AD8018
Following these generic guidelines will improve the performance
of the AD8018 in all applications.
Stability Enhancements
The CPE bridge hybrid circuit presents a complex impedance to
the drive amplifiers, particularly when transformer parasitics are
factored in. To ensure stable operation under the full range of
load conditions, a series R-C network (Zoebel Network) should
be connected between each amplifier’s output and ground. The
recommended values are 10 Ω for the resistor and 1 nF for the
capacitor to create a low impedance path to ground at frequen-
cies above 16 MHz (see Figure 2). R33 and R34 are added to
improve common-mode stability.
To optimize the AD8018’s performance as an ADSL differential
line driver, locate the transformer hybrid near the AD8018 drivers
and as close to the RJ11 jack as possible. Maintain differential
circuit symmetry into the differential driver and from the output
of the drivers through the transformer-coupled output of the bridge
circuit as much as possible.
CPE ADSL Application
The low-cost, high-output current dual AD8018 xDSL driver
amplifiers have been specifically designed to drive high fidelity
xDSL signals to within 0.5 V of the power rails, the performance
needed to provide CPE ADSL on a single 5 V supply. The
AD8018 may be used in transformer-coupled bridge hybrid cir-
cuits to drive modulated signals including Discrete MultiTone
(DMT) upstream to the central office.
Receive Channel Considerations
A transformer used at the output of the differential line driver to
step up the differential output voltage to the line has the inverse
effect on signals received from the line. A voltage reduction
or attenuation equal to the inverse of the turns ratio is realized
in the receive channel of a typical bridge hybrid. The turns ratio
of the transformer may also be dictated by the ability of the receive
circuitry to resolve low-level signals in the noisy twisted pair tele-
phone plant. Higher turns ratio transformers effectively reduce the
received signal-to-noise ratio due to the reduction in the received
signal strength.
Evaluation Board
The AD8018ARU-EVAL evaluation board circuit in Figure 12
offers the ability to evaluate the AD8018 in a typical xDSL bridge
hybrid circuit.
The receiver circuit on these boards is typically unpopulated.
Requesting samples of the AD8022AR with the AD8018ARU-
EVAL board will provide the capability to evaluate the
AD8018ARU along with other Analog Devices products in a typi-
cal transceiver circuit. The evaluation circuits have been designed
to replicate the CPE side analog transceiver hybrid circuits.
The AD8022, a dual amplifier with typical RTI voltage noise of
only 2.5 nV/√Hz and a low supply current of 4 mA/amplifier, is
recommended for the receive channel.
DMT Modulation, MultiTone Power Ratio (MTPR), and
Out-of-Band SFDR
ADSL systems rely on DMT modulation to carry digital data
over phone lines. DMT modulation appears in the frequency
domain as power contained in several individual frequency
subbands, sometimes referred to as tones or bins, each of which
is uniformly separated in frequency. A uniquely encoded, Quadra-
ture Amplitude Modulation (QAM)-like signal occurs at the center
frequency of each subband or tone. See Figure 9 for an example
of a DMT waveform in the frequency domain, and Figure 10 for
a time domain waveform. Difficulties will exist when decoding
these subbands if a QAM signal from one subband is corrupted
by the QAM signal(s) from other subbands, regardless of whether
the corruption comes from an adjacent subband or harmonics of
other subbands.
The circuit mentioned above is designed using a one-transformer
transceiver topology including a line receiver, line driver, line
matching network, an RJ11 jack for interfacing to line simulators,
and transformer-coupled inputs for single-ended-to-differential
input conversion.
AC-coupling capacitors of 0.01 µF, C8, and C10, in combina-
tion with 10 kΩ resistors R24 and R25, will form a zero frequency
at 1.6 kHz.
Transformer Selection
Customer premise ADSL requires the transmission of a +13 dBm
(20 mW) DMT signal. The DMT signal can have a crest factor
as high as 5.3, requiring the line driver to provide peak line power
of 27.5 dBm (560 mW). 27.5 dBm peak line power translates
into a 7.5 V peak voltage on the 100 Ω telephone line. Assuming
that the maximum low-distortion output swing available from
the AD8018 line driver on a 5 V supply is 4 V and, taking into
account the power lost due to the termination resistance, a step-up
transformer with turns ratio of 4.0 or greater is needed.
Conventional methods of expressing the output signal integrity
of line drivers, such as single-tone harmonic distortion or THD,
two-tone InterModulation Distortion (IMD), and third order
intercept (IP3), become significantly less meaningful when
amplifiers are required to process DMT and other heavily
modulated waveforms. A typical ADSL upstream DMT signal
can contain as many as 27 carriers (subbands or tones) of
QAM signals. MultiTone Power Ratio (MTPR) is the relative
difference between the measured power in a typical subband (at
one tone or carrier) versus the power at another subband spe-
cifically selected to contain no QAM data. In other words, a
selected subband (or tone) remains open or void of intentional
power (without a QAM signal), yielding an empty frequency bin.
MTPR, sometimes referred to as the “empty bin test,” is
typically expressed in dBc, similar to expressing the relative
difference between single-tone fundamentals and second or
third harmonic distortion components. Measurements of MTPR
are typically made on the line side or secondary side of the
transformer.
In the simplified differential drive circuit shown in Figure 2, the
AD8018 is coupled to the phone line through a step-up trans-
former with a 1:4 turns ratio. R1 and R2 are back-termination
or line-matching resistors, each 3.1 Ω (100 Ω/(2 × 42)), where
100 Ω is the approximate phone line impedance. The total dif-
ferential load for the AD8018, including the termination resistors,
is 12.5 Ω. Even under these conditions the AD8018 provides low
distortion signals to within 0.5 V of the power rails.
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ADI [ ADI ]
