LMV721_06 Datasheet, PDF (11/15 Pages) National Semiconductor (TI) – 10MHz, Low Noise, Low Voltage, and Low Power Operational Amplifier

Application Notes (Continued)
The capacitor C₁ is placed between the inverting input and
resistor R₁ to block the DC signal going into the AC signal
source, V_IN. The values of R₁ and C₁ affect the cutoff fre-
quency, fc = ¹â„2π R₁C₁.
As a result, the output signal is centered around mid-supply
(if the voltage divider provides V⁺/2 at the non-inverting
input). The output can swing to both rails, maximizing the
signal-to-noise ratio in a low voltage system.
10092223
FIGURE 8. Single-Supply Inverting Amplifier
4.4 Active Filter
4.4.1 Simple Low-Pass Active Filter
The simple low-pass filter is shown in Figure 9. Its low-pass
frequency gain (ω → o) is defined by −R₃/R₁. This allows
low-frequency gains other than unity to be obtained. The
filter has a −20dB/decade roll-off after its corner frequency
fc. R₂ should be chosen equal to the parallel combination of
R₁ and R₃ to minimize error due to bias current. The fre-
quency response of the filter is shown in Figure 10.
10092225
FIGURE 10. Frequency Response of Simple Low-pass
Active Filter in Figure 9
Note that the single-op-amp active filters are used in to the
applications that require low quality factor, Q(≤ 10), low
frequency (≤ 5KHz), and low gain (≤ 10), or a small value for
the product of gain times Q(≤ 100). The op amp should have
an open loop voltage gain at the highest frequency of inter-
est at least 50 times larger than the gain of the filter at this
frequency. In addition, the selected op amp should have a
slew rate that meets the following requirement:
Slew Rate ≥ 0.5 x (ω_H V_OPP) X 10 ^−6V/µsec
Where ω_H is the highest frequency of interest, and V_OPP is
the output peak-to-peak voltage.
10092224
FIGURE 9. Simple Low-Pass Active Filter
10092244
FIGURE 11. A Battery Powered Microphone
Preamplifier
Here is a LMV721 used as a microphone preamplifier. Since
the LMV721 is a low noise and low power op amp, it makes
it an ideal candidate as a battery powered microphone
preamplifier. The LMV721 is connected in an inverting con-
figuration. Resistors, R₁ = R₂ = 4.7kΩ, sets the reference
half way between V_CC = 3V and ground. Thus, this config-
ures the op amp for single supply use. The gain of the
preamplifier, which is 50 (34dB), is set by resistors R₃ =
10kΩ and R₄ = 500kΩ. The gain bandwidth product for the
LMV721 is 10 MHz. This is sufficient for most audio applica-
tion since the audio range is typically from 20 Hz to 20kHz. A
resistor R₅ = 5kΩ is used to bias the electret microphone.
Capacitors C₁ = C₂ = 4.7µF placed at the input and output of
the op amp to block out the DC voltage offset.
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