H(s) =

Post Amplifier gain

sRfCf+1

LMV1099

CORRECT

1.5~2.5 cm

OPTIMIZED

SPEECH

LMV1099

NEAR

SPEECH

WRONG

OPTIMIZED

SPEECH

LMV1099

SNAS490D – JULY 2010 – REVISED MAY 2013

www.ti.com

Figure 26. Broadside Array (WRONG)

Figure 27. Endfire Array (CORRECT)

Low-Pass Filter At The Output

At the output of the LMV1099 there is a provision to create a 1st 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:

(2)

This low-pass filter is created by connecting a capacitor between the LPF pin and the OUT pin of the LMV1099.

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 8.

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 Setting (dB)

Rf (kΩ)

Cf (nF)

6

20

3.9

12

40

2.0

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.

20

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