CF8223A datasheet(11/15 Pages) NPC

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SM8223A

NIPPON PRECISION CIRCUITS—11

Input Differential Ampliﬁer

The SM8223A uses an input differential ampliﬁer

for input gain adjustment of the tip/ring signal input

to the FSK detector or DTMF receiver. Differential

input conﬁguration and single-ended input conﬁgu-

ration circuits are shown in ﬁgure 3. A bypass capac-

itor should be connected between GND and AGND

in both circuit conﬁgurations.

The gain for single-ended conﬁgurations is given by

the following equation.

The gain for differential conﬁgurations is given by

the following equation,

and the input impedance is given by the following

equation.

FSK/DTMF Auto-discriminator

The SM8223A examines the tip/ring input signal and

determines the nature of the signal, FSK or DTMF,

and invokes the corresponding circuits, FSK decoder

or DTMF receiver, respectively. It determines

whether the input signal is an FSK signal or DTMF

signal by the presence or otherwise of the channel

seizure information in the FSK signal header. This

function automatically discriminates between the

input signals if the FSK signal conforms to the

Bellcore GR-30-CORE standard.

Figure 3. Input circuits

RING

AGND

TIP

Differntial Input

RING

AGND

TIP

Single-Ended Input

------

------ where R

2R4

-------------------

2 R

ωC

-----------



 2

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