UBA1707TS datasheet(13/36 Pages) PHILIPS

13 / 36 page

1999 Feb 17

Philips Semiconductors

Product speciﬁcation

Cordless telephone, answering machine

line interface

UBA1707

Fig.11 Equivalent circuit of an anti-sidetone network in a Wheatstone bridge configuration.

handbook, full pagewidth

MGL217

ZRXI

RXI

Zbal

Rast1

SLPE

RSLPE

GND

Zline

ZSET

AUTOMATIC GAIN CONTROL (PIN AGC; BITS RAGC1,

RAGC2, SAGC AND AGC)

The UBA1707 performs automatic line loss compensation.

The automatic gain control varies the gain of the transmit

amplifier and the gain of the receive amplifier in

accordance with the DC line current. The control range is

6.5 dB (which roughly corresponds to a line length of

5.5 km for a 0.5 mm diameter twisted-pair copper cable

with a DC resistance of 176

Ω/km and an average

attenuation of 1.2 dB/km).

When the line current is greater than Istop, the voltage

gains are minimum. When the line current is less than Istart,

the voltage gains are maximum. When the AGC pin is

connected to pin GND, the start line current (Istart) can be

chosen between 22.5 and 29.5 mA via bits RAGC1 and

RAGC2 through the serial interface. Two values for the

Istop/Istart ratio (slope of the AGC) are possible via the bit

SAGC through the serial interface. When bit SAGC is at

logic 0 then Istop = 2.7 × Istart (optimized for voltage

regulation mode). When SAGC is at logic 1 then

Istop = 1.9 × Istart (optimized for current regulation mode).

An external resistor RAGC (connected between pins GND

and AGC) enables the Istart and Istop line currents to be

increased (the ratio between Istart and Istop is not affected

by this external resistor). So internal and external

adjustments of the automatic gain control allow

optimization of the IC for many configurations of exchange

supply voltage and feeding bridge resistance. Part of the

line current flows into the ZSET impedance network. The IC

has been optimized for ZSET = 619 Ω. Changing this 619 Ω

value slightly modifies Istop and Istart line currents as well as

the value of the two AGC slopes.

The automatic gain control function can be disabled by

setting the AGC bit to logic 0 via the serial interface or

when pin AGC is left open-circuit. In this case both of the

voltage gains are maximum.

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