U4082B Datasheet, PDF(7/30 Page) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

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U4082B Datasheet, PDF (7/30 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

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U4082B

3.5 Volume Control

The volume control input at VCI (pin 13) is sensed as a voltage with respect to VB. It affects the

attenuators in receive mode only and has no effect during idle or transmit mode.

In receive mode, the attenuator receive gain, GR, is +6.0 dB, and attenuator transmit gain GT is

â46 dB under the condition that VCI = VB. When VCI < VB, the attenuator receive gain is

reduced (Figure 7-5 on page 19), whereas the attenuator transmit gain is increased; their sum,

however, remains constant. Voltage deviation at VCI changes the voltage at CT, which in turn

controls the attenuators (see section âAttenuator Control Blockâ on page 8).

The volume control setting does not affect the maximum attenuator input signal at which notice-

able distortion occurs.

The bias current at VCI is typically â60 nA. It does not vary significantly with the VCI voltage or

supply voltage VS.

3.6 Dial Tone Detector

The dial tone detector is a comparator with one side connected to the receive input (RI) and the

other to VB with a 15 mV offset (see Figure 3-3). If the circuit is in idle mode, and the incoming

signal is greater than 15 mV (10 mVrms), the comparator's output will change, disabling the

receive idle mode. The receive attenuator will then be at a setting determined mainly by the vol-

ume control.

This circuit prevents the dial tone (which would be considered as continuous noise) from fading

away as the circuit would have the tendency to switch to idle mode. By disabling the receive idle

mode, the dial tone remains at the normally-expected full level.

Figure 3-3. Dial Tone Detector

3.7 AGC

To R attenuator

15 mV

To attenuator

control

The AGC circuit affects the circuit only in receive mode, and only when the supply voltage is less

than 3.5V. As VS < 3.5V, the gain of the receive attenuator is reduced as seen in Figure 7-6 on

page 19. The transmit path attenuation changes such that the sum of the transmit and receive

gains remains constant.

The purpose of this feature is to reduce the power (and current) used by the speaker when a

line-powered speakerphone is connected to a long line where the available power is limited. By

reducing the speaker power, the voltage sag at VS is controlled, preventing possible erratic

operation.

4743DâCORDâ03/06

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