LMV721 datasheet(11/15 Pages) NSC

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Application Notes (Continued)

The capacitor C

1 is placed between the inverting input and

resistor R

1 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 = 1⁄2

π R

1C1.

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.

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 −R

3/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. R

2 should be chosen equal to the parallel combination of

1 and R3 to minimize error due to bias current. The fre-

quency response of the filter is shown in Figure 10.

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

≥ 0.5x(ω

H VOPP)X10

−6V/µsec

Where

H is the highest frequency of interest, and VOPP is

the output peak-to-peak voltage.

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, R

1 =R2 = 4.7k

Ω, 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 R

3 =

10k

Ω and R

4 = 500k

Ω. 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 R

5 =5k

Ω is used to bias the electret microphone.

Capacitors C

1 =C2 = 4.7µF placed at the input and output of

the op amp to block out the DC voltage offset.

10092223

FIGURE 8. Single-Supply Inverting Amplifier

10092224

FIGURE 9. Simple Low-Pass Active Filter

10092225

FIGURE 10. Frequency Response of Simple Low-pass

Active Filter in Figure 9

10092244

FIGURE 11. A Battery Powered Microphone

Preamplifier

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