MT88E45

Data Sheet

7

Zarlink Semiconductor Inc.

detecting CAS from Tip/Ring, in addition to detecting CAS from the hybrid/speech IC receive signal when it is off-

hook.

The MT88E45B offers an optimal solution which combines good speech immunity and MEI compatibility. Two input

op-amps allow the MT88E45B to be connected both to Tip/Ring and to the hybrid/speech IC receive pair. Both

connections can be differential or single ended. FSK demodulation is always on the Tip/Ring signal. CAS detection

can be from the Tip/Ring or hybrid/speech IC receive signal. Being able to detect CAS on Tip/Ring also makes the

MT88E45B suitable for BT on-hook CLIP applications.

For applications such as those in most European countries where Tip/Ring CAS detection is not needed, then the

Tip/Ring and Hybrid op-amp gains can be tailored independently to meet country specific FSK and CAS signal level

requirements respectively. Note that since the Hybrid op-amp is for 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.

Functional Description

3 to 5 V Operation

The MT88E45B’s FSK and CAS reject levels are proportional to Vdd. When operated at Vdd equal 3 V +/- 10%, to

keep the FSK and CAS reject levels as at 5 V (nominal) the Tip/Ring and Hybrid op-amp gains should be reduced

from those of 5 V. Gains for nominal Vdd (with a +/- 10% variation) other than 3 or 5 V can be chosen as

interpolation between the 3 and 5 V settings.

Input Configuration

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-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 33, or in the

differential input configuration shown in Figure 44.