UBA1706TS datasheet(13/28 Pages) PHILIPS

13 / 28 page

1999 Jun 04

Philips Semiconductors

Objective speciﬁcation

Cordless telephone line interface

UBA1706

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 UBA1706 performs automatic line loss compensation.

The AGC 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 corresponds

roughly 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 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). Therefore, internal and external

adjustments of the AGC 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 AGC function can be disabled by setting the AGC bit

to logic 0 via the serial interface or by leaving pin AGC

open-circuit. In this case, both of the voltage gains are

maximum.

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