Datasheet
-
+
V
S
/2
LMV641
V
S
C
3
2.2 nF
R
3
5.23 k:
V
OUT
VOICE IN
C
1
0.5 PF
R
1
5.23 k:
R
2
12.1 k:
C
2
15 nF
1
¨
¨
©
§
1 -
1
2
n
+1
©
- 1 = 0.062 x f
-3 dB
MAX FREQ =
= 0.062 x 380 kHz = 23.56 kHz
¨
¨
§
2
LMV641
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SNOSAW3C –SEPTEMBER 2007–REVISED FEBRUARY 2013
(5)
which is the highest frequency that can be measured with required accuracy.
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 signals that pass only certain frequencies
and eliminate unwanted 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 48 shows a multi-feedback, multi-pole filter (2
nd
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 R
1
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.
Figure 48. Low Power Voice In-Band Receive Filter for Battery-Powered Portable Use
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