AD8016 datasheet(16/20 Pages) AD

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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 N

≡

′

where:

Z' is the primary side impedance as seen by the differential

driver.

Z2 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 VS = ±12 V.

Figure 45. Dynamic Headroom vs. XFMR Turns Ratio, VS = ±12 V

1.0

1.2

1.4

2.0

–1

1.6

1.8

–2

1.1

1.3

1.5

1.7

1.9

DOWNSTREAM TURNS RATIO

VS = ±12V

PWDN1, PWDN0 = (1,1)

VS = ±11.4V

PWDN1, PWDN0 = (1,1)

VS = ±12V

PWDN1, PWDN0 = (1,0)

VS = ±11.4V

PWDN1, PWDN0 = (1,0)

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