|TEA1118AT||Versatile cordless transmisssion circuit|
|TEA1118AT Datasheet PDF : 24 Pages |
Versatile cordless transmisssion circuit
Automatic Gain Control (pin AGC)
The TEA1118 and TEA1118A perform 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 5.8 dB (which corresponds
approximately to a line length of 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).
The ICs can be used with different configurations of
feeding bridge (supply voltage and bridge resistance) by
connecting an external resistor RAGC between pins AGC
and VEE. This resistor enables the Istart and Istop line
currents to be increased (the ratio between Istart and Istop is
not affected by the resistor). The AGC function is disabled
when pin AGC is left open-circuit.
DTMF ampliﬁer (pin DTMF; TEA1118A only)
When the DTMF amplifier is enabled, dialling tones may
be sent on line. These tones can be heard in the earpiece
at a low level (confidence tone).
The TEA1118A has an asymmetrical DTMF input.
The input impedance between DTMF and VEE is 20 kΩ.
The voltage gain from pin DTMF to pin LN is 17.4 dB.
The automatic gain control has no effect on the DTMF
Mute function (pin MUTE; TEA1118A only)
The mute function performs the switching action between
the speech mode and the dialling mode. When MUTE is
LOW or open-circuit, the transmit and receive amplifiers
inputs are enabled while the DTMF input is disabled,
depending on the TMUTE level. When MUTE is HIGH, the
DTMF input is enabled and the transmit and receive
amplifiers inputs are disabled.
The TEA1118 and TEA1118A anti-sidetone network
comprising RCC//Zline, Rast1, Rast2, Rast3, RSLPE and Zbal
(see Fig.12) suppresses the transmitted signal in the
earpiece. Maximum compensation is obtained when the
following conditions are fulfilled:
RSLPE × Rast1 = RCC × (Rast2 + Rast3)
k = --[--R-----a---s--t-(-2--R--×--a---s-(--tR-1----a×---s--tR--3---S-+--L---RP---E-S---)-L---P---E---)---]--
Zbal = k × Zline
The scale factor k is chosen to meet the compatibility with
a standard capacitor from the E6 or E12 range for Zbal.
In practice, Zline varies considerably with the line type and
the line length. Therefore, the value chosen for Zbal should
be for an average line length which gives satisfactory
sidetone suppression with short and long lines.
The suppression also depends on the accuracy of the
match between Zbal and the impedance of the average
The anti-sidetone network for the TEA1118 and
TEA1118A (as shown in Fig.16) attenuates the receive
signal from the line by 32 dB before it enters the receive
The attenuation is almost constant over the whole audio
A Wheatstone bridge configuration (see Fig.13) may also
More information on the balancing of an anti-sidetone
bridge can be obtained in our publication “Applications
Handbook for Wired Telecom Systems, IC03b”, order
number 9397 750 00811.
MUTE and TMUTE levels for different modes (TEA1118A only)
Table 1 Required MUTE and TMUTE levels to enable the different possible modes
1. X = don’t care.
1997 Jul 14
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