MT88E45
CAS detection only, its gain can always be tailored
specifically for the CAS signal level.
The FSK demodulator is compatible with Bellcore,
ETSI and BT standards. The demodulated FSK data
is either output directly (bit stream mode) or stored in
a one byte buffer (buffer mode). In the buffer mode,
the stop bit immediately following a byte is also
stored and can be shifted out after the data byte.
This facility allows for framing error checking
required in Type 2 CPEs. In the bit stream mode, two
timing signals are provided. One indicates the bit
sampling instants of the data byte, the other the end
of byte. A carrier detector indicates presence of
signal and shuts off the data stream when there is no
signal.
The entire chip can be put into a virtually zero
current power down mode. The input op-amps, FSK
demodulator, CAS detector and the oscillator are all
shut off. Furthermore, power management has been
incorporated to minimize operating current. When
FSK is selected the CAS detector is powered down.
When CAS is selected the FSK demodulator is
powered down.
Advance Information
hook CLIP, while Hybrid CAS detection is needed for
optimal CAS speech immunity.
The feedback resistor connected between GS1 and
IN1- can be used to adjust the Tip/Ring signal gain.
The feedback resistor connected between GS2 and
IN2- can be used to adjust the hybrid receive signal
gain. When the Tip/Ring op-amp is selected, the
GS2 signal is ignored. When the Hybrid op-amp is
selected, the GS1 signal is ignored.
Either or both op-amps can be configured in the
single ended input configuration shown in Figure 3,
or in the differential input configuration shown in
Figure 4.
IN+
C
R
IN
IN-
Voltage Gain
(A
V
) = R
F
/ R
IN
Highpass Corner Frequency
f
-3dB
= 1/(2πR
IN
C)
R
F
GS
V
REF
Functional Description
3 to 5V Operation
Figure 3 - Single Ended Input Configuration
C1
R1
IN+
IN-
The MT88E45’s FSK and CAS reject levels are
proportional to Vdd. When operated at Vdd equal 3V
+/- 10%, to keep the FSK and CAS reject levels as at
5V (nominal) the Tip/Ring and Hybrid op-amp gains
should be reduced from those of 5V. Gains for
nominal Vdd (with a +/- 10% variation) other than 3
or 5V can be chosen as interpolation between the 3
and 5V settings.
Input Configuration
The MT88E45 provides an input arrangement
comprised of two op-amps and a bias source (V
REF
).
V
REF
is a low impedance voltage source which is
used to bias the op-amp inputs at Vdd/2. The Tip/
Ring op-amp (IN1+, IN1-, GS1 pins) is for connecting
to Tip and Ring. The Hybrid op-amp (IN2+, IN2-,
GS2 pins) is for connecting to the telephone hybrid
or speech IC receive pair.
Either FSK or CAS detection can be selected for the
Tip/Ring connection, while the hybrid connection is
for CAS detection only. Phrased in another way, FSK
demodulation is always on Tip/Ring, while CAS
detection can be on Tip/Ring or Hybrid Receive. Tip/
Ring CAS detection is required for MEI and BT on-
6
C2
R4
R5
GS
R3
R2
V
REF
Differential Input Amplifier
C1 = C2
R1 = R4 (For unity gain R5= R4)
R3 = (R2R5) / (R2 + R5)
Voltage Gain
Highpass Corner Frequency
(A
V
diff) = R5/R1
f
-3dB
= 1/(2πR1C1)
Input Impedance
(Z
IN
diff) = 2
R1
2
+ (1/ωC)
2
Figure 4 - Differential Input Configuration
CAS Detection
In North America, CAS is used in off-hook signalling
only. In Europe (ETSI) it is used in off-hook
signalling, and by BT in both on and off-hook
signalling. ETSI calls it the Dual Tone Alerting Signal
(DT-AS). Although the ETSI on-hook standard
contains a DT-AS specification, BT is the only
administration known to employ CAS in on-hook
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