Datasheet

AD8016 Data Sheet
Rev. C | Page 16 of 20
APPLICATIONS INFORMATION
The AD8016 dual amplifier forms an integrated single-channel
ADSL line driver. The AD8016 may be applied in driving mod-
ulated signals including discrete multitone (DMT) in either
direction; upstream from CPE to the CO and downstream
from CO to CPE. The most significant thermal management
challenge lies in driving downstream information from CO sites
to the CPE. Driving xDSL information downstream suggests
the need to locate many xDSL modems in a single CO site. The
implication is that several modems will be placed onto a single
printed circuit board residing in a card cage located in a variety
of ambient conditions. Environmental conditioners such as fans
or air conditioning may or may not be available, depending on
the density of modems and the facilities contained at the CO
site. To achieve long-term reliability and consistent modem
performance, designers of CO solutions must consider the wide
array of ambient conditions that exist within various CO sites.
MULTITONE POWER RATIO (MTPR)
ADSL systems rely on discrete multitone 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. (See Figure 6
for an example of downstream DMT signals used in evaluating
MTPR performance.) A uniquely encoded, quadrature ampli-
tude modulation (QAM) signal occurs at the center frequency
of each subband or tone. Difficulties arise 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. Conventional methods of expressing the output
signal integrity of line drivers, such as spurious-free dynamic
range (SFDR), single-tone harmonic distortion or THD, two-
tone intermodulation distortion (IMD), and third-order inter-
cept (IP3) become significantly less meaningful when amplifiers
are required to drive DMT and other heavily modulated
waveforms. A typical xDSL downstream DMT signal may
contain as many as 256 carriers (subbands or tones) of QAM
signals. MTPR is the relative difference between the measured
power in a typical subband (at one tone or carrier) vs. the power
at another subband specifically 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.
See Figure 6 for a sample of the ADSL downstream spectrum
showing MTPR results while driving 20.4 dBm of power onto
a 100 Ω line. Measurements of MTPR are typically made at
the output (line side) of ADSL hybrid circuits. MTPR can be
affected by the components contained in the hybrid circuit,
including the quality of the capacitor dielectrics, voltage ratings,
and the turns ratio of the selected transformers. Other compo-
nents aside, an ADSL driver hybrid containing the AD8016 can
be optimized for the best MTPR performance by selecting the
turns ratio of the transformers. The voltage and current demands
from the differential driver changes, depending on the trans-
former turns ratio. The point on the curve indicating maximum
dynamic headroom is achieved when the differential driver
delivers both the maximum voltage and current while maintaining
the lowest possible distortion. Below this point, the driver has
reserve current-driving capability and experiences voltage
clipping. Above this point, the amplifier runs out of current
drive capability before the maximum voltage drive capability
is reached. Because a transformer reflects the secondary load
impedance back to the primary side by the square of the turns
ratio, varying the turns ratio changes the load across the
differential driver. The following equation may be used to
calculate the load impedance across the output of the differen-
tial driver, reflected by the transformers, from the line side of
the xDSL driver hybrid.
( )
2
2
2 N
Z
Z
×
≡
′
where:
Z' is the primary side impedance as seen by the differential
driver.
Z
2
is the line impedance.
N is the transformer turns ratio.
Figure 45 shows the dynamic headroom in each subband of a
downstream DMT waveform vs. turns ratio running at 100%
and 60% of the quiescent power while maintaining −65 dBc
of MTPR at V
S
= ±12 V.
Figure 45. Dynamic Headroom vs. XFMR Turns Ratio, V
S
= ±12 V
4
1.0
3
2
1.2 1.4 2.0
1
0
–1
1.6 1.8
–2
1.1 1.3 1.5 1.7 1.9
DOWNSTREAM TURNS RATIO
DYNAMIC HEADROOM (dB)
V
S
= ±12V
PWDN1, PWDN0 = (1,1)
V
S
= ±11.4V
PWDN1, PWDN0 = (1,1)
V
S
= ±12V
PWDN1, PWDN0 = (1,0)
V
S
= ±11.4V
PWDN1, PWDN0 = (1,0)
01019-045