A-WEIGHTED

FILTER

Mic2+

Mic2-

Mic1+

Mic1-

470 nF

470 nF

470 nF

470 nF

OUT-

LPF

LMV1099

short

short

AC Voltmeter

OUT+

10

100

1k

10k

100k

FREQUENCY (Hz)

-70

-60

-50

-40

-30

-20

-10

0

10

LMV1099

www.ti.com

SNAS490D – JULY 2010 – REVISED MAY 2013

The use of this filter improves the correlation of the measured values to the way these ratios are perceived by

the human ear.

Figure 28. A-Weighted Filter

Measuring Uplink Noise and SNR

The overall noise of the LMV1099 is measured within the frequency band from 10Hz to 22kHz using an A-

weighted filter. The Mic+ and Mic- inputs of the LMV1099 are AC shorted between the input capacitors, see

Figure 29.

Figure 29. Noise Measurement Setup

For the signal to noise ratio (SNR) the signal level at the output is measured with a 1kHz input signal of 18mVP-P

using an A-weighted filter. This voltage represents the output voltage of a typical electret condenser microphone

at a sound pressure level of 94dB SPL, which is the standard level for these measurements. The LMV1099 is

programmed for 26dB of total gain (20dB preamplifier and 6dB postamplifier) with only mic1 or mic2 used. (See

also I2C Compatible Interface).

The input signal is applied differentially between the Mic+ and Mic-. Because the part is in Pass Through mode

the low-pass filter at the output of the LMV1099 is disabled.

Table 9. Revision History

Rev

Date

Description

1.0

08/12/10

Initial release.

1.01

12/10/10

Added the X1, X2, and X3 values of the mktg outline.

1.02

03/30/11

Edited Table 3 (Control Registers).

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