L3281 STMicroelectronics, L3281 Datasheet - Page 5

• Manufacturer Part Number: L3281
• Description: LOW VOLTAGE TELEPHONE SPEECH CIRCUITS
• Manufacturer: STMicroelectronics

LOGIC OF MUTE SWITCHING
CIRCUIT DESCRIPTION
TWO TO FOUR WIRE CONVERSION
The L3281AB is based on a Wheastone bridge
configuration. To balance the bridge the following
relation must be satisfied:
Figure 4: 2/4 Wire Conversion
(SPEECH)
MUTE
(DIAL)
OPEN
LOW
ZI / /
ACTIVE TO LINE
ACTIVE TO LINE
Zb
Zm
OUTPUT
OUTPUT
DTMF
R2
R3
ACTIVE TO EARPHONE
ACTIVE TO EARPHONE
OUTPUT
OUTPUT
BEEP
The AC signal from the microphone is sent to one
diagonal of the bridge (pins 1 and 3). A small per-
centage of the signal power is lost on Zb (being
Zb > (Zm//Zi)); the main part is sent to the line via
R2.
In receiving mode, the AC signal coming from the
LINE is sensed across the second diagonal of the
bridge (pins 12 and 2).
The impedance Zm and Zb can be complex.
MIC IMP
ACTIVE
MUTED
MUTED
ACTIVE
RX IMP
L3281
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