Datasheet

H(s) =
Post Amplifier gain
sR
f
C
f
+1
LMV1089
NEAR
SPEECH
OPTIMIZED
SPEECH
CORRECT
1.5 ~ 2.5 cm
LMV1089
www.ti.com SNAS441H –SEPTEMBER 2008–REVISED MAY 2010
Figure 34. Endfire Array (CORRECT)
Low-Pass Filter At The Output
At the output of the LMV1089 there is a provision to create a 1
st
order low-pass filter (only enabled in 'Noise
Cancelling' 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 resembles 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 between 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 Table 7.
Table 7. Low-Pass Filter Internal Impedance
Post Amplifier Gain Setting (dB
Feedback Resistance R
f
(kΩ)
)in Pass Through mode
6 20
9 29
12 40
15 57
18 80
This will result in the following values for a cutoff frequency of 2000 Hz:
Table 8. Low-Pass Filter Capacitor For 2kHz
Post Amplifier Gain R
f
(kΩ) C
f
(nF)
Setting (dB)
6 20 3.9
9 29 2.7
12 40 2.0
15 57 1.3
18 80 1.0
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