Datasheet
10
100
1k
10k
100k
FREQUENCY (Hz)
-70
-60
-50
-40
-30
-20
-10
0
10
dBV
LMV1089
SNAS441H –SEPTEMBER 2008–REVISED MAY 2010
www.ti.com
Measurement Setup
Because of the nature of the calibration system it is not possible to predict the absolute gain in the two
microphone channels of the Far Field Noise Cancelling System. This is because, after the calibration function
has been operated, the noise cancelling circuit will compensate for the difference in gain between the
microphones. In Noise Cancelling mode, this can result in a final gain offset of max 3dB between the gain set in
the registers (A[3:0] and B[2:0]) and the actual measured gain between input and output of the LMV1089. After
performing a calibration the frequency characteristic of the microphone channels will be matched for the two
microphones. As a result of this matching there can be a slight slope in the frequency characteristic in one or
both amplifiers.
A-WEIGHTED FILTER
The human ear is sensitive for acoustic signals within a frequency range from about 20Hz to 20kHz. Within this
range the sensitivity of the human ear is not equal for each frequency. To approach the hearing response,
weighting filters are introduced. One of those filters is the A-weighted filter.
The A-weighted filter is used in signal to noise 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.
Figure 35. A-Weighted Filter
MEASURING NOISE AND SNR
The overall noise of the LMV1089 is measured within the frequency band from 10Hz to 22kHz using an A-
weighted filter. The Mic+ and Mic- inputs of the LMV1089 are AC shorted between the input capacitors, see
Figure 36.
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