MTC-20144TQ-I AMI Semiconductor, Inc., MTC-20144TQ-I Datasheet - Page 2

• Manufacturer Part Number: MTC-20144TQ-I
• Description: Integrated ADSL CMOS Analog Front-End Circuit
• Manufacturer: AMI Semiconductor, Inc.

Functional Description
The MTC-20144 chip can be used on
the ATU-C side (LT), and on the ATU-R
(NT) side (defined by LTNT pin). The
selection consists mainly of a filter
interchange between the RX and TX
path . The filters (with a programmable
cutoff frequency) 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 3.3V supply and is
packaged in a 64 pins TQFP in order to
reduce PCB area. On the digital
interface, a alternative mode with lower
sampling rate (OSR=2; 4.4
Msamples/s) is provided. This allows
support of xDSL digital components
operating at different frequencies.
The Receiver (RX)
The DMT signal coming from the line to
the MTC-20144 is first filtered by the
two following external filters:
- POTS HP filter: Attenuation of speech
and POTS signalling.
- Channel filter: Attenuation of echo
signal to improve RX dynamic.
An analog multiplexer allows the
selection between two input ports which
can be used to select an attenuated
(0,10 dB for ex.) version of the signal in
case of short loop or large echo. The
signal is amplified by a low noise gain
stage (0-31 dB) then low-pass 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 (a 4.416 Ms/s
mode is also possible), transforms the
signal into a digital representation and
send it to the DMT signal processor via
the digital interface.
The Transmitter
(TX/TXE)
The 12 bits data at 8.832 Ms/s (or
4.416 Ms/s) coming from the DMT
signal processor through the digital
interface are transformed by a D/A
converter into a analog signal.
This signal is then filtered to decrease
DMT sidelobes level and meet the ANSI
transmitter spectral response but also to
reduce the 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
in case on short loop provide
attenuation provision (-15..0 dB).
The VCXO
The VCXO is divided in a XTAL driver
and a auxilliary 8 bits DAC for timing
recovery.
The XTAL driver is able to operate at
35.328 MHz and provides an
amplitude regulation mechanism to
avoid temperature/supply/technology
dependent frequency pulling.
The DAC which is driven by the CTRLIN
pin provides a current output with 8 bits
resolution and can be used to tune the
XTAL frequency with the help of external
components. A time constant between
DAC input and VCXO output can be
introduced (via the CTLIN interface) and
programmed with the help of an
external capacitor (on VCOCAP pin).
2
The Digital Interface
The digital part of the MTC-20144 can
be divided in 3 parts :
The data interface converts the
multiplexed data from/to the DMT
signal processor into valid
representation for the TX/TXE DAC and
RX ADC.
The control interface allows the board
processor to configure the MTC-20144
paths (RX/TX gains, filter band, ...) or
settings (OSR, vcodac enable,
digital/analog loopback, ...).
Package
The MTC-20144 is housed in a 64-pin
TQFP package.
MTC-20144