MTC20154 Datasheet, PDF (11/21 Pages) STMicroelectronics – INTEGRATED ADSL CMOS ANALOG FRONT-END CIRCUIT

MTC20154
MTC20154 RX PATH
Speech Filter
An external bidirectional LP filter for up and downstream POTS service splits out the speech signal to the
analog telephone circuit on both the NT and LT sides of the line. The ADSL analog front end integrated
circuit does not contain any circuitry for the POTS service but guarantees that the POTS bandwidth is not
disturbed by spurious signals from the ADSL spectrum.
Channel Filters
The purpose of these external analog circuits is to provide partial echo cancellation by analog filtering of
the receive signal for both ATU-R (reception of downstream channel) and ATU-C (reception of upstream
channel). This is feasible because the upstream and the downstream data can be modulated on separate
carriers (FDM).
Signal Attenuator (ATT) and Low Noise Amplifier
The attenuator needs to be DC decoupled from the external circuitry. In fact, it is also used to internally fix
the LNA input common mode voltage at the nominal value: AVDD/2. This is done by the use of an internal
biasing circuit. It is therefore mandatory to decouple the MTC20154 input from any external DC biasing
system. The Low Noise Amplifier (LNA) placed after the ATT will be used in combination with the attenu-
ation block. The goal is to obtain a range of RX path input level varying from –15 dB to 31 dB, while main-
taining the noise contribution negligible.
Figure 6. Signal Attenuator (ATT) and Low Noise Amplifier
Attenuation
2
RXP
Gain
5
Vref
LNA
RXN
Attenuator
MTC-20154
The input attenuator will have the following characteristics
Table 9. Attenuator Characteristics
Data
Input type
Common mode voltage (forced)
Maximum input differential voltage
Input impedance
Attenuation step
Attenuation range
Maximum attenuation error
Digital interface
Digital code
Min
AC only, DC decoupled
–
–
10
–
0
Typical
–
VDD/2
VDD
14.47
5
–
–
.05
–
2
‘11’ –> 0 dB attenuation
’00’ –> –15 dB attenuation
Max
–
VDD+1Vd
19
–
–15
–
–
Unity
V
Vpdiff
KOhms
dB
dB
dB
bits
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