-

+

VS/2

LMV641

VS

C3

2.2 nF

R3

5.23 k:

VOUT

VOICE IN

C1

0.5 PF

R1

5.23 k:

R2

12.1 k:

C2

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

1

¨

¨

©

§

1 -

1

2

n+1

2

- 1 x f-3 dB

22

LMV641

SNOSAW3D – SEPTEMBER 2007 – REVISED AUGUST 2016

www.ti.com

Product Folder Links: LMV641

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Copyright © 2007–2016, Texas Instruments Incorporated

Typical Applications (continued)

where

•

n is the bit resolution of the ADC

•

f−3 dB is the closed loop corner frequency.

(5)

Given that the LMV641 has a GBW of 10 MHz, and is operating with a closed loop gain of 26.3, its closed loop

bandwidth is 380 kHZ, therefore

(6)

which is the highest frequency that can be measured with required accuracy.

8.2.3 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 46 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 C1 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 VS via its non-inverting input.

This filter is also useful in applications for battery operated talking toys and games.

Figure 46. Low Power Voice In-Band Receive Filter for Battery-Powered Portable Use