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:

ZI / /

Zm

Zb

= R2

R3

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.

LOGIC OF MUTE SWITCHING

MUTE

DTMF

BEEP

MIC IMP

RX IMP

LOW

ACTIVE TO LINE

ACTIVE TO EARPHONE

MUTED

MUTED

(DIAL)

OUTPUT

OUTPUT

OPEN

ACTIVE TO LINE

ACTIVE TO EARPHONE

ACTIVE

ACTIVE

(SPEECH)

OUTPUT

OUTPUT

Figure 4: 2/4 Wire Conversion

L3281

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