LMV1088 Datasheet, PDF (23/30 Pages) National Semiconductor (TI) – Dual Input, Far Field Noise Suppression Microphone Amplifier with Automatic Calibration Ability

LMV1088
www.ti.com
SNAS385J – SEPTEMBER 2007 – REVISED APRIL 2013
The A-weighted filter is used in signal to noise measurements and THD+N measurements, where the wanted
audio signal is compared to device noise and distortion.
The use of this filter improves the correlation of the measured values to the way these ratios are perceived by
the human ear.
10
0
-10
-20
-30
-40
-50
-60
-70
10
100
1k
10k
FREQUENCY (Hz)
100k
Figure 28. A-Weighted Filter
MEASURING NOISE AND SNR
The overall noise of the LMV1088 is measured within the frequency band from 10Hz to 22kHz using an A-
weighted filter. The Mic+ and Mic- inputs of the LMV1088 are shorted for AC signals via a short between the
input capacitors , see Figure 29.
LMV1088
Mic2+
short ^{470 nF}
Mic2-
470 nF
Mic1+
short ^{470 nF}
Mic1-
470 nF
LPF
A-WEIGHTED
FILTER
OUT
AC Voltmeter
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 18mV_PP
using an A-weighted filter. This voltage represents the output voltage of a typical electret condenser microphone
at sound pressure level of 94dB SPL, which is the standard level for these measurements. The LMV1088 is
programmed for 17.5dB of total gain (20dB pre-amplifier and -2.5dB post-amplifier) with only Mic1 or Mic2 on.
(See also I²C Compatible Interface)
The input signal is applied differential between the corresponding Mic+ and Mic- . Because the part is in Pass
Through mode the Low-pass Filter at the output of the LMV1088 is disabled.
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