LMV1089RLEVAL National Semiconductor, LMV1089RLEVAL Datasheet - Page 28

• Manufacturer Part Number: LMV1089RLEVAL
• Description: BOARD EVALUATION LMV1089
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LMV1089RLEVAL

• Main Purpose: Audio, Microphone Amp
• Embedded: No
• Utilized Ic / Part: LMV1089
• Primary Attributes: Dual Input, Single Output
• Secondary Attributes: Graphical User Interface, I²C Interface
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

www.national.com
Low-Pass Filter At The Output
At the output of the LMV1089 there is a provision to create a
1
mode). This low-pass filter can be used to compensate for the
change in frequency response that results from the noise
cancellation process. The change in frequency response re-
sembles a first-order high-pass filter, and for many of the
applications it can be compensated by a first-order low-pass
filter with cutoff frequency between 1.5kHz and 2.5kHz.
The transfer function of the low-pass filter is derived as:
This low-pass filter is created by connecting a capacitor be-
tween the LPF pin and the OUT pin of the LMV1089. The
value of this capacitor also depends on the selected output
gain. For different gains the feedback resistance in the low-
pass filter network changes as shown in
This will result in the following values for a cutoff frequency of
2000 Hz:
st
Post Amplifier Gain Setting (dB)
order low-pass filter (only enabled in 'Noise Cancelling'
Setting (dB )in Pass
Post Amplifier Gain
TABLE 8. Low-Pass Filter Internal Impedance
TABLE 9. Low-Pass Filter Capacitor For 2kHz
Through mode
12
15
18
6
9
12
15
18
6
9
Feedback Resistance R
Table
R
(kΩ)
20
29
40
57
80
f
FIGURE 16. Endfire Array (CORRECT)
20
29
40
57
80
(kΩ)
8.
C
f
3.9
2.7
2.0
1.3
1.0
(nF)
f
28
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 (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 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,
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.
30047242