MTC20454

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Clock Driver Output CLKM

FUNCTIONAL DECRIPTION

The MTC20454 performs the analog portions of four ATU-C modems.

It has filters (with a programmable cut-off frequency) that use automatic continuous time tuning to avoid

time varying phase characteristics which can be of dramatic consequence for DMT modems. It requires

few external components, uses a 3.3 V supply and is packaged in a 100 pins TQFP in order to reduce

PCB area.

The following descriptions apply to each of the four analog front ends in the chip:

The Receiver (RX)

The DMT signal coming from the lines to the MTCMTC20454MTC2045420454 is first filtered by the two

following external filters:

The signal is amplified by a low noise gain stage (-15..+31 dB) then lowpass filtered to avoid anti-aliasing

and to ease further digital processing by removing unwanted high frequency out-of-band noise. A 12 bits

A/D converter samples the data at 8.832 MS/s, transforms the signal into a igital representation and sends

it to the DMT signal processor via the multiplexed digital interface.

The Transmitter (TX/TXE)

The 12 bits data at 8.832 Ms coming from the DMT signal processor through the multiplexed digital inter-

face are transformed by a D/A converter into an analog signal. This signal is then filtered to decrease DMT

sidelobes levels and meet the ANSI transmitter spectral response but also to reduce he out-of-band noise

(which can be echoed to the RX path) to an acceptable level. The pre-driver buffers The signal for the

external line driver and case of short loops provide attenuation provision

The Digital Interface

The digital part of the MTC20454 can be divided into two parts:

representation for the TX DAC and RX ADC for the requested line.

band, ...) or settings.

Symbol

Parameters

Test Cond

Min

Max

Unit

Low level output voltage

Iout = 4 mA

V

High level output voltage

Iout = 4 mA

V

Load capacitance

30

pF

Dcycle

Duty cycle

45

55

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