MTC20454 ST Microelectronics, Inc., MTC20454 Datasheet - Page 6

• Manufacturer Part Number: MTC20454
• Description: Quad Integrated Adsl CMOS Analog Front End Circuit
• Manufacturer: ST Microelectronics, Inc.

MTC20454
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:
■
■
6/15
Symbol
Dcycle
POTS HP filter: Attenuation of speech and POTS signalling.
Channel filter: Attenuation of echo signal to improve RX dynamic.
The data interface converts the multiplexed data from/to the DMT signal processor into a valid
representation for the TX DAC and RX ADC for the requested line.
The control interface allows the board processor to configure the MTC20454 paths (RX/TX gains, filter
band, ...) or settings.
C
V
V
load
OH
OL
Low level output voltage
High level output voltage
Load capacitance
Duty cycle
Parameters
Iout = 4 mA
Iout = 4 mA
Test Cond
.85*DV
Min
45
DD
.15*DV
Max
30
55
DD
Unit
pF
%
V
V