SM8223A Datasheet, PDF (11/15 Pages) Nippon Precision Circuits Inc – FSK Decoder and DTMF Receiver IC

SM8223A
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.
C1 R1
C1 R1
R3
TIP
RING
R2 _GS
R4 _AGND
C
Differntial Input
C1 R1
TIP
RING
R2 _GS
AGND
Single-Ended Input
C
Figure 3. Input circuits
The gain for single-ended configurations is given by
the following equation.
A_V
=
-^R---²--
R₁
and the input impedance is given by the following
equation.
Z_i = 2
R₁₂
+


ω^-----1C^----₁^{- }
2
The gain for differential configurations is given by
the following equation,
A_V
=
-^R---²--
R₁
where
R₃=
----^R---²----^R---⁴-----
R₂ + R₄
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.
NIPPON PRECISION CIRCUITS—11