winding resistances—a performance deficiency of the
transformer.)
Figure 46. THD+Noise Performance of LMC6035 and “Benchmark” per Circuit of Figure 43
Figure 46 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.
APPLICATION CIRCUITS
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 47 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 component values in parenthesis which are obtainable
from standard 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 47 provide a cutoff frequency of
3kHz. An example of the scaling procedure follows Figure 47.
Figure 47. 2-Pole, 3kHz, Active, Sallen and Key, Lowpass Filter with Butterworth Response
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