Measurement Setup
Because of the nature of the calibration system it is not pos-
sible 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 (RA[3:0] and RB[2:0]) and the actual
measured gain between input and output of the LMV1088.
After performing a calibration the frequency characteristic of
the microphone channels will be matched for the two micro-
phones. 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 fre-
quency range from about 20Hz to 20kHz. Within this range
the sensitivity of the human ear is not equal for each frequen-
cy. 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
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.
20213010
FIGURE 11. A-Weighted Filter
21 www.national.com
LMV1088
