LMV641 datasheet(15/18 Pages) NSC

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VOICEBAND FILTER

The majority of the energy of recognizable speech is within a

band of frequencies between 200 Hz and 4 kHz. Therefore it

is beneficial to design circuits which transmit telephone sig-

nals that pass only certain frequencies and eliminate unwant-

ed signals (noise) that could interfere with conversations and

introduce error into control signals. The pass band of these

circuits is defined as the ranges of frequencies that are

passed. A telephone system voice frequency (VF) channel

has a pass band of 0 Hz to 4 kHz. Specifically for human

voices most of the energy content is found from 300 Hz to 3

kHz and any signal within this range is considered an in-band

signal. Alternatively, any signal outside this range but within

the VF channel is considered an out-of-band signal.

To properly recover a voice signal in applications such as

cellular phones, cordless phones, and voice pagers, a low

power bandpass filter that is matched to the human voice

spectrum can be implemented using an LMV641 op amp.

Figure 9 shows a multi-feedback, multi-pole filter (2nd order

response) with a gain of −1. The lower 3 dB cutoff frequency

which is set by the DC blocking capacitor C

1 and resistor R1

is 60 Hz and the upper cutoff frequency is 3.5 kHz.

The total current consumption is a mere 138 µA. The LV641

is operating with a gain of −1, but the circuit is easily modified

to add gain. The op amp is powered from a single supply,

hence the need for offset (common-mode) adjustment of its

output, which is set to ½ V

S via its non-inverting input.

This filter is also useful in applications for battery operated

talking toys and games.

20203373

FIGURE 9. Low Power Voice In-Band Receive Filter for

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