AD45048ARZ Analog Devices Inc, AD45048ARZ Datasheet - Page 7

• Manufacturer Part Number: AD45048ARZ
• Description: IC ADSL LINE DRIVER R-R 8SOIC
• Manufacturer: Analog Devices Inc
• Type: Driverr

Specifications of AD45048ARZ

• Number Of Drivers/receivers: 2/0
• Protocol: xDSL
• Voltage - Supply: 6 V ~ 12 V
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Op Amp Type: Rail To Rail
• No. Of Amplifiers: 2
• Bandwidth: 65MHz
• Slew Rate: 55V/µs
• Supply Voltage Range: 3.3V To 24V
• Amplifier Case Style: SOIC
• No. Of Pins: 8
• Operating Temperature Range: -40°C To
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

GENERAL DESCRIPTION
The AD45048 is a voltage feedback, rail-to-rail output amplifier
with high output current capability. Fabricated on Analog
Devices’ proprietary high speed eXtra fast complementary
bipolar high voltage process (XFCB-HV), the high bandwidth
and fast slew rate of the AD45048 keep distortion to a
minimum while dissipating minimum power. The XFCB-HV,
silicon-on-insulator (SOI) process prevents latch-up problems
and enables the construction of high frequency, low distortion
amplifiers, such as the AD45048.
POWER SUPPLY AND DECOUPLING
The AD45048 can be powered with a good quality, well
regulated, low noise supply anywhere in the range from +3 V to
±12.6 V. In order to optimize the AD45048 in standard ADSL
CPE line driver applications (see Figure 8), power the amplifier
with a well regulated 12 V supply. Careful attention should be
paid to decoupling the power supply. High quality capacitors
with low equivalent series resistance (ESR), such as multilayer
ceramic capacitors (MLCCs), should be used to minimize the
supply voltage ripple and power dissipation. A 0.1 μF MLCC
decoupling capacitor(s) should be located no more than
1/8-inch away from the power supply pin(s). A large, usually
tantalum, 10 μF to 47 μF capacitor is recommended to provide
good decoupling for lower frequency signals and to supply
current for fast, large signal changes at the AD45048 outputs.
LAYOUT CONSIDERATIONS
As is the case with all high speed applications, careful attention
to printed circuit board layout details prevents associated board
parasitics from becoming problematic. Proper RF layout and
printed circuit board design techniques are strongly
recommended. The PCB should have a low impedance return
path (or ground) to the supply. Removing the ground plane
from all layers in the immediate area of the amplifier reduces
stray capacitances. The signal routing should be short and
direct in order to minimize the parasitic inductance and
capacitance associated with these traces. Termination resistors
and loads should be located as close as possible to their
respective inputs and outputs. Input traces should be kept as far
apart as possible from the output traces to minimize coupling
(crosstalk) though the board.
Wherever there are complementary signals, a symmetrical
layout should be provided to the extent possible in order to
maximize balanced performance. When running differential
signals over a long distance, the traces on the PCB should be
close together or any differential wiring should be twisted
together to minimize the area of the loop that is formed. This
reduces the radiated energy and makes the circuit less
susceptible to RF interference. Adherence to stripline design
techniques for long signal traces (greater than about 1 inch) is
recommended.
Rev. A | Page 7 of 8
CPE ADSL APPLICATION
The low cost, high output current dual AD45048 xDSL line
driver amplifiers have been specifically designed to drive high
fidelity xDSL signals to within 0.5 V of the power rails on a
single 12 V supply. The AD45048 can be used in transformer-
coupled bridge hybrid circuits designed to drive modulated
signals, including discrete multitone (DMT), upstream to the
central office.
TRANSFORMER SELECTION
Customer premise ADSL applications require the transmission
of a 13 dBm DMT signal (20 mW into 100 Ω). DMT signals can
have a crest factor (V peak/V rms ratio) as high as 5.3, requiring
the line driver to provide a peak power of 560 mW. The line
driver is required to drive a 7.5 V peak onto the 100 Ω
telephone line while maintaining about −65 dBc to −70 dBc of
MTPR. Since the maximum low distortion output swing
available from the AD45048 line driver is approximately 11.5 V
on a 12 V supply (depending on the load), and taking into
account the power lost in the transformer and termination
resistors, a step-up transformer with a minimum turns ratio of
1.5 or greater is needed. In the simplified differential driver
circuit shown in Figure 8, the AD45048 is driving a 25 Ω
impedance reflected by 1:2 step-up transformer. R3 and R6 are
12.5 Ω each and are back-termination or load-matching
resistors whose values can be calculated by
where 100 Ω is the approximate phone line impedance and N is
the transformer turns ratio. In Figure 8, the total differential
load including the termination resistors is 50 Ω, and under
these conditions, the AD45048 is capable of driving low
distortion signals to within 0.5 V of the power rails.
RECEIVE CHANNEL CONSIDERATIONS
A step-up transformer of N turns used at the output of the
differential line driver increases the differential output voltage
to the line (see Figure 8). However, the inverse effect is seen in
the receive channel as the amplitude of signal on the driver side
of the transformer is divided by N turns. The decision to use a
particular transformer turns ratio may be impacted by the
ability of the receive circuitry to resolve low level signals in the
noisy twisted pair telephone plant. Higher turns ratio
transformers reduce the effective receive channel SNR (signal-
to-noise ratio) due to the reduction in the received signal
strength.
An amplifier with low RTI noise, such as the AD8022
(2.5 nV/√Hz), is recommended for the receive channel. For a
complete selection of amplifiers and other related components,
see www.analog.com.
(100 Ω/(N
2
))/2
AD45048